DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

NONRESIDENTTRAININGCOURSE

July 1990

Fluid PowerNAVEDTRA 14105

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

Although the words “he,” “him,” and“his” are used sparingly in this course toenhance communication, they are notintended to be gender driven or to affront ordiscriminate against anyone.

COMMANDING OFFICERNETPDTC

6490 SAUFLEY FIELD RDPENSACOLA, FL 32509-5237

ERRATA #3 19 Oct 99

Specific Instructions and Errata

FLUID POWER

1. This errata supersedes all previous erratas. No attempt has been made toissue corrections for errors in typing, punctuation, etc., that do not affectyour ability to answer the question or questions.

2. To receive credit for deleted questions, show this errata to your localcourse administrator (ESO/scorer). The local course administrator is directedto correct the course and the answer key by indicating the question deleted.

3. Assignment Booklet

Delete the following questions, and leave the corresponding spaces blankon the answer sheets:

Questions2-62-9

2-153-5

Questions4-525-225-67

Make the following changes:

Question1-19

1-52

3-32

4-154-184-285-8

5-52/5-55

5-67

ChangeIn the questions, change the question to read "In themetric system, the density of a substance is expressedas..."In the question, line 5, "60 cubic centimeters" isequivalent to 60 milliliters.In the blurb before the question, line 2, delete "and3-33."In alternative 3, change "form" to ‘from."In the question. line 2, change "instead" to "installed."In alternative 2, change "el" to "element."In the blurb preceding the question, line 1, change "1-8"to "5-8."In the column under "COMPONENTS", in alternative 3, add"mover" after "prime."In the blurb preceding the question, line 2, change"5-71" to " 5-70."

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PREFACE

By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy.Remember, however, this self-study course is only one part of the total Navy training program. Practicalexperience, schools, selected reading, and your desire to succeed are also necessary to successfully roundout a fully meaningful training program.

COURSE OVERVIEW : In completing this nonresident training course, you will demonstrate aknowledge of the subject matter by correctly answering questions on the following: fundamental physics asappropriate to fluids at rest and in motion; types and characteristics of hydraulic and pneumatic fluids; majorcomponents of basic fluid power systems and diagrams used to illustrate these systems; proper proceduresand precautions for handling and replacing lines, connectors, and sealing devices; proper procedures foreliminating contaminants; purpose, operation, application of pumps, reservoirs, strainers, filters,accumulators, flow control and measuring devices, directional control valves, and actuators; arrangementand operation of representative fluid power systems including the function and interrelationship of majorcomponents.

THE COURSE: This self-study course is organized into subject matter areas, each containing learningobjectives to help you determine what you should learn along with text and illustrations to help youunderstand the information. The subject matter reflects day-to-day requirements and experiences ofpersonnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers(ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational ornaval standards, which are listed in theManual of Navy Enlisted Manpower Personnel Classificationsand Occupational Standards, NAVPERS 18068.

THE QUESTIONS: The questions that appear in this course are designed to help you understand thematerial in the text.

VALUE : In completing this course, you will improve your military and professional knowledge.Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you arestudying and discover a reference in the text to another publication for further information, look it up.

1990 Edition Prepared byMMC Albert Beasley, Jr.

Published byNAVAL EDUCATION AND TRAINING

PROFESSIONAL DEVELOPMENTAND TECHNOLOGY CENTER

NAVSUP Logistics Tracking Number0504-LP-026-7730

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Sailor’s Creed

“ I am a United States Sailor.

I will support and defend theConstitution of the United States ofAmerica and I will obey the ordersof those appointed over me.

I represent the fighting spirit of theNavy and those who have gonebefore me to defend freedom anddemocracy around the world.

I proudly serve my country’s Navycombat team with honor, courageand commitment.

I am committed to excellence andthe fair treatment of all.”

C O N T E N T S

CHAPTER

1. Introduction to Fluid Power.. . . . . . . . . . . . . . . . . . . . . . .

2. Forces in Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. Hydraulic Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Fluid Lines and Fittings . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6. Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7. Sealing Devices and Materials . . . . . . . . . . . . . . . . . . . . . . .

8. Measurement and Pressure Control Devices . . . . . . . . . .

9. Reservoirs, Strainers, Filters, and Accumulators . . . . . .

10. Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. Pneumatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12. Basic Diagrams and Systems . . . . . . . . . . . . . . . . . . . . . . . .

APPENDIX

I. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

II. Mechanical Symbols Other than Aeronauticalfor Fluid Power Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . .

III. Aeronautical Mechanical Symbols for FluidPower Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page

1-1

2-1

3-1

4-1

5-1

6-1

7-1

8-1

9-1

10-1

11-1

12-1

AI-1

AII-1

AIII-1

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . INDEX-1

. . .iii

CREDITS

The companies listed below have provided permission to use certaintradenames/trademarks in this edition of Fluid Power. Permission to use thesetradenames/trademarks is gratefully acknowledged. Permission to reproduceor use these tradenames/trademarks must be obtained from the source.

SOURCE TEXT ON PAGE

DuPont

Greene, Tweed and Company

Minnesota Rubber

5-8

7-5

7-15

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v

INSTRUCTIONS FOR TAKING THE COURSE

ASSIGNMENTS

The text pages that you are to study are listed atthe beginning of each assignment. Study thesepages carefully before attempting to answer thequestions. Pay close attention to tables andillustrations and read the learning objectives.The learning objectives state what you should beable to do after studying the material. Answeringthe questions correctly helps you accomplish theobjectives.

SELECTING YOUR ANSWERS

Read each question carefully, then select theBEST answer. You may refer freely to the text.The answers must be the result of your ownwork and decisions. You are prohibited fromreferring to or copying the answers of others andfrom giving answers to anyone else taking thecourse.

SUBMITTING YOUR ASSIGNMENTS

To have your assignments graded, you must beenrolled in the course with the NonresidentTraining Course Administration Branch at theNaval Education and Training ProfessionalDevelopment and Technology Center(NETPDTC). Following enrollment, there aretwo ways of having your assignments graded:(1) use the Internet to submit your assignmentsas you complete them, or (2) send all theassignments at one time by mail to NETPDTC.

Grading on the Internet: Advantages toInternet grading are:

• you may submit your answers as soon asyou complete an assignment, and

• you get your results faster; usually by thenext working day (approximately 24 hours).

In addition to receiving grade results for eachassignment, you will receive course completionconfirmation once you have completed all the

assignments. To submit your assignmentanswers via the Internet, go to:

http://courses.cnet.navy.mil

Grading by Mail: When you submit answersheets by mail, send all of your assignments atone time. Do NOT submit individual answersheets for grading. Mail all of your assignmentsin an envelope, which you either provideyourself or obtain from your nearest EducationalServices Officer (ESO). Submit answer sheetsto:

COMMANDING OFFICERNETPDTC N3316490 SAUFLEY FIELD ROADPENSACOLA FL 32559-5000

Answer Sheets: All courses include one“scannable” answer sheet for each assignment.These answer sheets are preprinted with yourSSN, name, assignment number, and coursenumber. Explanations for completing the answersheets are on the answer sheet.

Do not use answer sheet reproductions:Useonly the original answer sheets that weprovide—reproductions will not work with ourscanning equipment and cannot be processed.

Follow the instructions for marking youranswers on the answer sheet. Be sure that blocks1, 2, and 3 are filled in correctly. Thisinformation is necessary for your course to beproperly processed and for you to receive creditfor your work.

COMPLETION TIME

Courses must be completed within 12 monthsfrom the date of enrollment. This includes timerequired to resubmit failed assignments.

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PASS/FAIL ASSIGNMENT PROCEDURES

If your overall course score is 3.2 or higher, youwill pass the course and will not be required toresubmit assignments. Once your assignmentshave been graded you will receive coursecompletion confirmation.

If you receive less than a 3.2 on any assignmentand your overall course score is below 3.2, youwill be given the opportunity to resubmit failedassignments. You may resubmit failedassignments only once. Internet students willreceive notification when they have failed anassignment--they may then resubmit failedassignments on the web site. Internet studentsmay view and print results for failedassignments from the web site. Students whosubmit by mail will receive a failing result letterand a new answer sheet for resubmission of eachfailed assignment.

COMPLETION CONFIRMATION

After successfully completing this course, youwill receive a letter of completion.

ERRATA

Errata are used to correct minor errors or deleteobsolete information in a course. Errata mayalso be used to provide instructions to thestudent. If a course has an errata, it will beincluded as the first page(s) after the front cover.Errata for all courses can be accessed andviewed/downloaded at:

http://www.advancement.cnet.navy.mil

STUDENT FEEDBACK QUESTIONS

We value your suggestions, questions, andcriticisms on our courses. If you would like tocommunicate with us regarding this course, weencourage you, if possible, to use e-mail. If youwrite or fax, please use a copy of the StudentComment form that follows this page.

For subject matter questions:

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Address: COMMANDING OFFICERNETPDTC N3146490 SAUFLEY FIELD ROADPENSACOLA FL 32509-5237

For enrollment, shipping, grading, orcompletion letter questions

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Address: COMMANDING OFFICERNETPDTC N3316490 SAUFLEY FIELD ROADPENSACOLA FL 32559-5000

NAVAL RESERVE RETIREMENT CREDIT

If you are a member of the Naval Reserve, youmay earn retirement points for successfullycompleting this course, if authorized undercurrent directives governing retirement of NavalReserve personnel. For Naval Reserveretirement, this course is evaluated at 8 points.(Refer to Administrative Procedures for NavalReservists on Inactive Duty,BUPERSINST1001.39, for more information about retirementpoints.)

vii

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NETPDTC 1550/41 (Rev 4-00

CHAPTER 1

INTRODUCTION TO FLUID POWER

Fluid power is a term which was created toinclude the generation, control, and applicationof smooth, effective power of pumped orcompressed fluids (either liquids or gases) whenthis power is used to provide force and motionto mechanisms. This force and motion maybe inthe form of pushing, pulling, rotating, regulating,or driving. Fluid power includes hydraulics, whichinvolves liquids, and pneumatics, which involvesgases. Liquids and gases are similar in manyrespects. The differences are pointed out in theappropriate areas of this manual.

This manual presents many of the funda-mental concepts in the fields of hydraulics andpneumatics. It is intended as a basic reference forall personnel of the Navy whose duties andresponsibilities require them to have a knowledgeof the fundamentals of fluid power. Conse-quently, emphasis is placed primarily on thetheory of operation of typical fluid power systemsand components that have applications in navalequipment. Many applications of fluid power arepresented in this manual to illustrate the functionsand operation of different systems and com-ponents. However, these are only representativeof the many applications of fluid power in navalequipment. Individual training manuals for eachrate provide information concerning the applica-tion of fluid power to specific equipment forwhich the rating is responsible.

A brief summary of the contents of thistraining manual is given in the followingparagraphs:

Chapter 2 covers the characteristics of liquidsand the factors affecting them. It also explainsthe behavior of liquids at rest, identifies thecharacteristics of liquids in motion, and explainsthe operation of basic hydraulic components.

Chapter 3 discusses the qualities of fluidsacceptable for hydraulic systems and the types offluids used. Included are sections on safetyprecautions to follow when handling potentially

hazardous fluids, liquid contamination, andcontrol of contaminants.

Chapter 4 covers the hydraulic pump, thecomponent in the hydraulic system whichgenerates the force required for the system toperform its design function. The informationprovided covers classifications, types, operation,and construction of pumps.

Chapter 5 deals with the piping, tubing andflexible hoses, and connectors used to carry fluidsunder pressure.

Chapter 6 discusses the classification, types,and operation of valves used in the control offlow, pressure, and direction of fluids.

Chapter 7 covers the types and purposes ofsealing devices used in fluid power systems,including the different materials used in theirconstruction. Additionally, the guidelines forselecting, installing, and removing O-rings areincluded.

Chapter 8 discusses the operation of devicesused to measure and regulate the pressure of fluidsand to measure the temperature of fluids.

Chapter 9 describes the functions and typesof reservoirs, strainers, filters, and accumulators,and their uses in fluid power systems.

Chapter 10 discusses the types and operationof actuators used to transform the energygenerated by hydraulic systems into mechanicalforce and motion.

Chapter 11 deals with pneumatics. It discussesthe origin of pneumatics, the characteristics andcompressibility of gases, and the most commonlyused gases in pneumatic systems. Also, sectionsare included to cover safety precautions and thepotential hazards of compressed gases.

Chapter 12 identifies the types of diagramsencountered in fluid power systems. This chapteralso discusses how components of chapters 4, 5,6, 8, 9, and 10 are combined to form and operatetogether as a system.

A glossary of terms commonly used in fluidpower is provided in appendix I. Appendix IIprovides symbols used in aeronautical mechanical

1-1

systems, and appendix III provides symbols usedin nonaeronautical mechanical systems.

The remainder of chapter 1 is devoted to theadvantages and problems of fluid power appli-cations. Included are brief sections on the history,development, and applications of hydraulics,the states of matter.

ADVANTAGES OF FLUID POWER

and

The extensive use of hydraulics and pneuma-tics to transmit power is due to the fact thatproperly constructed fluid power systems possessa number of favorable characteristics. Theyeliminate the need for complicated systems ofgears, cams, and levers. Motion can be trans-mitted without the slack inherent in the use ofsolid machine parts. The fluids used are notsubject to breakage as are mechanical parts, andthe mechanisms are not subjected to great wear.

The different parts of a fluid power systemcan be conveniently located at widely separatedpoints, since the forces generated are rapidlytransmitted over considerable distances with smallloss. These forces can be conveyed up and downor around corners with small loss in efficiency andwithout complicated mechanisms. Very largeforces can be controlled by much smaller ones andcan be transmitted through comparatively smalllines and orifices.

If the system is well adapted to the work it isrequired to perform, and if it is not misused, itcan provide smooth, flexible, uniform actionwithout vibration, and is unaffected by variationof load. In case of an overload, an automaticrelease of pressure can be guaranteed, so that thesystem is protected against breakdown or strain.Fluid power systems can provide widely variablemotions in both rotary and straight-line trans-mission of power. The need for control by handcan be minimized. In addition, fluid powersystems are economical to operate.

The question may arise as to why hydraulicsis used in some applications and pneumatics inothers. Many factors are considered by the userand/or the manufacturer when determining whichtype of system to use in a specific application.There are no hard and fast rules to follow;however, past experience has provided somesound ideas that are usually considered when suchdecisions are made. If the application requiresspeed, a medium amount of pressure, and onlyfairly accurate control, a pneumatic system maybe used. If the application requires only a medium

amount of pressure and a more accurate control,a combination of hydraulics and pneumatics maybe used. If the application requires a great amountof pressure and/or extremely accurate control, ahydraulic system should be used.

SPECIAL PROBLEMS

The extreme flexibility of fluid power elementspresents a number of problems. Since fluids haveno shape of their own, they must be positivelyconfined throughout the entire system. Specialconsideration must be given to the structuralintegrity of the parts of a fluid power system.Strong pipes and containers must be provided.Leaks must be prevented. This is a seriousproblem with the high pressure obtained in manyfluid power installations.

The operation of the system involves constantmovement of the fluid within the lines andcomponents. This movement causes frictionwithin the fluid itself and against the containingsurfaces which, if excessive, can lead to seriouslosses in efficiency. Foreign matter must not beallowed to accumulate in the system, where it willclog small passages or score closely fitted parts.Chemical action may cause corrosion. Anyoneworking with fluid power systems must know howa fluid power system and its components operate,both in terms of the general principles commonto all physical mechanisms and of the peculiaritiesof the particular arrangement at hand.

HYDRAULICS

The word hydraulics is based on the Greekword for water, and originally covered the studyof the physical behavior of water at rest and inmotion. Use has broadened its meaning to includethe behavior of all liquids, although it is primarilyconcerned with the motion of liquids.

Hydraulics includes the manner in whichliquids act in tanks and pipes, deals with theirproperties, and explores ways to take advantageof these properties.

DEVELOPMENT OF HYDRAULICS

Although the modern development ofhydraulics is comparatively recent, the ancientswere familiar with many hydraulic principles andtheir applications. The Egyptians and the ancientpeople of Persia, India, and China conveyed water

1-2

along channels for irrigation and domesticpurposes, using dams and sluice gates to controlthe flow. The ancient Cretans had an elaborateplumbing system. Archimedes studied the laws offloating and submerged bodies. The Romansconstructed aqueducts to carry water to theircities.

After the breakup of the ancient world, therewere few new developments for many centuries.Then, over a comparatively short period,beginning near the end of the seventeenth century,Italian physicist, Evangelista Torricelle, Frenchphysicist, Edme Mariotte, and later, DanielBernoulli conducted experiments to study theelements of force in the discharge of waterthrough small openings in the sides of tanks andthrough short pipes. During the same period,Blaise Pascal, a French scientist, discovered thefundamental law for the science of hydraulics.

Pascal’s law states that increase in pressure onthe surface of a confined fluid is transmittedundiminished throughout the confining vessel orsystem (fig. 1-1). (This is the basic principle ofhydraulics and is covered in detail in chapter 2of this manual.)

For Pascal’s law to be made effective forpractical applications, it was necessary to have apiston that “fit exactly.” It was not until the latterpart of the eighteenth century that methods werefound to make these snugly fitted parts requiredin hydraulic systems. This was accomplished bythe invention of machines that were used to cutand shape the necessary closely fitted parts and,particularly, by the development of gaskets andpackings. Since that time, components such asvalves, pumps, actuating cylinders, and motorshave been developed and refined to makehydraulics one of the leading methods of trans-mitting power.

Figure 1-1.—Force transmitted through fluid.

Use of Hydraulics

The hydraulic press, invented by EnglishmanJohn Brahmah, was one of the first work-able pieces of machinery developed that usedhydraulics in its operation. It consisted of aplunger pump piped to a large cylinder and a ram.This press found wide use in England because itprovided a more effective and economical meansof applying large forces in industrial uses.

Today, hydraulic power is used to operatemany different tools and mechanisms. In agarage, a mechanic raises the end of an auto-mobile with a hydraulic jack. Dentists and barbersuse hydraulic power, through a few strokes of acontrol lever, to lift and position their chairs toa convenient working height. Hydraulic doorstopskeep heavy doors from slamming. Hydraulicbrakes have been standard equipment on auto-mobiles since the 1930s. Most automobiles areequipped with automatic transmissions that arehydraulically operated. Power steering is anotherapplication of hydraulic power. Constructionworkers depend upon hydraulic power for theoperation of various components of theirequipment. For example, the blade of a bulldozeris normally operated by hydraulic power.

During the period preceding World War II,the Navy began to apply hydraulics to navalmechanisms extensively. Since then, navalapplications have increased to the point wheremany ingenious hydraulic devices are used in thesolution of problems of gunnery, aeronautics, andnavigation. Aboard ship, hydraulic power is usedto operate such equipment as anchor windlasses,cranes, steering gear, remote control devices, andpower drives for elevating and training guns androcket launchers. Elevators on aircraft carriers usehydraulic power to transfer aircraft from thehangar deck to the flight deck and vice versa.

Hydraulics and pneumatics (chapter 11) arecombined for some applications. This combina-tion is referred to as hydropneumatics. A nexample of this combination is the lift used ingarages and service stations. Air pressure isapplied to the surface of hydraulic fluid in areservoir. The air pressure forces the hydraulicfluid to raise the lift.

STATES OF MATTER

The material that makes up the universe isknown as matter. Matter is defined as anysubstance that occupies space and has weight.

1-3

Matter exists in three states: solid, liquid, and gas;each has distinguishing characteristics. Solids havea definite volume and a definite shape; liquidshave a definite volume, but take the shape of theircontaining vessels; gases have neither a definiteshape nor a definite volume. Gases not only takethe shape of the containing vessel, but also expandand fill the vessel, regardless of its volume.Examples of the states of matter are iron, water,and air.

Matter can change from one state to another.Water is a good example. At high temperaturesit is in the gaseous state known as steam. Atmoderate temperatures it is a liquid, and at lowtemperatures it becomes ice, which is definitelya solid state. In this example, the temperature isthe dominant factor in determining the state thesubstance assumes.

Pressure is another important factor that willaffect changes in the state of matter. At pressureslower than atmospheric pressure, water will boiland thus change into steam at temperatures lowerthan 212° Fahrenheit (F). Pressure is also a criticalfactor in changing some gases to liquids or solids.Normally, when pressure and chilling are bothapplied to a gas, the gas assumes a liquid state.Liquid air, which is a mixture of oxygen andnitrogen, is produced in this manner.

In the study of fluid power, we are concernedprimarily with the properties and characteristicsof liquids and gases. However, you should keepin mind that the properties of solids also affectthe characteristics of liquids and gases. The linesand components, which are solids, enclose andcontrol the liquid or gas in their respectivesystems.

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CHAPTER 2

FORCES IN LIQUIDS

The study of liquids is divided into two mainparts: liquids at rest (hydrostatics) and liquids inmotion (hydraulics).

The effects of liquids at rest can oftenbe expressed by simple formulas. The effectsof liquids in motion are more difficult toexpress due to frictional and other factorswhose actions cannot be expressed by simplemathematics.

In chapter 1 we learned that liquids have adefinite volume but take the shape of theircontaining vessel. There are two additionalcharacteristics we must explore prior to pro-ceeding.

Liquids are almost incompressible. Forexample, if a pressure of 100 pounds per squareinch (psi) is applied to a given volume of waterthat is at atmospheric pressure, the volume willdecrease by only 0.03 percent. It would take aforce of approximately 32 tons to reduce itsvolume by 10 percent; however, when this forceis removed, the water immediately returns to itsoriginal volume. Other liquids behave in aboutthe same manner as water.

Another characteristic of a liquid is thetendency to keep its free surface level. If thesurface is not level, liquids will flow in thedirection which will tend to make the surfacelevel.

LIQUIDS AT REST

In studying fluids at rest, we are con-cerned with the transmission of force andthe factors which affect the forces in liquids.Additionally, pressure in and on liquids andfactors affecting pressure are of great im-portance.

PRESSURE AND FORCE

The terms force and pressure are usedextensively in the study of fluid power. Itis essential that we distinguish between theterms. Force means a total push or pull.It is the push or pull exerted against thetotal area of a particular surface and is expressedin pounds or grams. Pressure means the amountof push or pull (force) applied to each unit areaof the surface and is expressed in pounds persquare inch (lb/in2) or grams per squarecentimeter (gm/cm2). Pressure maybe exerted inone direction, in several directions, or in alldirections.

Computing Force, Pressure, and Area

A formula is used in computing force,pressure, and area in fluid power systems. In thisformula, P refers to pressure, F indicates force,and A represents area.

Force equals pressure times area. Thus, theformula is written

Equation 2-1.

Pressure equals force divided by area. Byrearranging the formula, this statement may becondensed into

Equation 2-2.

Since area equals force divided by pressure,the formula is written

Equation 2-3.

2-1

Figure 2-1.—Device for determining the arrangement of theforce, pressure, and area formula.

Figure 2-1 illustrates a memory device forrecalling the different variations of this formula.Any letter in the triangle may be expressed as theproduct or quotient of the other two, dependingon its position within the triangle.

For example, to find area, consider the letterA as being set off to itself, followed by an equalsign. Now look at the other two letters. The letterF is above the letter P; therefore,

NOTE: Sometimes the area may not beexpressed in square units. If the surface isrectangular, you can determine its area bymultiplying its length (say, in inches) by its width(also in inches). The majority of areas you willconsider in these calculations are circular in shape.Either the radius or the diameter may be given,but you must know the radius in inches to findthe area. The radius is one-half the diameter. Todetermine the area, use the formula for findingthe area of a circle. This is written A = whereA is the area, is 3.1416 (3.14 or 3 1/7 for mostcalculations), and r2 indicates the radius squared.

Atmospheric Pressure

The atmosphere is the entire mass of air thatsurrounds the earth. While it extends upward forabout 500 miles, the section of primary interestis the portion that rests on the earth’s surface andextends upward for about 7 1/2 miles. This layeris called the troposphere.

If a column of air 1-inch square extending allthe way to the “top” of the atmosphere couldbe weighed, this column of air would weighapproximately 14.7 pounds at sea level. Thus,atmospheric pressure at sea level is approximately14.7 psi.

As one ascends, the atmospheric pressuredecreases by approximately 1.0 psi for every 2,343feet. However, below sea level, in excavations anddepressions, atmospheric pressure increases.Pressures under water differ from those under aironly because the weight of the water must beadded to the pressure of the air.

Atmospheric pressure can be measured by anyof several methods. The common laboratorymethod uses the mercury column barometer. Theheight of the mercury column serves as anindicator of atmospheric pressure. At sea level andat a temperature of 0° Celsius (C), the height ofthe mercury column is approximately 30 inches,or 76 centimeters. This represents a pressure ofapproximately 14.7 psi. The 30-inch column isused as a reference standard.

Another device used to measure atmosphericpressure is the aneroid barometer. The aneroidbarometer uses the change in shape of anevacuated metal cell to measure variations inatmospheric pressure (fig. 2-2). The thin metal ofthe aneroid cell moves in or out with the variationof pressure on its external surface. This movementis transmitted through a system of levers to apointer, which indicates the pressure.

The atmospheric pressure does not varyuniformly with altitude. It changes more rapidlyat lower altitudes because of the compressibilityof the air, which causes the air layers close to theearth’s surface to be compressed by the air massesabove them. This effect, however, is partiallycounteracted by the contraction of the upper

Figure 2-2.—Simple diagram of the aneroid barometer.

2-2

layers due to cooling. The cooling tends toincrease the density of the air.

Atmospheric pressures are quite large, but inmost instances practically the same pressure ispresent on all sides of objects so that no singlesurface is subjected to a great load.

Atmospheric pressure acting on the surface ofa liquid (fig. 2-3, view A) is transmitted equallythroughout the liquid to the walls of the container,but is balanced by the same atmospheric pressureacting on the outer walls of the container. In viewB of figure 2-3, atmospheric pressure acting onthe surface of one piston is balanced by the samepressure acting on the surface of the other piston.The different areas of the two surfaces make nodifference, since for a unit of area, pressures arebalanced.

TRANSMISSION OF FORCESTHROUGH LIQUIDS

When the end of a solid bar is struck, the mainforce of the blow is carried straight through thebar to the other end (fig. 2-4, view A). Thishappens because the bar is rigid. The directionof the blow almost entirely determines thedirection of the transmitted force. The more rigid

Figure 2-4.—Transmission of force: (A) solid; (B) fluid.

the bar, the less force is lost inside the bar ortransmitted outward at right angles to thedirection of the blow.

When a force is applied to the end of a columnof confined liquid (fig. 2-4, view B), it istransmitted straight through to the other end andalso equally and undiminished in every directionthroughout the column—forward, backward, andsideways—so that the containing vessel is literallyfilled with pressure.

An example of this distribution of force isillustrated in figure 2-5. The flat hose takes on

Figure 2-3.—Effects of atmospheric pressure. Figure 2-5.—Distribution of force.

2-3

a circular cross section when it is filled with waterunder pressure. The outward push of the wateris equal in every direction.

So far we have explained the effects ofatmospheric pressure on liquids and how externalforces are distributed through liquids. Let us nowfocus our attention on forces generated by theweight of liquids themselves. To do this, we mustfirst discuss density, specific gravity, and Pascal’slaw.

Density and Specific Gravity

The density of a substance is its weight per unitvolume. The unit volume in the English systemof measurement is 1 cubic foot. In the metricsystem it is the cubic centimeter; therefore, densityis expressed in pounds per cubic foot or in gramsper cubic centimeter.

To find the density of a substance, you mustknow its weight and volume. You then divide itsweight by its volume to find the weight per unitvolume. In equation form, this is written as

Equation 2-4.

EXAMPLE: The liquid that fills a certaincontainer weighs 1,497.6 pounds. Thecontainer is 4 feet long, 3 feet wide, and2 feet deep. Its volume is 24 cubic feet(4 ft x 3 ft x 2 ft). If 24 cubic feet of thisliquid weighs 1,497.6 pounds, then 1 cubicfoot weighs

or 62.4 pounds. Therefore, the density ofthe liquid is 62.4 pounds per cubic foot.

This is the density of water at 4°C and isusually used as the standard for comparingdensities of other substances. The temperature of4°C was selected because water has its maximumdensity at this temperature. In the metric system,the density of water is 1 gram per cubiccentimeter. The standard temperature of 4°C isused whenever the density of liquids and solidsis measured. Changes in temperature will notchange the weight of a substance but will changethe volume of the substance by expansion orcontraction, thus changing the weight per unitvolume.

In physics, the word specific implies a ratio.Weight is the measure of the earth’s attraction fora body. The earth’s attraction for a body is calledgravity. Thus, the ratio of the weight of a unitvolume of some substance to the weight of anequal volume of a standard substance, measuredunder standard pressure and temperature con-ditions, is called specific gravity. The termsspecific weight and specific density are sometimesused to express this ratio.

The following formulas are used to find thespecific gravity (sp gr) of solids and liquids, withwater used as the standard substance.

or,

The same formulas are used to find the specificgravity of gases by substituting air, oxygen, orhydrogen for water.

If a cubic foot of a certain liquid weighs 68.64pounds, then its specific gravity is 1.1,

Thus, the specific gravity of the liquid is theratio of its density to the density of water. If thespecific gravity of a liquid or solid is known, thedensity of the liquid or solid maybe obtained bymultiplying its specific gravity by the density ofwater. For example, if a certain hydraulic liquidhas a specific gravity of 0.8, 1 cubic foot of theliquid weighs 0.8 times as much as a cubic footof water—0.8 times 62.4, or 49.92 pounds. In themetric system, 1 cubic centimeter of a substancewith a specific gravity of 0.8 weighs 1 times 0.8,or 0.8 grams. (Note that in the metric system thespecific gravity of a liquid or solid has the samenumerical value as its density, because waterweighs 1 gram per cubic centimeter.)

Specific gravity and density are independentof the size of the sample under consideration anddepend only on the substance of which it is made.

A device called a hydrometer is used formeasuring the specific gravity of liquids.

2-4

Pascal’s Law

Recall from chapter 1 that the foundation ofmodern hydraulics was established when Pascaldiscovered that pressure in a fluid acts equally inall directions. This pressure acts at right anglesto the containing surfaces. If some type ofpressure gauge, with an exposed face, is placedbeneath the surface of a liquid (fig. 2-6) at aspecific depth and pointed in different directions,the pressure will read the same. Thus, we can saythat pressure in a liquid is independent ofdirection.

Pressure due to the weight of a liquid, at anylevel, depends on the depth of the fluid from thesurface. If the exposed face of the pressure gauges,figure 2-6, are moved closer to the surface of theliquid, the indicated pressure will be less. Whenthe depth is doubled, the indicated pressure isdoubled. Thus the pressure in a liquid is directlyproportional to the depth.

Consider a container with vertical sides(fig. 2-7) that is 1 foot long and 1 foot wide. Letit be filled with water 1 foot deep, providing 1cubic foot of water. We learned earlier in thischapter that 1 cubic foot of water weighs 62.4pounds. Using this information and equation 2-2,P = F/A, we can calculate the pressure on thebottom of the container.

Since there are 144 square inches in 1 square foot,

This can be stated as follows: the weight of acolumn of water 1 foot high, having a cross-sectional area of 1 square inch, is 0.433 pound.

If the depth of the column is tripled, theweight of the column will be 3 x 0.433, or 1.299pounds, and the pressure at the bottom will be1.299 lb/in2 (psi), since pressure equals the forcedivided by the area. Thus, the pressure at anydepth in a liquid is equal to the weight of thecolumn of liquid at that depth divided by the

Figure 2-6.—Pressure of a liquid is independent of direction.

cross-sectional area of the column at that depth.The volume of a liquid that produces the pressureis referred to as the fluid head of the liquid. Thepressure of a liquid due to its fluid head is alsodependent on the density of the liquid.

If we let A equal any cross-sectional area ofa liquid column and h equal the depth of thecolumn, the volume becomes Ah. Using equation2-4, D = W/V, the weight of the liquid above areaA is equal to AhD.

Figure 2-7.—Water pressure in a 1-cubic-foot container.

2-5

Since pressure is equal to the force per unit area,set A equal to 1. Then the formula pressurebecomes

P = h D Equation 2-5.

It is essential that h and D be expressed in similarunits. That is, if D is expressed in pounds percubic foot, the value of h must be expressed infeet. If the desired pressure is to be expressed inpounds per square inch, the pressure formula,equation 2-5, becomes

Equation 2-6.

Pascal was also the first to prove byexperiment that the shape and volume of acontainer in no way alters pressure. Thus in figure2-8, if the pressure due to the weight of the liquidat a point on horizontal line H is 8 psi, thepressure is 8 psi everywhere at level H in thesystem. Equation 2-5 also shows that the pressureis independent of the shape and volume of acontainer.

Pressure and Force in Fluid Power Systems

Figure 2-9.—Force transmitted through fluid.

of the shape of the container. Consider the effectof this in the system shown in figure 2-9. If thereis a resistance on the output piston and the inputpiston is pushed downward, a pressure is createdthrough the fluid, which acts equally at rightangles to surfaces in all parts of the container.

If force 1 is 100 pounds and the area of theinput piston is 10 square inches, then the pressurein the fluid is 10 psi

Recall that, according to Pascal’s law, anyforce applied to a confined fluid is transmittedin all directions throughout the fluid regardless

NOTE: Fluid pressure cannot be createdwithout resistance to flow. In this case, resistance

Figure 2-8.—Pressure relationship

2-6

with shape.

is provided by the equipment to which theoutput piston is attached. The force of re-sistance acts against the top of the outputpiston. The pressure created in the systemby the input piston pushes on the underside ofthe output piston with a force of 10 pounds oneach square inch.

In this case, the fluid column has a uniformcross section, so the area of the output pistonis the same as the area of the input piston,or 10 square inches. Therefore, the upwardforce on the output piston is 100 pounds(10 psi x 10 sq. in.), the same as the force appliedto the input piston. All that was accomplished inthis system was to transmit the 100-pound forcearound the bend. However, this principle under-lies practically all mechanical applications of fluidpower.

At this point you should note that sincePascal’s law is independent of the shape ofthe container, it is not necessary that thetube connecting the two pistons have the samecross-sectional area of the pistons. A connectionof any size, shape, or length will do, as long asan unobstructed passage is provided. Therefore,the system shown in figure 2-10, with a relativelysmall, bent pipe connecting two cylinders,will act exactly the same as the system shown infigure 2-9.

MULTIPLICATION OF FORCES.— Con-sider the situation in figure 2-11, where the inputpiston is much smaller than the output piston.Assume that the area of the input piston is 2square inches. With a resistant force on the outputpiston a downward force of 20 pounds acting on

the input piston creates a pressure of or 10 psi

Figure 2-10.—Transmitting force through a small pipe.

Figure 2-11.—Multiplication of forces.

in the fluid. Although this force is much smallerthan the force applied in figures 2-9 and 2-10, thepressure is the same. This is because the force isapplied to a smaller area.

This pressure of 10 psi acts on all parts of thefluid container, including the bottom of theoutput piston. The upward force on the outputpiston is 200 pounds (10 pounds of pressure oneach square inch). In this case, the original forcehas been multiplied tenfold while using the samepressure in the fluid as before. In any system withthese dimensions, the ratio of output force toinput force is always ten to one, regardless of theapplied force. For example, if the applied forceof the input piston is 50 pounds, the pressure inthe system will be 25 psi. This will support aresistant force of 500 pounds on the output piston.

The system works the same in reverse. If wechange the applied force and place a 200-poundforce on the output piston (fig. 2-11), making itthe input piston, the output force on the inputpiston will be one-tenth the input force, or 20pounds. (Sometimes such results are desired.)Therefore, if two pistons are used in a fluid powersystem, the force acting on each piston is directlyproportional to its area, and the magnitude ofeach force is the product of the pressure and thearea of each piston.

Note the white arrows at the bottom of figure2-11 that indicate up and down movement. Themovement they represent will be explained laterin the discussion of volume and distance factors.

2-7

DIFFERENTIAL AREAS.— Consider thespecial situation shown in figure 2-12. Here, asingle piston (1) in a cylinder (2) has a piston rod(3) attached to one of its sides. The piston rodextends out of one end of the cylinder. Fluid underpressure is admitted equally to both ends of thecylinder. The opposed faces of the piston (1)behave like two pistons acting against each other.The area of one face is the full cross-sectional areaof the cylinder, say 6 square inches, while the areaof the other face is the area of the cylinder minusthe area of the piston rod, which is 2 squareinches. This leaves an effective area of 4 squareinches on the right face of the piston. The pressureon both faces is the same, in this case, 20 psi.Applying the rule just stated, the force pushingthe piston to the right is its area times the pressure,or 120 pounds (20 x 6). Likewise, the forcepushing the piston to the left is its area times thepressure, or 80 pounds (20 x 4). Therefore, thereis a net unbalanced force of 40 pounds acting tothe right, and the piston will move in thatdirection. The net effect is the same as if the pistonand the cylinder had the same cross-sectional areaas the piston rod.

VOLUME AND DISTANCE FACTORS.—You have learned that if a force is applied to asystem and the cross-sectional areas of the inputand output pistons are equal, as in figures 2-9 and2-10, the force on the input piston will support

an equal resistant force on the output piston. Thepressure of the liquid at this point is equal to theforce applied to the input piston divided by thepiston’s area. Let us now look at what happenswhen a force greater than the resistance is appliedto the input piston.

In the system illustrated in figure 2-9, assumethat the resistance force on the output piston is100 psi. If a force slightly greater than 100 poundsis applied to the input piston, the pressure in thesystem will be slightly greater than 10 psi. Thisincrease in pressure will overcome the resistanceforce on the output piston. Assume that the inputpiston is forced downward 1 inch. The movementdisplaces 10 cubic inches of fluid. The fluid mustgo somewhere. Since the system is closed and thefluid is practically incompressible, the fluid willmove to the right side of the system. Because theoutput piston also has a cross-sectional area of10 square inches, it will move 1 inch upward toaccommodate the 10 cubic inches of fluid. Youmay generalize this by saying that if two pistonsin a closed system have equal cross-sectional areasand one piston is pushed and moved, the otherpiston will move the same distance, though in theopposite direction. This is because a decrease involume in one part of the system is balanced byone equal increase in volume in another part ofthe system.

Apply this reasoning to the system in figure2-11. If the input piston is pushed down a distance

Figure 2-12.—Differential areas on a piston.

2-8

of 1 inch, the volume of fluid in the left cylinderwill decrease by 2 cubic inches. At the same time,the volume in the right cylinder will increase by2 cubic inches. Since the diameter of the rightcylinder cannot change, the piston must moveupward to allow the volume to increase. Thepiston will move a distance equal to the volumeincrease divided by the surface area of the piston(equal to the surface area of the cylinder). In thisexample, the piston will move one-tenth of an inch(2 cu. in. ÷ 20 sq. in.). This leads to the secondbasic rule for a fluid power system that containstwo pistons: The distances the pistons move areinversely proportional to the areas of the pistons.Or more simply, if one piston is smaller than theother, the smaller piston must move a greaterdistance than the larger piston any time the pistonsmove.

LIQUIDS IN MOTION

In the operation of fluid power systems, theremust be a flow of fluid. The amount of flow willvary from system to system. To understand fluidpower systems in action, it is necessary tounderstand some of the characteristics of liquidsin motion.

Liquids in motion have characteristics dif-ferent from liquids at rest. Frictional resistanceswithin a fluid (viscosity) and inertia contribute tothese differences. (Viscosity is discussed in chapter3.) Inertia, which means the resistance a massoffers to being set in motion, will be discussedlater in this section. There are other relationshipsof liquids in motion with which you must becomefamiliar. Among these are volume and velocityof flow, flow rate and speed, laminar andturbulent flow, and more importantly, the forceand energy changes which occur in flow.

VOLUME AND VELOCITY OF FLOW

The volume of a liquid passing a point in agiven time is known as its volume of flow or flowrate. The volume of flow is usually expressed ingallons per minute (gpm) and is associated withrelative pressures of the liquid, such as 5 gpm at40 psi.

The velocity of flow or velocity of the fluidis defined as the average speed at which the fluidmoves past a given point. It is usually expressedin feet per second (fps) or feet per minute (fpm).Velocity of flow is an important consideration insizing the hydraulic lines. (Hydraulic lines arediscussed in chapter 5.)

Volume and velocity of flow are oftenconsidered together. With other conditionsunaltered—that is, with volume of inputunchanged—the velocity of flow increases as thecross section or size of the pipe decreases, and thevelocity of flow decreases as the cross sectionincreases. For example, the velocity of flow is slowat wide parts of a stream and rapid at narrowparts, yet the volume of water passing each partof the stream is the same.

In figure 2-13, if the cross-sectional area ofthe pipe is 16 square inches at point A and 4square inches at point B, we can calculate therelative velocity of flow using the flow equation

Q = v A Equation 2-7.

where Q is the volume of flow, v is the velocityof flow and A is the cross-sectional area of theliquid. Since the volume of flow at point A, Q1,is equal to the volume of flow at point B, Q2, wecan use equation 2-7 to determine the ratio of the

Figure 2-13.—Volume and velocity of flow.

2-9

velocity of flow at point A, v1, to the velocity offlow at point B, v2.

Since Q1 = Q2, A1v 1 = A2v 2

From figure 2-13; A1 = 16sq. in., A2 = 4sq. in.

Substituting: 16v1 = 4V 2 or v2 = 4vI

Therefore, the velocity of flow at point B is fourtimes the velocity of flow at point A.

VOLUME OF FLOW AND SPEED

If you consider the cylinder volume you mustfill and the distance the piston must travel, youcan relate the volume of flow to the speed of thepiston. The volume of the cylinder is found bymultiplying the piston area by the length the pistonmust travel (stroke).

Suppose you have determined that twocylinders have the same volume and that onecylinder is twice as long as the other. In this case,the cross-sectional area of the longer tube will behalf of the cross-sectional area of the other tube.If fluid is pumped into each cylinder at the samerate, both pistons will reach their full travel at thesame time. However, the piston in the smallercylinder must travel twice as fast because it hastwice as far to go.

There are two ways of controlling the speedof the piston, (1) by varying the size of the cylinderand (2) by varying the volume of flow (gpm) tothe cylinders. (Hydraulic cylinders are discussedin detail in chapter 10. )

STREAMLINE ANDTURBULENT FLOW

At low velocities or in tubes of small diameter,flow is streamlined. This means that a givenparticle of fluid moves straight forward withoutbumping into other particles and without crossingtheir paths. Streamline flow is often referred toas laminar flow, which is defined as a flowsituation in which fluid moves in parallel laminaor layers. As an example of streamline flow,consider figure 2-14, which illustrates an openstream flowing at a slow, uniform rate with logsfloating on its surface. The logs represent particlesof fluid. As long as the stream flows at a slow,uniform rate, each log floats downstream in its

Figure 2-14.—Streamline flow.

own path, without crossing or bumping into theother.

If the stream narrows, however, and thevolume of flow remains the same, the velocityof flow increases. If the velocity increasessufficiently, the water becomes turbulent. (Seefig. 2-15.) Swirls, eddies, and cross-motions areset up in the water. As this happens, the logs arethrown against each other and against the banksof the stream, and the paths followed by differentlogs will cross and recross.

Particles of fluid flowing in pipes act in thesame manner. The flow is streamlined if the fluidflows slowly enough, and remains streamlined atgreater velocities if the diameter of the pipe issmall. If the velocity of flow or size of pipe isincreased sufficiently, the flow becomes turbulent.

While a high velocity of flow will produceturbulence in any pipe, other factors contributeto turbulence. Among these are the roughness ofthe inside of the pipe, obstructions, the degree ofcurvature of bends, and the number of bends inthe pipe. In setting up or maintaining fluid powersystems, care should be taken to eliminate or

Figure 2-15.—Turbulent flow.

2-10

minimize as many causes of turbulence aspossible, since the energy consumed by turbulenceis wasted. Limitations related to the degreeand number of bends of pipe are discussed inchapter 5.

While designers of fluid power equipment dowhat they can to minimize turbulence, it cannotbe avoided. For example, in a 4-inch pipe at 68°F,flow becomes turbulent at velocities over approxi-mately 6 inches per second or about 3 inches persecond in a 6-inch pipe. These velocities are farbelow those commonly encountered in fluid powersystems, where velocities of 5 feet per second andabove are common. In streamlined flow, lossesdue to friction increase directly with velocity. Withturbulent flow these losses increase much morerapidly.

FACTORS INVOLVED IN FLOW

An understanding of the behavior of fluids inmotion, or solids for that matter, requires anunderstanding of the term inertia. Inertia is theterm used by scientists to describe the propertypossessed by all forms of matter that makes thematter resist being moved if it is at rest, andlikewise, resist any change in its rate of motionif it is moving.

The basic statement covering inertia isNewton’s first law of motion—inertia. Sir IsaacNewton was a British philosopher and mathe-matician. His first law states: A body at rest tendsto remain at rest, and a body in motion tends toremain in motion at the same speed and direction,unless acted on by some unbalanced force.This simply says what you have learned byexperience—that you must push an object to startit moving and push it in the opposite directionto stop it again.

A familiar illustration is the effort a pitchermust exert to make a fast pitch and the oppositionthe catcher must put forth to stop the ball.Similarly, considerable work must be performedby the engine to make an automobile beginto roll; although, after it has attained a certainvelocity, it will roll along the road at uniformspeed if just enough effort is expended toovercome friction, while brakes are necessary tostop its motion. Inertia also explains the kick orrecoil of guns and the tremendous striking forceof projectiles.

Inertia

To

and Force

overcome the tendency of an object toresist any change in its state of rest or motion,some force that is not otherwise canceled orunbalanced must act on the object. Someunbalanced force must be applied whenever fluidsare set in motion or increased in velocity; whileconversely, forces are made to do work elsewherewhenever fluids in motion are retarded orstopped.

There is a direct relationship between themagnitude of the force exerted and the inertiaagainst which it acts. This force is dependenton two factors: (1) the mass of the object(which is proportional to its weight), and (2)the rate at which the velocity of the objectis changed. The rule is that the force inpounds required to overcome inertia is equalto the weight of the object multiplied by thechange in velocity, measured in feet per second,and divided by 32 times the time in secondsrequired to accomplish the change. Thus, the rateof change in velocity of an object is proportionalto the force applied. The number 32 appearsbecause it is the conversion factor between weightand mass.

There are five physical factors that can act ona fluid to affect its behavior. All of the physicalactions of fluids in all systems are determined bythe relationships of these five factors to eachother. Summarizing, these five factors are asfollows:

1. Gravity, which acts at all times on allbodies, regardless of other forces

2. Atmospheric pressure, which acts onany part of a system exposed to the openair

3. Specific applied forces, which mayor maynot be present, but which, in any event, areentirely independent of the presence or absenceof motion

4. Inertia, which comes into play wheneverthere is a change from rest to motion or theopposite, or whenever there is a change indirection or in rate of motion

5. Friction, which is always present wheneverthere is motion

2-11

Figure 2-16 illustrates a possible relationshipof these factors with respect to a particle of fluid(P) in a system. The different forces are shownin terms of head, or in other words, in terms ofvertical columns of fluid required to providethe forces. At the particular moment underconsideration, a particle of water (P) is being actedon by applied force (A), by atmospheric pressure(B), and by gravity (C) produced by the weightof the fluid standing over it. The particle possessessufficient inertia or velocity head to rise to levelP1, since head equivalent to F was lost in frictionas P passed through the system. Since atmosphericpressure (B) acts downward on both sides of thesystem, what is gained on one side is lost on theother.

If all the pressure acting on P to force itthrough the nozzle could be recovered in the formof elevation head, it would rise to level Y. Ifaccount is taken of the balance in atmosphericpressure, in a frictionless system, P would rise tolevel X, or precisely as high as the sum of thegravity head and the head equivalent to theapplied force.

Kinetic Energy

It was previously pointed out that a force mustbe applied to an object in order to give it a velocity

or to increase the velocity it already has. Whetherthe force begins or changes velocity, it acts overa certain distance. A force acting over a certaindistance is work. Work and all forms into whichit can be changed are classified as energy.Obviously then, energy is required to give anobject velocity. The greater the energy used, thegreater the velocity will be.

Disregarding friction, for an object to bebrought to rest or for its motion to be sloweddown, a force opposed to its motion must beapplied to it. This force also acts over somedistance. In this way energy is given up by theobject and delivered in some form to whateveropposes its continuous motion. The moving objectis therefore a means of receiving energy at oneplace (where its motion is increased) and deliveringit to another point (where it is stopped orretarded). While it is in motion, it is said tocontain this energy as energy of motion or kineticenergy.

Since energy can never be destroyed, it followsthat if friction is disregarded the energy deliveredto stop the object will exactly equal the energythat was required to increase its speed. At all timesthe amount of kinetic energy possessed by anobject depends on its weight and the velocity atwhich it is moving.

Figure 2-16.—Physical factors governing fluid flow.

2-12

The mathematical relationship for kineticenergy is stated in the rule: “Kinetic energy infoot-pounds is equal to the force in pounds whichcreated it, multiplied by the distance throughwhich it was applied, or to the weight of themoving object in pounds, multiplied by the squareof its velocity in feet per second, and divided by64.s”

The relationship between inertia forces,velocity, and kinetic energy can be illustrated byanalyzing what happens when a gun fires aprojectile against the armor of an enemy ship. (Seefig. 2-17.) The explosive force of the powder inthe breach pushes the projectile out of the gun,giving it a high velocity. Because of its inertia,the projectile offers opposition to this suddenvelocity and a reaction is set up that pushes thegun backward (kick or recoil). The force of theexplosion acts on the projectile throughout itsmovement in the gun. This is force acting througha distance producing work. This work appears askinetic energy in the speeding projectile. Theresistance of the air produces friction, which usessome of the energy and slows down the projectile.Eventually, however, the projectile hits its targetand, because of the inertia, tries to continuemoving. The target, being relatively stationary,tends to remain stationary because of its inertia.The result is that a tremendous force is set up thateither leads to the penetration of the armor orthe shattering of the projectile. The projectileis simply a means of transferring energy, inthis instance for destructive purpose, from thegun to the enemy ship. This energy is transmittedin the form of energy of motion or kineticenergy.

A similar action takes place in a fluid powersystem in which the fluid takes the place of theprojectile. For example, the pump in a hydraulic

Figure 2-17.—Relationship of inertia, velocity, and kineticenergy.

system imparts energy to the fluid, whichovercomes the inertia of the fluid at rest andcauses it to flow through the lines. The fluid flowsagainst some type of actuator that is at rest. Thefluid tends to continue flowing, overcomes theinertia of the actuator, and moves the actuatorto do work. Friction uses up a portion of theenergy as the fluid flows through the lines andcomponents.

RELATIONSHIP OF FORCE,PRESSURE, AND HEAD

In dealing with fluids, forces are usuallyconsidered in relation to the areas over which theyare applied. As previously discussed, a forceacting over a unit area is a pressure, and pressurecan alternately be stated in pounds per square inchor in terms of head, which is the vertical heightof the column of fluid whose weight wouldproduce that pressure.

In most of the applications of fluid power inthe Navy, applied forces greatly outweigh all otherforces, and the fluid is entirely confined. Underthese circumstances it is customary to think of theforces involved in terms of pressures. Since theterm head is encountered frequently in the studyof fluid power, it is necessary to understand whatit means and how it is related to pressure andforce.

All five of the factors that control the actionsof fluids can, of course, be expressed either asforce, or in terms of equivalent pressures or head.In each situation, the different factors are referredto in the same terms, since they can be added andsubtracted to study their relationship to eachother.

At this point you need to review some termsin general use. Gravity head, when it is importantenough to be considered, is sometimes referredto as head. The effect of atmospheric pressure isreferred to as atmospheric pressure. (Atmosphericpressure is frequently and improperly referred toas suction.) Inertia effect, because it is alwaysdirectly related to velocity, is usually calledvelocity head; and friction, because it representsa loss of pressure or head, is usually referred toas friction head.

STATIC AND DYNAMIC FACTORS

Gravity, applied forces, and atmosphericpressure are static factors that apply equally to

2-13

fluids at rest or in motion, while inertia andfriction are dynamic factors that apply only tofluids in motion. The mathematical sum ofgravity, applied force, and atmospheric pressureis the static pressure obtained at any one pointin a fluid at any given time. Static pressure existsin addition to any dynamic factors that may alsobe present at the same time.

Remember, Pascal’s law states that a pressureset up in a fluid acts equally in all directions andat right angles to the containing surfaces. Thiscovers the situation only for fluids at rest orpractically at rest. It is true only for the factorsmaking up static head. Obviously, when velocitybecomes a factor it must have a direction, andas previously explained, the force related to thevelocity must also have a direction, so thatPascal’s law alone does not apply to the dynamicfactors of fluid power.

The dynamic factors of inertia and friction arerelated to the static factors. Velocity head andfriction head are obtained at the expense of statichead. However, a portion of the velocity head canalways be reconverted to static head. Force, whichcan be produced by pressure or head when dealingwith fluids, is necessary to start a body movingif it is at rest, and is present in some form whenthe motion of the body is arrested; therefore,whenever a fluid is given velocity, some part ofits original static head is used to impart thisvelocity, which then exists as velocity head.

BERNOULLI’S PRINCIPLE

Consider the system illustrated in figure 2-18.Chamber A is under pressure and is connected bya tube to chamber B, which is also under pressure.The pressure in chamber A is static pressure of100 psi. The pressure at some point (X) along theconnecting tube consists of a velocity pressure of

Figure 2-18.—Relation of static and dynamic factors—Bernoulli’s principle.

10 psi exerted in a direction parallel to the lineof flow, plus the unused static pressure of 90 psi,which still obeys Pascal’s law and operates equallyin all directions. As the fluid enters chamber Bit is slowed down, and its velocity is changed backto pressure. The force required to absorb itsinertia equals the force required to start the fluidmoving originally, so that the static pressure inchamber B is equal to that in chamber A.

This situation (fig. 2-18) disregards friction;therefore, it would not be encountered in actualpractice. Force or head is also required toovercome friction but, unlike inertia effect, thisforce cannot be recovered again, although theenergy represented still exists somewhere as heat.Therefore, in an actual system the pressure inchamber B would be less than in chamber A bythe amount of pressure used in overcomingfriction along the way.

At all points in a system the static pressure isalways the original static pressure, less any velocityhead at the point in question and less the frictionhead consumed in reaching that point. Since boththe velocity head and the friction head representenergy that came from the original static head,and since energy cannot be destroyed, the sum ofthe static head, the velocity head, and the frictionhead at any point in the system must add up tothe original static head. This is known asBernoulli's principle, which states: For thehorizontal flow of fluid through a tube, the sumof the pressure and the kinetic energy per unitvolume of the fluid is constant. This principlegoverns the relations of the static and dynamicfactors concerning fluids, while Pascal’s law statesthe manner in which the static factors behavewhen taken by themselves.

MINIMIZING FRICTION

Fluid power equipment is designed to reducefriction to the lowest possible level. Volume andvelocity of flow are made the subject of carefulstudy. The proper fluid for the system is chosen.Clean, smooth pipe of the best dimensions for theparticular conditions is used, and it is installedalong as direct a route as possible. Sharp bendsand sudden changes in cross-sectional areas areavoided. Valves, gauges, and other componentsare designed to interrupt flow as little as possible.Careful thought is given to the size and shape ofthe openings. The systems are designed so they

2-14

can be kept clean inside and variations fromnormal operation can easily be detected andremedied.

OPERATION OF HYDRAULICCOMPONENTS

To transmit and control power throughpressurized fluids, an arrangement of inter-connected components is required. Such anarrangement is commonly referred to as a system.The number and arrangement of the componentsvary from system to system, depending on theparticular application. In many applications, onemain system supplies power to several subsystems,which are sometimes referred to as circuits. Thecomplete system may be a small compact unit;more often, however, the components are locatedat widely separated points for convenient controland operation of the system.

The basic components of a fluid power systemare essentially the same, regardless of whether thesystem uses a hydraulic or a pneumatic medium.There are five basic components used in a system.These basic components are as follows:

1.

2.

3.

4.

5.

Reservoir or receiver

Pump or compressor

Lines (pipe, tubing, or flexible hose)

Directional control valve

Actuating device

Several applications of fluid power requireonly a simple system; that is, a system which usesonly a few components in addition to the fivebasic components. A few of these applications arepresented in the following paragraphs. We willexplain the operation of these systems briefly atthis time so you will know the purpose of eachcomponent and can better understand howhydraulics is used in the operation of thesesystems. More complex fluid power systems aredescribed in chapter 12.

HYDRAULIC JACK

The hydraulic jack is perhaps one of thesimplest forms of a fluid power system. Bymoving the handle of a small device, an individual

can lift a load weighing several tons. A smallinitial force exerted on the handle is transmittedby a fluid to a much larger area. To understandthis better, study figure 2-19. The small inputpiston has an area of 5 square inches and isdirectly connected to a large cylinder with anoutput piston having an area of 250 square inches.The top of this piston forms a lift platform.

If a force of 25 pounds is applied to the inputpiston, it produces a pressure of 5 psi in the fluid,that is, of course, if a sufficient amount ofresistant force is acting against the top of theoutput piston. Disregarding friction loss, thispressure acting on the 250 square inch area of theoutput piston will support a resistance force of1,250 pounds. In other words, this pressure couldovercome a force of slightly under 1,250 pounds.An input force of 25 pounds has been transformedinto a working force of more than half a ton;however, for this to be true, the distance traveledby the input piston must be 50 times greater thanthe distance traveled by the output piston. Thus,for every inch that the input piston moves, theoutput piston will move only one-fiftieth of ani n c h .

This would be ideal if the output piston neededto move only a short distance. However, in mostinstances, the output piston would have to becapable of moving a greater distance to serve apractical application. The device shown in figure2-19 is not capable of moving the output pistonfarther than that shown; therefore, some othermeans must be used to raise the output piston toa greater height.

Figure 2-19.—Hydraulic jack.

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The output piston can be raised higher andmaintained at this height if additional componentsare installed as shown in figure 2-20. In thisillustration the jack is designed so that it can beraised, lowered, or held at a constant height.These results are attained by introducing a numberof valves and also a reserve supply of fluid to beused in the system.

Notice that this system contains the five basiccomponents—the reservoir; cylinder 1, whichserves as a pump; valve 3, which serves as adirectional control valve; cylinder 2, which servesas the actuating device; and lines to transmit thefluid to and from the different components. Inaddition, this system contains two valves, 1 and2, whose functions are explained in the followingdiscussion.

As the input piston is raised (fig. 2-20, viewA), valve 1 is closed by the back pressure fromthe weight of the output piston. At the same time,valve 2 is opened by the head of the fluid in thereservoir. This forces fluid into cylinder 1. Whenthe input piston is lowered (fig. 2-20, view B), apressure is developed in cylinder 1. When thispressure exceeds the head in the reservoir, it closesvalve 2. When it exceeds the back pressure fromthe output piston, it opens valve 1, forcing fluidinto the pipeline. The pressure from cylinder 1 is

Figure 2-20.—Hydraulic jack; (A) up stroke; (B) downstroke.

thus transmitted into cylinder 2, where it acts toraise the output piston with its attached liftplatform. When the input piston is again raised,the pressure in cylinder 1 drops below that incylinder 2, causing valve 1 to close. This preventsthe return of fluid and holds the output pistonwith its attached lift platform at its new level.During this stroke, valve 2 opens again allowinga new supply of fluid into cylinder 1 for the nextpower (downward) stroke of the input piston.Thus, by repeated strokes of the input piston, thelift platform can be progressively raised. To lowerthe lift platform, valve 3 is opened, and the fluidfrom cylinder 2 is returned to the reservoir.

HYDRAULIC BRAKES

The hydraulic brake system used in theautomobile is a multiple piston system. A multiplepiston system allows forces to be transmitted totwo or more pistons in the manner indicated infigure 2-21. Note that the pressure set up by theforce applied to the input piston (1) is transmittedundiminished to both output pistons (2 and 3),and that the resultant force on each piston isproportional to its area. The multiplication offorces from the input piston to each output pistonis the same as that explained earlier.

The hydraulic brake system from the mastercylinders to the wheel cylinders on most

Figure 2-21.—Multiple piston system.

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automobiles operates in a way similar to thesystem illustrated in figure 2-22.

When the brake pedal is depressed, thepressure on the brake pedal moves the pistonwithin the master cylinder, forcing the brake fluidfrom the master cylinder through the tubing andflexible hose to the wheel cylinders. The wheelcylinders contain two opposed output pistons,each of which is attached to a brake shoe fittedinside the brake drum. Each output piston pushesthe attached brake shoe against the wall of thebrake drum, thus retarding the rotation of thewheel. When pressure on the pedal is released, thesprings on the brake shoes return the wheel

cylinder pistons to their released positions. Thisaction forces the displaced brake fluid backthrough the flexible hose and tubing to the mastercylinder.

The force applied to the brake pedal producesa proportional force on each of the outputpistons, which in turn apply the brake shoesfrictionally to the turning wheels to retardrotation.

As previously mentioned, the hydraulic brakesystem on most automobiles operates in a similarway, as shown in figure 2-22. It is beyond thescope of this manual to discuss the various brakesystems.

Figure 2-22.—An automobile brake system.

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CHAPTER 3

HYDRAULIC FLUIDS

During the design of equipment that requiresfluid power, many factors are considered inselecting the type of system to be used—hydraulic,pneumatic, or a combination of the two. Someof the factors are required speed and accuracy ofoperation, surrounding atmospheric conditions,economic conditions, availability of replacementfluid, required pressure level, operating tempera-ture range, contamination possibilities, cost oftransmission lines, limitations of the equipment,lubricity, safety to the operators, and expectedservice life of the equipment.

After the type of system has been selected,many of these same factors must be consideredin selecting the fluid for the system. This chapteris devoted to hydraulic fluids. Included in it aresections on the properties and characteristicsdesired of hydraulic fluids; types of hydraulicfluids; hazards and safety precautions for workingwith, handling, and disposing of hydraulicliquids; types and control of contamination; andsampling.

PROPERTIES

If fluidity (the physical property of a substancethat enables it to flow) and incompressibility werethe only properties required, any liquid not toothick might be used in a hydraulic system.However, a satisfactory liquid for a particularsystem must possess a number of other properties.The most important properties and some charac-teristics are discussed in the following paragraphs.

VISCOSITY

Viscosity is one of the most importantproperties of hydraulic fluids. It is a measure ofa fluid’s resistance to flow. A liquid, such asgasoline, which flows easily has a low viscosity;

and a liquid, such as tar, which flows slowly hasa high viscosity. The viscosity of a liquid isaffected by changes in temperature and pressure.As the temperature of a liquid increases, itsviscosity decreases. That is, a liquid flows moreeasily when it is hot than when it is cold. Theviscosity of a liquid increases as the pressure onthe liquid increases.

A satisfactory liquid for a hydraulic systemmust be thick enough to give a good seal atpumps, motors, valves, and so on. These com-ponents depend on close fits for creating andmaintaining pressure. Any internal leakagethrough these clearances results in loss of pressure,instantaneous control, and pump efficiency.Leakage losses are greater with thinner liquids(low viscosity). A liquid that is too thin will alsoallow rapid wearing of moving parts, or of partsthat operate under heavy loads. On the otherhand, if the liquid is too thick (viscosity too high),the internal friction of the liquid will cause anincrease in the liquid’s flow resistance throughclearances of closely fitted parts, lines, andinternal passages. This results in pressure dropsthroughout the system, sluggish operationof the equipment, and an increase in powerconsumption.

Measurement of Viscosity

Viscosity is normally determined by measuringthe time required for a fixed volume of a fluid(at a given temperature) to flow through acalibrated orifice or capillary tube. The instru-ments used to measure the viscosity of a liquidare known as viscometers or viscosimeters.

Several types of viscosimeters are in use today.The Saybolt viscometer, shown in figure 3-1,measures the time required, in seconds, for 60milliliters of the tested fluid at 100°F to passthrough a standard orifice. The time measured is

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Figure 3-1.—Saybolt viscometer.

used to express the fluid’s viscosity, in Sayboltuniversal seconds or Saybolt furol seconds.

The glass capillary viscometers, shown infigure 3-2, are examples of the second type ofviscometer used. These viscometers are used to

measure kinematic viscosity. Like the Sayboltviscometer, the glass capillary measures the timein seconds required for the tested fluid to flowthrough the capillary. This time is multiplied bythe temperature constant of the viscometer in useto provide the viscosity, expressed in centistrokes.

The following formulas may be used toconvert centistrokes (cSt units) to approximateSaybolt universal seconds (SUS units).

For SUS values between 32 and 100:

For SUS values greater than 100:

Although the viscometers discussed above areused in laboratories, there are other viscometersin the supply system that are available for localuse. These viscometers can be used to test theviscosity of hydraulic fluids either prior to theirbeing added to a system or periodically after theyhave been in an operating system for a while.

Figure 3-2.–Various styles of glass capillary viscometers.

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Additional information on the various typesof viscometers and their operation can be foundin the Physical Measurements Training Manual,NAVAIR 17-35QAL-2.

Viscosity Index

The viscosity index (V.I.) of an oil is a numberthat indicates the effect of temperature changeson the viscosity of the oil. A low V.I. signifiesa relatively large change of viscosity with changesof temperature. In other words, the oil becomesextremely thin at high temperatures and extremelythick at low temperatures. On the other hand, ahigh V.I. signifies relatively little change inviscosity over a wide temperature range.

An ideal oil for most purposes is onethat maintains a constant viscosity throughouttemperature changes. The importance of the V.I.can be shown easily by considering automotivelubricants. An oil having a high V.I. resistsexcessive thickening when the engine is cold and,consequently, promotes rapid starting and promptcirculation; it resists excessive thinning when themotor is hot and thus provides full lubrication andprevents excessive oil consumption.

Another example of the importance of the V.I.is the need for a high V.I. hydraulic oil for militaryaircraft, since hydraulic control systems may beexposed to temperatures ranging from below–65°F at high altitudes to over 100°F on theground. For the proper operation of the hydrauliccontrol system, the hydraulic fluid must have asufficiently high V.I. to perform its functions atthe extremes of the expected temperature range.

Liquids with a high viscosity have a greaterresistance to heat than low viscosity liquids whichhave been derived from the same source. Theaverage hydraulic liquid has a relatively lowviscosity. Fortunately, there is a wide choice ofliquids available for use in the viscosity rangerequired of hydraulic liquids.

The V.I. of an oil may be determined if itsviscosity at any two temperatures is known.Tables, based on a large number of tests, areissued by the American Society for Testingand Materials (ASTM). These tables permitcalculation of the V.I. from known viscosities.

LUBRICATING POWER

If motion takes place between surfaces incontact, friction tends to oppose the motion.When pressure forces the liquid of a hydraulicsystem between the surfaces of moving parts, the

liquid spreads out into a thin film which enablesthe parts to move more freely. Different liquids,including oils, vary greatly not only in theirlubricating ability but also in film strength. Filmstrength is the capability of a liquid to resist beingwiped or squeezed out from between the surfaceswhen spread out in an extremely thin layer. Aliquid will no longer lubricate if the film breaksdown, since the motion of part against part wipesthe metal clean of liquid.

Lubricating power varies with temperaturechanges; therefore, the climatic and workingconditions must enter into the determination ofthe lubricating qualities of a liquid. Unlikeviscosity, which is a physical property, thelubricating power and film strength of a liquidis directly related to its chemical nature.Lubricating qualities and film strength can beimproved by the addition of certain chemicalagents.

CHEMICAL STABILITY

Chemical stability is another property whichis exceedingly important in the selection of ahydraulic liquid. It is defined as the liquid’s abilityto resist oxidation and deterioration for longperiods. All liquids tend to undergo unfavorablechanges under severe operating conditions. Thisis the case, for example, when a system operatesfor a considerable period of time at hightemperatures.

Excessive temperatures, especially extremelyhigh temperatures, have a great effect on the lifeof a liquid. The temperature of the liquid in thereservoir of an operating hydraulic system doesnot always indicate the operating conditionsthroughout the system. Localized hot spots occuron bearings, gear teeth, or at other points wherethe liquid under pressure is forced through smallorifices. Continuous passage of the liquid throughthese points may produce local temperatures highenough to carbonize the liquid or turn it intosludge, yet the liquid in the reservoir may notindicate an excessively high temperature.

Liquids may break down if exposed to air,water, salt, or other impurities, especially if theyare in constant motion or subjected to heat. Somemetals, such as zinc, lead, brass, and copper, haveundesirable chemical reactions with certainliquids.

These chemical reactions result in the forma-tion of sludge, gums, carbon, or other depositswhich clog openings, cause valves and pistons tostick or leak, and give poor lubrication to moving

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parts. Once a small amount of sludge or otherdeposits is formed, the rate of formation generallyincreases more rapidly. As these deposits areformed, certain changes in the physical andchemical properties of the liquid take place. Theliquid usually becomes darker, the viscosityincreases and damaging acids are formed.

The extent to which changes occur in differentliquids depends on the type of liquid, type ofrefining, and whether it has been treated toprovide further resistance to oxidation. Thestability of liquids can be improved by theaddition of oxidation inhibitors. Inhibitorsselected to improve stability must be compatiblewith the other required properties of the liquid.

FREEDOM FROM ACIDITY

An ideal hydraulic liquid should be free fromacids which cause corrosion of the metals in thesystem. Most liquids cannot be expected to remaincompletely noncorrosive under severe operatingconditions. The degree of acidity of a liquid, whennew, may be satisfactory; but after use, the liquidmay tend to become corrosive as it begins todeteriorate.

Many systems are idle for long periods afteroperating at high temperatures. This permitsmoisture to condense in the system, resulting inrust formation.

Certain corrosion- and rust-preventive addi-tives are added to hydraulic liquids. Some of theseadditives are effective only for a limited period.Therefore, the best procedure is to use the liquidspecified for the system for the time specified bythe system manufacturer and to protect the liquidand the system as much as possible fromcontamination by foreign matter, from abnormaltemperatures, and from misuse.

FLASHPOINT

Flashpoint is the temperature at which a liquidgives off vapor in sufficient quantity to ignitemomentarily or flash when a flame is applied. Ahigh flashpoint is desirable for hydraulic liquidsbecause it provides good resistance to combustionand a low degree of evaporation at normaltemperatures. Required flashpoint minimumsvary from 300°F for the lightest oils to 510°F forthe heaviest oils.

FIRE POINT

Fire point is the temperature at which asubstance gives off vapor in sufficient quantityto ignite and continue to burn when exposed toa spark or flame. Like flashpoint, a high fire pointis required of desirable hydraulic liquids.

MINIMUM TOXICITY

Toxicity is defined as the quality, state, ordegree of being toxic or poisonous. Some liquidscontain chemicals that are a serious toxic hazard.These toxic or poisonous chemicals may enter thebody through inhalation, by absorption throughthe skin, or through the eyes or the mouth. Theresult is sickness and, in some cases, death.Manufacturers of hydraulic liquids strive toproduce suitable liquids that contain no toxicchemicals and, as a result, most hydraulic liquidsare free of harmful chemicals. Some fire-resistantliquids are toxic, and suitable protection and carein handling must be provided.

DENSITY AND COMPRESSIBILITY

A fluid with a specific gravity of less than 1.0is desired when weight is critical, although withproper system design, a fluid with a specificgravity greater than one can be tolerated. Whereavoidance of detection by military units is desired,a fluid which sinks rather than rises to the surfaceof the water is desirable. Fluids having a specificgravity greater than 1.0 are desired, as leakingfluid will sink, allowing the vessel with the leakto remain undetected.

Recall from chapter 2 that under extremepressure a fluid may be compressed up to 7percent of its original volume. Highly com-pressible fluids produce sluggish system operation.This does not present a serious problem in small,low-speed operations, but it must be consideredin the operating instructions.

FOAMING TENDENCIES

Foam is an emulsion of gas bubbles in thefluid. Foam in a hydraulic system results fromcompressed gases in the hydraulic fluid. A fluidunder high pressure can contain a large volumeof air bubbles. When this fluid is depressurized,as when it reaches the reservoir, the gas bubblesin the fluid expand and produce foam. Anyamount of foaming may cause pump cavitationand produce poor system response and spongy

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control. Therefore, defoaming agents are oftenadded to fluids to prevent foaming. Minimizingair in fluid systems is discussed later in thischapter.

CLEANLINESS

Cleanliness in hydraulic systems has receivedconsiderable attention recently. Some hydraulicsystems, such as aerospace hydraulic systems, areextremely sensitive to contamination. Fluidcleanliness is of primary importance becausecontaminants can cause component malfunction,prevent proper valve seating, cause wear incomponents, and may increase the response timeof servo valves. Fluid contaminants are discussedlater in this chapter.

The inside of a hydraulic system can only bekept as clean as the fluid added to it. Initial fluidcleanliness can be achieved by observing stringentcleanliness requirements (discussed later in thischapter) or by filtering all fluid added to thesystem.

TYPES OF HYDRAULIC FLUIDS

There have been many liquids tested for usein hydraulic systems. Currently, liquids being usedinclude mineral oil, water, phosphate ester,water-based ethylene glycol compounds, andsilicone fluids. The three most common types ofhydraulic liquids are petroleum-based, syntheticfire-resistant, and water-based fire-resistant.

PETROLEUM-BASED FLUIDS

The most common hydraulic fluids used inshipboard systems are the petroleum-based oils.These fluids contain additives to protect the fluidfrom oxidation (antioxidant), to protect systemmetals from corrosion (anticorrosion), to reducetendency of the fluid to foam (foam suppressant),and to improve viscosity.

Petroleum-based fluids are used in surfaceships’ electrohydraulic steering and deckmachinery systems, submarines’ hydraulicsystems, and aircraft automatic pilots, shockabsorbers, brakes, control mechanisms, and otherhydraulic systems using seal materials compatiblewith petroleum-based fluids.

SYNTHETIC FIRE-RESISTANT FLUIDS

Petroleum-based oils contain most of thedesired properties of a hydraulic liquid. However,they are flammable under normal conditions andcan become explosive when subjected to highpressures and a source of flame or high tempera-tures. Nonflammable synthetic liquids have beendeveloped for use in hydraulic systems where firehazards exist.

Phosphate Ester Fire-Resistant Fluid

Phosphate ester fire-resistant fluid forshipboard use is covered by specification MIL-H-19457. There are certain trade names closelyassociated with these fluids. However, the onlyacceptable fluids conforming to MIL-H-19457 arethe ones listed on the current Qualified ProductsList (QPL) 19457. These fluids will be deliveredin containers marked MIL-H-19457C or a laterspecification revision. Phosphate ester incontainers marked by a brand name without aspecification identification must not be used inshipboard systems, as they may contain toxicchemicals.

These fluids will burn if sufficient heat andflame are applied, but they do not supportcombustion. Drawbacks of phosphate ester fluidsare that they will attack and loosen commonlyused paints and adhesives, deteriorate many typesof insulations used in electrical cables, anddeteriorate many gasket and seal materials.Therefore, gaskets and seals for systems in whichphosphate ester fluids are used are manufacturedof specific materials. Naval Ships’ TechnicalManual, chapter 262, specifies paints to be usedon exterior surfaces of hydraulic systems andcomponents in which phosphate ester fluid is usedand on ship structure and decks in the immediatevicinity of this equipment. Naval Ships’ TechnicalManual, chapter 078, specifies gasket and sealmaterials used. NAVAIR 01-1A-17 also containsa list of materials resistant to phosphate esterfluids.

Trade names for phosphate ester fluids, whichdo not conform to MIL-H-19457 include Pydraul,Skydrol, and Fyre Safe.

PHOSPHATE ESTER FLUID SAFETY.—As a maintenance person, operator, supervisor,or crew member of a ship, squadron, or navalshore installation, you must understand thehazards associated with hydraulic fluids to whichyou may be exposed.

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Phosphate ester fluid conforming to specifi-cation MIL-H-19457 is used in aircraft elevators,ballast valve operating systems, and replenish-ment-at-sea systems. This type of fluid containsa controlled amount of neurotoxic material.Because of the neurotoxic effects that can resultfrom ingestion, skin absorption, or inhalation ofthese fluids, be sure to use the followingprecautions:

1. Avoid contact with the fluids by wearingprotective clothing.

2. Use chemical goggles or face shields toprotect your eyes.

3. If you are expected to work in anatmosphere containing a fine mist or spray,wear a continuous-flow airline respirator.

4. Thoroughly clean skin areas contaminatedby this fluid with soap and water.

5. If you get any fluid in your eyes, flush themwith running water for at least 15 minutesand seek medical attention.

If you come in contact with MIL-H-19457fluid, report the contact when you seek medicalaid and whenever you have a routine medicalexamination.

Naval Ships’ Technical Manual, chapter 262,contains a list of protective clothing, along withnational stock numbers (NSN), for use with fluidsconforming to MIL-H-19457. It also containsprocedures for repair work and for low-levelleakage and massive spills cleanup.

PHOSPHATE ESTER FLUID DISPOSAL.—Waste MIL-H-19457 fluids and refuse (rags andother materials) must not be dumped at sea. Fluidshould be placed in bung-type drums. Rags andother materials should be placed in open topdrums for shore disposal. These drums should bemarked with a warning label stating their content,safety precautions, and disposal instructions.Detailed instructions for phosphate ester fluidsdisposal can be found in Naval Ships’ TechnicalManual, chapter 262, and OPNAVINST 5090.1.

Silicone Synthetic Fire-Resistant Fluids

Silicone synthetic fire-resistant fluids arefrequently used for hydraulic systems whichrequire fire resistance, but which have onlymarginal requirements for other chemical orphysical properties common to hydraulic fluids.Silicone fluids do not have the detrimentalcharacteristics of phosphate ester fluids, nor

do they provide the corrosion protection andlubrication of phosphate ester fluids, but they areexcellent for fire protection. Silicone fluidconforming to MIL-S-81087 is used in the missileholddown and lockout system aboard submarines.

Lightweight Synthetic Fire-Resistant Fluids

In applications where weight is critical,lightweight synthetic fluid is used in hydraulicsystems. MIL-H-83282 is a synthetic, fire-resistanthydraulic fluid used in military aircraft andhydrofoils where the requirement to minimizeweight dictates the use of a low-viscosity fluid.It is also the most commonly used fluid in aviationsupport equipment. NAVAIR 01-1A-17 containsadditional information on fluids conforming tospecification MIL-H-83282.

WATER-BASED FIRE-RESISTANTFLUIDS

The most widely used water-based hydraulicfluids may be classified as water-glycol mixturesand water-synthetic base mixtures. The water-glycol mixture contains additives to protect it fromoxidation, corrosion, and biological growth andto enhance its load-carrying capacity.

Fire resistance of the water mixture fluidsdepends on the vaporization and smotheringeffect of steam generated from the water. Thewater in water-based fluids is constantly beingdriven off while the system is operating. There-fore, frequent checks to maintain the correct ratioof water are important.

The water-based fluid used in catapultretracting engines, jet blast deflectors, andweapons elevators and handling systems conformsto MIL-H-22072.

The safety precautions outlined for phosphateester fluid and the disposal of phosphate esterfluid also apply to water-based fluid conformingto MIL-H-22072.

CONTAMINATION

Hydraulic fluid contamination may bedescribed as any foreign material or substancewhose presence in the fluid is capable of adverselyaffecting system performance or reliability. It mayassume many different forms, including liquids,gases, and solid matter of various composition,sizes, and shapes. Solid matter is the type mostoften found in hydraulic systems and is generally

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referred to as particulate contamination. Con-tamination is always present to some degree, evenin new, unused fluid, but must be kept below alevel that will adversely affect system operation.Hydraulic contamination control consists ofrequirements, techniques, and practices necessaryto minimize and control fluid contamination.

CLASSIFICATION

There are many types of contaminants whichare harmful to hydraulic systems and liquids.These contaminants may be divided into twodifferent classes—particulate and fluid.

Particulate Contamination

This class of contaminants includes organic,metallic solid, and inorganic solid contaminants.These contaminants are discussed in the followingparagraphs.

ORGANIC CONTAMINATION.— Organicsolids or semisolids found in hydraulic systemsare produced by wear, oxidation, or polymeriza-tion. Minute particles of O-rings, seals, gaskets,and hoses are present, due to wear or chemicalreactions. Synthetic products, such as neoprene,silicones, and hypalon, though resistant tochemical reaction with hydraulic fluids, producesmall wear particles. Oxidation of hydraulic fluidsincreases with pressure and temperature, althoughantioxidants are blended into hydraulic fluids tominimize such oxidation. The ability of ahydraulic fluid to resist oxidation or poly-merization in service is defined as its oxidationstability. Oxidation products appear as organicacids, asphaltics, gums, and varnishes. Theseproducts combine with particles in the hydraulicfluid to form sludge. Some oxidation products areoil soluble and cause the hydraulic fluid toincrease in viscosity; other oxidation products arenot oil soluble and form sediment.

METALLIC SOLID CONTAMINATION.—Metallic contaminants are almost always presentin a hydraulic system and will range in size frommicroscopic particles to particles readily visibleto the naked eye. These particles are the result ofwearing and scoring of bare metal parts andplating materials, such as silver and chromium.Although practically all metals commonly usedfor parts fabrication and plating may be foundin hydraulic fluids, the major metallic materialsfound are ferrous, aluminum, and chromium

particles. Because of their continuous high-speedinternal movement, hydraulic pumps usuallycontribute most of the metallic particulatecontamination present in hydraulic systems. Metalparticles are also produced by other hydraulicsystem components, such as valves and actuators,due to body wear and the chipping and wearingaway of small pieces of metal plating materials.

INORGANIC SOLID CONTAMINA-TION.— This contaminant group includes dust,paint particles, dirt, and silicates. Glass particlesfrom glass bead peening and blasting may alsobe found as contaminants. Glass particles are veryundesirable contaminants due to their abrasiveeffect on synthetic rubber seals and the very finesurfaces of critical moving parts. Atmosphericdust, dirt, paint particles, and other materials areoften drawn into hydraulic systems from externalsources. For example, the wet piston shaft of ahydraulic actuator may draw some of theseforeign materials into the cylinder past the wiperand dynamic seals, and the contaminant materialsare then dispersed in the hydraulic fluid.Contaminants may also enter the hydraulic fluidduring maintenance when tubing, hoses, fittings,and components are disconnected or replaced. Itis therefore important that all exposed fluid portsbe sealed with approved protective closures tominimize such contamination.

Fluid Contamination

Air, water, solvent, and other foreign fluidsare in the class of fluid contaminants.

AIR CONTAMINATION.— Hydraulic fluidsare adversely affected by dissolved, entrained, orfree air. Air may be introduced through impropermaintenance or as a result of system design. Anymaintenance operation that involves breaking intothe hydraulic system, such as disconnecting orremoving a line or component will invariablyresult in some air being introduced into thesystem. This source of air can and must beminimized by prebilling replacement componentswith new filtered fluid prior to their installation.Failing to prefill a filter element bowl with fluidis a good example of how air can be introducedinto the system. Although prebilling will minimizeintroduction of air, it is still important to vent thesystem where venting is possible.

Most hydraulic systems have built-in sourcesof air. Leaky seals in gas-pressurized accumulatorsand reservoirs can feed gas into a system faster

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than it can be removed, even with the best ofmaintenance. Another lesser known but majorsource of air is air that is sucked into the systempast actuator piston rod seals. This usually occurswhen the piston rod is stroked by some externalmeans while the actuator itself is not pressurized.

WATER CONTAMINATION.— Water is aserious contaminant of hydraulic systems.Hydraulic fluids are adversely affected bydissolved, emulsified, or free water. Watercontamination may result in the formation of ice,which impedes the operation of valves, actuators,and other moving parts. Water can also cause theformation of oxidation products and corrosionof metallic surfaces.

SOLVENT CONTAMINATION.— Solventcontamination is a special form of foreign fluidcontamination in which the original contami-nating substance is a chlorinated solvent. Chlori-nated solvents or their residues may, whenintroduced into a hydraulic system, react with anywater present to form highly corrosive acids.

Chlorinated solvents, when allowed to com-bine with minute amounts of water often foundin operating hydraulic systems, change chemicallyinto hydrochloric acids. These acids then attackinternal metallic surfaces in the system,particularly those that are ferrous, and producea severe rust-like corrosion. NAVAIR 01-1A-17and NSTM, chapter 556, contain tables ofsolvents for use in hydraulic maintenance.

FOREIGN-FLUIDS CONTAMINATION.—Hydraulic systems can be seriously contaminatedby foreign fluids other than water and chlorinatedsolvents. This type of contamination is generallya result of lube oil, engine fuel, or incorrecthydraulic fluid being introduced inadvertently intothe system during servicing. The effects of suchcontamination depend on the contaminant, theamount in the system, and how long it has beenpresent.

NOTE: It is extremely important that thedifferent types of hydraulic fluids are not mixedin one system. If different type hydraulic fluidsare mixed, the characteristics of the fluid requiredfor a specific purpose are lost. Mixing thedifferent types of fluids usually will result in aheavy, gummy deposit that will clog passages andrequire a major cleaning. In addition, seals andpacking installed for use with one fluid usually

are not compatible with other fluids and damageto the seals will result.

ORIGIN OF CONTAMINATION

Recall that contaminants are produced fromwear and chemical reactions, introduced byimproper maintenance, and inadvertently intro-duced during servicing. These methods of con-taminant introduction fall into one of the fourmajor areas of contaminant origin.

1. Particles originally contained in the system.These particles originate during the fabricationand storage of system components. Weld spatterand slag may remain in welded system com-ponents, especially in reservoirs and pipeassemblies. The presence is minimized by properdesign. For example, seam-welded overlappingjoints are preferred, and arc welding of opensections is usually avoided. Hidden passages invalve bodies, inaccessible to sand blasting or othermethods of cleaning, are the main source ofintroduction of core sand. Even the most carefullydesigned and cleaned casting will almost invari-ably free some sand particles under the action ofhydraulic pressure. Rubber hose assemblies alwayscontain some loose particles. Most of theseparticles can be removed by flushing the hosebefore installation; however, some particleswithstand cleaning and are freed later by theaction of hydraulic pressure.

Particles of lint from cleaning rags cancause abrasive damage in hydraulic systems,especially to closely fitted moving parts. Inaddition, lint in a hydraulic system packs easilyinto clearances between packing and contactingsurfaces, leading to component leakage anddecreased efficiency. Lint also helps clog filtersprematurely. The use of the proper wipingmaterials will reduce or eliminate lint contamina-tion. The wiping materials to be used for a givenapplication will be determined by

a.b.

c.

substances being wiped or absorbed,the amount of absorbency required,and/orthe required degree of cleanliness.

These wiping materials are categorized forcontamination control by the degree of lint ordebris that they may deposit during use. Forinternal hydraulic repairs, this factor itselfwill determine the choice of wiping material.

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NAVAIR 01-1A-17 and NSTM, chapter 556,provides information on low-lint wiping cloths.

Rust or corrosion initially present in ahydraulic system can usually be traced toimproper storage of materials and componentparts. Particles can range in size from large flakesto abrasives of microscopic dimensions. Properpreservation of stored parts is helpful in elimi-nating corrosion.

2. Particles introduced from outside sources.Particles can be introduced into hydraulic systemsat points where either the liquid or certain workingparts of the system (for example, piston rods) areat least in temporary contact with the atmosphere.The most common contaminant introductionareas are at the refill and breather openings,cylinder rod packings, and open lines wherecomponents are removed for repair or replace-ment. Contamination arising from carelessnessduring servicing operations is minimized by theuse of filters in the system fill lines and fingerstrainers in the filler adapter of hydraulicreservoirs. Hydraulic cylinder piston rodsincorporate wiper rings and dust seals to preventthe dust that settles on the piston rod during itsoutward stroke from entering the system when thepiston rod retracts. Caps and plugs are availableand should be used to seal off the open lines whena component is removed for repair orreplacement.

3. Particles created within the system duringoperation. Contaminants created during systemoperation are of two general types—mechanicaland chemical. Particles of a mechanical nature areformed by wearing of parts in frictional contact,such as pumps, cylinders, and packing glandcomponents. These wear particles can vary fromlarge chunks of packings down to steel shavingsthat are too small to be trapped by filters.

The major source of chemical contami-nants in hydraulic liquid is oxidation. Thesecontaminants are formed under high pressure andtemperatures and are promoted by the chemicalaction of water and air and of metals like copperand iron oxides. Liquid-oxidation products appearinitially as organic acids, asphaltines, gums,and varnishes—sometimes combined with dustparticles as sludge. Liquid-soluble oxidationproducts tend to increase liquid viscosity, whileinsoluble types separate and form sediments,especially on colder elements such as heatexchanger coils.

Liquids containing antioxidants have littletendency to form gums and sludge under normaloperating conditions. However, as the tempera-ture increases, resistance to oxidation diminishes.Hydraulic liquids that have been subjected toexcessively high temperatures (above 250°F formost liquids) will break down, leaving minuteparticles of asphaltines suspended in the liquids.The liquid changes to brown in color and isreferred to as decomposed liquid. This explainsthe importance of keeping the hydraulic liquidtemperature below specific levels.

The second contaminant-producing chemi-cal action in hydraulic liquids is one that permitsthese liquids to react with certain types of rubber.This reaction causes structural changes in therubber, turning it brittle, and finally causing itscomplete disintegration. For this reason, thecompatibility of system liquid with seals and hosematerial is a very important factor.

4. Particles introduced by foreign liquids. Oneof the most common foreign-fluid contaminantsis water, especially in hydraulic systems thatrequire petroleum-based liquids. Water, whichenters even the most carefully designed system bycondensation of atmospheric moisture, normallysettles to the bottom of the reservoir. Oilmovement in the reservoir disperses the water intofine droplets, and agitation of the liquid inthe pump and in high-speed passages forms anoil-water-air emulsion. This emulsion normallyseparates during the rest period in the systemreservoir; but when fine dust and corrosionparticles are present, the emulsion is chemicallychanged by high pressures into sludge. Thedamaging action of sludge explains the need foreffective filtration, as well as the need for waterseparation qualities in hydraulic liquids.

CONTAMINATION CONTROL

Maintaining hydraulic fluid within allowablecontamination limits for both water and particu-late matter is crucial to the care and protectionof hydraulic equipment.

Filters (discussed in chapter 9) will provideadequate control of the particular contaminationproblem during all normal hydraulic systemoperations if the filtration system is installedproperly and filter maintenance is performedproperly. Filter maintenance includes changingelements at proper intervals. Control of the sizeand amount of contamination entering the systemfrom any other source is the responsibility

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of the personnel who service and maintain theequipment. During installation, maintenance, andrepair of hydraulic equipment, the retention ofcleanliness of the system is of paramountimportance for subsequent satisfactory per-formance.

The following maintenance and servicingprocedures should be adhered to at all times toprovide proper contamination control:

1. All tools and the work area (workbenchesand test equipment) should be kept in a clean,dirt-free condition.

2. A suitable container should always beprovided to receive the hydraulic liquid that isspilled during component removal or disassembly.

NOTE: The reuse of drained hydraulicliquid is prohibited in most hydraulic systems. Insome large-capacity systems the reuse of fluid ispermitted. When liquid is drained from thesesystems for reuse, it must be stored in a clean andsuitable container. The liquid must be strainedand/or filtered when it is returned to the systemreservoir.

3. Before hydraulic lines or fittings aredisconnected, the affected area should be cleanedwith an approved dry-cleaning solvent.

4. All hydraulic lines and fittings should becapped or plugged immediately after discon-nection.

5. Before any hydraulic components areassembled, their parts should be washed with anapproved dry-cleaning solvent.

6. After the parts have been cleaned indry-cleaning solvent, they should be driedthoroughly with clean, low-lint cloths andlubricated with the recommended preservative orhydraulic liquid before assembly.

NOTE: Only clean, low lint type I or IIcloths as appropriate should be used to wipe ordry component parts.

7. All packings and gaskets should be replacedduring the assembly procedures.

8. All parts should be connected with care toavoid stripping metal slivers from threaded areas.All fittings and lines should be installed andtorqued according to applicable technicalinstructions.

9. All hydraulic servicing equipment shouldbe kept clean and in good operating condition.

Some hydraulic fluid specifications, such asMIL-H-6083, MIL-H-46170, and MIL-H-83282,contain particle contamination limits that are solow that the products are packaged under cleanroom conditions. Very slight amounts of dirt,rust, and metal particles will cause them tofail the specification limit for contamination.Since these fluids are usually all packaged inhermetically sealed containers, the act of openinga container may allow more contaminants into thefluid than the specification allows. Therefore,extreme care should be taken in the handling ofthese fluids. In opening the container for use,observation, or tests, it is extremely important thatthe can be opened and handled in a cleanenvironment. The area of the container to beopened should be flushed with filtered solvent(petroleum ether or isopropyl alcohol), and thedevice used for opening the container should bethoroughly rinsed with filtered solvent. After thecontainer is opened, a small amount of thematerial should be poured from the container anddisposed of prior to pouring the sample foranalysis. Once a container is opened, if thecontents are not totally used, the unused portionshould be discarded. Since the level of con-tamination of a system containing these fluidsmust be kept low, maintenance on the system’scomponents must be performed in a cleanenvironment commonly known as a controlledenvironment work center. Specific informationabout the controlled environment work center canbe found in the Aviation Hydraulics Manual,NAVAIR 01-1A-17.

HYDRAULIC FLUID SAMPLING

The condition of a hydraulic system, as wellas its probable future performance, can best bedetermined by analyzing the operating fluid. Ofparticular interest are any changes in the physicaland chemical properties of the fluid and excessiveparticulate or water contamination, either ofwhich indicates impending trouble.

Excessive particulate contamination of thefluid indicates that the filters are not keeping thesystem clean. This can result from improper filtermaintenance, inadequate filters, or excessiveongoing corrosion and wear.

Operating equipment should be sampledaccording to instructions given in the operating

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and maintenance manual for the particularequipment or as directed by the MRCs.

1. All samples should be taken from circu-lating systems, or immediately upon shutdown,while the hydraulic fluid is within 5°C (9°F) ofnormal system operating temperature. Systemsnot up to temperature may provide nonrepre-sentative samples of system dirt and watercontent, and such samples should either beavoided or so indicated on the analysis report. Thefirst oil coming from the sampling point shouldbe discarded, since it can be very dirty and doesnot represent the system. As a general rule, avolume of oil equivalent to one to two times thevolume of oil contained in the sampling line andvalve should be drained before the sample istaken.

2. Ideally, the sample should be taken froma valve installed specifically for sampling. Whensampling valves are not installed, the taking ofsamples from locations where sediment or watercan collect, such as dead ends of piping, tankdrains, and low points of large pipes and filterbowls, should be avoided if possible. If samplesare taken from pipe drains, sufficient fluid shouldbe drained before the sample is taken to ensurethat the sample actually represents the system.Samples are not to be taken from the tops ofreservoirs or other locations where the contami-nation levels are normally low.

3. Unless otherwise specified, a minimum ofone sample should be taken for each system

located wholly within one compartment. Forship’s systems extending into two or morecompartments, a second sample is required. Anexception to this requirement is submarineexternal hydraulic systems, which require only onesample. Original sample points should be labeledand the same sample points used for successivesampling. If possible, the following samplinglocations should be selected:

a. A location that provides a samplerepresentative of fluid being suppliedto system components

b. A return line as close to the supply tankas practical but upstream of any returnline filter

c. For systems requiring a second sample,a location as far from the pump aspractical

Operation of the sampling point should notintroduce any significant amount of externalcontaminants into the collected fluid. Additionalinformation on hydraulic fluid sampling can befound in NAVAIR 01-1A-17.

Most fluid samples are submitted to shorelaboratories for analysis. NAVAIR 17-15-50-1and NSTM, chapter 556, contain details oncollecting, labeling, and shipping samples.

NAVAIR 01-1A-17 contains procedures forunit level, both aboard ship and ashore, testingof aviation hydraulic fluids for water, particulate,and chlorinated solvent contamination.

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CHAPTER 4

PUMPS

Pumps are used for some essential services inthe Navy. Pumps supply water to the boilers, drawcondensation from the condensers, supply seawater to the firemain, circulate cooling water forcoolers and condensers, pump out bilges, transferfuel, supply water to the distilling plants, andserve many other purposes. Although the pumpsdiscussed in this chapter are used primarily inhydraulic systems, the principles of operationapply as well to the pumps used in other systems.

PURPOSE

The purpose of a hydraulic pump is to supplya flow of fluid to a hydraulic system. The pumpdoes not create system pressure, since pressure canbe created only by a resistance to the flow. As thepump provides flow, it transmits a force to thefluid. As the fluid flow encounters resistance, thisforce is changed into a pressure. Resistance toflow is the result of a restriction or obstructionin the path of the flow. This restriction is normallythe work accomplished by the hydraulic system,but can also be restrictions of lines, fittings, andvalves within the system. Thus, the pressure iscontrolled by the load imposed on the system orthe action of a pressure-regulating device.

OPERATION

A pump must have a continuous supply offluid available to the inlet port to supply fluid tothe system. As the pump forces fluid through theoutlet port, a partial vacuum or low-pressure areais created at the inlet port. When the pressure atthe inlet port of the pump is lower than the localatmospheric pressure, atmospheric pressure actingon the fluid in the reservoir forces the fluid intothe pump’s inlet. If the pump is located ata level lower than the reservoir, the force ofgravity supplements atmospheric pressure on thereservoir. Aircraft and missiles that operate at

high altitudes are equipped with pressurizedhydraulic reservoirs to compensate for lowatmospheric pressure encountered at highaltitudes.

PERFORMANCE

Pumps are normally rated by their volumetricoutput and pressure. Volumetric output is theamount of fluid a pump can deliver to its outletport in a certain period of time at a given speed.Volumetric output is usually expressed in gallonsper minute (gpm). Since changes in pump speedaffect volumetric output, some pumps are ratedby their displacement. Pump displacement is theamount of fluid the pump can deliver per cycle.Since most pumps use a rotary drive, displacementis usually expressed in terms of cubic inches perrevolution.

As we stated previously, a pump does notcreate pressure. However, the pressure developedby the restrictions in the system is a factor thataffects the volumetric output of the pump. As thesystem pressure increases, the volumetric outputdecreases. This drop in volumetric output is theresult of an increase in the amount of internalleakage from the outlet side to the inlet side ofthe pump. This leakage is referred to as pumpslippage and is a factor that must be consideredin all pumps. This explains why most pumps arerated in terms of volumetric output at a givenpressure.

CLASSIFICATION OF PUMPS

Many different methods are used to classifypumps. Terms such as nonpositive displacement,positive displacement, fixed displacement,variable displacement, fixed delivery, variabledelivery, constant volume, and others are used todescribe pumps. The first two of these termsdescribe the fundamental division of pumps; that

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is, all pumps are either nonpositive displacementor positive displacement.

Basically, pumps that discharge liquid in acontinuous flow are referred to as nonpositivedisplacement, and those that discharge volumesseparated by a period of no discharge are referredto as positive displacement.

Although the nonpositive-displacement pumpnormally produces a continuous flow, it does notprovide a positive seal against slippage; therefore,the output of the pump varies as system pressurevaries. In other words, the volume of fluiddelivered for each cycle depends on the resistanceto the flow. This type of pump produces a forceon the fluid that is constant for each particularspeed of the pump. Resistance in the dischargeline produces a force in a direction opposite thedirection of the force produced by the pump.When these forces are equal, the fluid is in a stateof equilibrium and does not flow.

If the outlet of a nonpositive-displacementpump is completely closed, the discharge pressurewill increase to the maximum for that particularpump at a specific speed. Nothing more willhappen except that the pump will churn the fluidand produce heat.

In contrast to the nonpositive-displacementpump, the positive-displacement pump providesa positive internal seal against slippage. Therefore,this type of pump delivers a definite volume offluid for each cycle of pump operation, regardlessof the resistance offered, provided the capacityof the power unit driving the pump is notexceeded. If the outlet of a positive-displacementpump were completely closed, the pressure wouldinstantaneously increase to the point at which theunit driving the pump would stall or somethingwould break.

Positive-displacement pumps are furtherclassified as fixed displacement or variabledisplacement. The fixed-displacement pumpdelivers the same amount of fluid on each cycle.The output volume can be changed only bychanging the speed of the pump. When a pumpof this type is used in a hydraulic system, apressure regulator (unloading valve) must beincorporated in the system. A pressure regulatoror unloading valve is used in a hydraulic systemto control the amount of pressure in the systemand to unload or relieve the pump when thedesired pressure is reached. This action of apressure regulator keeps the pump from workingagainst a load when the hydraulic system is atmaximum pressure and not functioning. Duringthis time the pressure regulator bypasses the fluid

from the pump back to the reservoir. (See chapter6 for more detailed information concerningpressure regulators.) The pump continues todeliver a fixed volume of fluid during each cycle.Such terms as fixed delivery, constant delivery,and constant volume are all used to identify thefixed-displacement pump.

The variable-displacement pump is con-structed so that the displacement per cycle can bevaried. The displacement is varied through the useof an internal controlling device. Some of thesecontrolling devices are described later in thischapter.

Pumps may also be classified according to thespecific design used to create the flow of fluid.Practically all hydraulic pumps fall within threedesign classifications-centrifugal, rotary, andreciprocating. The use of centrifugal pumps inhydraulics is limited and will not be discussed inthis text.

ROTARY PUMPS

All rotary pumps have rotating parts whichtrap the fluid at the inlet (suction) port and forceit through the discharge port into the system.Gears, screws, lobes, and vanes are commonlyused to move the fluid. Rotary pumps are positivedisplacement of the fixed displacement type.

Rotary pumps are designed with very smallclearances between rotating parts and stationaryparts to minimize slippage from the dischargeside back to the suction side. They are designedto operate at relatively moderate speeds.Operating at high speeds causes erosion andexcessive wear which results in increasedclearances.

There are numerous types of rotary pumpsand various methods of classification. They maybe classified by the shaft position—eithervertically or horizontally mounted; the type ofdrive—electric motor, gasoline engine, and soforth; their manufacturer’s name; or their serviceapplication. However, classification of rotarypumps is generally made according to the type ofrotating element. A few of the most commontypes of rotary pumps are discussed in thefollowing paragraphs.

GEAR PUMPS

Gear pumps are classified as either externalor internal gear pumps. In external gear pumpsthe teeth of both gears project outward from their

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centers (fig, 4-1). External pumps may use spurgears, herringbone gears, or helical gears to movethe fluid. In an internal gear pump, the teeth ofone gear project outward, but the teeth of theother gear project inward toward the center of thepump (fig. 4-2, view A). Internal gear pumps maybe either centered or off-centered.

Spur Gear Pump

The spur gear pump (fig. 4-1) consists of twomeshed gears which revolve in a housing. Thedrive gear in the illustration is turned by a driveshaft which is attached to the power source. Theclearances between the gear teeth as they mesh andbetween the teeth and the pump housing are verysmall.

The inlet port is connected to the fluid supplyline, and the outlet port is connected to thepressure line. In figure 4-1 the drive gear is turningin a counterclockwise direction, and the driven(idle) gear is turning in a clockwise direction. As

Figure 4-2.—Off-centered internal gear pump.

the teeth pass the inlet port, liquid is trappedbetween the teeth and the housing. This liquid iscarried around the housing to the outlet port. Asthe teeth mesh again, the liquid between the teethis pushed into the outlet port. This actionproduces a positive flow of liquid into the system.A shearpin or shear section is incorporated in thedrive shaft. This is to protect the power source

Figure 4-1.—Gear-type rotary pump.

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or reduction gears if the pump fails because of is pumped in the same manner as in the spur gearexcessive load or jamming of parts. pump. However, in the herringbone pump, each

set of teeth begins its fluid discharge phase beforethe previous set of teeth has completed its

Herringbone Gear Pump discharge phase. This overlapping and therelatively larger space at the center of the gears

The herringbone gear pump (fig. 4-3) is a tend to minimize pulsations and give a steadiermodification of the spur gear pump. The liquid flow than the spur gear pump.

Figure 4-3.—Herringbone gear pump.

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Helical Gear Pump

The helical gear pump (fig. 4-4) is stillanother modification of the spur gear pump.Because of the helical gear design, theoverlapping of successive discharges fromspaces between the teeth is even greater than itis in the herringbone gear pump; therefore, thedischarge flow is smoother. Since the dischargeflow is smooth in the helical pump, the gearscan be designed with a small number of largeteeth—thus allowing increased capacity withoutsacrificing smoothness of flow.

The pumping gears of this type of pump aredriven by a set of timing and driving gears thathelp maintain the required close clearanceswithout actual metallic contact of the pumpinggears. (Metallic contact between the teeth of thepumping gears would provide a tighter sealagainst slippage; however, it would cause rapidwear of the teeth, because foreign matter in theliquid would be present on the contactsurfaces.)

Roller bearings at both ends of the gear shaftsmaintain proper alignment and minimize thefriction loss in the transmission of power. Suitable

packings are used to prevent leakage around theshaft.

Off-centered Internal Gear Pump

This pump is illustrated in figure 4-2, view B.The drive gear is attached directly to the drive shaftof the pump and is placed off-center in relation tothe internal gear. The two gears mesh on one sideof the pump, between the suction (inlet) anddischarge ports. On the opposite side of thechamber, a crescent-shaped form fitted to a closetolerance fills the space between the two gears.

The rotation of the center gear by the driveshaft causes the outside gear to rotate, since thetwo are meshed. Everything in the chamber rotatesexcept the crescent. This causes liquid to betrapped in the gear spaces as they pass thecrescent. The liquid is carried from the suction portto the discharge port where it is forced out of thepump by the meshing of the gears. The size of thecrescent that separates the internal and externalgears determines the volume delivery of the pump.A small crescent allows more volume of liquid perrevolution than a larger crescent.

Figure 4-4.—Helical gear pump.

4-5

Centered Internal Gear Pump

Another design of internal gear pump isillustrated in figures 4-5 and 4-6. This pumpconsists of a pair of gear-shaped elements, onewithin the other, located in the pump chamber.The inner gear is connected to the drive shaft ofthe power source.

The operation of this type of internal gearpump is illustrated in figure 4-6. To simplify theexplanation, the teeth of the inner gear and thespaces between the teeth of the outer gear arenumbered. Note that the inner gear has one lesstooth than the outer gear. The tooth form of eachgear is related to that of the other in such a waythat each tooth of the inner gear is always insliding contact with the surface of the outer gear.Each tooth of the inner gear meshes with the outergear at just one point during each revolution. Inthe illustration, this point is at the X. In view A,tooth 1 of the inner gear is meshed with space 1of the outer gear. As the gears continue to rotatein a clockwise direction and the teeth approachpoint X, tooth 6 of the inner gear will mesh withspace 7 of the outer gear, tooth 5 with space 6,and so on. During this revolution, tooth 1 willmesh with space 2; and during the followingrevolution, tooth 1 will mesh with space 3. As aresult, the outer gear will rotate at just six-seventhsthe speed of the inner gear.

At one side of the point of mesh, pockets ofincreasing size are formed as the gears rotate,while on the other side the pockets decrease in size.In figure 4-6, the pockets on the right-hand sideof the drawings are increasing in size toward thebottom of the illustration, while those on theleft-hand side are decreasing in size toward thetop of the illustration. The intake side ofthe pump would therefore be on the right and thedischarge side on the left. In figure 4-5, since theright-hand side of the drawing was turned overto show the ports, the intake and discharge appear

Figure 4-5.—Centered internal gear pump.

Figure 4-6.—Principles of operation of the internal gearpump.

reversed. Actually, A in one drawing covers A inthe other.

LOBE PUMP

The lobeoperation as

pump uses the same principle ofthe external gear pump described

4-6

Figure 4-7.—Lobe pump.

previously. The lobes are considerably larger thangear teeth, but there are only two or three lobeson each rotor. A three-lobe pump is illustratedin figure 4-7. The two elements are rotated, onedirectly driven by the source of power, and theother through timing gears. As the elementsrotate, liquid is trapped between two lobes of eachrotor and the walls of the pump chamber andcarried around from the suction side to thedischarge side of the pump. As liquid leaves thesuction chamber, the pressure in the suction

chamber is lowered, and additional liquid is forcedinto the chamber from the reservoir.

The lobes are constructed so there is acontinuous seal at the points where they meet atthe center of the pump. The lobes of the pumpillustrated in figure 4-7 are fitted with small vanesat the outer edge to improve the seal of the pump.Although these vanes are mechanically held intheir slots, they are, to some extent, free to moveoutward. Centrifugal force keeps the vanes snugagainst the chamber and the other rotatingmembers.

SCREW PUMP

Screw pumps for power transmission systemsare generally used only on submarines. Althoughlow in efficiency and expensive, the screw pumpis suitable for high pressures (3000 psi), anddelivers fluid with little noise or pressurepulsation.

Screw pumps are available in several differentdesigns; however, they all operate in a similarmanner. In a fixed-displacement rotary-type screwpump (fig. 4-8, view A), fluid is propelled axially

Figure 4-8.—Screw pumps.

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in a constant, uniform flow through the actionof just three moving parts-a power rotor and twoidler rotors. The power rotor is the only drivenelement, extending outside the pump casing forpower connections to an electrical motor. Theidler rotors are turned by the power rotor throughthe action of the meshing threads. The fluidpumped between the meshing helical threads ofthe idler and power rotors provides a protectivefilm to prevent metal-to-metal contact. The idlerrotors perform no work; therefore, they do notneed to be connected by gears to transmit power.The enclosures formed by the meshing of therotors inside the close clearance housing containthe fluid being pumped. As the rotors turn, theseenclosures move axially, providing a continuousflow. Effective performance is based on thefollowing factors:

1. The rolling action obtained with the threaddesign of the rotors is responsible for the veryquiet pump operation. The symmetrical pressureloading around the power rotor eliminates theneed for radial bearings because there are noradial loads. The cartridge-type ball bearing in thepump positions the power rotor for proper sealoperation. The axial loads on the rotors createdby discharge pressure are hydraulically balanced.

2. The key to screw pump performance is theoperation of the idler rotors in their housingbores. The idler rotors generate a hydrodynamicfilm to support themselves in their bores likejournal bearings. Since this film is self-generated,it depends on three operating characteristics ofthe pump—speed, discharge pressure, and fluidviscosity. The strength of the film is increased byincreasing the operating speed, by decreasingpressure, or by increasing the fluid viscosity. Thisis why screw pump performance capabilities arebased on pump speed, discharge pressure, andfluid viscosity.

The supply line is connected at the center ofthe pump housing in some pumps (fig. 4-8, viewB). Fluid enters into the pump’s suction port,which opens into chambers at the ends of thescrew assembly. As the screws turn, the fluid flowsbetween the threads at each end of the assembly.The threads carry the fluid along within thehousing toward the center of the pump to thedischarge port.

VANE PUMP

Vane-type hydraulic pumps generally havecircularly or elliptically shaped interior and flat

end plates. (Figure 4-9 illustrates a vane pumpwith a circular interior.) A slotted rotor is fixedto a shaft that enters the housing cavity throughone of the end plates. A number of smallrectangular plates or vanes are set into the slotsof the rotor. As the rotor turns, centrifugal forcecauses the outer edge of each vane to slide alongthe surface of the housing cavity as the vanes slidein and out of the rotor slots. The numerouscavities, formed by the vanes, the end plates, thehousing, and the rotor, enlarge and shrink as therotor and vane assembly rotates. An inlet port isinstalled in the housing so fluid may flow into thecavities as they enlarge. An outlet port is providedto allow the fluid to flow out of the cavities asthey become small.

The pump shown in figure 4-9 is referred toas an unbalanced pump because all of thepumping action takes place on one side of therotor. This causes a side load on the rotor. Somevane pumps are constructed with an ellipticallyshaped housing that forms two separate pumpingareas on opposite sides of the rotor. This cancelsout the side loads; such pumps are referred to asbalanced vane.

Usually vane pumps are fixed displacementand pump only in one direction. There are,however, some designs of vane pumps thatprovide variable flow. Vane pumps are generallyrestricted to service where pressure demand doesnot exceed 2000 psi. Wear rates, vibration, andnoise levels increase rapidly in vane pumps aspressure demands exceed 2000 psi.

RECIPROCATING PUMPS

The term reciprocating is defined as back-and-forth motion. In the reciprocating pump it is this

Figure 4-9.—Vane pump.

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back-and-forth motion of pistons inside ofcylinders that provides the flow of fluid. Recipro-cating pumps, like rotary pumps, operate onthe positive principle—that is, each strokedelivers a definite volume of liquid to thesystem.

The master cylinder of the automobile brakesystem, which is described and illustrated inchapter 2, is an example of a simple reciprocatingpump. Several types of power-operated hydraulicpumps, such as the radial piston and axial piston,are also classified as reciprocating pumps. Thesepumps are sometimes classified as rotary pumps,because a rotary motion is imparted to the pumpsby the source of power. However, the actualpumping is performed by sets of pistons recipro-cating inside sets of cylinders.

HAND PUMPS

There are two types of manually operatedreciprocating pumps—the single-action andthe double-action. The single-action pumpprovides flow during every other stroke, while thedouble-action provides flow during each stroke.Single-action pumps are frequently used inhydraulic jacks.

A double-action hand pump is illustrated infigure 4-10. This type of pump is used in someaircraft hydraulic systems as a source of hydraulicpower for emergencies, for testing certainsubsystems during preventive maintenanceinspections, and for determining the causes ofmalfunctions in these subsystems.

This pump (fig. 4-10) consists of a cylinder,a piston containing a built-in check valve (A), apiston rod, an operating handle, and a check valve(B) at the inlet port. When the piston is moved

Figure 4-10.—Hydraulic hand pump.

to the left, the force of the liquid in the outletchamber and spring tension cause valve A to close.This movement causes the piston to force theliquid in the outlet chamber through the outletport and into the system. This same pistonmovement causes a low-pressure area in the inletchamber. The difference in pressure between theinlet chamber and the liquid (at atmosphericpressure) in the reservior acting on check valveB causes its spring to compress; thus, opening thecheck valve. This allows liquid to enter the inletchamber.

When the piston completes this stroke to theleft, the inlet chamber is full of liquid. Thiseliminates the pressure difference between the inletchamber and the reservior, thereby allowingspring tension to close check valve B.

When the piston is moved to the right, theforce of the confined liquid in the inlet chamberacts on check valve A. This action compressesthe spring and opens check valve A whichallows the liquid to flow from the intakechamber to the outlet chamber. Because of thearea occupied by the piston rod, the outletchamber cannot contain all the liquid dischargedfrom the inlet chamber. Since liquids do notcompress, the extra liquid is forced out of theoutlet port into the system.

PISTON PUMPS

Piston pumps are made in a variety oftypes and configurations. A basic distinctionis made between axial and radial pumps. Theaxial piston pump has the cylinders parallelto each other and the drive shaft. The radialpiston design has the cylinders extendingradially outward from the drive shaft likethe spokes of a wheel. A further distinctionis made between pumps that provide a fixeddelivery and those able to vary the flow of thefluid. Variable delivery pumps can be furtherdivided into those able to pump fluid from zeroto full delivery in one direction of flow and thoseable to pump from zero the full delivery in eitherdirection.

All piston pumps used in Navy shipboardsystems have the cylinders bored in a cylinderblock that is mounted on bearings within ahousing. This cylinder block assembly rotates withthe pump drive shaft.

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Radial Piston Pumps

Figure 4-11 illustrates the operation of theradial piston pump. The pump consists of a pintle,which remains stationary and acts as a valve; a

Figure 4-11.—Principles of operation of the radial pistonpump.

cylinder block, which revolves around the pintleand contains the cylinders in which the pistonsoperate; a rotor, which houses the reaction ringof hardened steel against which the piston headspress; and a slide block, which is used to controlthe length of the piston strokes. The slide blockdoes not revolve but houses and supports therotor, which does revolve due to the friction setup by the sliding action between the piston headsand the reaction ring. The cylinder block isattached to the drive shaft.

Referring to view A of figure 4-11, assume thatspace X in one of the cylinders of the cylinderblock contains liquid and that the respective pistonof this cylinder is at position 1. When the cylinderblock and piston are rotated in a clockwisedirection, the piston is forced into its cylinder asit approaches position 2. This action reduces thevolumetric size of the cylinder and forces aquantity of liquid out of the cylinder and into theoutlet port above the pintle. This pumping actionis due to the rotor being off-center in relation tothe center of the cylinder block.

In figure 4-11 view B, the piston has reachedposition 2 and has forced the liquid out of theopen end of the cylinder through the outlet abovethe pintle and into the system. While the pistonmoves from position 2 to position 3, the open endof the cylinder passes over the solid part of thepintle; therefore, there is no intake or dischargeof liquid during this time. As the piston andcylinder move from position 3 to position 4,centrifugal force causes the piston to moveoutward against the reaction ring of the rotor.During this time the open end of the cylinder isopen to the intake side of the pintle and, therefore,fills with liquid. As the piston moves fromposition 4 to position 1, the open end of thecylinder is against the solid side of the pintle andno intake or discharge of liquid takes place. Afterthe piston has passed the pintle and starts towardposition 2, another discharge of liquid takes place.Alternate intake and discharge continues as therotor revolves about its axis-intake on one sideof the pintle and discharge on the other, as thepiston slides in and out.

Notice in views A and B of figure 4-11 thatthe center point of the rotor is different from thecenter point of the cylinder block. The differenceof these centers produces the pumping action. Ifthe rotor is moved so that its center point is thesame as that of the cylinder block, as shown infigure 4-11, view C, there is no pumping action,since the piston does not move back and forth inthe cylinder as it rotates with the cylinder block.

4-10

The flow in this pump can be reversed bymoving the slide block, and therefore the rotor,to the right so the relation of the centers of therotor and the cylinder block is reversed from theposition shown in views A and B of figure 4-11.View D shows this arrangement. Liquid enters thecylinder as the piston travels from position 1 toposition 2 and is discharged from the cylinder asthe piston travels from position 3 to 4.

In the illustrations the rotor is shown in thecenter, the extreme right, or the extreme left inrelation to the cylinder block. The amount ofadjustment in distance between the two centersdetermines the length of the piston stroke, whichcontrols the amount of liquid flow in and out ofthe cylinder. Thus, this adjustment determines thedisplacement of the pump; that is, the volume ofliquid the pump delivers per revolution. Thisadjustment may be controlled in different ways.Manual control by a handwheel is the simplest.The pump illustrated in figure 4-11 is controlledin this way. For automatic control of delivery

to accommodate varying volume requirementsduring the operating cycle, a hydraulicallycontrolled cylinder may be used to position theslide block. A gear-motor controlled by a pushbutton or a limit switch is sometimes used for thispurpose.

Figure 4-11 is shown with four pistons for thesake of simplicity. Radial pumps are actuallydesigned with an odd number of pistons (fig.4-12). This is to ensure that no more than onecylinder is completely blocked by the pintle at anyone time. If there were an even number of pistonsspaced evenly around the cylinder block (forexample, eight), there would be occasions whentwo of the cylinders would be blocked by thepintle, while at other times none would beblocked. This would cause three cylinders to dis-charge at one time and four at one time, causingpulsations in flow. With an odd number of pistonsspaced evenly around the cylinder block, only onecylinder is completely blocked by the pintle at anyone time. This reduces pulsations of flow.

Figure 4-12.—Nine-piston radial piston pump.

4-11

Axial Piston Pumps

In axial piston pumps of the in-line type,where the cylinders and the drive shaft are parallel(fig. 4-13), the reciprocating motion is created bya cam plate, also known as a wobble plate, tiltingplate, or swash plate. This plate lies in a planethat cuts across the center line of the drive shaftand cylinder barrel and does not rotate. In afixed-displacement pump, the cam plate will berigidly mounted in a position so that it intersectsthe center line of the cylinder barrel at an angleapproximately 25 degrees from perpendicular.Variable-delivery axial piston pumps are designedso that the angle that the cam plate makes witha perpendicular to the center line of the cylinderbarrel may be varied from zero to 20 or 25 degreesto one or both sides. One end of each piston rodis held in contact with the cam plate as the cylinderblock and piston assembly rotates with the driveshaft. This causes the pistons to reciprocate withinthe cyIinders. The length of the piston stroke isproportional to the angle that the cam plate is setfrom perpendicular to the center line of thecylinder barrel.

A variation of axial piston pump is thebent-axis type shown in figure 4-14. This type doesnot have a tilting cam plate as the in-line pumpdoes. Instead, the cylinder block axis is variedfrom the drive shaft axis. The ends of the

Figure 4-14.—Bent-axis axial piston pump.

connecting rods are retained in sockets on a discthat turns with the drive shaft. The cylinder blockis turned with the drive shaft by a universal jointassembly at the intersection of the drive shaft andthe cylinder block shaft. In order to vary the pumpdisplacement, the cylinder block and valve plateare mounted in a yoke and the entire assemblyis swung in an are around a pair of mountingpintles attached to the pump housing.

The pumping action of the axial piston pumpis made possible by a universal joint or link.

Figure 4-13.—In-line axial piston pump.

4-12

Figure 4-15 is a series of drawings that illustrateshow the universal joint is used in the operationof this pump.

First, a rocker arm is installed on a horizontalshaft. (See fig. 4-15, view A.) The arm is joinedto the shaft by a pin so that it can be swung backand forth, as indicated in view B. Next, a ring isplaced around the shaft and secured to the rockerarm so the ring can turn from left to right asshown in view C. This provides two rotarymotions in different planes at the same time andin varying proportions as may be desired. Therocker arm can swing back and forth in one arc,and the ring can simultaneously move from left

Figure 4-15.–Relationship of the universal joint in operationof the axial piston pump.

to right in another arc, in a plane at right anglesto the plane in which the rocker arm turns.

Next, a tilting plate is added to the assembly.The tilting plate is placed at a slant to the axisof the shaft, as depicted in figure 4-15, view D.The rocker arm is then slanted at the same angleas the tilting plate, so that it lies parallel to thetilting plate. The ring is also parallel to, and incontact with, the tilting plate. The position of thering in relation to the rocker arm is unchangedfrom that shown in figure 4-15, view C.

Figure 4-15, view E, shows the assembly afterthe shaft, still in a horizontal position, has beenrotated a quarter turn. The rocker arm is still inthe same position as the tilting plate and is nowperpendicular to the axis of the shaft. The ringhas turned on the rocker pins, so that it haschanged its position in relation to the rocker arm,but it remains parallel to, and in contact with, thetilting plate.

View F of figure 4-15 shows the assembly afterthe shaft has been rotated another quarter turn.The parts are now in the same position as shownin view D, but with the ends of the rocker armreversed. The ring still bears against the tiltingplate.

As the shaft continues to rotate, the rockerarm and the ring turn about their pivots, with eachchanging its relation to the other and with the ringalways bearing on the plate.

Figure 4-15, view G, shows a wheel added tothe assembly. The wheel is placed upright andfixed to the shaft, so that it rotates with the shaft.In addition, two rods, A and B, are looselyconnected to the tilting ring and extend throughtwo holes standing opposite each other in the fixedwheel. As the shaft is rotated, the fixed wheelturns perpendicular to the shaft at all times. Thetilting ring rotates with the shaft and alwaysremains tilted, since it remains in contact with thetilting plate. Referring to view G, the distancealong rod A, from the tilting ring to the fixedwheel, is greater than the distance along rod B.As the assembly is rotated, however, the distancealong rod A decreases as its point of attachmentto the tilting ring moves closer to the fixed wheel,while the distance along rod B increases. Thesechanges continue until after a half revolution, atwhich time the initial positions of the rods havebeen reversed. After another half revolution, thetwo rods will again be in their original positions.

As the assembly rotates, the rods move in andout through the holes in the fixed wheel. This isthe way the axial piston pump works. To get apumping action, place pistons at the ends of the

4-13

rods, beyond the fixed wheel, and insert them intocylinders. The rods must be connected to thepistons and to the wheel by ball and socket joints.As the assembly rotates, each piston moves backand forth in its cylinder. Suction and dischargelines can be arranged so that liquid enters thecylinders while the spaces between the pistonheads and the bases of the cylinders are increasing,and leaves the cylinders during the other half ofeach revolution when the pistons are moving inthe opposite direction.

The main parts of the pump are the driveshaft, pistons, cylinder block, and valve and swashplates. There are two ports in the valve plate.These ports connect directly to openings in theface of the cylinder block. Fluid is drawn into oneport and forced out the other port by thereciprocating action of the pistons.

IN-LINE VARIABLE-DISPLACEMENTAXIAL PISTON PUMP.— When the drive shaftis rotated, it rotates the pistons and the cylinderblock with it. The swash plate placed at an anglecauses the pistons to move back and forth in thecylinder block while the shaft, piston, cylinderblock, and swash plate rotate together. (The shaft,piston, cylinder block, and swash plate togetheris sometimes referred to as the rotating group orassembly.) As the pistons reciprocate in thecylinder block, fluid enters one port and is forcedout the other.

Figure 4-13 shows piston A at the bottom ofits stroke. When piston A has rotated to theposition held by piston B, it will have movedupward in its cylinder, forcing fluid through theoutlet port during the entire distance. During theremainder of the rotation back to it originalposition, the piston travels downward in thecylinder. This action creates a low-pressure areain the cylinder. The difference in pressure betweenthe cylinder inlet and the reservoir causes fluidto flow into the inlet port to the cylinder. Sinceeach one of the pistons performs the sameoperation in succession, fluid is constantly beingtaken into the cylinder bores through the inlet portand discharged from the cylinder bores intothe system. This action provides a steady,nonpulsating flow of fluid.

The tilt or angle of the swash plate determinesthe distance the pistons move back and forth intheir cylinders; thereby, controlling the pumpoutput.

When the swash plate is at a right angle to theshaft, and the pump is rotating, the pistons donot reciprocate; therefore, no pumping action

takes place. When the swash plate is tilted awayfrom a right angle, the pistons reciprocate andfluid is pumped.

Since the displacement of this type of pumpis varied by changing the angle of the tilting box,some means must be used to control the changesof this angle. Various methods are used to controlthis movement—manual, electric, pneumatic, orhydraulic.

STRATOPOWER PUMP.— Another type ofaxial piston pump, sometimes referred to as anin-line pump, is commonly referred to as aStratopower pump. This pump is availablein either the fixed-displacement type or thevariable-displacement type.

Two major functions are performed by theinternal parts of the fixed-displacement Strato-power pump. These functions are mechanicaldrive and fluid displacement.

The mechanical drive mechanism is shown infigure 4-16. In this type of pump, the pistons andblock do not rotate. Piston motion is caused byrotating the drive cam displacing each piston thefull height of the drive cam during each revolutionof the shaft. The ends of the pistons are attachedto a wobble plate supported by a freed center pivotand are held inconstant contact with the cam face.As the high side of the rotating drive camdepresses one side of the wobble plate, the otherside of the wobble plate is withdrawn an equalamount, moving the pistons with it. The two creepplates are provided to decrease wear on therevolving cam.

A schematic diagram of the displacement offluid is shown in figure 4-17. Fluid is displacedby axial motion of the pistons. As each pistonadvances in its respective cylinder block bore,pressure opens the check valve and a quantity offluid is forced past it. Combined back pressureand check valve spring tension close the check

Figure 4-16.—Mechanical drive—Stratopower pump.

4-14

Figure 4-17.—Fluid displacement—Stratopower pump.

valve when the piston advances to its foremostposition. The low-pressure area occurring in thecylinder during the piston return causes fluid toflow from the reservoir into the cylinder.

The internal features of the variable-displacement Stratopower pump are illustrated infigure 4-18. This pump operates similarly to the

fixed-displacement Stratopower pump; however,this pump provides the additional function ofautomatically varying the volume output.

This function is controlled by the pressure inthe hydraulic system. For example, let us take apump rated at 3000 psi, and providing flow to a3000 psi system. As system pressure approaches,say 2850 psi, the pump begins to unload (deliverless flow to the system) and is fully unloaded (zeroflow) at 3000 psi.

The pressure regulation and flow arecontrolled by internal bypasses that automaticallyadjust fluid delivery to system demands.

The bypass system is provided to supplyself-lubrication, particularly when the pump is innonflow operation. The ring of bypass holes inthe pistons are aligned with the bypass passageeach time a piston reaches the very end of itsforward travel. This pumps a small quantity offluid out of the bypass passage back to the supplyreservoir and provides a constant changing offluid in the pump. The bypass is designed to pumpagainst a considerable back pressure for use withpressurized reservoirs.

Figure 4-18.—Internal features of Stratopower variable-displacement pump.

4-15

CHAPTER 5

FLUID LINES AND FITTINGS

The control and application of fluid powerwould be impossible without suitable means oftransferring the fluid between the reservoir, thepower source, and the points of application. Fluidlines are used to transfer the fluid, and fittingsare used to connect the lines to the power sourceand the points of application.

This chapter is devoted to fluid lines andfittings. After studying this chapter, you shouldhave the knowledge to identify themonly used lines and fittings, andexplain the procedure for fabricating,labeling the lines.

TYPES OF LINES

The three types of lines used insystems are pipe (rigid), tubingand hose (flexible). A number ofconsidered when the type of line is

most com-be able totesting, and

fluid power(semirigid),factors areselected for

a particular fluid system. These factors includethe type of fluid, the required system pressure,and the location of the system. For example,heavy pipe might be used for a large stationaryfluid power system, but comparatively lightweighttubing must be used in aircraft and missilesystems because weight and space are criticalfactors. Flexible hose is required in installationswhere units must be free to move relative to eachother.

PIPES AND TUBING

There are three important dimensions of anytubular product—outside diameter (OD), insidediameter (ID), and wall thickness. Sizes of pipeare listed by the nominal (or approximate) ID andthe wall thickness. Sizes of tubing are listed bythe actual OD and the wall thickness.

SELECTION OF PIPES AND TUBING

The material, ID, and wall thickness arethe three primary considerations in the selec-tion of lines for a particular fluid powersystem.

The ID of a line is important, since itdetermines how much fluid can pass through theline in a given time period (rate of flow)without loss of power due to excessive frictionand heat. The velocity of a given flow is lessthrough a large opening than through a smallopening. If the ID of the line is too small for theamount of flow, excessive turbulence and frictionheat cause unnecessary power loss and overheatedfluid.

Sizing of Pipes and Tubing

Pipes are available in three different weights:standard (STD), or Schedule 40; extra strong(XS), or Schedule 80; and double extra strong(XXS). The schedule numbers range from 10to 160 and cover 10 distinct sets of wallthickness. (See table 5-1.) Schedule 160 wallthickness is slightly thinner than the double extrastrong.

As mentioned earlier, the size of pipes isdetermined by the nominal (approximate) ID. Forexample, the ID for a 1/4-inch Schedule 40 pipeis 0.364 inch, and the ID for a 1/2-inch Schedule40 pipe is 0.622 inch.

It is important to note that the IDs of all pipesof the same nominal size are not equal. This isbecause the OD remains constant and the wallthickness increases as the schedule numberincreases. For example, a nominal size 1-inchSchedule 40 pipe has a 1.049 ID. The same sizeSchedule 80 pipe has a 0.957 ID, while Schedule

5-1

Table 5-1.—Wall Thickness Schedule Designations for Pipe

160 pipe has a 0.815 ID. In each case the OD is1.315 (table 5-1) and the wall thicknesses are

0.133 0.179

and 0.250 respectively. Note

that the difference between the OD and IDincludes two wall thicknesses and must be dividedby 2 to obtain the wall thickness.

Tubing differs from pipe in its size classi-fication. Tubing is designated by its actual OD.(See table 5-2.) Thus, 5/8-inch tubing has an ODof 5/8 inch. As indicated in the table, tubing isavailable in a variety of wall thicknesses. Thediameter of tubing is often measured andindicated in 16ths. Thus, No. 6 tubing is 6/16 or3/8 inch, No. 8 tubing is 8/16 or 1/2 inch, andso forth.

The wall thickness, material used, and IDdetermine the bursting pressure of a line or fitting.The greater the wall thickness in relation to theID and the stronger the metal, the higher thebursting pressure. However, the greater the ID fora given wall thickness, the lower the burstingpressure, because force is the product of area andpressure.

Materials

The pipe and tubing used in fluid powersystems are commonly made from steel, copper,brass, aluminum, and stainless steel. Each of these

metals has its own distinct advantages ordisadvantages in certain applications.

Steel pipe and tubing are relatively inexpensiveand are used in many hydraulic and pneumaticsystems. Steel is used because of its strength,suitability for bending and flanging, andadaptability to high pressures and temperatures.Its chief disadvantage is its comparatively lowresistance to corrosion.

Copper pipe and tubing are sometimes usedfor fluid power lines. Copper has high resistanceto corrosion and is easily drawn or bent. However,it is unsatisfactory for high temperatures and hasa tendency to harden and break due to stress andvibration.

Aluminum has many of the characteristics andqualities required for fluid power lines. It has highresistance to corrosion and is easily drawn or bent.In addition, it has the outstanding characteristicof light weight. Since weight elimination is a vitalfactor in the design of aircraft, aluminum alloytubing is used in the majority of aircraft fluidpower systems.

Stainless-steel tubing is used in certain areasof many aircraft fluid power systems. As a generalrule, exposed lines and lines subject to abrasionor intense heat are made of stainless steel.

An improperly piped system can lead toserious power loss and possible harmful fluid

5-2

Table 5-2.—Tubing Size Designation

contamination. Therefore in maintenance and PREPARATION OF PIPESrepair of fluid power system lines, the basic design AND TUBINGrequirements must be kept in mind. Two primaryrequirements are as follows:

1. The lines must have the correct ID toprovide the required volume and velocity of flowwith the least amount of turbulence during alldemands on the system.

2. The lines must be made of the propermaterial and have the wall thickness to providesufficient strength to both contain the fluid at therequired pressure and withstand the surges ofpressure that may develop in the system.

Fluid power systems are designed as compactlyas possible, to keep the connecting lines short.Every section of line should be anchored securelyin one or more places so that neither the weightof the line nor the effects of vibration are carriedon the joints. The aim is to minimize stressthroughout the system.

Lines should normally be kept as short andfree of bends as possible. However, tubing shouldnot be assembled in a straight line, because a bendtends to eliminate strain by absorbing vibrationand also compensates for thermal expansion and

5-3

contraction. Bends are preferred to elbows,because bends cause less of a power loss. A fewof the correct and incorrect methods of installingtubing are illustrated in figure 5-1.

Bends are described by their radius measure-ments. The ideal bend radius is 2 1/2 to 3 timesthe ID, as shown in figure 5-2. For example, ifthe ID of a line is 2 inches, the radius of the bendshould be between 5 and 6 inches.

While friction increases markedly for sharpercurves than this, it also tends to increase up toa certain point for gentler curves. The increasesin friction in a bend with a radius of more than3 pipe diameters result from increased turbulencenear the outside edges of the flow. Particles offluid must travel a longer distance in making thechange in direction. When the radius of the bendis less than 2 1/2 pipe diameters, the increasedpressure loss is due to the abrupt change in thedirection of flow, especially for particles near theinside edge of the flow.

During your career in the Navy, you may berequired to fabricate new tubing to replacedamaged or failed lines. Fabrication of tubingconsists of four basic operations: cutting,deburring, bending, and joint preparation.

Tube Cutting and Deburring

The objective of cutting tubing is to producea square end that is free from burrs. Tubing maybe cut using a standard tube cutter (fig. 5-3), achipless cutter (fig. 5-4), or a fine-toothedhacksaw if a tube cutter is not available.

When you use the standard tube cutter, placethe tube in the cutter with the cutting wheel at thepoint where the cut is to be made. Apply lightpressure on the tube by tightening the adjusting

Figure 5-2.—Ideal bend radius.

knob. Too much pressure applied to the cuttingwheel at onetime may deform the tubing or causeexcessive burrs. Rotate the cutter toward its openside (fig. 5-3). As you rotate the cutter, adjust thetightening knob after each complete turn tomaintain light pressure on the cutting wheel.

When you use the chipless cutter, take thefollowing steps:

1. Select the chipless cutter according totubing size.

2. Rotate the cutter head to accept the tubingin the cutting position. Check that the cutterratchet is operating freely and that the cutter wheelis clear of the cutter head opening (fig. 5-4).

3. Center the tubing on two rollers and thecutting blade.

4. Use the hex key provided with theturn the drive screw in until the cuttertouches the tube.

Figure 5-1.—Correct and incorrect methods of installing tubing.

5-4

kit towheel

Figure 5-3.—Tube cutting.

5. Tighten the drive screw 1/8 to 1/4 turn. Donot overtighten the drive screw. Overtighteningcan damage soft tubing or cause excessive wearor breakage of the cutter wheel in hard tubing.

6. Swing the ratchet handle back and forththrough the available clearance until there is anoticeable ease of rotation. Avoid putting sideforce on the cutter handle. Side force will causethe cutter wheel to break.

7. Tighten the drive screw an additional 1/8to 1/4 turn and swing the ratchet handle back andforth, retightening the drive screw as needed untilthe cut is completed. The completed cut shouldbe 1/2 degree square to the tube centerline.

Figure 5-4.—Chipless cutter.

After the tubing is cut, remove all burrs andsharp edges from inside and outside of the tube(fig. 5-5) with deburring tools. Clean out thetubing. Make sure no foreign particles remain.

A convenient method for cutting tubing witha hacksaw is to place the tube in a flaring blockand clamp the block in a vice. After cutting thetubing with a hacksaw, remove all saw marks byfiling.

Tube Bending

The objective in tube bending is to obtain asmooth bend without flattening the tube. Tubebending is usually done with either a hand tubebender or a mechanically operated bender.

Figure 5-5.—Properly burred tubing.

5-5

Figure 5-6.—Bending tubing with hand-operated tube bender.

HAND TUBE BENDER.— The hand tubetubing. The radius block is marked in degrees ofbender shown in figure 5-6 consists of a handle,bend ranging from 0 to 180 degrees. The slide bara radius block, a clip, and a slide bar. The handlehas a mark which is lined up with the zero markand slide bar are used as levers to provide theon the radius block. The tube is inserted in themechanical advantage necessary to bend thetube bender, and after the marks are lined up, the

5-6

Figure 5-7.—Mechanically operated tube bender.

slide bar is moved around until the mark on theslide bar reaches the desired degree of bend onthe radius block. See figure 5-6 for the sixprocedural steps in tube bending with thehand-operated tube bender.

MECHANICAL TUBE BENDER.— Thetube bender shown in figure 5-7 is issued as a kit.The kit contains the equipment necessary forbending tubing from 1/4 inch to 3/4 inch indiameter.

This tube bender is designed for use withaircraft grade, high-strengths stainless-steel

tubing, as well as all other metal tubing. It isdesigned to be fastened to a bench or tripod. Thebase is formed to provide a secure grip in a vise.

This type of tube bender uses a hand crankand gears. The forming die is keyed to the drivegear and is secured by a screw.

The forming die on the mechanical tubebender is calibrated in degrees, similarly to theradius block of the hand bender. A length ofreplacement tubing may be bent to a specifiednumber of degrees or it may be bent to duplicatea bend either in a damaged tube or in a pattern.Duplicating a bend of a damaged tube or of apattern is done by laying the sample or patternon top of the tube being bent and slowly bendingthe new tube to the required bend.

Tube Flaring

Tube flaring is a method of forming the endof a tube into a funnel shape so it can be held bya threaded fitting. When a flared tube is prepared,a flare nut is slipped onto the tube and the endof the tube is flared. During tube installation, theflare is seated to a fitting with the inside of theflare against the cone-shaped end of the fitting,and the flare nut is screwed onto the fitting,pulling the inside of the flare against the seatingsurface of the fitting.

Either of two flaring tools (fig. 5-8) may beused. One gives a single flare and the other givesa double flare. The flaring tool consists of a splitdie block that has holes for various sizes of tubing,

Figure 5-8.—Flaring tools.

5-7

a clamp to lock the end of the tubing inside thedie block, and a yoke with a compressor screwand cone that slips over the die block and formsthe 45-degree flare on the end of the tube. Thescrew has a T-handle. A double flaring tube hasadaptors that turn in the edge of the tube beforea regular 45-degree double flare is made.

To use the single flaring tool, first check tosee that the end of the tubing has been cut offsquarely and has had the burrs removed fromboth inside and outside. Slip the flare nut ontothe tube before you make the flare. Then, openthe die block. Insert the end of the tubing intothe hole corresponding to the OD of the tubingso that the end protrudes slightly above the topface of the die blocks. The amount by which thetubing extends above the blocks determines thefinished diameter of the flare. The flare must belarge enough to seat properly against the fitting,but small enough that the threads of the flare nutwill slide over it. Close the die block and securethe tool with the wing nut. Use the handle of theyoke to tighten the wing nut. Then place the yokeover the end of the tubing and tighten the handleto force the cone into the end of the tubing. Thecompleted flare should be slightly visible abovethe face of the die blocks.

FLEXIBLE HOSE

Shock-resistant, flexible hose assemblies arerequired to absorb the movements of mountedequipment under both normal operating condi-tions and extreme conditions. They are alsoused for their noise-attenuating properties andto connect moving parts of certain equipment.The two basic hose types are synthetic rubberand polytetrafluoroethylene (PTFE), such asDu Pont’s Teflon®fluorocarbon resin.

Figure 5-9.—Synthetic rubber hoses.

pressure ranges: low, medium, and high. Theouter cover is designed to withstand external abuseand contains identification markings.

Synthetic rubber hoses with rubber covers areidentified with the military specification number,the size by dash number, the quarter and year ofcure or manufacture, and the manufacturer’s codeidentification number or federal supply codenumber printed along their layline (fig. 5-10, viewA). The layline is a legible marking parallel to thelongitudinal axis of a hose used in determiningthe straightness or lay of the hose.

Synthetic rubber hoses with wire braid coverare identified by bands (fig. 5-10, view B) wrappedaround the hose ends and at intervals along thelength of the hose.

Sizing

Rubber hoses are designed for specific fluid,temperature, and pressure ranges and areprovided in various specifications. Rubber hoses(fig. 5-9) consist of a minimum three layers; aseamless synthetic rubber tube reinforced with oneor more layers of braided or spiraled cotton, wire,or synthetic fiber; and an outer cover. The innertube is designed to withstand the attack of thefluid that passes through it. The braided orspiraled layers determine the strength of the hose.The greater the number of these layers, the greateris the pressure rating. Hoses are provided in three

5-8

The size of a flexible hose is identified by thedash (-) number, which is the ID of the hoseexpressed in 16ths of an inch. For example, theID of a -64 hose is 4 inches. For a few hose stylesthis is the nominal and not the true ID.

Cure Date

Synthetic rubber hoses will deteriorate fromaging. A cure date is used to ensure that they donot deteriorate beyond material and performancespecifications. The cure date is the quarter andyear the hose was manufactured. For example,

Technical Directive for Piping Devices andFlexible Hose Assemblies, NAVSEA S6430-AE-TED-010. volume 1. provide detailed instructionson discarding and downgrading of rubber hosesexceeding their shelf life.

PFTE

1Q89 orthe first

Figure 5-10.—Hose identification.

1/89 means the hose was made duringquarter (1 Jan to 31 Mar) of 1989.

The cure date limits the length of time a rubberhose can be stored, in bulk or as an assembly,prior to being placed into service. The storage orshelf life for rubber hose is 4 years. For the hosemanufactured in 1Q89, the storage or shelf lifewill end on the 31st of March 1993. At this point,the hose is no longer considered usable and shouldbe discarded or downgraded. The Aviation Hoseand Tube Manual, NAVAIR 01-1A-20, and the

5-9

PFTE hose is a flexible hose designed to meetthe requirements of higher operating pressures andtemperatures in present fluid power systems. Thistype of hose is made from a chemical resin, whichis processed and extruded into a tube shaped toa desired size. It is reinforced with one or morelayers of braided stainless-steel wire or with aneven number of spiral wrap layers with an outerwire braid layer.

PTFE hose is unaffected by all fluids presentlyused in fluid power systems. It is inert to acids,both concentrated and diluted. Certain PFTEhose may be used in systems where operatingtemperatures range from –100°F to +500°F.PTFE is nonflammable; however, where thepossibility of open flame exists, a special asbestosfire sleeve should be used.

PFTE hose will not absorb moisture. This,together with its chemical inertness and anti-adhesive characteristics, makes it ideal for missilefluid power systems where noncontamination andcleanliness are essential.

In lieu of layline marking, PTFE hoses areidentified by metal or pliable plastic bands at theirends and at intervals along their length. Figure5-10, view C, shows a hose label for a PTFE hose.Usually the only condition that will shorten thelife of PTFE hose is excessive temperature. Forthis reason there is no manufacture date listed onthe identification tag.

APPLICATION

As mentioned earlier, flexible hose is availablein three pressure ranges: low, medium, and high.When replacing hoses, it is important to ensurethat the replacement hose is a duplicate of the oneremoved in length, OD, material, type andcontour, and associated markings. In selectinghose, several precautions must be observed. Theselected hose must

1.2.

be compatible with the system fluid,have a rated pressure greater than the designpressure of the system,

3. be designed to give adequate performance andservice for infrequent transient pressure peaksup to 150 percent of the working pressure ofthe hose, and

4. have a safety factor with a burst pressure ata minimum of 4 times the rated workingpressure.

There are temperature restrictions applied tothe use of hoses. Rubber hose must not be usedwhere the operating temperature exceeds 200°F.PTFE hoses in high-pressure air systems must notbe used where the temperature exceeds 350°F.PTFE hoses in water and steam drain applicationsmust not be used where the operating temperatureexceeds 380°F.

FABRICATION AND TESTING

The fabrication of flexible hose assemblies iscovered in applicable training manuals, technicalpublications, and NAVAIR 01-1A-20. After ahose assembly has been completely fabricated itmust be cleaned, visually inspected for foreignmaterials, and proof tested.

A hose assembly is proof tested by theapplication of a nondestructive pressure for aminimum of 1 minute but not longer than 5

minutes to ensure that it will withstand normalworking pressures. The test pressure, known asnormal proof pressure, is twice the rated workingpressure. While the test pressure is being applied,the hose must not burst, leak, or show signsof fitting separation. NAVAIR 01-1A-20 andNAVSEA S6430-AE-TED-010, volume 1, providedetailed instructions on cleaning of hoses, cleaningand test media, proof pressure and proof testing.

After proof testing is completed, the hose mustbe flushed and dried and the ends capped orplugged to keep dirt and other contaminants outof the hose.

IDENTIFICATION

The final step after fabrication and satisfac-tory testing of a hose assembly is the attachmentof identification tags as shown in figure 5-11 (forships) and in figure 5-12 (for aircraft). The tagshown in figure 5-12, view B, is used in areaswhere a tag maybe drawn into an engine intake.Hose assemblies to be installed in aircraft fuel andoil tanks are marked with an approved electricengraver on the socket-wrench flats with therequired information.

Figure 5-11.—Hose assembly identification tags (ships).

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Figure 5-12.—Hose assembly

INSTALLATION

Flexible hose must

identification tags (aircraft).

not be twisted duringinstallation, since this reduces the life of the hoseconsiderably and may cause the fittings to loosenas well. You can determine whether or not a hoseis twisted by looking at the layline that runs alongthe length of the hose. If the layline does not spiralaround the hose, the hose is not twisted. If thelayline does spiral around the hose, the hose istwisted (fig. 5-13, view B) and must be untwisted.

Flexible hose should be protected from chafingby using a chafe-resistant covering wherevernecessary.

The minimum bend radius for flexible hosevaries according to the size and construction ofthe hose and the pressure under which thesystem operates. Current applicable technicalpublications contain tables and graphs showingminimum bend radii for the different types ofinstallations. Bends that are too sharp will reducethe bursting pressure of flexible hose considerablybelow its rated value.

Flexible hose should be installed so that it willbe subjected to a minimum of flexing duringoperation. Support clamps are not necessary withshort installations; but for hose of considerablelength (48 inches for example), clamps should beplaced not more than 24 inches apart. Closer

5-11

Figure 5-13.—Correct and incorrect installation of flexiblehose.

supports are desirable and in some cases may berequired.

A flexible hose must never be stretched tightlybetween two fittings. About 5 to 8 percent of thetotal length must be allowed as slack to providefreedom of movement under pressure. Whenunder pressure, flexible hose contracts in lengthand expands in diameter. Examples of correct andincorrect installations of flexible hose areillustrated in figure 5-13.

PFTE hose should be handled carefully duringremoval and installation. Some PFTE hose is pre-formed during fabrication. This type of hose tendsto form itself to the installed position in the sys-tem. To ensure its satisfactory function and reducethe likelihood of failure, anyone who works withPFTE hose should observe the following rules:

1.2.3.

4.

Do not exceed recommended bend limits.Do not exceed twisting limits.Do not straighten a bent hose that hastaken a permanent set.Do not hang, lift, or support objects fromPFTE hose.

Once flexible hose assemblies are installed,there are no servicing or maintenance require-ments other than periodic inspections. Theseinspections are conducted according to mainte-nance instruction manuals (MIMs), maintenancerequirement cards (MRCs), and depot-levelspecifications.

TYPES OF FITTINGSAND CONNECTORS

Some type of connector or fitting must beprovided to attach the lines to the components ofthe system and to connect sections of line toeach other. There are many different types ofconnectors and fittings provided for this purpose.The type of connector or fitting required for aspecific system depends on several factors. Onedetermining factor, of course, is the type of fluidline (pipe, tubing, or flexible hose) used inthe system. Other determining factors are thetype of fluid medium and the maximum operatingpressure of the system. Some of the most commontypes of fittings and connectors are described inthe following paragraphs.

THREADED CONNECTORS

There are several different types of threadedconnectors. In the type discussed in this section,both the connector and the end of the fluid line(pipe) are threaded. These connectors are used insome low-pressure fluid power systems and areusually made of steel, copper, or brass, and areavailable in a variety of designs.

Threaded connectors are made with standardpipe threads cut on the inside surface. The endof the pipe is threaded with outside threads.Standard pipe threads are tapered slightly toensure tight connections. The amount of taper isapproximately 3/4 inch in diameter per foot ofthread.

Metal is removed when a pipe is threaded,thinning the pipe and exposing new and roughsurfaces. Corrosion agents work more quickly atsuch points than elsewhere. If pipes are assembledwith no protective compound on the threads,corrosion sets in at once and the two sectionsstick together so that the threads seize whendisassembly is attempted. The result is damagedthreads and pipes.

To prevent seizing, a suitable pipe threadcompound is sometimes applied to the threads.The two end threads must be kept free of

5-12

compound so that it will not contaminate thefluid. Pipe compound, when improperly applied,may get inside the lines and components anddamage pumps and control equipment.

Another material used on pipe threads issealant tape. This tape, which is made of PTFE,provides an effective means of sealing pipeconnections and eliminates the necessity oftorquing connections to excessively high valuesin order to prevent pressure leaks. It also providesfor ease of maintenance whenever it is necessaryto disconnect pipe joints. The tape is applied overthe male threads, leaving the first thread exposed.After the tape is pressed firmly against thethreads, the joint is connected.

FLANGE CONNECTORS

Bolted flange connectors (fig. 5-14) aresuitable for most pressures now in use. Theflanges are attached to the piping by welding,brazing, tapered threads (for some low-pressuresystems), or rolling and bending into recesses.Those illustrated are the most common types offlange joints used. The same types of standardfitting shapes (tee, cross, elbow, and so forth) aremanufactured for flange joints. Suitable gasketmaterial must be used between the flanges.

WELDED CONNECTORS

The subassemblies of some fluid powersystems are connected by welded joints, especiallyin high-pressure systems which use pipe for fluidlines. The welding is done according to standard

Figure 5-14.—Four types of bolted flange connectors.

specifications which define the materials andtechniques.

BRAZED CONNECTORS

Silver-brazed connectors are commonly usedfor joining nonferrous (copper, brass, and soon)piping in the pressure and temperature rangewhere their use is practical. Use of this type ofconnector is limited to installations in which thepiping temperature will not exceed 425°F and thepressure in cold lines will not-exceed 3,000 psi.The alloy is melted by heating the joint with anoxyacetylene torch. This causes the alloy insertto melt and fill the few thousandths of an inchannular space between the pipe and the fitting.

A fitting of this type which has been removedfrom a piping system can be rebrazed into asystem, as in most cases sufficient alloy remainsin the insert groove for a second joint. New alloyinserts may be obtained for fittings which do nothave sufficient alloy remaining in the insert formaking a new joint.

FLARED CONNECTORS

Flared connectors are commonly used in fluidpower systems containing lines made of tubing.These connectors provide safe, strong, dependableconnections without the need for threading,welding, or soldering the tubing. The connectorconsists of a fitting, a sleeve, and a nut (fig. 5-15).

The fittings are made of steel, aluminum alloy,or bronze. The fitting used in a connection shouldbe made of the same material as that of the sleeve,the nut, and the tubing. For example, use steelconnectors with steel tubing and aluminum alloy

Figure 5-15.—Flared-tube fitting.

connectors with aluminum alloy tubing. Fittingsare made in union, 45-degree and 90-degreeelbow, tee, and various other shapes (fig. 5-16).

Tees, crosses, and elbows are self-explanatory.Universal and bulkhead fittings can be mountedsolidly with one outlet of the fitting extendingthrough a bulkhead and the other outlet(s) posi-tioned at any angle. Universal means the fittingcan assume the angle required for the specificinstallation. Bulkhead means the fitting is longenough to pass through a bulkhead and isdesigned so it can be secured solidly to thebulkhead.

For connecting to tubing, the ends of thefittings are threaded with straight machine threadsto correspond with the female threads of the nut.In some cases, however, one end of the fitting maybe threaded with tapered pipe threads to fit

Figure 5-16.—Flared-tube fittings.

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threaded ports in pumps, valves, and othercomponents. Several of these thread combinationsare shown in figure 5-16.

Tubing used with flare connectors must beflared prior to assembly. The nut fits over thesleeve and when tightened, it draws the sleeve andtubing flare tightly against the male fitting to forma seal.

The male fitting has a cone-shaped surfacewith the same angle as the inside of the flare. Thesleeve supports the tube so vibration does notconcentrate at the edge of the flare, and distributesthe shearing action over a wider area for addedstrength. Tube flaring is covered in Tools andTheir Uses, NAVEDTRA 10085 (series), andother applicable training manuals.

Correct and incorrect methods of installingflared-tube connectors are illustrated in figure5-17. Tubing nuts should be tightened with atorque wrench to the value specified in applicabletechnical publications.

If an aluminum alloy flared connector leaksafter being tightened to the required torque, itmust not be tightened further. Overtightening mayseverely damage or completely cut off the tubingflare or may result in damage to the sleeve or nut.The leaking connection must be disassembled andthe fault corrected.

If a steel tube connection leaks, it may betightened 1/6 turn beyond the specified torque inan attempt to stop the leakage; then if it still leaks,it must be disassembled and repaired.

Undertightening of connections may beserious, as this can allow the tubing to leak at theconnector bemuse of insufficient grip on the flareby the sleeve. The use of a torque wrench willprevent undertightening.

CAUTION

A nut should never be tightened whenthere is pressure in the line, as this will tendto damage the connection without addingany appreciable torque to the connection.

Figure 5-17.—Correct and incorrect methods of installing flared fittings.

5-14

FLARELESS-TUBE CONNECTORS

This type of connector eliminates all tubeflaring, yet provides a safe, strong, and depend-able tube connection. This connector consistsof a fitting, a sleeve or ferrule, and a nut.(See fig. 5-18.)

NOTE

Although the use of flareless tubeconnectors is widespread, NAVSEA policyis to reduce or eliminate use of flarelessfittings in newly designed ships; the extentto which flareless fittings are approved foruse in a particular ship is reflected inapplicable ship drawings.

Flareless-tube fittings are available in manyof the same shapes and thread combinations asflared-tube fittings. (See fig. 5-16.) The fitting hasa counterbore shoulder for the end of the tubingto rest against. The angle of the counterborecauses the cutting edge of the sleeve or ferrule tocut into the outside surface of the tube when thetwo are assembled.

The nut presses on the bevel of the sleeve andcauses it to clamp tightly to the tube. Resistanceto vibration is concentrated at this point ratherthan at the sleeve cut. When fully tightened, thesleeve or ferrule is bowed slightly at the midsectionand acts as a spring. This spring action of thesleeve or ferrule maintains a constant tensionbetween the body and the nut and thus preventsthe nut from loosening.

Prior to the installation of a new flareless-tubeconnector, the end of the tubing must be square,

Figure 5-18. —Flareless-tube connector.

concentric, and free of burrs. For the connectionto be effective, the cutting edge of the sleeve orferrule must bite into the periphery of the tube(fig. 5-19). This is ensured by presetting the sleeveor ferrule on the tube.

Presetting

Presetting consists of deforming the ferrule tobite into the tube OD and deforming the end ofthe tube to form a shallow conical ring seatingsurface. The tube and ferrule assembly should bepreset in a presetting tool that has an end sectionidentical to a fitting body but which is made ofspecially hardened steel. This tool hardness isneeded to ensure that all deformation at the tubeend seat goes into the tube.

Presetting is done with a hydraulic presettingtool or a manual presetting tool, either in the shopor aboard ship. The tool vendor’s instructionsmust be followed for the hydraulic presetting tool.If a presetting tool is not available, the fittingbody intended for installation is used in the samemanner as the manual presetting tool. (If analuminum fitting is used, it should not be reusedin the system.) The manual tool is used as follows:

WARNING

Failure to follow these instructions mayresult in improperly preset ferrules withinsufficient bite into the tube. Improperlypreset ferrules have resulted in joints thatpassed hydrostatic testing and operated forweeks or years, then failed catastrophicallyunder shock, vibration, or normal operat-ing loads. Flareless fitting failures have

Figure 5-19.—Unused ferrules.

5-15

caused personnel injury, damage to equip-ment, and unnecessary interruption ofpropulsion power.

1. Cut the tubing square and lightly deburrthe inside and outside corners. For corrosionresisting steel (CRES) tubing, use a hacksaw ratherthan a tubing cutter to avoid work hardening thetube end. For CRES, and if necessary for othermaterials, dress the tube end smooth and squarewith a file. Tube ends with irregular cutting markswill not produce satisfactory seating surfaceimpressions.

2. Test the hardness of the ferrule by makinga light scratch on the tubing at least 1/2 inch backfrom the tube end, using a sharp corner on theferrule. If the ferrule will not scratch the tube,no bite will be obtained. This test maybe omittedfor flush-type ferrules where the bite will bevisible. Moderate hand pressure is sufficient forproducing the scratch.

3. Lubricate the nut threads, the ferruleleading and trailing edges, and the preset toolthreads with a thread lubricant compatible withthe system. Slide the nut onto the tubing so thethreads face the tube end. Note whether theferrule is a flush type or recessed type (fig. 5-19),and slide the ferrule onto the tube so the cuttingedge is toward the tube end (large end toward thenut).

4. Bottom the end of the tubing in thepresetting tool. Slide the ferrule up into thepresetting tool, and confirm that the nut can bemoved down the tube sufficiently to expose atleast 1/8 inch of tubing past the ferrule after thepresetting operation (fig. 5-20) to allow forinspection of the ferrule.

5. While keeping the tube bottomed in thepresetting tool, tighten the nut onto the fittingbody until the ferrule just grips the tube byfriction. This ring grip point may be identified bylightly turning the tube or the presetting tool andslowly tightening the nut until the tube cannotbe turned in the presetting tool by hand.Mark the nut and the presetting tool at thisposition.

6. Tighten the nut according to the numberof turns given in table 5-3, depending on tubesize.

5-16

Figure 5-20.—Tube and ferrule assembled for preset-ting, showing nut position required for inspectingferrule.

Inspection

Disassemble and inspect the fitting as follows(mandatory):

1. Ensure that the end of the tubing has animpression of the presetting tool seat surface(circular appearing ring) for 360 degrees. A partialcircle, a visibly off-center circle, or a circle brokenby the roughness of the tube end is unsatisfactory.

2. Check for proper bite:

a. For flush-type ferrules, a raised ridge(fig. 5-21) of tube metal must be visible completelyaround the tube at the leading edge of the ferrule.The best practice is to obtain a ridge about 50percent of the ferrule edge thickness.

Table 5-3.—Number of Turns

Figure 5-21.—Ferrules installed on tube, preset and removedfor inspection.

b. For recessed-type ferrules, the leadingedge must be snug against the tube OD. Determinethis visually and by attempting to rock the ferruleon the tube.

3. Ensure that the nut end of the ferrule (bothtypes) is collapsed around the tube to providesupport against bending loads and vibration.

4. The ferrule (both types) must have little orno play along the direction of the tube run. Checkthis by trying to move the ferrule back and forthby hand. The ferrule will often be free to rotateon the tubing; this does not affect its function.

5. For flush-type ferrules, check that the gapbetween the raised metal ridge and the cutting endof the ferrule stays the same while the ferrule isrotated. (Omit this check for recessed-type ferrulesor if the flush-type ferrule will not rotate on thetube).

6. Check that the middle portion of the ferrule(both types) is bowed or sprung into an arc. Theleading edge of the ferrule may appear flattenedinto a cone shape; this is acceptable as long asthere is a bowed section near the middle of theferrule. If the whole leading section of the ferruleis flattened into a cone with no bowed section,the ferrule (and possibly the fitting body, if used)has been damaged by overtightening and will notseal reliably.

Final Assembly

When you make a final assembly in thesystem, use the following installation procedure:

1. Lubricate all threads with a liquid that iscompatible with the fluid to be used in the system.

2. Place the tube assembly in position andcheck for alignment.

3. Tighten the nut by hand until you feel anincrease in resistance to turning. This indicatesthat the sleeve or ferrule pilot has contacted thefitting.

4. If possible, use a torque wrench to tightenflareless tubing nuts. Torque values for specificinstallations are usually listed in the applicabletechnical publications. If it is not possible to usea torque wrench, use the following procedures fortightening the nuts:

After the nut is handtight, turn the nut 1/6turn (one flat on a hex nut) with a wrench. Usea wrench on the connector to prevent it fromturning while tightening the nut. After you installthe tube assembly, have the system pressure tested.Should a connection leak, you may tighten thenut an additional 1/6 turn (making a total of 1/3turn). If, after tightening the nut a total of 1/3turn, leakage still exists, remove the assembly andinspect the components of the assembly for scores,cracks, presence of foreign material, or damagefrom overtightening.

NOTE: Overtightening a flareless-tube nutdrives the cutting edge of the sleeve or ferruledeeply into the tube, causing the tube to beweakened to the point where normal vibrationcould cause the tube to shear. After you completethe inspection (if you do not find any dis-crepancies), reassemble the connection and repeatthe pressure test procedures.

CAUTION: Do not in any case tighten thenut beyond 1/3 turn (two flats on the hex nut);this is the maximum the fitting may be tightenedwithout the possibility of permanently damagingthe sleeve or the tube.

CONNECTORS FORFLEXIBLE HOSE

As stated previously, the fabrication of flexiblehose assemblies is covered in applicable trainingmanuals, technical publications, and NAVAIR01-1A-20. There are various types of end fittingsfor both the piping connection side and the hose

5-17

connection side of hose fittings. Figure 5-22 showscommonly used fittings.

Piping Connection Side of Hose Fitting

The piping side of an end fitting comes withseveral connecting variations: flange, JIC 37°flare, O-ring union, and split clamp, to name afew. Not all varieties are available for each hose.Therefore, installers must consult the militaryspecification and manufacturer’s data todetermine the specific end fittings available.

Hose Connection Side of Hose Fitting

Hose fittings are attached to the hose byseveral methods. Each method is determined by

the fitting manufacturer and takes into con-sideration such things as size, construction, wallthickness, and pressure rating. Hoses used forflexible connections use one of the followingmethods for attachment of the fitting to thehose.

ONE-PIECE REUSABLE SOCKET.— Thesocket component of the fitting is fabricated asa single piece. One-piece reusable sockets arescrewed or rocked onto the hose OD, followedby insertion of the nipple component.

SEGMENTED, BOLTED SOCKET.— Thesegmented, bolted socket consists of two or moresegments which are bolted together on the hoseafter insertion of the nipple component.

Figure 5-22.—End fittings and hose fittings.

5-18

SEGMENTED SOCKET, RING ANDBAND ATTACHED.— The segmented, ring andband attached socket consists of three or moresegments. As with the bolt-together segments, thesegments, ring and band are put on the hose afterinsertion of the nipple. A special tool is requiredto compress the segments.

SEGMENTED SOCKET, RING AND BOLTATTACHED.— The segmented, ring and boltattached socket consists of three or moresegments. As with other segmented socket-typefittings, the segments, ring, and nuts and boltsare put on the hose after insertion of the nipple.

SOLID SOCKET, PERMANENTLYATTACHED.— This type of socket is perma-nently attached to the hose by crimping orswaging. It is not reusable and is only foundon hose assemblies where operating conditionspreclude the use of other fitting types. Hoseassemblies with this type of fitting attachment arepurchased as complete hose assemblies from themanufacturer.

QUICK-DISCONNECT COUPLINGS

Self-sealing, quick-disconnect couplings areused at various points in many fluid powersystems. These couplings are installed at locationswhere frequent uncoupling of the lines is requiredfor inspection, test, and maintenance. Quick-disconnect couplings are also commonly used inpneumatic systems to connect sections of air hoseand to connect tools to the air pressure lines. Thisprovides a convenient method of attaching anddetaching tools and sections of lines without losingpressure.

Quick-disconnect couplings provide a meansfor quickly disconnecting a line without the lossof fluid from the system or the entrance offoreign matter into the system. Several types ofquick-disconnect couplings have been designed foruse in fluid power systems. Figure 5-23 illustrates

Figure 5-23.—Quick-disconnect coupling for air lines.

a coupling that is used with portable pneumatictools. The male section is connected to the toolor to the line leading from the tool. The femalesection, which contains the shutoff valve, isinstalled in the pneumatic line leading fromthe pressure source. These connectors can beseparated or connected by very little effort on thepart of the operator.

The most common quick-disconnect couplingfor hydraulic systems consists of two parts, heldtogether by a union nut. Each part contains avalve which is held open when the coupling isconnected, allowing fluid to flow in eitherdirection through the coupling. When thecoupling is disconnected, a spring in each partcloses the valve, preventing the loss of fluid andentrance of foreign matter.

MANIFOLDS

Some fluid power systems are equipped withmanifolds in the pressure supply and/or returnlines. A manifold is a fluid conductor thatprovides multiple connection ports. Manifoldseliminate piping, reduce joints, which are oftena source of leakage, and conserve space. Forexample, manifolds may be used in systems thatcontain several subsystems. One common lineconnects the pump to the manifold. There areoutlet ports in the manifold to provide con-nections to each subsystem. A similar manifoldmay be used in the return system. Lines from thecontrol valves of the subsystem connect to the inletports of the manifold, where the fluid combinesinto one outlet line to the reservoir. Somemanifolds are equipped with the check valves,relief valves, filters, and so on, required for thesystem. In some cases, the control valves aremounted on the manifold in such a manner thatthe ports of the valves are connected directly tothe manifold.

Manifolds are usually one of three types—sandwich, cast, or drilled. The sandwich type isconstructed of three or more flat plates. Thecenter plate (or plates) is machined for passages,and the required inlet and outlet ports are drilledinto the outer plates. The plates are then bondedtogether to provide a leakproof assembly. The casttype of manifold is designed with cast passagesand drilled ports. The casting may be iron, steel,bronze, or aluminum, depending upon the typeof system and fluid medium. In the drilled typeof manifold, all ports and passages are drilled ina block of metal.

5-19

A simple manifold is illustrated in figure 5-24.This manifold contains one pressure inlet port andseveral pressure outlet ports that can be blockedoff with threaded plugs. This type of manifoldcan be adapted to systems containing variousnumbers of subsystems. A thermal relief valvemay be incorporated in this manifold. In this case,the port labeled T is connected to the return lineto provide a passage for the relieved fluid to flowto the reservoir.

Figure 5-25 shows a flow diagram in amanifold which provides both pressure and returnpassages. One common line provides pressurizedfluid to the manifold, which distributes the fluidto any one of five outlet ports. The return sideof the manifold is similar in design. This manifoldis provided with a relief valve, which is connectedto the pressure and return passages. In the eventof excessive pressure, the relief valve opens andallows the fluid to flow from the pressure side ofthe manifold to the return side.

Figure 5-25.—Fluid manifold—flow diagram.

PRECAUTIONARY MEASURES

The fabrication, installation, and maintenanceof all fluid lines and connectors are beyond thescope of this training manual. However, there aresome general precautionary measures that applyto the maintenance of all fluid lines.

Regardless of the type of lines or connectorsused to make up a fluid power system, makecertain they are the correct size and strength and

perfectly clean on the inside. All lines must beabsolutely clean and free from scale and otherforeign matter. Iron or steel pipes, tubing, andfittings can be cleaned with a boiler tubewire brush or with commercial pipe cleaningapparatus. Rust and scale can be removed fromshort, straight pieces by sandblasting, providedthere is no danger that sand particles will remainlodged in blind holes or pockets after the piece

Figure 5-24 .—Fluid manifold.

5-20

is flushed. In the case of long pieces or pieces bent Open ends of pipes, tubing, hose, and fittingsto complex shapes, rust and scale can be removed should be capped or plugged when they are to beby pickling (cleaning metal in a chemical bath). stored for any considerable period. Rags or wasteParts must be degreased prior to pickling. The must not be used for this purpose, because theymanufacturer of the parts should provide deposit harmful lint which can cause severecomplete pickling instructions. damage to the fluid power system.

5-21

CHAPTER 6

VALVES

It is all but impossible to design a practicalfluid power system without some means ofcontrolling the volume and pressure of the fluidand directing the flow of fluid to the operatingunits. This is accomplished by the incorporationof different types of valves. A valve is defined asany device by which the flow of fluid may bestarted, stopped, or regulated by a movable partthat opens or obstructs passage. As appliedin fluid power systems, valves are used forcontrolling the flow, the pressure, and thedirection of the fluid flow.

Valves must be accurate in the control of fluidflow and pressure and the sequence of operation.Leakage between the valve element and the valveseat is reduced to a negligible quantity byprecision-machined surfaces, resulting in carefullycontrolled clearances. This is one of the veryimportant reasons for minimizing contaminationin fluid power systems. Contamination causesvalves to stick, plugs small orifices, and causesabrasions of the valve seating surfaces, whichresults in leakage between the valve element andvalve seat when the valve is in the closed position.Any of these can result in inefficient operationor complete stoppage of the equipment.

Valves may be controlled manually, electri-cally, pneumatically, mechanically, hydraulically,or by combinations of two or more of thesemethods. Factors that determine the method ofcontrol include the purpose of the valve, thedesign and purpose of the system, the location ofthe valve within the system, and the availabilityof the source of power.

The different types of valves used in fluidpower systems, their classification, and theirapplication are discussed in this chapter.

CLASSIFICATIONS

Valves are classified according to their use:flow control, pressure control, and directional

control. Some valves have multiple functions thatfall into more than one classification.

FLOW CONTROL VALVES

Flow control valves are used to regulate theflow of fluids in fluid-power systems. Control offlow in fluid-power systems is important becausethe rate of movement of fluid-powered machinesdepends on the rate of flow of the pressurizedfluid. These valves may be manually, hydrau-lically, electrically, or pneumatically operated.

Some of the different types of flow controlvalves are discussed in the following paragraphs.

BALL VALVES

Ball valves, as the name implies, are stopvalves that use a ball to stop or start a flow offluid. The ball, shown in figure 6-1, performs the

Figure 6-1.—Typical ball valve.

6-1

same function as the disk in other valves. As thevalve handle is turned to open the valve, the ballrotates to a point where part or all of the holethrough the ball is in line with the valve body inletand outlet, allowing fluid to flow through thevalve. When the ball is rotated so the hole isperpendicular to the flow openings of the valvebody, the flow of fluid stops.

Most ball valves are the quick-acting type.They require only a 90-degree turn to eithercompletely open or close the valve. However,many are operated by planetary gears. This typeof gearing allows the use of a relatively small

handwheel and operating force to operate a fairlylarge valve. The gearing does, however, increasethe operating time for the valve. Some ball valvesalso contain a swing check located within the ballto give the valve a check valve feature. Figure 6-2shows a ball-stop, swing-check valve with aplanetary gear operation.

In addition to the ball valves shown in figures6-1 and 6-2, there are three-way ball valves thatare used to supply fluid from a single source toone component or the other in a two-componentsystem (fig. 6-3).

Figure 6-2.—Typical ball-stop, swing-check valve.

6-2

Figure 6-3.—Three-way ball valve.

GATE VALVES

Gate valves are used when a straight-line flowof fluid and minimum flow restriction are needed.Gate valves are so-named because the part thateither stops or allows flow through the valveacts somewhat like a gate. The gate is usuallywedge-shaped. When the valve is wide open thegate is fully drawn up into the valve bonnet. Thisleaves an opening for flow through the valve thesame size as the pipe in which the valve is installed

(fig. 6-4). Therefore, there is little pressure dropor flow restriction through the valve.

Gate valves are not suitable for throttlingpurposes. The control of flow is difficult becauseof the valve’s design, and the flow of fluidslapping against a partially open gate cancause extensive damage to the valve. Except asspecifically authorized, gate valves should not beused for throttling.

Gate valves are classified as either rising-stemor nonrising-stem valves. The nonrising-stemvalve is shown in figure 6-4. The stem is threadedinto the gate. As the handwheel on the stem isrotated, the gate travels up or down the stem onthe threads while the stem remains verticallystationary. This type of valve will almost alwayshave a pointer indicator threaded onto the upperend of the stem to indicate the position of the gate.

Valves with rising stems (fig. 6-5) are usedwhen it is important to know by immediateinspection whether the valve is open or closed andwhen the threads (stem and gate) exposed to thefluid could become damaged by fluid contami-nants. In this valve, the stem rises out of the valvewhen the valve is opened.

GLOBE VALVES

Globe valves are probably the most commonvalves in existence. The globe valve gets its name

Figure 6-4.—Operation of a gate valve.

6-3

Figure 6-5.—Rising stem gate valve.

Figure 6-6.—Types of globe valve bodies.

from the globular shape of the valve body. Othertypes of valves may also have globular-shapedbodies. Thus, it is the internal structure of thevalve that identifies the type of valve.

The inlet and outlet openings for globe valvesare arranged in a way to satisfy the flowrequirements. Figure 6-6 shows straight-, angle-,and cross-flow valves.

The moving parts of a globe valve consist ofthe disk, the valve stem, and the handwheel. Thestem connects the handwheel and the disk. It isthreaded and fits into the threads in the valvebonnet.

The part of the globe valve that controls flowis the disk, which is attached to the valve stem.(Disks are available in various designs.) The valveis closed by turning the valve stem in until the diskis seated into the valve seat. This prevents fluidfrom flowing through the valve (fig. 6-7, view A).The edge of the disk and the seat are veryaccurately machined so that they forma tight seal

when the valve is closed. When the valve is open(fig. 6-7, view B), the fluid flows through the spacebetween the edge of the disk and the seat. Sincethe fluid flows equally on all sides of the centerof support when the valve is open, there is nounbalanced pressure on the disk to cause unevenwear. The rate at which fluid flows through thevalve is regulated by the position of the disk inrelation to the seat. The valve is commonly usedas a fully open or fully closed valve, but it maybe used as a throttle valve. However, since theseating surface is a relatively large area, it is notsuitable as a throttle valve, where fine adjustmentsare required in controlling the rate of flow.

The globe valve should never be jammed inthe open position. After a valve is fully opened,the handwheel should be turned toward the closedposition approximately one-half turn. Unless thisis done, the valve is likely to seize in the openposition, making it difficult, if not impossible, toclose the valve. Many valves are damaged in this

Figure 6-7.—Operation of a globe valve.

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manner. Another reason for not leaving globevalves in the fully open position is that it issometimes difficult to determine if the valve isopen or closed. If the valve is jammed in the openposition, the stem may be damaged or broken bysomeone who thinks the valve is closed, andattempts to open it.

It is important that globe valves be installedwith the pressure against the face of the disk tokeep the system pressure away from the stempacking when the valve is shut.

NEEDLE VALVES

Needle valves are similar in design andoperation to the globe valve. Instead of a disk,a needle valve has a long tapered point at the endof the valve stem. A cross-sectional view of aneedle valve is illustrated in figure 6-8.

The long taper of the valve element permitsa much smaller seating surface area than that ofthe globe valve; therefore, the needle valve is moresuitable as a throttle valve. Needle valves are usedto control flow into delicate gauges, whichmight be damaged by sudden surges of fluid under

pressure. Needle valves are also used to controlthe end of a work cycle, where it is desirable formotion to be brought slowly to a halt, and at otherpoints where precise adjustments of flow arenecessary and where a small rate of flow isdesired.

Although many of the needle valves used influid power systems are the manually operatedtype (fig. 6-8), modifications of this type of valveare often used as variable restrictors. This valve isconstructed without a handwheel and is adjustedto provide a specific rate of flow. This rate of flowwill provide a desired time of operation for aparticular subsystem. Since this type of valve canbe adjusted to conform to the requirements of aparticular system, it can be used in a variety ofsystems. Figure 6-9 illustrates a needle valve thatwas modified as a variable restrictor.

HYDRAULIC AND PNEUMATICGLOBE VALVES

The valve consists of a valve body and a stemcartridge assembly. The stem cartridge assemblyincludes the bonnet, gland nut, packing, packingretainer, handle, stem, and seat. On small valves(1/8 and 1/4 inch) the stem is made in one piece,but on larger sizes it is made of a stem, guide,and stem retainer. The valve disk is made of nylonand is swaged into either the stem, for 1/8- and1/4-inch valves, or the guide, for larger valves.The bonnet screws into the valve body withleft-hand threads and is sealed by an O-ring(including a back-up ring).

Figure 6-8.—Cross-sectional view of a needle valve. Figure 6-9.—Variable restrictor.

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The valve is available with either a rising stemor a non-rising stem. The rising stem valve usesthe same port body design as does the non-risingstem valve. The stem is threaded into the glandnut and screws outward as the valve is opened.

This valve does not incorporate provisions fortightening the stem packing nor replacing thepacking while the valve is in service; therefore,complete valve disassembly is required formaintenance. Figure 6-10 illustrates a rising stemhydraulic and pneumatic globe valve. Additionalinformation on this valve is available in StandardNavy Valves, NAVSHIPS 0948-012-5000.

PRESSURE CONTROL VALVES

The safe and efficient operation of fluidpower systems, system components, and relatedequipment requires a means of controllingpressure. There are many types of automaticpressure control valves. Some of them merelyprovide an escape for pressure that exceeds a setpressure; some only reduce the pressure to a lowerpressure system or subsystem; and some keep thepressure in a system within a required range.

RELIEF VALVES

Some fluid power systems, even when operat-ing normally, may temporarily develop excessivepressure; for example, when an unusually strongwork resistance is encountered. Relief valves areused to control this excess pressure.

Relief valves are automatic valves used onsystem lines and equipment to prevent over-pressurization. Most relief valves simply lift (open)at a preset pressure and reset (shut) when thepressure drops slightly below the lifting pressure.They do not maintain flow or pressure at a givenamount, but prevent pressure from rising abovea specific level when the system is temporarilyoverloaded.

Main system relief valves are generallyinstalled between the pump or pressure source andthe first system isolation valve. The valve mustbe large enough to allow the full output of thehydraulic pump to be delivered back to thereservoir. In a pneumatic system, the relief valvecontrols excess pressure by discharging the excessgas to the atmosphere.

Figure 6-10.—Hydraulic and pneumatic globe valve (rising stem).

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Smaller relief valves, similar in design andoperation to the main system relief valve, are oftenused in isolated parts of the system where a checkvalve or directional control valve prevents pressurefrom being relieved through the main system reliefvalve and where pressures must be relieved at aset point lower than that provided by the mainsystem relief. These small relief valves are alsoused to relieve pressures caused by thermalexpansion (see glossary) of the fluids.

Figure 6-11 shows a typical relief valve. Systempressure simply acts under the valve disk at theinlet to the valve. When the system pressureexceeds the force exerted by the valve spring, thevalve disk lifts off of its seat, allowing some ofthe system fluid to escape through the valve outletuntil the system pressure is reduced to just belowthe relief set point of the valve.

All relief valves have an adjustment forincreasing or decreasing the set relief pressure.Some relief valves are equipped with an adjustingscrew for this purpose. This adjusting screw isusually covered with a cap, which must beremoved before an adjustment can be made. Sometype of locking device, such as a lock nut, isusually provided to prevent the adjustment fromchanging through vibration. Other types of reliefvalves are equipped with a handwheel for makingadjustments to the valve. Either the adjustingscrew or the handwheel is turned clockwise toincrease the pressure at which the valve will open.In addition, most relief valves are also provided

Figure 6-11 .—Relief valve.

with an operating lever or some type of device toallow manual cycling or gagging the valve openfor certain tasks.

Various modifications of the relief valveshown in figure 6-11 are used to efficiently servethe requirements of some fluid power systems;however, this relief valve is unsatisfactory forsome applications. To give you a better under-standing of the operation of relief valves, we willdiscuss some of the undesirable characteristics ofthis valve.

A simple relief valve, such as the oneillustrated in figure 6-11, with a suitable springadjustment can be set so that it will open whenthe system pressure reaches a certain level, 500psi for example. When the valve does open, thevolume of flow to be handled may be greater thanthe capacity of the valve; therefore, pressure inthe system may increase to several hundred psiabove the set pressure before the valve brings thepressure under control. A simple relief valve willbe effective under these conditions only if it is verylarge. In this case, it would operate stiffly and thevalve element would chatter back and forth. Inaddition, the valve will not close until the systempressure decreases to a point somewhat below theopening pressure.

The surface area of the valve element must belarger than that of the pressure opening if thevalve is to seat satisfactorily as shown in figure6-12. The pressure in the system acts on the valveelement open to it. In each case in figure 6-12,the force exerted directly upward by systempressure when the valve is closed depends on thearea (A) across the valve element where theelement seats against the pressure tube. Themoment the valve opens, however, the upwardforce exerted depends on the horizontal area (B)of the entire valve element, which is greater thanarea A. This causes an upward jump of the valveelement immediately after it opens, because the

Figure 6-12.—Pressure acting on different areas.

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same pressure acting over different areas producesforces proportional to the areas. It also requiresa greater force to close the valve than was requiredto open it. As a result, the valve will not close untilthe system pressure has decreased to a certainpoint below the pressure required to open it.

Let us assume that a valve of this type is setto open at 500 psi. (Refer to fig. 6-12.) When thevalve is closed, the pressure acts on area A. If thisarea is 0.5 square inch, an upward force of 250pounds (500 0.5) will be exerted on the valveat the moment of opening. With the valve open,however, the pressure acts on area B. If area Bis 1 square inch, the upward force is 500 pounds,or double the force at which the valve actuallyopened. For the valve to close, pressure in thesystem would have to decrease well below thepoint at which the valve opened. The exactpressure would depend on the shape of the valveelement.

In some hydraulic systems, there is a pressurein the return line. This back pressure is causedby restrictions in the return line and will vary inrelation to the amount of fluid flowing in thereturn line. This pressure creates a force on theback of the valve element and will increase theforce necessary to open the valve and relievesystem pressure.

It follows that simple relief valves have atendency to open and close rapidly as they “hunt”above and below the set pressure, causingpressure pulsations and undesirable vibrationsand producing a noisy chatter. Because of theunsatisfactory performance of the simple reliefvalve in some applications, compound relief valveswere developed.

Compound relief valves use the principles ofoperation of simple relief valves for one stage oftheir action—that of the pilot valve. Provision ismade to limit the amount of fluid that the pilotvalve must handle, and thereby avoid theweaknesses of simple relief valves. (A pilotvalve is a small valve used for operating anothervalve.)

The operation of a compound relief valve isillustrated in figure 6-13. In view A, the mainvalve, which consists of a piston, stem, and spring,is closed, blocking flow from the high-pressureline to the reservoir. Fluid in the high-pressure lineflows around the stem of the main valves as itflows to the actuating unit. The stem of the mainvalve is hollow (the stem passage) and containsthe main valve spring, which forces the main valveagainst its seat. When the pilot valve is open thestem passage allows fluid to flow from the pilot

Figure 6-13.—Operation of compound relief valve,

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valve, around the main valve spring, and downto the return line.

There is also a narrow passage (piston passage)through the main valve piston. This passageconnects the high-pressure line to the valvechamber.

The pilot valve is a small, ball-type, spring-loaded check valve, which connects the top of thepassage from the valve chamber with the passagethrough the main valve stem. The pilot valve isthe control unit of the relief valve because thepressure at which the relief valve will opendepends on the tension of the pilot valve spring.The pilot valve spring tension is adjusted byturning the adjusting screw so that the ball willunseat when system pressure reaches the presetlimit.

Fluid at line pressure flows through thenarrow piston passage to fill the chamber.Because the line and the chamber are connected,the pressure in both are equal. The top andbottom of the main piston have equal areas;therefore, the hydraulic forces acting upwardand downward are equal, and there is no tendencyfor the piston to move in either direction.The only other force acting on the main valveis that of the main valve spring, which holds itclosed.

When the pressure in the high-pressure lineincreases to the point at which the pilot valveis set, the ball unseats (fig. 6-13, view B).This opens the valve chamber through thevalve stem passage to the low-pressure returnline. Fluid immediately begins to flow out of thechamber, much faster than it can flow throughthe narrow piston passage. As a result thechamber pressure immediately drops, and thepilot valve begins to close again, restrictingthe outward flow of fluid. Chamber pressuretherefore increases, the valve opens, and the cyclerepeats.

So far, the only part of the valve that hasmoved appreciably is the pilot, which functionsjust like any other simple spring-loaded reliefvalve. Because of the small size of the pistonpassage, there is a severe limit on the amountof overpressure protection the pilot can providethe system. All the pilot valve can do is limitfluid pressure in the valve chamber above themain piston to a preset maximum pressure,

by allowing excess fluid to flow through thepiston passage, through the stem passage, andinto the return line. When pressure in the systemincreases to a value that is above the flow capacityof the pilot valve, the main valve opens,permitting excess fluid to flow directly to thereturn line. This is accomplished in the followingmanner.

As system pressure increases, the upward forceon the main piston overcomes the downwardforce, which consists of the tension of the mainpiston spring and the pressure of the fluid in thevalve chamber (fig. 6-13, view C). The piston thenrises, unseating the stem, and allows the fluid toflow from the system pressure line directly intothe return line. This causes system pressure todecrease rapidly, since the main valve is designedto handle the complete output of the pump. Whenthe pressure returns to normal, the pilot springforces the ball onto the seat. Pressures are equalabove and below the main piston, and the mainspring forces the valve to seat.

As you can see, the compound valve over-comes the greatest limitation of a simple reliefvalve by limiting the flow through the pilot valveto the quantity it can satisfactorily handle. Thislimits the pressure above the main valve andenables the main line pressure to open the mainvalve. In this way, the system is relieved when anoverload exists.

PRESSURE REGULATORS

Pressure regulators, often referred to asunloading valves, are used in fluid power systemsto regulate pressure. In pneumatic systems, thevalve, commonly referred to as a pressureregulator, simply reduces pressure. This type ofvalve is discussed later in this chapter underpressure-reducing valves. In hydraulic systems thepressure regulator is used to unload the pump andto maintain and regulate system pressure at thedesired values. All hydraulic systems do notrequire pressure regulators. The open-centersystem (discussed in chapter 12) does not requirea pressure regulator. Many systems are equippedwith variable-displacement pumps (discussed inchapter 4), which contain a pressure-regulatingdevice.

Pressure regulators are made in a variety oftypes and by various manufacturers; however, the

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basic operating principles of all regulators aresimilar to the one illustrated in figure 6-14.

A regulator is open when it is directing fluidunder pressure into the system (fig. 6-14, view A).In the closed position (fig. 6-14, view B), the fluidin the part of the system beyond the regulator istrapped at the desired pressure, and the fluid fromthe pump is bypassed into the return line and backto the reservoir. To prevent constant opening andclosing (chatter), the regulator is designed to openat a pressure somewhat lower than the closingpressure. This difference is known as differentialor operating range. For example, assume that apressure regulator is set to open when the systempressure drops below 600 psi, and close when thepressure rises above 800 psi. The differential oroperating range is 200 psi.

Referring to figure 6-14, assume that thepiston has an area of 1 square inch, the pilot valvehas a cross-sectional area of one-fourth squareinch, and the piston spring provides 600 poundsof force pushing the piston down. When thepressure in the system is less than 600 psi, fluidfrom the pump will enter the inlet port, flow tothe top of the regulator, and then to the pilotvalve. When the pressure of the fluid at the inletincreases to the point where the force it createsagainst the front of the check valve exceeds theforce created against the back of the check valveby system pressure and the check valve spring, thecheck valve opens. This allows fluid to flow intothe system and to the bottom of the regulatoragainst the piston. When the force created by the

system pressure exceeds the force exerted by thespring, the piston moves up, causing the pilotvalve to unseat. Since the fluid will take the pathof least resistance, it will pass through theregulator and back to the reservoir through thereturn line.

When the fluid from the pump is suddenlyallowed a free path to return, the pressure on theinput side of the check valve drops and the checkvalve closes. The fluid in the system is thentrapped under pressure. This fluid will remainpressurized until a power unit is actuated, or untilpressure is slowly lost through normal internalleakage within the system.

When the system pressure decreases to a pointslightly below 600 psi, the spring forces the pistondown and closes the pilot valve. When the pilotvalve is closed, the fluid cannot flow directly tothe return line. This causes the pressure to increasein the line between the pump and the regulator.This pressure opens the check valve, causing thefluid to enter the system.

In summary, when the system pressuredecreases a certain amount, the pressure regulatorwill open, sending fluid to the system. When thesystem pressure increases sufficiently, theregulator will close, allowing the fluid from thepump to flow through the regulator and back tothe reservoir. The pressure regulator takes the loadoff of the pump and regulates system pressure.

Figure 6-14.—Hydraulic pressure regulator.

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Figure 6-15 .—Installation

SEQUENCE VALVES

of sequence valves.

Sequence valves control the sequence ofoperation between two branches in a circuit; thatis, they enable one unit to automatically setanother unit into motion. An example of the useof a sequence valve is in an aircraft landing gearactuating system.

In a landing gear actuating system, the landinggear doors must open before the landing gearstarts to extend. Conversely, the landing gear mustbe completely retracted before the doors close. A

sequence valve installed in each landing gearactuating line performs this function.

A sequence valve is somewhat similar to arelief valve except that, after the set pressure hasbeen reached, the sequence valve diverts the fluidto a second actuator or motor to do work inanother part of the system. Figure 6-15 shows aninstallation of two sequence valves that controlthe sequence of operation of three actuatingcylinders. Fluid is free to flow into cylinder A.The first sequence valve (1) blocks the passage offluid until the piston in cylinder A moves to theend of its stroke. At this time, sequence valve 1opens, allowing fluid to enter cylinder B. Thisaction continues until all three pistons completetheir strokes.

There are various types of sequence valves.Some are controlled by pressure and some arecontrolled mechanically.

Pressure-Controlled Sequence Valve

The operation of a typical pressure-controlledsequence valve is illustrated in figure 6-16. Theopening pressure is obtained by adjusting thetension of the spring that normally holds thepiston in the closed position. (Note that the toppart of the piston has a larger diameter than thelower part.) Fluid enters the valve through theinlet port, flows around the lower part of thepiston and exits the outlet port, where it flows tothe primary (first) unit to be operated (fig. 6-16,view A). This fluid pressure also acts against thelower surface of the piston.

Figure 6-16.—Operation of a pressure-controlled sequence valve.

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When the primary actuating unit completes itsoperation, pressure in the line to the actuating unitincreases sufficiently to overcome the force of thespring, and the piston rises. The valve is then inthe open position (fig. 6-16, view B). The fluidentering the valve takes the path of least resistanceand flows to the secondary unit.

A drain passage is provided to allow any fluidleaking past the piston to flow from the top ofthe valve. In hydraulic systems, this drain line isusually connected to the main return line.

Mechanically Operated Sequence Valve

The mechanically operated sequence valve(fig. 6-17) is operated by a plunger that extendsthrough the body of the valve. The valve ismounted so that the plunger will be operated bythe primary unit.

A check valve, either a ball or a poppet, isinstalled between the fluid ports in the body. Itcan be unseated by either the plunger or fluidpressure.

Port A (fig. 6-17) and the actuator of theprimary unit are connected by a common line.Port B is connected by a line to the actuator ofthe secondary unit. When fluid under pressureflows to the primary unit, it also flows into thesequence valve through port A to the seated checkvalve in the sequence valve. In order to operatethe secondary unit, the fluid must flow throughthe sequence valve. The valve is located so thatthe primary unit depresses the plunger as itcompletes its operation. The plunger unseatsthe check valve and allows the fluid to flow

Figure 6-17.—Mechanically operated sequence valve.

through the valve, out port B, and to thesecondary unit.

This type of sequence valve permits flow inthe opposite direction. Fluid enters port B andflows to the check valve. Although this is returnflow from the actuating unit, the fluid overcomesspring tension, unseats the check valve, and flowsout through port A.

PRESSURE-REDUCING VALVES

Pressure-reducing valves provide a steadypressure into a system that operates at a lowerpressure than the supply system. A reducing valvecan normally be set for any desired downstreampressure within the design limits of the valve. Oncethe valve is set, the reduced pressure will bemaintained regardless of changes in supplypressure (as long as the supply pressure is at leastas high as the reduced pressure desired) andregardless of the system load, providing the loaddoes not exceed the design capacity of the reducer.

Figure 6-18.—Spring-loaded pressure-reducing valve.

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There are various designs and types ofpressure-reducing valves. The spring-loadedreducer and the pilot-controlled valve arediscussed in this text.

Spring-Loaded Reducer

The spring-loaded pressure-reducing valve(fig. 6-18) is commonly used in pneumaticsystems. It is often referred to as a pressureregulator.

The valve simply uses spring pressure againsta diaphragm to open the valve. On the bottomof the diaphragm, the outlet pressure (the pressurein the reduced-pressure system) of the valve forcesthe diaphragm upward to shut the valve. Whenthe outlet pressure drops below the set point ofthe valve, the spring pressure overcomes the outletpressure and forces the valve stem downward,opening the valve. As the outlet pressure increases,

approaching the desired value, the pressureunder the diaphragm begins to overcome springpressure, forcing the valve stem upwards, shuttingthe valve. You can adjust the downstreampressure by turning the adjusting screw, whichvaries the spring pressure against the diaphragm.This particular spring-loaded valve will fail in theopen position if a diaphragm rupture occurs.

Pilot-Controlled Pressure-Reducing Valve

Figure 6-19 illustrates the operation of apilot-controlled pressure-reducing valve. Thisvalve consists of an adjustable pilot valve, whichcontrols the operating pressure of the valve, anda spool valve, which reacts to the action of thepilot valve.

The pilot valve consists of a poppet (1), aspring (2), and an adjusting screw (3). The valve

Figure 6-19.—Pilot-controlled pressure-reducing valve.

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spool assembly consists of a valve spool (10) anda spring (4).

Fluid under main pressure enters the inlet port(11) and under all conditions is free to flowthrough the valve and the outlet port (5). (Eitherport 5 or port 11 maybe used as the high-pressureport.)

Figure 6-19, view A, shows the valve in theopen position. In this position, the pressure in thereduced-pressure outlet port (6) has not reachedthe preset operating pressure of the valve. Thefluid also flows through passage 8, through smallerpassage 9 in the center of the valve spool, and intochamber 12. The fluid pressure at outlet port 6is therefore distributed to both ends of the spool.When these pressures are equal the spool is hydrau-lically balanced. Spring 4 is a low-tension springand applies only a slight downward force on thespool. Its main purpose is to position the spooland to maintain opening 7 at its maximum size.

As the pressure increases in outlet port 6 (fig.16, view B), this pressure is transmitted throughpassages 8 and 9 to chamber 12. This pressure alsoacts on the pilot valve poppet (1). When thispressure increases above the preset operatingpressure of the valve, it overcomes the force ofpilot valve spring 2 and unseats the poppet. Thisallows fluid to flow through the drain port (15).Because the small passage (9) restricts flow intochamber 12, the fluid pressure in the chamberdrops. This causes a momentary difference inpressure across the valve spool (10) which allowsfluid pressure acting against the bottom area ofthe valve spool to overcome the downward forceof spring 4. The spool is then forced upward untilthe pressures across its ends are equalized. As thespool moves upward, it restricts the flow throughopening 7 and causes the pressure to decrease inthe reduced pressure outlet port 6. If the pressurein the outlet port continues to increase to a valueabove the preset pressure, the pilot valve will openagain and the cycle will repeat. This allows thespool valve to move up higher into chamber 12;thus further reducing the size of opening 7.These cycles repeat until the desired pressure ismaintained in outlet 6.

When the pressure in outlet 6 decreases to avalue below the preset pressure, spring 4 forcesthe spool downward, allowing more fluid to flowthrough opening 7.

COUNTERBALANCE VALVE

The counterbalance valve is normally locatedin the line between a directional control valve andthe outlet of a vertically mounted actuatingcylinder which supports weight or must be held

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in position for a period of time. This valve servesas a hydraulic resistance to the actuating cylinder.For example, counterbalance valves are used insome hydraulically operated forklifts. The valveoffers a resistance to the flow from the actuatingcylinder when the fork is lowered. It also helpsto support the fork in the UP position.

Counterbalance valves are also used in air-launched weapons loaders. In this case the valveis located in the top of the lift cylinder. The valverequires a specific pressure to lower the load. Ifadequate pressure is not available, the load cannotbe lowered. This prevents collapse of the load dueto any malfunction of the hydraulic system.

One type of counterbalance valve is illustratedin figure 6-20. The valve element is a balancedspool (4). The spool consists of two pistonspermanently fixed on either end of a shaft. Theinner surface areas of the pistons are equal;therefore, pressure acts equally on both areasregardless of the position of the valve and has noeffect on the movement of the valve—hence, theterm balanced. The shaft area between the twopistons provides the area for the fluid to flow

Figure 6-20.—Counterbalance valve.

when the valve is open. A small piston (9) isattached to the bottom of the spool valve.

When the valve is in the closed position, thetop piston of the spool valve blocks the dischargeport (8). With the valve in this position, fluidflowing from the actuating unit enters the inletport (5). The fluid cannot flow through the valvebecause discharge port 8 is blocked. However,fluid will flow through the pilot passage (6) to thesmall pilot piston. As the pressure increases, it actson the pilot piston until it overcomes the presetpressure of spring 3. This forces the valve spool(4) up and allows the fluid to flow around theshaft of the valve spool and out discharge port8. Figure 6-20 shows the valve in this position.During reverse flow, the fluid enters port 8. Thespring (3) forces valve spool 4 to the closedposition. The fluid pressure overcomes the springtension of the check valve (7). The check valveopens and allows free flow around the shaft ofthe valve spool and out through port 5.

The operating pressure of the valve can beadjusted by turning the adjustment screw (1),which increases or decreases the tension of thespring. This adjustment depends on the weightthat the valve must support.

It is normal for a small amount of fluid to leakaround the top piston of the spool valve and intothe area around the spring. An accumulationwould cause additional pressure on top of thespool valve. This would require additionalpressure to open the valve. The drain (2) providesa passage for this fluid to flow to port 8.

DIRECTIONAL CONTROL VALVES

Directional control valves are designed todirect the flow of fluid, at the desired time, to thepoint in a fluid power system where it will dowork. The driving of a ram back and forth in itscylinder is an example of when a directionalcontrol valve is used. Various other terms are usedto identify directional valves, such as selectorvalve, transfer valve, and control valve. Thismanual will use the term directional control valveto identify these valves.

Directional control valves for hydraulicand pneumatic systems are similar in designand operation. However, there is one majordifference. The return port of a hydraulic valveis ported through a return line to the reservoir,while the similar port of a pneumatic valve,commonly referred to as the exhaust port, isusually vented to the atmosphere. Any otherdifferences are pointed out in the discussion ofthe valves.

Directional control valves may be operated bydifferences in pressure acting on opposite sidesof the valving element, or they maybe positionedmanually, mechanically, or electrically. Often twoor more methods of operating the same valve willbe used in different phases of its action.

CLASSIFICATION

Directional control valves may be classified inseveral ways. Some of the different ways are bythe type of control, the number of ports in thevalve housing, and the specific function of thevalve. The most common method is by the typeof valving element used in the construction of thevalve. The most common types of valvingelements are the ball, cone or sleeve, poppet,rotary spool, and sliding spool. The basicoperating principles of the poppet, rotary spool,and sliding spool valving elements are discussedin this text.

Poppet

The poppet fits into the center bore of the seat(fig. 6-21). The seating surfaces of the poppet andthe seat are lapped or closely machined so thatthe center bore will be sealed when the poppet is

Figure 6-21.—Operation of a simple poppet valve.

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seated (shut). The action of the poppet is similarto that of the valves in an automobile engine. Inmost valves the poppet is held in the seatedposition by a spring.

The valve consists primarily of a movablepoppet which closes against the valve seat. In theclosed position, fluid pressure on the inlet sidetends to hold the valve tightly closed. A smallamount of movement from a force applied to thetop of the poppet stem opens the poppet andallows fluid to flow through the valve.

The use of the poppet as a-valving element isnot limited to directional control valves.

Rotary Spool

The rotary spool directional control valve(fig. 6-22) has a round core with one or morepassages or recesses in it. The core is mountedwithin a stationary sleeve. As the core is rotatedwithin the stationary sleeve, the passages orrecesses connect or block the ports in the sleeve.The ports in the sleeve are connected to theappropriate lines of the fluid system.

Sliding spool

The operation of a simple sliding spooldirectional control valve is shown in figure 6-23.The valve is so-named because of the shape of thevalving element that slides back and forth to blockand uncover ports in the housing. (The slidingelement is also referred to as a piston.) The innerpiston areas (lands) are equal. Thus fluid underpressure which enters the valve from the inlet ports

CHECK VALVE

Figure 6-22.—Parts of a rotary spool directional controlvalve.

Figure 6-23.—Two-way, sliding spool directional controlvalve.

acts equally on both inner piston areas regardlessof the position of the spool. Sealing is usuallyaccomplished by a very closely machined fitbetween the spool and the valve body or sleeve.For valves with more ports, the spool is designedwith more pistons or lands on a common shaft.The sliding spool is the most commonly used typeof valving element used in directional controlvalves.

Check valves are used in fluid systems topermit flow in one direction and to prevent flowin the other direction. They are classified asone-way directional control valves.

The check valve may be installed inde-pendently in a line to allow flow in one directiononly, or it may be used as an integral part ofglobe, sequence, counterbalance, and pressure-reducing valves.

Check valves are available in various designs.They are opened by the force of fluid in motionflowing in one direction, and are closed by fluidattempting to flow in the opposite direction. Theforce of gravity or the action of a spring aids inclosing the valve.

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Figure 6-24.—Swing check valve.

Figure 6-24 shows a swing check valve. In theopen position, the flow of fluid forces the hingeddisk up and allows free flow through the valve.Flow in the opposite direction with the aid ofgravity, forces the hinged disk to close the passageand blocks the flow. This type of valve issometimes designed with a spring to assist inclosing the valve.

The most common type of check valve,installed in fluid-power systems, uses either a ballor cone for the sealing element (fig. 6-25). As fluidpressure is applied in the direction of the arrow,the cone (view A) or ball (view B) is forced off

its seat, allowing fluid to flow freely through thevalve. This valve is known as a spring-loadedcheck valve.

The spring is installed in the valve to hold thecone or ball on its seat whenever fluid is notflowing. The spring also helps to force the coneor ball on its seat when the fluid attempts to flowin the opposite direction. Since the opening andclosing of this type of valve is not dependent ongravity, its location in a system is not limited tothe vertical position.

A modification of the spring-loaded checkvalve is the orifice check valve (fig. 6-26). This

Figure 6-25.—Spring-loaded check valves. Figure 6-26.—Typical orifice check valves.

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valve allows normal flow in one direction andrestricted flow in the other. It is often referredto as a one-way restrictor.

Figure 6-26, view A, shows a cone-type orificecheck valve. When sufficient fluid pressure isapplied at the inlet port, it overcomes springtension and moves the cone off of its seat. Thetwo orifices (2) in the illustration represent severalopenings located around the slanted circumferenceof the cone. These orifices allow free flow of fluidthrough the valve while the cone is off of its seat.When fluid pressure is applied through the outletport, the force of the fluid and spring tensionmove the cone to the left and onto its seat. Thisaction blocks the flow of fluid through the valve,except through the orifice (1) in the center of thecone. The size of the orifice (in the center of thecone) determines the rate of flow through thevalve as the fluid flows from right to left.

Figure 6-26, view B, shows a ball-type orificecheck valve. Fluid flow through the valve fromleft to right forces the ball off of its seat andallows normal flow. Fluid flow through the valvein the opposite direction forces the ball onto itsseat. Thus, the flow is restricted by the size of theorifice located in the housing of the valve.

NOTE: The direction of free flow through theorifice check valve is indicated by an arrowstamped on the housing.

SHUTTLE VALVE

In certain fluid power systems, the supply offluid to a subsystem must be from more than onesource to meet system requirements. In somesystems an emergency system is provided as asource of pressure in the event of normal systemfailure. The emergency system will usually actuateonly essential components.

The main purpose of the shuttle valve is toisolate the normal system from an alternate oremergency system. It is small and simple; yet, itis a very important component.

Figure 6-27 is a cutaway view of a typicalshuttle valve. The housing contains three ports—normal system inlet, alternate or emergencysystem inlet, and outlet. A shuttle valve used tooperate more than one actuating unit may containadditional unit outlet ports. Enclosed in thehousing is a sliding part called the shuttle. Itspurpose is to seal off either one or the other inletports. There is a shuttle seat at each inlet port.

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Figure 6-27.—Shuttle valve.

When a shuttle valve is in the normaloperation position, fluid has a free flow from thenormal system inlet port, through the valve, andout through the outlet port to the actuating unit.The shuttle is seated against the alternate systeminlet port and held there by normal systempressure and by the shuttle valve spring. Theshuttle remains in this position until the alternatesystem is activated. This action directs fluid underpressure from the alternate system to the shuttlevalve and forces the shuttle from the alternatesystem inlet port to the normal system inlet port.Fluid from the alternate system then has a freeflow to the outlet port, but is prevented fromentering the normal system by the shuttle, whichseals off the normal system port.

The shuttle may be one of four types: (1)sliding plunger, (2) spring-loaded piston, (3)spring-loaded ball, or (4) spring-loaded poppet.In shuttle valves that are designed with a spring,the shuttle is normally held against the alternatesystem inlet port by the spring.

TWO-WAY VALVES

The term two-way indicates that the valvecontains and controls two functional flow controlports-an inlet and an outlet. A two-way, slidingspool directional control valve is shown in figure6-23. As the spool is moved back and forth, iteither allows fluid to flow through the valve orprevents flow. In the open position, the fluidenters the inlet port, flows around the shaft ofthe spool, and through the outlet port. The spoolcannot move back and forth by difference of

forces set up within the cylinder, since the forcesthere are equal. As indicated by the arrows againstthe pistons of the spool, the same pressure actson equal areas on their inside surfaces. In theclosed position, one of the pistons of the spoolsimply blocks the inlet port, thus preventing flowthrough the valve.

A number of features common to most slidingspool valves are shown in figure 6-23. The smallports at either end of the valve housing providea path for any fluid that leaks past the spool toflow to the reservoir. This prevents pressure frombuilding up against the ends of the pistons, whichwould hinder the movement of the spool. Whenspool valves become worn, they may lose balancebecause of greater leakage on one side of the spoolthan on the other. In that event, the spool wouldtend to stick when it is moved back and forth.Small grooves are therefore machined around thesliding surface of the piston; and in hydraulicvalves, leaking liquid will encircle the pistons andkeep the contacting surfaces lubricated andcentered.

THREE-WAY VALVES

Three-way valves contain a pressure port, acylinder port, and a return or exhaust port. Thethree-way directional control valve is designed tooperate an actuating unit in one direction; itpermits either the load on the actuating unit ora spring to return the unit to its original position.

Cam-Operated Three-Way Valves

Figure 6-28 shows the operation of a cam-operated, three-way, poppet-type directionalcontrol valve. View A shows fluid under pressureforcing the piston outward against a load. Theupper poppet (2) is unseated by the inside cam(5), permitting fluid to flow from the line (3) intothe cylinder to actuate the piston. The lowerpoppet (1) is seated, sealing off the flow into thereturn line (4). As the force of the pressurized fluidextends the piston rod, it also compresses thespring in the cylinder.

View B shows the valve with the controlhandle turned to the opposite position. In thisposition, the upper poppet (2) is seated, blockingthe flow of fluid from the pressure line (3). Thelower poppet (1) is unseated by the outside cam(6). This releases the pressure in the cylinder andallows the spring to expand, which forces thepiston rod to retract. The fluid from the cylinderflows through the control valve and out the return

Figure 6-28.—Three-way, poppet-type directional controlvalve (cam-operated).

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port (4). In hydraulic systems, the return port isconnected by a line to the reservoir. In pneumaticsystems, the return port is usually open to theatmosphere.

Pilot-Operated Three-Way Valves

A pilot-operated, poppet-type, three-waydirectional control valve is shown in figure 6-29.Valves of this design are often used in pneumaticsystems. This valve is normally closed and isforced open by fluid pressure entering thepilot chamber. The valve contains two poppetsconnected to each other by a common stem. Thepoppets are connected to diaphragms which holdthem in a centered position.

The movement of the poppet is controlled bythe pressure in the pilot port and the chamberabove the upper diaphragm. When the pilotchamber is not pressurized, the lower poppet isseated against the lower valve seat. Fluid can flowfrom the supply line through the inlet port andthrough the holes in the lower diaphragm to fillthe bottom chamber. This pressure holds thelower poppet tightly against its seat and blocksflow from the inlet port through the valve. At thesame time, due to the common stem, the upperpoppet is forced off of its seat. Fluid from theactuating unit flows through the open passage,around the stem, and through the exhaust portto the atmosphere.

When the pilot chamber is pressurized, theforce acting against the diaphragm forces thepoppet down. The upper poppet closes against itsseat, blocking the flow of fluid from the cylinderto the exhaust port. The lower poppet opens, andthe passage from the supply inlet port to thecylinder port is open so that the fluid can flowto the actuating unit.

The valve in figure 6-29 is a normally closedvalve. Normally open valves are similar in design.When no pressure is applied to the pilot chamber,the upper poppet is forced off of its seat and thelower poppet is closed. Fluid is free to flow fromthe inlet port through the cylinder to the actuatingunit. When pilot pressure is applied, the poppetsare forced downward, closing the upper poppetand opening the lower poppet. Fluid can now flowfrom the cylinder through the valve and out theexhaust port to the atmosphere.

FOUR-WAY VALVES

Most actuating devices require system pressurefor operation in either direction. The four-waydirectional control valve, which contains fourports, is used to control the operation of suchdevices. The four-way valve is also used in somesystems to control the operation of other valves.It is one of the most widely used directionalcontrol valves in fluid power systems.

The typical four-way directional control valvehas four ports: a pressure port, a return or exhaustport, and two cylinder or working ports. Thepressure port is connected to the main systempressure line and the return line is connected tothe reservoir in hydraulic systems. In pneumaticsystems the return port is usually vented to theatmosphere. The two cylinder ports are connectedby lines to the actuating units.

Poppet-Type Four-Way Valves

Figure 6-30 shows atypical four-way, poppet-type directional control valve. This is a manuallyoperated valve and consists of a group ofconventional spring-loaded poppets. The poppetsare enclosed in a common housing and areinterconnected by ducts to direct the flow of fluidin the desired direction.

Figure 6-29.—Three-way, poppet-type, normally closed directional control valve (pilot-operated).

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The poppets are actuated by cams on acamshaft (fig. 6-30). The camshaft is controlledby the movement of the handle. The valve maybe operated by manually moving the handle, or,in some cases, the handle may be connected bymechanical linkage to a control handle which islocated in a convenient place for the operatorsome distance from the valve.

The camshaft may be rotated to any oneof three positions (neutral and two workingpositions). In the neutral position the camshaftlobes are not contacting any of the poppets. Thisassures that the poppet springs will hold all fourpoppets firmly seated. With all poppets seated,there is no fluid flow through the valve. This alsoblocks the two cylinder ports; so when the valveis in neutral, the fluid in the actuating unit istrapped. Relief valves are installed in bothworking lines to prevent overpressurization causedby thermal expansion.

NOTE: In some versions of this type of valve,the cam lobes are designed so that the tworeturn/exhaust poppets are open when the valveis in the neutral position. This compensates forthermal expansion, because both working lines areopen to the return/exhaust when the valve is inthe neutral position.

The poppets are arranged so that rotation ofthe camshaft will open the proper combinationof poppets to direct the flow of fluid through thedesired working line to an actuating unit. At thesame time, fluid will be directed from theactuating unit through the opposite working line,through the valve, and back to the reservoir(hydraulic) or exhausted to the atmosphere(pneumatic).

To stop rotation of the camshaft at an exactposition, a stop pin is secured to the body andextends through a cutout section of the camshaftflange. This stop pin prevents overtravel byensuring that the camshaft stops rotating at thepoint where the cam lobes have moved thepoppets the greatest distance from their seats andwhere any further rotation would allow thepoppets to start returning to their seats.

O-rings are spaced at intervals along the lengthof the shaft to prevent external leakage aroundthe ends of the shaft and internal leakage fromone of the valve chambers to another. Thecamshaft has two lobes, or raised portions. Theshape of these lobes is such that when the shaftis placed in the neutral position the lobes will notcontact any of the poppets.

When the handle is moved in either directionfrom neutral, the camshaft is rotated. This rotates

Figure 6-30.—Cutaway view of poppet-type, four-way directional control valve.

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the lobes, which unseat one pressure poppet andone return/exhaust poppet (fig. 6-31). The valveis now in the working position. Fluid underpressure, entering the pressure port, flows throughthe vertical fluid passages in both pressure poppetsseats. Since only one pressure poppet, IN (2), isunseated by the cam lobe, the fluid flows past theopen poppet to the inside of the poppet seat. Fromthere it flows through the diagonal passages, outone cylinder port, C2, and to the actuating unit.

Return fluid from the actuating unit enters theother cylinder port, C1. It then flows through thecorresponding fluid passage, past the unseatedreturn poppet, OUT (1), through the vertical fluidpassages, and out the return/exhaust port. Whenthe camshaft is rotated in the opposite directionto the neutral position, the two poppets seat andthe flow stops. When the camshaft is furtherrotated in this direction until the stop pins hits,the opposite pressure and return poppets areunseated. This reverses the flow in the workinglines, causing the actuating unit to move in theopposite direction.

Rotary Spool Valve

Four-way directional control valves of thistype are frequently used as pilot valves to directflow to and from other valves (fig. 6-32). Fluidis directed from one source of supply through therotary valve to another directional control valve,where it positions the valve to direct flow fromanother source to one side of an actuating unit.Fluid from the other end of the main valve flowsthrough a return line, through the rotary valveto the return or exhaust port.

The principal parts of a rotary spool direc-tional control valve are shown in figure 6-22.

Figure 6-31.—Working view of a poppet-type, four-waydirectional control valve.

Figure 6-32.—Sliding spool valve controlled by a rotary spoolvalve.

Figure 6-33 shows the operation of a rotary spoolvalve. Views A and C show the valve in a positionto deliver fluid to another valve, while view Bshows the valve in the neutral position, with allpassages through the valve blocked.

Rotary spool valves can be operated manually,electrically, or by fluid pressure.

Sliding Spool Valve

The sliding spool four-way directional controlvalve is similar in operation to the two-wayvalve previously described in this chapter. It issimple in its principle of operation and is themost durable and trouble-free of all four-waydirectional control valves.

The valve described in the following para-graphs is a manually operated type. The sameprinciple is used in many remotely controlleddirectional control valves.

The valve (fig. 6-34) consists of a valve bodycontaining four fluid ports—pressure (P),

Figure 6-33.—Operation of a rotary spool, four-waydirectional control valve.

6-22

Figure 6-34.—Operation of a sliding spool, four-way directional control valve.

6-23

return/exhaust (R), and two cylinder ports (C/1and C2). A hollow sleeve fits into the main boreof the body. There are O-rings placed at intervalsaround the outside diameter of the sleeve. TheseO-rings form a seal between the sleeve and thebody, creating chambers around the sleeve. Eachof the chambers is lined up with one of the fluidports in the body. The drilled passage in the bodyaccounts for a fifth chamber which results inhaving the two outboard chambers connected tothe return/exhaust port. The sleeve has a patternof holes drilled through it to allow fluid to flowfrom one port to another. A series of holes aredrilled into the hollow center sleeve in eachchamber.

The sleeve is prevented from turning by asleeve retainer bolt or pin which secures it to thevalve body.

The sliding spool fits into the hollow centersleeve. This spool is similar to the spool in thetwo-way valve, except that this spool has threepistons or lands. These lands are lapped ormachine fitted to the inside of the sleeve.

One end of the sliding spool is connected toa handle either directly or by mechanical linkageto a more desirable location. When the controlhandle is moved, it will position the spool withinthe sleeve. The lands of the spool then line updifferent combinations of fluid ports thusdirecting a flow of fluid through the valve.

The detent spring is a clothespin-type spring,secured to the end of the body by a springretaining bolt. The two legs of the spring extenddown through slots in the sleeve and fit into thedetents. The spool is gripped between the two legsof the spring. To move the spool, enough forcemust be applied to spread the two spring legs andallow them to snap back into the next detent,which would be for another position.

Figure 6-34, view A, shows a manuallyoperated sliding spool valve in the neutralposition. The detent spring is in the center detentof the sliding spool. The center land is lined upwith the pressure port (P) preventing fluid fromflowing into the valve through this port. Thereturn/exhaust port is also blocked, preventingflow through that port. With both the pressureand return ports blocked, fluid in the actuatinglines is trapped. For this reason, a relief valve isusually installed in each actuating line when thistype of valve is used.

Figure 6-34, view B, shows the valve in theworking position with the end of the sliding spoolretracted. The detent spring is in the outboarddetent, locking the sliding spool in this position.

The lands have shifted inside the sleeve, and theports are opened. Fluid under pressure enters thesleeve, passes through it by way of the drilledholes, and leaves through cylinder port C2. Returnfluid, flowing from the actuator enters port C1,flows through the sleeve, and is directed out thereturn port back to the reservoir or exhausted tothe atmosphere. Fluid cannot flow past the spoollands because of the lapped surfaces.

Figure 6-34, view C, shows the valve in theopposite working position with the sliding spoolextended. The detent spring is in the inboarddetent. The center land of the sliding spool is nowon the other side of the pressure port, and thefluid under pressure is directed through the sleeveand out port C1. Return fluid flowing in the othercylinder port is directed to the drilled passage inthe body. It flows along this passage to the otherend of the sleeve where it is directed out of thereturn/exhaust port.

The directional control valves previouslydiscussed are for use in closed-center fluid powersystems. Figure 6-35 shows the operation of

Figure 6-35.—Open center, sliding spool directional controlvalve.

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a representative open-center, sliding spool When the spool is moved to the right of thedirectional control valve. neutral position, view B, one working line (C1)

is aligned to system pressure and the otherWhen this type of valve is in the neutral working line (C2) is open through the hollow

position (fig. 6-35, view A), fluid flows into the spool to the return port. View C shows the flowvalve through the pressure port (P) through the of fluid through the valve with the spool movedhollow spool, and return to the reservoir. to the left of neutral.

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CHAPTER 7

SEALING DEVICES AND MATERIALS

Recall from chapter 1 that Pascal’s theorem,from which the fundamental law for the scienceof hydraulics evolved, was proposed in theseventeenth century. One stipulation to make thelaw effective for practical applications was apiston that would “fit” the opening in the vessel“exactly.” However, it was not until the lateeighteenth century that Joseph Brahmah inventedan effective piston seal, the cup packing. This ledto Brahmah's development of the hydraulic press.

The packing was probably the most importantinvention in the development of hydraulics as aleading method of transmitting power. Thedevelopment of machines to cut and shape closelyfitted parts was also very important in thedevelopment of hydraulics. However, regardlessof how precise the machining process is, some typeof packing is usually required to make the piston,and many other parts of hydraulic components,“fit exactly.” This also applies to the componentsof pneumatic systems.

Through years of research and experiments,many different materials and designs have beencreated in attempts to develop suitable packingdevices. Suitable materials must be durable, mustprovide effective sealing, and must be compatiblewith the fluid used in the system.

The packing materials are commonly referredto as seals or sealing devices. The seals used influid power systems and components are dividedinto two general classes-static seals and dynamicseals.

The static seal is usually referred to as a gasket.The function of a gasket is to provide a materialthat can flow into the surface irregularities ofmating areas that require sealing. To do this, thegasket material must be under pressure. Thisrequires that the joint be tightly bolted orotherwise held together.

The dynamic seal, commonly referred to asa packing, is used to provide a seal between twoparts that move in relation to each other.

These two classifications of seals—gasketsand packing—apply in most cases; however,

deviations are found in some technical publi-cations. Certain types of seals (for example, theO-ring, which is discussed later) may be usedeither as a gasket or a packing.

Many of the seals in fluid power systemsprevent external leakage. These seals serve twopurposes—to seal the fluid in the system and tokeep foreign matter out of the system. Other sealssimply prevent internal leakage within a system.

NOTE: Although leakage of any kind resultsin a loss of efficiency, some leakage, especiallyinternal leakage, is desired in hydraulic systemsto provide lubrication of moving parts. This alsoapplies to some pneumatic systems in which dropsof oil are introduced into the flow of air in thesystem.

The first part of this chapter deals primarilywith the different types of materials used in theconstruction of seals. The next section is devotedto the different shapes and designs of seals andtheir application as gaskets and/or packings influid power systems. Also included in this chapterare sections concerning the functions of wipersand backup washers in fluid power systems andthe selection, storage, and handling of sealingdevices.

SEAL MATERIALS

As mentioned previously, many differentmaterials have been used in the development ofsealing devices. The material used for a particularapplication depends on several factors: fluidcompatibility, resistance to heat, pressure, wearresistance, hardness, and type of motion.

The selection of the correct packings andgaskets and their proper installation are importantfactors in maintaining an efficient fluid powersystem. The types of seals to be used in aparticular piece of equipment is specified by theequipment manufacturer.

7-1

Often the selection of seals is limited to sealscovered by military specifications. However, thereare occasions when nonstandard or proprietaryseals reflecting the advancing state of the art maybe approved. Thus, it is important to follow themanufacturer’s instructions when you replaceseals. If the proper seal is not available, youshould give careful consideration in the selectionof a suitable substitute. Consult the Naval Ships’Technical Manual, military standards, militarystandardization handbooks, and other applicabletechnical manuals if you have any doubts inselecting the proper seal.

Seals are made of materials that havebeen carefully chosen or developed for spe-cific applications. These materials includetetrafluoroethylene (TFE), commonly calledTeflon; synthetic rubber (elastomers); cork;leather; metal; and asbestos. Some of the mostcommon materials used to make seals for fluidpower systems are discussed in the followingparagraphs.

CORK

Cork has several of the required properties,which makes it ideally suited as a sealing materialin certain applications. The compressibility ofcork seals makes them well suited for confinedapplications in which little or no spread of thematerial is allowed. The compressibility of corkalso makes a good seal that can be cut to anydesired thickness and shape to fit any surface andstill provide an excellent seal.

One of the undesirable characteristics of corkis its tendency to crumble. If cork is used aspacking or in areas where there is a high fluidpressure and/or high flow velocity, small particleswill be cast off into the system. Cork use in fluidpower systems is therefore limited. It is sometimesused as gasket materials for inspection plates ofhydraulic reservoirs.

Cork is generally recommended for use wheresustained temperatures do not exceed 2750F.

CORK AND RUBBER

Cork and rubber seals are made by combiningsynthetic rubber and cork. This combination hasthe properties of both of the two materials.This means that seals can be made with thecompressibility of cork, but with a resistance tofluid comparable to the synthetic rubber on whichthey are based. Cork and rubber composition is

sometimes used to make gaskets for applicationssimilar to those described for cork gaskets.

LEATHER

Leather is a closely knit material that isgenerally tough, pliable, and relatively resistantto abrasion, wear, stress, and the effects oftemperature changes. Because it is porous, it isable to absorb lubricating fluids. This porositymakes it necessary to impregnate leather for mostuses. In general, leather must be tanned andtreated in order to make it useful as a gasketmaterial. The tanning processes are thosenormally used in the leather industry.

Leather is generally resistant to abrasionregardless of whether the grain side or the fleshside is exposed to abrasive action. Leather remainsflexible at low temperatures and can be forcedwith comparative ease into contact with metalflanges. When properly impregnated, it isimpermeable to most liquids and some gases,and capable of withstanding the effects oftemperatures ranging from –700F to +2200F.

Leather has four basic limitations. First, thesize of the typical hide limits the size of the sealsthat can be made from leather. A secondlimitation is the number of seals that areacceptable. Another limitation is that under heavymechanical pressures leather tends to extrude.Finally, many of the properties (such asimpermeability, tensile strength, high- andlow-temperature resistance, pliability, andcompatibility with environment) depend upon thetype of leather and impregnation. Leathers nottanned and impregnated for specific conditionsand properties will become brittle, dry, andcompletely degreased by exposure to particularchemicals. Leather is never used with steampressure of any type, nor with acid or alkalisolutions.

Leather may be used as packing. Whenmolded into V’s and U’s, and cups, and othershapes, it can be applied as dynamic packing,while in its flat form it can be used as straightcompression packing.

METAL

One of the most common metal seals used inNavy equipment is copper. Flat copper rings aresometimes used as gaskets under adjusting screwsto provide a fluid seal. Molded copper rings aresometimes used as packing with speed gearsoperating under high pressures. Either type is

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Figure 7-1.—Spiral-wouna metallic-asbestos gasket.

easily bent and requires careful handling. Inaddition, copper becomes hard when used overlong periods and when subjected to compression.Whenever a unit or component is disassembled,the copper sealing rings should be replaced.However, if new rings are not available and thepart must be repaired, the old ring should besoftened by annealing. (Annealing is the processof heating a metal, then cooling it, to make itmore pliable and less brittle.)

Metallic piston rings are used as packing insome fluid power actuating cylinders. These ringsare similar in design to the piston rings inautomobile engines.

Metal is also used with asbestos to formspiral-wound metallic-asbestos gaskets (fig. 7-1).These gaskets are composed of interlocked pliesof preformed corrugated metal and asbestosstrips, called a filler.

The filler may or may not be encased in a solidmetal outer ring. These gaskets are used in flangedconnections and for connecting the body to thebonnet in some valves, and are usually requiredin specific high-pressure, high-temperatureapplications.

RUBBER

The term rubberand synthetic rubbers,

covers many naturaleach of which can be

compounded into numerous varieties. Thecharacteristics of these varieties have a wide range,as shown in table 7-1. The table shows, with theexception of a few basic similarities, that rubbershave diverse properties and limitations; therefore,specific applications require careful study beforethe sealing material is selected.

Natural rubbers have many of the charac-teristics required in an effective seal. However,their very poor resistance to petroleum fluids andrapid aging when exposed to oxygen or ozone limittheir use. Today their use has almost ceased.

There are two general classes of syntheticrubber seals. One class is made entirely of a certainsynthetic rubber. The term homogeneous, whichmeans having uniform structure or compositionthroughout, is frequently used to describe thisclass of seal. The other class of seal is made byimpregnating woven cotton duck or fine-weaveasbestos with synthetic rubber. This class issometimes referred to as fabricated seals.

Additional information on sealing materialsis provided in the Military Handbook, GasketMaterials (Nonmetalic), MIL-HDBK-212; andthe Naval Ships’ Technical Manual, chapter 078.

TYPES OF SEALS

Fluid power seals are usually typed accordingto their shape or design. These types includeT-seals, V-rings, O-rings, U-cups and so on. Someof the most commonly used seals are discussedin the remainder of this chapter.

T-SEALS

The T-seal has an elastomeric bidirectionalsealing element resembling an inverted letter T.This sealing element is always paired with twospecial extrusion-resisting backup rings, one oneach side of the T. The basic T-seal configurationis shown in figure 7-2, view A. The backup rings

F i g u r e 7 - 2 . – T - s e a l s .

7-3

Table 7-l.—Comparison of Physical Properties for Some Hydraulic Fluid Seal Materials

Figure 7-3.—V-rings.

7-4

are single turn, bias cut, and usually made of TFE,molybdenum-disulfide-impregnated nylon, or acombination of TFE and nylon. Nylon is widelyused for T-seal backup rings because it providesexcellent resistance to extrusion and has lowfriction characteristics.

The special T-ring configuration adds stabilityto the seal, eliminating spiraling and rolling.

T-seals are used in applications where largeclearances could occur as a result of the expansionof the thin-walled hydraulic cylinder. The T-ringis installed under radial compression and providesa positive seal at zero or low pressure. Backuprings, one on each side, ride free of T-ring flangesand the rod or cylinder wall (fig. 7-2, view B).These clearances keep seal friction to a minimumat low pressure. When pressure is applied (fig. 7-2,view C), the T-ring acts to provide positive sealingaction as fluid pressure increases. One frequentlyused T-ring, manufactured by Greene, Tweed andCompany, (called a G-Tring®1), incorporates aunique, patented backup ring feature. One corneron the ID of each radius-styled backup ring onthe G-Tring® set has been rounded to mate withthe inside corner of the rubber T. Figure 7-2, viewsB and C, shows the G-Tring®.

There is no military standard part numberingsystem by which T-seals can be identified. Ingeneral, each manufacturer issues proprietary partnumbers to identify seals. However, it is commonpractice to identify T-seal sizes by the samedash numbers used for equivalent O-ring sizes(discussed later in this chapter) as defined byAS568 and MS28775 dimension standards.Typically, an O-ring groove that accepts a certainO-ring dash number will accept the same dashnumber T-seal.

In the absence of an existing military standardfor identifying T-seals, a new and simple

1G-Tring® is a Greene, Tweed Trademark,

numbering system was created to identify T-sealsrequired for hydraulic actuators (piston seals only)without reference to a particular manufacturer’spart number. The Navy number is composed ofthe letters G-T followed by a dash number of threedigits and one letter, R, S, or T (for example,G-T-217T). The three digits are the appropriateO-ring size dash number according to AS568 orMS28775. The letters R, S, and T designate thenumber of backup rings that the groove of theT-seal is designed to accommodate: none, one,or two, respectively.

V-RINGS

The V-ring is one of the most frequently useddynamic seals in ship service although itsidentification, installation, and performance areprobably most misunderstood. Properly selectedand installed, V-rings can provide excellent servicelife; otherwise, problems associated with friction,rod and seal wear, noise, and leakage can beexpected.

The V-ring is the part of the packing set thatdoes the sealing. It has a cross section resemblingthe letter V, (fig. 7-3) from which its name isderived. To achieve a seal, the V-ring must beinstalled as part of a packing set or stack, whichincludes one male adapter, one female adapter,and several V-rings (fig. 7-4). The male adapteris the first ring on the pressure end of the packingstack and is flat on one side and wedge-shapedon the other to contain the V of the adjacentV-ring. The female adapter, the last ring of the

Figure 7-4.—Outside packed V-ring installations.

7-5

packing stack, is flat on one side and V-shapedon the other to properly support the adjacentV-ring. Proper design and installation of thefemale adapter has significant impact on theservice life and performance of the V-ringsbecause the female adapter bridges the clearancegap between the moving surfaces and resistsextrusion.

The packing set is installed in a cavity that isslightly deeper than the free stack height (thenominal overall height of a V-ring packing set,including the male and female adapters asmeasured before installation) and as wide as thenominal cross section of the V-rings. This cavity,called a packing gland or stuffing box, containsand supports the packing around the shaft, rod,or piston. Adjustment of the packing gland depththrough the use of shims or spacers is usuallynecessary to obtain the correct squeeze orclearance on the packing stack for good servicelife.

Two basic installations apply to V-ringpackings. The more common is referred to as anoutside packed installation, in which the packingseals against a shaft or rod, as shown in figure7-4. The inside packed installation, is shown asa piston seal in figure 7-5. When V-ring packingis to be used in an inside packed installation, onlyendless ring packing should be used. Wherepressures exist in both directions, as on adouble-acting piston, opposing sets of packing

Figure 7-5.—Inside packed V-ring installation.

should always be installed so the sealing lips faceaway from each other as in figure 7-5. Thisprevents trapping pressure between the sets ofpackings. The female adapters in inside packedinstallations should always be located adjacent toa fixed or rigid part of the piston.

O-RINGS

An O-ring is doughnut-shaped. O-rings areusually molded from rubber compounds; how-ever, they can be molded or machined from plasticmaterials. The O-ring is usually fitted into arectangular groove (usually called a gland)machined into the mechanism to be sealed. AnO-ring seal consists of an O-ring mountedin the gland so that the O-ring’s cross sectionis compressed (squeezed) when the gland isassembled (fig. 7-6).

An O-ring sealing system is often one of thefirst sealing systems considered when a fluidclosure is designed because of the followingadvantages of such a system:

1.2.3.4.5.6.7.

SimplicityRuggednessLow costEase of installationEase of maintenanceNo adjustment requiredNo critical torque in clamping

Figure 7-6.—O-ring installed in a gland.

7-6

8.

9.

10.

11.

Low distortion of structure

Small space requirement

Reliability

Effectiveness over wide pressure andtemperature ranges

As stated previously, O-rings are used in bothstatic (as gaskets) and dynamic (as packing)applications. An O-ring will almost always be themost satisfactory choice of seals in staticapplications if the fluids, temperatures, pressure,and geometry permit.

Standard O-ring packings are not specificallydesigned to be used as rotary seals. Wheninfrequent rotary motion or low peripheralvelocity is involved standard O-ring packings maybe used, provided consistent surface finishes overthe entire gland are used and eccentricities areaccurately controlled. O-rings cannot compensatefor out-of-round or eccentrically rotating shafts.

As rotary seals, O-rings perform satisfactorilyin two application areas:

1. In low-speed applications where the surfacespeed of the shaft does not exceed 200 ft/min

2. In high-speed moderate-pressure appli-cations, between 50 and 800 psi

The use of low-friction extrusion-resistantdevices is helpful in prolonging the life andimproving the performance of O-rings used asrotary seals.

O-rings are often used as reciprocating sealsin hydraulic and pneumatic systems. While bestsuited for short-stroke, relatively small diameterapplications, O-rings have been used successfullyin long-stroke, large diameter applications.Glands for O-rings used as reciprocating seals areusually designed according to MIL-G-5514 toprovide a squeeze that varies from 8 to 10 percentminimum and 13.5 to 16 percent maximum. Asqueeze of 20 percent is allowed on O-rings witha cross section of 0.070-inch or less. In somereciprocating pneumatic applications, a floatingO-ring design may simultaneously reduce frictionand wear by maintaining no squeeze by the glandon the O-ring. When air pressure enters thecylinder, the air pressure flattens the O-ring,causing sufficient squeeze to seal during the

stroke. If the return stroke does not use pneumaticpower, the O-ring returns to its round crosssection, minimizing drag and wear on the returnstroke.

Identification

As a maintenance person or supervisorworking with fluid power systems, you must beable to positively identify, inspect, and install thecorrect size and type of O-ring to ensure the bestpossible service. These tasks can be difficult sincepart numbers cannot be put directly on the sealsand because of the continual introduction of newtypes of seals and obsolescence of others. (NavalShips’ Technical Manual, chapter 078, containsa table that cross-references obsolete and currentO-ring specifications for ship applications.)

O-rings are packaged in individually sealedenvelopes. O-ring seals manufactured to govern-ment specifications are marked according to therequirements of the specific military specificationand standard. The required marking for eachpackage is as follows:

1.

2.

3.

4.

5.

6.

7.

8.

9.

National stock number (NSN)

Nomenclature

Military part number

Material specification

Manufacturer’s

Manufacturer’s

Manufacturer’s

name

compound number

batch number

Contract number

Cure date

NOTE: Keep preformed packings in theiroriginal envelopes, which provide preservation,protection, identification, and cure date.

When you select an O-ring for installation,carefully observe the information on the package.If you cannot positively identify an O-ring,discard it. The part number on the sealedpackage provides the most reliable and completeidentification.

7-7

Sizes

A standardized dash number system forO-ring sizes is used in many military and industrialspecifications. The O-ring size is identified by adash number rather than the actual dimensionsfor convenience. The basis for the dash numbersis contained in Aerospace Standard AS568. Fornongasket O-rings (packing), the dash numbersare divided into groups of one hundred. Eachhundred group identifies the cross section size ofthe O-rings within the group (table 7-2).

The 900 series dash numbers contained inAS568 identify all the presently standardizedstraight thread tube fitting boss gaskets. With theexception of -901, the last two digits of the dashdesignate the tube size in 16ths of an inch. Forexample, the -904 size is for a 1/4-inch tube.

Dimensions

The critical dimensions of an O-ring are its ID,its cross sectional diameter (W), and the heightand width of the residual molding flash (seefig. 7-7).

Nominal dimensions have been used todescribe O-ring sizes, although this practice israpidly being replaced by the use of dash numbers.The actual inside diameter of a seal will be slightlyless than the nominal ID, but the actual OD will

Table 7-2.—O-Ring Dash Numbers Versus Cross SectionSizes

be slightly larger than the nominal OD. Forexample, an AS568-429 O-ring is described innominal dimensions as 5 inches ID by 5-1/2 inchesOD by 1/4-inch W. Actual dimensions are 4.975inches ID by 5.525 inches OD by 0.275 inches W.

Specifications

Material and performance requirementsfor O-rings are often identified in militaryspecifications. The dimensions of these O-ringswill usually be found in accompanying slash sheets(which bear the specification number and are apart of the specification) or will be identified byvarious drawings and standards that relate to thespecification. Included among the specificationsare Air Force-Navy Standards (AN), Mili-tary Standards (MS), and National AerospaceStandards (NAS). If the specification does notidentify sizes, the sizes should be identified by theAS568 dash number. Usually, you can usedrawings, technical manuals, and allowance partslists (APLs) to identify replacement O-rings.(Notes 2 and 3 of table 7-1 list some of thefrequently used military specifications).

Cure Date

A cure date is as applicable to natural orsynthetic O-rings as it is to rubber hoses. This dateis the basis for determining the age of O-rings.It is extremely important that the cure date benoted on all packages.

Shelf Life and Expiration Date

All elastomers change gradually with age;some change more rapidly than others. Theshelf life for rubber products is contained inMIL-HDBK-695.

Check the age of natural or synthetic rubberpreformed packings before installation todetermine whether they are acceptable for use.Make a positive identification, indicating thesource, cure date, and expiration date. Ensure thatthis information is available for all packing used.Shelf life requirements do not apply once thepacking is installed in a component.

The expiration date is the date after whichpacking should not be installed. The expirationdate of all packings can be determined by addingthe shelf life to the cure date.

7-8

Replacement

Figure 7-8 shows a typical O-ring installation.When such an installation shows signs of internalor external leakage, the component must bedisassembled and the seals replaced. Sometimescomponents must be resealed because of the agelimitations of the seals. The O-ring should alsobe replaced whenever a gland that has been inservice is disassembled and reassembled.

Often a poor O-ring installation begins whenan old seal is removed. O-ring removal involvesworking with parts that have critical surfacefinishes. If hardened-steel, pointed, or sharp-edged tools are used for removal of O-rings orbackup rings, scratches, abrasions, dents, andother deformities on critical sealing surfaces canresult in seal failure which, in turn, can result in

Figure 7-7.—Critical dimensions of an O-ring.

might scratch or mar component surfaces ordamage the O-ring. An O-ring tool kit isavailable in the supply system for O-ring in-stallation or removal. If these tools are not onhand, special tools can be made for this purpose.A few examples of tools used in the removaland installation of O-rings are illustrated in

functional failure of

When removingnot use pointed or

the equipment.

or installing O-rings, dosharp-edged tools which Figure 7-8.–Typical O-ring instalation.

7-9

figure 7-9. These tools should be fabricated fromsoft metal such as brass or aluminum; however,tools made from phenolic rod, wood, or plasticmay also be used.

Tool surfaces must be well rounded, polished,and free of burrs. Check the tools often, especiallythe surfaces that come in contact with O-ringgrooves and critical polished surfaces.

Notice in figure 7-9, view A, how thehook-type removal tool is positioned under theO-ring and then lifted to allow the extractor tool,as well as the removal tool, to pull the O-ring fromits cavity. View B shows the use of another typeof extractor tool in the removal of internallyinstalled O-rings.

In view C, notice the extractor tool positionedunder both O-rings at the same time. This methodof manipulating the tool positions both O-rings,which allows the hook-type removal tool to

extract both O-rings with minimum effort. ViewD shows practically the same removal as view C,except for the use of a different type of extractortool.

The removal of external O-rings is less difficultthan the removal of internally installed O-rings.Views E and F show the use of a spoon-typeextractor, which is positioned under the seal. Afterthe O-ring is dislodged from its cavity, thespoon is held stationary while the piston issimultaneously rotated and withdrawn. View Fis similar to view E, except that only one O-ringis installed, and a different type of extractor toolis used. The wedge-type extractor tool is insertedbeneath the O-ring; the hook-type removal toolhooks the O-ring. A slight pull on the latter toolremoves the O-ring from its cavity.

After removing all O-rings, cleaning of theaffected parts that will receive new O-rings is

Figure 7-9.—O-ring tools and O-ring removal.

7-10

mandatory. Ensure that the area used for suchinstallations is clean and free from allcontamination.

Remove each O-ring that is to be installedfrom its sealed package and inspect it for defectssuch as blemishes, abrasions, cuts, or punctures.Although an O-ring may appear perfect at firstglance, slight surface flaws may exist. These areoften capable of preventing satisfactory O-ringperformance. O-rings should be rejected for flawsthat will affect their performance.

By rolling the ring on an inspection cone ordowel, the inner diameter surface can be checkedfor small cracks, particles of foreign material, andother irregularities that will cause leakage orshorten its life. The slight stretching of the ringwhen it is rolled inside out will help to reveal somedefects not otherwise visible. A further check ofeach O-ring should be made by stretching itbetween the fingers, but care must be taken not toexceed the elastic limits of the rubber. Followingthese inspection practices will prove to be amaintenance economy. It is far more desirable to

take care identifying and inspecting O-rings than torepeatedly overhaul components with faulty seals.

After inspection and prior to installation,lubricate the O-ring, and all the surfaces that itmust slide over with a light coat of the system fluidor a lubricant approved for use in the system.Consult the applicable technical instruction orNaval Ships’ Technical Manual for the correctlubricant for pneumatic systems.

Assembly must be made with care so that theO-ring is properly placed in the groove and notdamaged as the gland is closed. During someinstallations, such as on a piston, it will benecessary to stretch the O-ring. Stretch the O-ringas little and as uniformly as possible. Avoid rollingor twisting the O-ring when maneuvering it intoplace. Keep the position of the O-ring mold lineconstant. O-rings should not be left in a twistedcondition after installation.

If the O-ring installation requires spanning orinserting through sharp-threaded areas, ridges,slots, and edges, use protective measures, such asthe O-ring entering sleeve (fig. 7-10, view A). If

Figure 7-10.–O-ring installation.

7-11

the recommended O-ring entering sleeve (a soft,thin wall, metallic sleeve) is not available, papersleeves and covers may be fabricated by using theseal package (glossy side out) or lint-free bondpaper (see views B and C of fig. 7-10).

After you place the O-ring in the cavityprovided, gently roll the O-ring with your fingersto remove any twist that might have occurredduring the installation. After installation, anO-ring should seat snugly but freely in its groove.If backup rings are installed in the groove, becertain the backup rings are installed on thecorrect side of the ring.

BACKUP RINGS

Backup rings, also referred to as retainer rings,antiextrusion devices, and nonextrusion rings, arewasher-like devices that are installed on thelow-pressure side of packing to prevent extrusionof the packing material. Backup rings in dynamicseals minimize erosion of the packing materialsand subsequent failure of the seal. At lowerpressures, backup rings will prolong the normalwear life of the packing. At higher pressures,backup rings permit greater clearances betweenthe moving parts. Normally, backup rings arerequired for operating pressures over 1500 psi.

Backup rings can be made of polytetra-fluoroethylene, hard rubber, leather, and othermaterials. The most common material currentlyused is tetrafluoroethylene (TFE). Backup ringsare available as single-turn continuous (uncut orsolid), single-turn (bias) cut, and spiral cut. Seefigure 7-11. Leather rings are always furnished insolid ring form (unsplit). Rings of TFE areavailable in all three types.

Packaging and Storing

Backup rings are not color-coded or otherwisemarked and must be identified from the packaging

labels. The dash number following the militarystandard number found on the package indicatesthe size, and usually relates directly to the dashnumber of the O-rings for which the backup ringis dimensionally suited. Backup rings made ofTFE do not deteriorate with age and do not haveshelf life limitations. TFE backup rings areprovided by manufacturer either in individuallysealed packages or on mandrels. If unpackagedrings are stored for a long time without the useof mandrels, a condition of overlap may develop.Overlap occurs when the ID of the backup ringbecomes smaller and its ends overlap each other.To correct this overlap condition, stack TFE ringson a mandrel of the correct diameter, and clampthe rings with their coils flat and parallel. Placethe rings in an oven at a maximum temperatureof 1770C (3500F) for approximately 10 minutes.Do not overheat them because fumes fromdecomposing TFE are toxic. Remove andwater-quench the rings. Store the rings at roomtemperature before you use them (preferably for48 hours).

Installation

Care must be taken in handling and installingbackup rings. Do not insert them with sharptools. Backup rings must be inspected priorto using them for evidence of compressiondamage, scratches, cuts, nicks, or frayed con-ditions. If O-rings are to be replaced wherebackup rings are installed in the same groove,never replace the O-ring without replacingthe backup rings, or vice versa. Many sealsuse two backup rings, one on either side of theO-ring (fig. 7-12). Two backup rings are usedprimarily in situations (such as a reciprocatingpiston seal) where alternating pressure directioncan cause packing to be extruded on both sidesof the gland.

Figure 7-11.—Types of backup rings.

7-12

Figure 7-12.—Backup ring configuration.

If only one backup ring is used, place thebackup ring on the low-pressure side of thepacking (fig. 7-13, view A). When a backup ringis placed on the high-pressure side of the packing,the pressure against the relatively hard surface ofthe backup ring forces the softer packing againstthe low-pressure side of the gland, resulting in arapid failure due to extrusion (fig. 7-13, view B).

When dual backup rings are installed, staggerthe split scarfed ends as shown in figure 7-14.When installing a spiral cut backup ring (MS28782or MS28783), be sure to wind the ring correctlyto ease installation and ensure optimum per-formance.

When TFE spiral rings are being installed ininternal grooves, the ring must have a right-hand

Figure 7-13.–Location of a single backup ring.

Figure 7-14.—Installation of cut dual backup rings.

7-13

Figure 7-15.–Installation of TFE back up rings (internal).

7-14

spiral. Figure 7-15, view A, shows how to changethe direction of the spiral. The ring is thenstretched slightly, as shown in view B prior toinstallation into the groove. While the TFE ringis being inserted into the groove, rotate thecomponent in a clockwise direction. This will tendto expand the ring diameter and reduce thepossibility of damaging the ring.

When TFE spiral rings are being installed inexternal grooves, the ring should have a left-handspiral. As the ring is being inserted into thegroove, rotate the component in a clockwisedirection. This action will tend to contract the ringdiameter and reduce the possibility of damagingthe ring.

In applications where a leather backup ringis called for, place the smooth-grained side of theleather next to the ring. Do not cut leather backuprings. Use a leather backup ring as one continuousring and lubricate the ring prior to installing it,particularly the smaller sizes. If stretching isnecessary for proper installation, soak the backupring in the system fluid or in an acceptablelubricant at room temperature for at least 30minutes.

or two backup rings, depending upon the specificseal groove application and width. The Quad-Ring® seal works well in, both hydraulic andpneumatic systems.

Many Quad-Ring® seal sizes have beenassigned NSNs and are stocked in the FederalSupply System. Quad-Ring® seals in manu-facturer’s sizes designated as Q1 through Q88 areinterchangeable with O-rings conforming toAN6227. Likewise, Quad-Ring® seals in com-mercial sizes Q101 through Q152 are inter-changeable with O-rings conforming to AN6230in the respective dash sizes from –1 through–52.Therefore, the Quad-Ring® seal stock partnumber uses the AN standard O-ring designationsAN6227 and AN6230 and the commercial Q dashnumber designation. For example, NSNs arefound under such reference part numbers asAN6227Q10 and AN6230Q103. If the letter Qdoes not follow AN6227 or AN6230, the partnumber is an O-ring not a Quad-Ring® seal.

If Quad-Ring® seals are not available formaintenance actions, appropriate sized O-ringscan be installed and they work satisfactorily.

QUAD-O-DYN® SEALSQUAD-RINGS

The Quad-Ring® seal is a special configura-tion ring packing, manufactured by the MinnesotaRubber. As opposed to an O-ring, a Quad-Ring® seal has a more square cross-sectionalshape with rounded corners (fig. 7-16). The Quad-Ring® seal design offers more stability than theO-ring design and practically eliminates thespiraling or twisting that is sometimes encounteredwith the O-ring.

Quad-Rings® seals are completely inter-changeable with O-rings in the sizes offered bythe manufacturer. They may be installed with one

The Quad-O-Dyn®, also manufactured byMinnesota Rubber, is a special form of theQuad-Ring. The Quad-O-Dyn differs from theQuad-Ring in configuration (fig. 7-17), is harder,is subject to greater squeeze, and is made of adifferent material. The Quad-O-Dyn® seal alsoworks well in O-rings glands.

The Quad-O-Dyn® is used in relatively fewapplications. However, for difficult dynamicsealing applications, the Quad-O-Dyn® canperform better than the Quad-Ring. Quad-O-Dyn® rings are installed in submarine hydraulicsystems plant accumulators.

Figure 7-16.—Quad-Ring.

7-15

Figure 7-17.—Quad-O-Dyn® seal.

U-CUPS AND U-PACKINGS

The distinction between U-cups andU-packings results from the difference in materialsused in their fabrication. The U-cup is usuallymade of homogeneous synthetic rubber;U-packings are usually made of leather or fabric-reinforced rubber. Special aspects of each type willbe discussed separately. However, all U-cups andU-packings have cross sections resembling theletter U. Both types are balanced packings, bothseal on the ID and the OD, and both are appliedindividually, not in stacks like V-rings. Sizedifferences between U-cups and U-packings areusually substantial enough to prevent inter-changeability. There are a few sizes with smallerdiameters and cross sections that may appearto be dimensionally equivalent but are not.Therefore, U-packings should not be substitutedfor U-cups (or vice versa) in any installation.

U-CUPS

The U-cup (fig. 7-18) has been a popularpacking in the past because of installation easeand low friction. U-cups are used primarily forpressures below 1500 psi, but higher pressures arepossible with the use of antiextrusion rings. Fordouble-acting pistons, two U-cups are installedin separate grooves, back-to-back or heel-to-heel.Two U-cups are never used in the same groove.This heel-to-heel type of installation is commonfor single-acting (monodirectional) seals, such asU-cups and V-rings, and is necessary to preventa pressure trap (hydraulic lock) between twopackings. Installation of two U-cups with sealinglips facing each other can result in hydraulic lockand must be avoided.

Leather U-Packings

As a rule, leather U-packings are made withstraight side walls (no flared sealing lips). See

figure 7-19. The leather may be chemically treatedor otherwise impregnated to improve its per-formance. Leather U-packings are available instandard sizes conforming to industrial specifica-tions. For support, the cavity of the U-packingshould contain a metal pedestal ring or should befilled with a suitable material. Leather U-packingswith an integral pedestal support have beeninstalled in some submarine steering and divingram piston seals.

CUP PACKINGS

Cup packings resemble a cup or deep dish witha hole in the center for mounting (fig. 7-20). Cupseals are used exclusively to seal pistons in bothlow- and high-pressure hydraulic and pneu-matic service. They are produced in leather,homogeneous synthetic rubber, and fabric-reinforced synthetic rubber. Although the cuppacking lip flares outward, the rubbing contactis made at the lip only when the fluid pressure islow. As the fluid pressure increases, the cup heelexpands outward until it contacts the cylinderwall, at which point high-pressure sealing is ineffect. As the pressure loading shifts the sealingline to the cup heel, the lip is actually pulled intothe cup and away from the cylinder wall. On thereturn stroke when the pressure is relaxed, the heelwill shrink slightly, leaving only the lip in contactwith the wall, avoiding unnecessary wear at theheel.

For reciprocating pistons, two cups installedback-to-back in separate glands are required.

FLANGE PACKINGS

Flange packings are used exclusively in low-pressure, outside-packed installations, such as rod

Figure 7-18.—Typical U-cup seal. Figure 7-19.—U-packing.

7-16

Figure 7-20.–Cup packing.

seals. The flangemade of leather,

(sometimes calledfabric-reinforced

the hat) isrubber, or

homogeneous rubber. Lip sealing occurs only onthe packing ID (fig. 7-21). Flange packings aregenerally used only for rod seals when otherpackings such as V-rings or U-seals cannot beused.

DIRT EXCLUSION SEALS(WIPERS AND SCRAPERS)

Dirt exclusion devices are essential if asatisfactory life is to be obtained from most rodseals. The smooth finished moving rod surface,if not enclosed or protected by some sort ofcovering, will accumulate a coating of dust orabrasive material that will be dragged or carriedinto the packing assembly area on the return rodstroke. Exclusion devices called wipers or scrapersare designed to remove this coating. While theterms wiper and scraper are often usedinterchangeably, it is useful to reserve scraper

Figure 7-21.—Typical flange packing cross section.

for metal lip-type devices that remove heavilyencrusted deposits of dirt or other abrasivematerial that would merely deflect a softer lip andbe carried into the cylinder. Sometimes a rod willhave both a scraper and a wiper, the former toremove heavy deposits and the latter to excludeany dust particles that remain. Whenever metallicscrapers are used with felt wipers in the samegroove, the felt wiper must not be compressed norrestricted in any way that affects its function asa lubricator. A wiper installed in a seal assemblyin a pneumatic application may remove too muchoil from the rod, requiring some method ofreplacing the oil. A common remedy is to providea periodically oiled felt ring between the wiperand the seal. Felt wipers provide lubricationto extended operating rods, thus increasingcomponent wear life. These wipers are only usedto provide lubrication to parts.

Much longer life could be obtained from mostseals if proper attention were given to wipers andscrapers. Often, wiper or scraper failure is notnoticed when a seal packing fails. As a result, onlythe packing is replaced, and the same worn wiperor scraper is reinstalled to destroy anotherpacking. Check the wiper or scraper conditionupon its removal. If the wiper is worn, dirty, orembedded with metallic particles, replace it witha new one. It is usually good practice to replacethe wiper every time you replace the seal and evenmore frequently if the wiper is readily accessiblewithout component disassembly. If replacementsare not available, wash dirty wipers that are stillin good condition with suitable solvent andreinstall them. Remember that a wiper or scraperis deliberately installed as a sacrificial partto protect and preserve the sealing packing.Therefore, from a user’s standpoint, wipers andscrapers should be inspected and replaced asnecessary.

STORAGE OF SEALS

Proper storage practices must be observed toprevent deformation and deterioration of seals.Most synthetic rubbers are not damaged bystorage under ideal conditions. However, mostsynthetic rubbers will deteriorate when exposedto heat, light, oil, grease, fuels, solvents, thinners,moisture, strong drafts, or ozone (form of oxygenformed from an electrical discharge). Damage byexposure is magnified when rubber is undertension, compression, or stress. There are several

7-17

conditions to be avoided, which include thefollowing:

1. Deformation as a result of improperstacking of parts and storage containers.

2. Creasing caused by force applied to cornersand edges, and by squeezing between boxes andstorage containers.

3. Compression and flattening, as a result ofstorage under heavy parts.

4. Punctures caused by staples used to attachidentification.

5. Deformation and contamination due tohanging the seals from nails or pegs. Seals shouldbe kept in their original envelopes, which providepreservation, protection, identification, and curedate.

6. Contamination by piercing the sealedenvelope to store O-rings on rods, nails, or wirehanging devices.

7. Contamination by fluids leaking from partsstored above and adjacent to the seal surfaces.

8. Contamination caused by adhesive tapesapplied to seal surfaces. A torn seal packageshould be secured with a pressure-sensitivemoistureproof tape, but the tape must not contactthe seal surfaces.

9. Retention of overage parts as a resultof improper storage arrangement or illegibleidentification. Seals should be arranged so theolder seals are used first.

7-18

CHAPTER 8

MEASUREMENT AND PRESSURECONTROL DEVICES

For safe and efficient operation, fluid powersystems are designed to operate at a specificpressure and/or temperature, or within a pressureand/or temperature range.

You have learned that the lubricating powerof hydraulic fluids varies with temperature andthat excessively high temperatures reduce the lifeof hydraulic fluids. Additionally, you havelearned that the materials, dimensions, andmethod of fabrication of fluid power componentslimit the pressure and temperature at which asystem operates. You have also learned of meansof automatically controlling pressure in bothhydraulic and pneumatic systems.

Most fluid power systems are provided withpressure gauges and thermometers for measuringand indicating the pressure and/or the tempera-ture in the system. Additionally, various tempera-ture and pressure switches are used to warn of anadverse pressure or temperature condition. Someswitches will even shut the system off when anadverse condition occurs. These devices will bediscussed in this chapter.

PRESSURE GAUGES

Many pressure-measuring instruments arecalled gauges. However, this section will berestricted to two mechanical instruments thatcontain elastic elements that respond to pressuresfound in fluid power systems—the Bourdon-tubeand bellows gauges.

BOURDON TUBE GAUGES

The majority of pressure gauges in use havea Bourdon-tube as a measuring element. (Thegauge is named for its inventor, Eugene Bourdon,a French engineer.) The Bourdon tube is a devicethat senses pressure and converts the pressure todisplacement. Since the Bourdon-tube displace-ment is a function of the pressure applied, it maybe mechanically amplified and indicated by a

pointer. Thus, the pointer position indirectlyindicates pressure.

The Bourdon-tube gauge is available invarious tube shapes: curved or C-shaped, helical,and spiral. The size, shape, and material of thetube depend on the pressure range and the typeof gauge desired. Low-pressure Bourdon tubes(pressures up to 2000 psi) are often made ofphosphor bronze. High-pressure Bourdon tubes(pressures above 2000 psi) are made of stainlesssteel or other high-strength materials. High-pressure Bourdon tubes tend to have more circularcross sections than their lower-range counterparts,which tend to have oval cross sections. TheBourdon tube most commonly used is theC-shaped metal tube that is sealed at one end andopen at the other (fig. 8-1).

Figure 8-1.—Simplex Bourdon-tube pressure gauge.

8-1

C-shaped Bourdon Tube

The C-shaped Bourdon tube has a hollow,elliptical cross section. It is closed at one end andis connected to the fluid pressure at the other end.When pressure is applied, its cross sectionbecomes more circular, causing the tube tostraighten out, like a garden hose when the wateris first turned on, until the force of the fluidpressure is balanced by the elastic resistance ofthe tube material. Since the open end of the tubeis anchored in a fixed position, changes in pressuremove the closed end. A pointer is attached to theclosed end of the tube through a linkage arm anda gear and pinion assembly, which rotates thepointer around a graduated scale.

Bourdon-tube pressure gauges are oftenclassified as simplex or duplex, depending uponwhether they measure one pressure or twopressures. A simplex gauge has only one Bourdontube and measures only one pressure. The pressuregauge shown in figure 8-1 is a simplex gauge. Ared hand is available on some gauges. This handis manually positioned at the maximum operatingpressure of the system or portion of the systemin which the gauge is installed.

When two Bourdon tubes are mounted ina single case, with each mechanism acting

independently but with the two pointers mountedon a common dial, the assembly is called a duplexgauge. Figure 8-2 shows a duplex gauge with viewsof the dial and the operating mechanism. Notethat each Bourdon tube has its own pressureconnection and its own pointer. Duplex gaugesare used to give a simultaneous indication of thepressure from two different locations. Forexample, it may be used to measure the inlet andoutlet pressures of a strainer to obtain thedifferential pressure across it.

Differential pressure may also be measuredwith Bourdon-tube gauges. One kind of Bourdon-tube differential pressure gauge is shown infigure 8-3. This gauge has two Bourdon tubesbut only one pointer. The Bourdon tubes areconnected in such a way that they indicate thepressure difference, rather than either of twoactual pressures.

As mentioned earlier, Bourdon-tube pressuregauges are used in many hydraulic systems. In thisapplication they are usually referred to ashydraulic gauges. Bourdon-tube hydraulic gaugesare not particularly different from other types ofBourdon-tube gauges in how they operate;however, they do sometimes have special designfeatures because of the extremely high systempressures to which they may be exposed. For

Figure 8-2.—Duplex Bourdon-tube pressure gauge.

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Figure 8-3.—Bourdon-tube differential pressure gauge.

example, some hydraulic gauges have a specialtype of spring-loaded linkage that is capable oftaking overpressure and underpressure withoutdamage to the movement and that keeps thepointer from slamming back to zero when thepressure is suddenly changed. A hydraulic gaugethat does not have such a device must be protectedby a suitable check valve. Some hydraulic gaugesmay also have special dials that indicate both thepressure (in psi) and the corresponding total forcebeing applied, for example tons of force producedby a hydraulic press.

Spiral and Helical Bourdon Tubes

Spiral and helical Bourdon tubes (figs. 8-4 and8-5) are made from tubing with a flattened cross

Figure 8-4.—Spiral Bourdon tube.

section. Both were designed to provide more travelof the tube tip, primarily for moving the recordingpen of pressure recorders.

BELLOWS ELASTIC ELEMENTS

A bellows elastic element is a convoluted unitthat expands and contracts axially with changesin pressure. The pressure to be measured can beapplied to either the outside or the inside of thebellows; in practice, most bellows measuring

Figure 8-5.—Helical Bourdon tube.

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Figure 8-6.—Simple bellows gauge.

devices have the pressure applied to the outsideof the bellows (fig. 8-6).

Simple Bellows Elements

Bellows elastic elements are made of brass,phosphor bronze, stainless steel, beryllium-copper, or other metal suitable for the intendedservice of the gauge. Motion of the element(bellows) is transmitted by suitable linkage andgears to a dial pointer. Most bellows gauges arespring-loaded—that is, a spring opposes thebellows and thus prevents full expansion of thebellows. Limiting the expansion of the bellows inthis way protects the bellows and prolongs its life.Because of the elasticity in both the bellows andthe spring in a spring-loaded bellows element, therelationship between the applied pressure andbellows movement is linear.

Dual Bellows Indicators

Another type of bellows element is the dual-bellows element. Figure 8-7 is a schematic diagramof this indicator. Dual-bellows element pressureindicators are used throughout the Navy as flow-measuring, level-indicating, or pressure-indicatingdevices.

Figure 8-7.–Differential pressure sensor dual bellows.

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Figure 8-8.–Dual bellows assembly.

When in operation, the bellows will move inproportion to the difference in pressure appliedacross the bellows unit assembly. The linearmotion of the bellows is picked up by a drive armand transmitted as a rotary motion through atorque tube assembly (fig. 8-8). The indicatingmechanism multiplies rotation of the torque tubethrough a gear and pinion to the indicatingpointer.

Bellows elements are used in various appli-cations where the pressure-sensitive device mustbe powerful enough to operate not only theindicating pointer but also some type of recordingdevice.

PRESSURE SWITCHES

Often when a measured pressure reaches acertain maximum or minimum value, it is desir-able to have an alarm sound a warning, a lightto give a signal, or an auxiliary control system toenergize or de-energize. A pressure switch is thedevice commonly used for this purpose.

One of the simplest pressure switches is thesingle-pole, single-throw, quick-acting type shownin figure 8-9. This switch is contained in a metal

Figure 8-9.—Typical pressure switch.

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case that has a removable cover, an electricalconnection, and a pressure-sensing connection.The switch contains a seamless metallic bellowslocated in its housing. Changes in the measuredpressure causes the bellows to work against anadjustable spring. This spring determines thepressure required to actuate the switch. Throughsuitable linkage, the spring causes the contacts toopen or close the electrical circuit automaticallywhen the operating pressure falls below or risesabove a specified value. A permanent magnet inthe switch mechanism provides a positive snap onboth the opening and closing of the contacts. Theswitch is constantly energized. However, it is theclosing of the contacts that energizes the entireelectrical circuit.

Another pressure switch is an electric-hydraulic assembly that is used for shutting offthe pump’s motor whenever the system pressureexceeds a pre-determined maximum value (fig.8-10). The switch is mounted on the pump housingso that the former’s low pressure ports draindirectly into the pump housing.

This pressure switch principally consists of aflange-mounted hydraulic valve to which is fixeda normally closed electrical limit switch.

The valve consists of two hydraulicallyinterconnected components, the pilot valve sub-assembly, which bolts on the bottom of thebody (l), functions to sense system pressurecontinuously and initiates pressure switch actionwhenever this pressure exceeds the adjusted settingof the pilot adjustment. System pressure isdirected into the bottom port and is appliedagainst the exposed tip of the pilot piston (5). Thispiston is held on its seat by compression from thepiston spring (6) which is dependent on theposition of the adjusting screw (8). Whenever thepressure causes a force sufficiently large enoughto raise the pilot piston from its seat, fluidflows through an interconnecting passage to theactuating piston (2) chamber. The accompanyingfluid force raises the actuating piston against theforce of spring 3 and causes depression ofthe extended switch plunger. This, in turn,disconnects the contained electrical switch, whichmay be connected into the pump motor’s electricsupply system.

Pressure switches come in many sizes andconfigurations depending on how they will beused.

Figure 8-10.—Electric-hydraulic pressure switch.

TEMPERATURE-MEASURINGINSTRUMENTS

Temperature is the degree of hotness orcoldness of a substance measured on a definitescale. Temperature is measured when a measuringinstrument, such as a thermometer, is broughtinto contact with the medium being measured.

All temperature-measuring instruments usesome change in a material to indicate temperature.Some of the effects that are used to indicatetemperature are changes in physical properties andaltered physical dimensions. One of the moreimportant physical properties used in temperature-measuring instruments is the change in the lengthof a material in the form of expansion andcontraction.

Consider the uniform homogeneous barillustrated in figure 8-11. If the bar has a given

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Figure 8-11.—Expansion of a bar.

length (LO) at some temperature and is heated, itwill expand (Lf). The amount of expansion is a function of the original length and thetemperature increase. The amount a materialchanges in length with temperature is called thelinear coefficient of expansion.

The linear coefficient of expansion for amaterial is a physical property of that materialand describes its behavior with respect totemperature.

BIMETALLIC EXPANSIONTHERMOMETER

If two materials with different linear coef-ficients are bonded together, as the temperaturechanges their rate of expansion will be different.This will cause the entire assembly to bend in anarc as shown in figure 8-12.

When the temperature is raised, an arc isformed around the material with the smallerexpansion coefficient. Since this assembly isformed by joining two dissimilar materials, it isknown as a bimetallic element.

A modification of this bimetallic strip servesas the basis for one of the simplest and mostcommonly encountered temperature-measuringinstruments, the bimetallic thermometer.

Figure 8-13 shows a bimetallic thermometer.In it, a bimetallic strip is wound in the form ofa long helix. One end of the helix is held rigid.As the temperature varies, the helix tries to windor unwind. This causes the free end to rotate. The

Figure 8-12.—Effect of unequal expansion of a bimetallicstrip.

free end is connected to a pointer. The pointeractually indicates angular rotation of the helix;however, since the rotation is linear and a functionof temperature, the scale is marked in units oftemperature.

DISTANT-READING THERMOMETERS

Distant-reading dial thermometers are usedwhen the indicating portion of the instrumentmust be placed at a distance from where thetemperature is being measured. The distant-reading thermometer has a long capillary, some

Figure 8-13.—Bimetallic thermometer.

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as long as 125 feet, which separates the sensingbulb from the Bourdon tube and dial (fig. 8-14).

There are three basic types of distant-readingthermometers: the liquid filled, the gas filled,and the combination liquid-vapor filled. Thethermometers are filled with fluid (liquid or gas)at some temperature and sealed. Almost the entirevolume of the fluid is in the sensing bulb.

As the temperature of the bulb changes, thevolume of the fluid tries to change. Sincethe volume of the thermometer (sensing bulb,capillary, and Bourdon tube) is constant, apressure change occurs within the thermometer.This pressure change causes the Bourdon tube tostraighten out (with an increase in pressure),working a system of levers and gears, which causesthe thermometer pointer to move over the dial andregister temperature.

TEMPERATURE SWITCHES

Temperature switches operate from tempera-ture changes occurring in an enclosure, or in theair surrounding the temperature-sensing element.The operation of the temperature switch is similarto the operation of the pressure switch shown infigure 8-9; both switches are operated by changesin pressure. The temperature element is arrangedso a change in temperature causes a change in theinternal pressure of a sealed-gas or air-filled bulb

Figure 8-14.—Distant-reading, Bourdon-tube thermometers.

or helix, which is connected to the actuating deviceby a small tube or pipe. Figure 8-15 shows atemperature switch and two types of sensingelements.

A temperature change causes a change in thevolume of the sealed-in gas, which causesmovement of a bellows. The movement istransmitted by a plunger to the switch arm. Themoving contact is on the arm. A fixed contact maybe arranged so the switch will open or close ona temperature rise. This allows the switch contactsto be arranged to close when the temperaturedrops to a predetermined value and to open whenthe temperature rises to the desired value. Thereverse action can be obtained by a change in thecontact positions.

GAUGE SNUBBERS

The irregularity of impulses applied to thefluid power system by some pumps or aircompressors causes the gauge pointer to oscillateviolently. This makes reading of the gauge notonly difficult but often impossible. Pressureoscillations and other sudden pressure changesexisting in fluid power systems will also affect thedelicate internal mechanism of gauges and causeeither damage to or complete destruction of the

Figure 8-15.—Temperature switch with two types of sensingelements. A. Bulb unit. B. Helix unit.

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gauge. A pressure gauge snubber is thereforeinstalled in the line that leads to the pressuregauge.

The purpose of the snubber is to dampen theoscillations and thus provide a steady reading andprotection for the gauge. The basic componentsof a snubber are the housing, fitting assembly witha fixed orifice diameter, and a pin and plungerassembly (fig. 8-16). The snubbing action isobtained by metering fluid through the snubber.The fitting assembly orifice restricts the amountof fluid that flows to the gauge, thereby snubbingthe force of a pressure surge. The pin is pushedand pulled through the orifice of the fittingassembly by the plunger, keeping it clean and ata uniform size.

Figure 8-16.—Pressure gauge snubber.

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CHAPTER 9

RESERVOIRS, STRAINERS, FILTERS,AND ACCUMULATORS

Fluid power systems must have a sufficientand continuous supply of uncontaminated fluidto operate efficiently. As stated in chapter 3 andemphasized throughout this manual, the fluidmust be kept free of all foreign matter.

This chapter covers hydraulic reservoirs,various types of strainers and filters, andaccumulators installed in fluid power systems.

RESERVOIRS

A hydraulic system must have a reserve offluid in addition to that contained in the pumps,actuators, pipes, and other components of thesystem. This reserve fluid must be readily availableto make up losses of fluid from the system, tomake up for compression of the fluid underpressure, and to compensate for the loss ofvolume as the fluid cools. This extra fluid iscontained in a tank usually called a reservoir. Areservoir may sometimes be referred to as a sumptank, service tank, operating tank, supply tank,or base tank.

In addition to providing storage for the reservefluid needed for the system, the reservoir acts asa radiator for dissipating heat from the fluid andas a settling tank where heavy particles ofcontamination may settle out of the fluid andremain harmlessly on the bottom until removedby cleaning or flushing of the reservoir. Also, thereservoir allows entrained air to separate from thefluid.

Most reservoirs have a capped opening forfilling, an air vent, an oil level indicator or dipstick, a return line connection, a pump inlet orsuction line connection, a drain line connection,and a drain plug (fig. 9-1). The inside of thereservoir generally will have baffles to preventexcessive sloshing of the fluid and to put apartition between the fluid return line and thepump suction or inlet line. The partition forcesthe returning fluid to travel farther around thetank before being drawn back into the active

Figure 9-1.—Nonpressurized reservoir (ground or shipinstallation).

system through the pump inlet line. This aids insettling the contamination and separating the airfrom the fluid.

Large reservoirs are desirable for cooling. Alarge reservoir also reduces recirculation whichhelps settle contamination and separate air. Asa ‘‘thumb rule,” the ideal reservoir should be twoto three times the pump output per minute.However, due to space limitations in mobile andaerospace systems, the benefits of a large reservoirmay have to be sacrificed. But, they must be largeenough to accommodate thermal expansion of thefluid and changes in fluid level due to systemoperation. Reservoirs are of two general types—nonpressurized and pressurized.

NONPRESSURIZED RESERVOIRS

Hydraulic systems designed to operateequipment at or near sea level are normallyequipped with nonpressurized reservoirs. Thisincludes the hydraulic systems of ground and ship

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installations. A typical reservoir for use withground and ship installations is shown in figure9-1. This type of reservoir is made of hot rolledsteel plates and has welded seams. The ends extendbelow the bottom of the reservoir and serve assupports. The bottom of the reservoir is convex,and a drain plug is incorporated at the lowestpoint.

Nonpressurized reservoirs are also used inseveral transport-, patrol-, and utility-typeaircraft. These aircraft are not designed for violentmaneuvers and, in some cases, do not fly at highaltitude. Those aircraft that have nonpressurizedreservoirs installed and that fly at high altitudeshave the reservoirs installed within a pressurizedarea. (High altitude in this situation means analtitude where atmospheric pressure is inadequateto maintain sufficient flow of fluid to thehydraulic pumps.)

Most nonpressurized aircraft reservoirs areconstructed in a cylindrical shape (fig. 9-2). Theouter housing is manufactured from a strongcorrosion-resistant metal. Filter elements arenormally installed internally within the reservoirto clean returning system hydraulic fluid. Someof the older aircraft have a filter bypass valveinstalled to allow fluid to bypass the filter if thefilter becomes clogged. Reservoirs that are filledby pouring fluid directly into them have a filler(finger) strainer assembly installed in the filler wellto strain out impurities as the fluid enters thereservoir.

Figure 9-2.—Nonpressurized aircraft reservoir.

The quantity of fluid in the reservoir isindicated by either a glass tube, a directing gauge,or a float-type rod, which is visible through atransparent dome installed on the reservoir.

PRESSURIZED RESERVOIRS

A pressurized reservoir is required in hydraulicsystems where atmospheric pressure is insufficientto maintain a net positive suction head (NPSH)to the pump. There are two common types ofpressurized reservoirs—fluid-pressurized andair-pressurized.

Fluid-Pressurized Reservoir

Some aircraft hydraulic systems use fluidpressure for pressurizing the reservoir. Thereservoir shown in figure 9-3 is of this type. Thisreservoir is divided into two chambers by afloating piston. The piston is forced downwardin the reservoir by a compression spring withinthe pressurizing cylinder and by system pressureentering the pressurizing port of the cylinder.

The pressurizing port is connected directly tothe pressure line. When the system is pressurized,pressure enters the pressure port, thus pressurizingthe reservoir. This pressurizes the pump suctionline and the reservoir return line to the samepressure.

The reservoir shown in figure 9-3 has fiveports—pump suction, return, pressurizing,overboard drain, and bleed. Fluid is supplied tothe pump through the pump suction port. Fluidreturns to the reservoir from the system throughthe return port. Pressure from the pump entersthe pressurizing cylinder in the top of the reservoirthrough the pressurizing port. The overboarddrain port is used to drain the reservoir whileperforming maintenance, and the bleed port isused as an aid when servicing the reservoir.

Air-Pressurized Reservoirs

Air-pressurized reservoirs, such as the oneshown in figure 9-4, are currently used in manyhigh-performance naval aircraft. The reservoir iscylindrical in shape and has a piston installedinternally to separate the air and fluid chambers.Air pressure is usually provided by engine bleedair. The piston rod end protrudes through thereservoir end cap and indicates the fluid quantity.The quantity indication may be seen by inspectingthe distance the piston rod protrudes from thereservoir end cap. The reservoir is provided with

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Figure 9-3.—Typical fluid-pressurized reservoir.

threaded openings for connecting fittings andcomponents. Figure 9-4 shows several componentsinstalled in lines leading to and from the reservoir;however, this may not be the case in actualinstallation. The air relief valve, bleeder valve, andsoon, may reinstalled directly on the reservoir.

Because the reservoir is pressurized, it cannormally be installed at any altitude and stillmaintain a positive flow of fluid to the pump.

Figure 9-4.—Air-pressurized reservoir.

Some air-pressurized reservoirs also havedirect contact of fluid to gas. These reservoirs areinstalled in large systems and may be cylindricalor rectangular in shape. They contain an oil levelindicator, a pump inlet or suction line connection,a return line, a gas pressurization and ventingconnection, and a drain line connection or a drainplug. These reservoirs are pressurized by air fromthe ship’s service air system or nitrogen banks.These reservoirs are found on board aircraftcarriers and submarines.

ACCUMULATORS

An accumulator is a pressure storage reservoirin which hydraulic fluid is stored under pressurefrom an external source. The storage of fluidunder pressure serves several purposes in hydraulicsystems.

In some hydraulic systems it is necessary tomaintain the system pressure within a specificpressure range for long periods of time. It is verydifficult to maintain a closed system without someleakage, either external or internal. Even a smallleak can cause a decrease in pressure. By usingan accumulator, leakage can be compensated for

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Figure 9-5.–Cross-section view of a piston-type accumulator with a tailrod.

9-4

and the system pressure can be maintained withinan acceptable range for long periods of time.Accumulators also compensate for thermalexpansion and contraction of the liquid due tovariations in temperature.

A liquid, flowing at a high velocity in a pipewill create a backward surge when stoppedsuddenly by the closing of a valve. This suddenstoppage causes instantaneous pressures two tothree times the operating pressure of the system.These pressures, or shocks, produce objectionalnoise and vibrations which can cause considerabledamage to piping, fittings, and components. Theincorporation of an accumulator enables suchshocks and surges to be absorbed or cushionedby the entrapped gas, thereby reducing theireffects. The accumulator also dampens pressuresurges caused by pulsating delivery from thepump.

There are times when hydraulic systemsrequire large volumes of liquid for short periodsof time. This is due to either the operation of largecylinders or the necessity of operating two or morecircuits simultaneously. It is not economical toinstall a pump of such large capacity in the systemfor only intermittent usage, particularly if there

is sufficient time during the working cycle for anaccumulator to store up enough liquid to aid thepump during these peak demands.

The energy stored in accumulators maybe alsoused to actuate hydraulically operated units ifnormal hydraulic system failure occurs.

Four types of accumulators used in Navyhydraulic systems are as follows:

1. Piston type2. Bag or bladder type3. Direct-contact gas-to-fluid type4. Diaphragm type

PISTON-TYPE ACCUMULATORS

Piston-type accumulators consist of acylindrical body called a barrel, closures on eachend called heads, and an internal piston. Thepiston may be fitted with a tailrod, which extendsthrough one end of the cylinder (fig. 9-5), or itmay not have a tailrod at all (fig. 9-6). In the lattercase, it is referred to as a floating piston.Hydraulic fluid is pumped into one end of thecylinder and the piston is forced toward theopposite end of the cylinder against a captive

Figure 9-6.—Floating piston-type accumulator.

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charge of air or an inert gas such as nitrogen.Sometimes the amount of air charge is limited tothe volume within the accumulator; otherinstallations may use separate air flasks which arepiped to the air side of the accumulator. Pistonaccumulators may be mounted in any position.

The gas portion of the accumulator may belocated on either side of the piston. For example,in submarine hydraulic systems with tailrodpistons, the gas is usually on the bottom and thefluid on top; in surface ships with floating pistons,the gas is usually on the top. The orientation ofthe accumulator and the type of accumulator arebased upon such criteria as available space,maintenance accessibility, size, need for externalmonitoring of the piston’s location (tailrodindication), contamination tolerance, seal life, andsafety. The purpose of the piston seals is to keepthe fluid and the gas separate.

Usually, tailrod accumulators use two pistonseals, one for the air side and one for the oil side,with the space between them vented to theatmosphere through a hole drilled the length ofthe tailrod. When the piston seals fail in this typeof accumulator, air or oil leakage is apparent.However, seal failure in floating piston ornonvented tailrod accumulators will not be asobvious. Therefore, more frequent attention toventing or draining the air side is necessary. Anindication of worn and leaking seals can bedetected by the presence of significant amountsof oil in the air side.

BLADDER-TYPE ACCUMULATORS

Bladder- or bag-type accumulators consist ofa shell or case with a flexible bladder inside theshell. See figure 9-7. The bladder is larger indiameter at the top (near the air valve) andgradually tapers to a smaller diameter at thebottom. The synthetic rubber is thinner at the topof the bladder than at the bottom. The operationof the accumulator is based on Barlow’s formulafor hoop stress, which states: “The stress in acircle is directly proportional to its diameter andwall thickness.” This means that for a certainthickness, a large diameter circle will stretch fasterthan a small diameter circle; or for a certaindiameter, a thin wall hoop will stretch faster thana thick wall hoop. Thus, the bladder will stretcharound the top at its largest diameter and thinnestwall thickness, and then will gradually stretchdownward and push itself outward against thewalls of the shell. As a result, the bladder iscapable of squeezing out all the liquid from.

Figure 9-7.—Bladder-type accumulator.

the accumulator. Consequently, the bladderaccumulator has a very high volumetric efficiency.In other words, this type of accumulator iscapable of supplying a large percentage of thestored fluid to do work.

The bladder is precharged with air or inert gasto a specified pressure. Fluid is then forced intothe area around the bladder, further compressingthe gas in the bladder. This type of accumulatorhas the advantage that as long as the bladder isintact there is no exposure of fluid to the gascharge and therefore less danger of an explosion.

DIRECT-CONTACT GAS-TO-FLUIDACCUMULATORS

Direct-contact gas-to-fluid accumulatorsgenerally are used in very large installations whereit would be very expensive to require a piston-or bladder-type accumulator. This type ofaccumulator consists of a fully enclosed cylinder,mounted in a vertical position, containing a liquid

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port on the bottom and a pneumatic charging portat the top (fig. 9-8). This type of accumulator isused in some airplane elevator hydraulic systemswhere several thousand gallons of fluid are neededto supplement the output of the hydraulic pumpsfor raising the elevator platform. The directcontact between the air or gas and the hydraulicfluid tends to entrain excessive amounts of gasin the fluid. For this reason, direct contactaccumulators are generally not used for pressuresover 1200 psi. The use of this type of accumulatorwith flammable fluid is dangerous because thereis a possibility of explosion if any oxygen ispresent in the gas, and pressure surges generateexcessive heat. For this reason, safety fluids areused in this type of installation.

DIAPHRAGM ACCUMULATORS

The diaphragm-type accumulator is con-structed in two halves which are either screwedor bolted together. A synthetic rubber diaphragmis installed between both halves, making twochambers. Two threaded openings exist in theassembled component. The opening at the top,as shown in figure 9-9, contains a screen discwhich prevents the diaphragm from extrudingthrough the threaded opening when systempressure is depleted, thus rupturing the dia-phragm. On some designs the screen is replacedby a button-type protector fastened to the center

Figure 9-8.—Direct-contact gas-to-fluid accumulator.

Figure 9-9.—Diaphragm accumulator.

of the diaphragm. An air valve for pressurizingthe accumulator is located in the gas chamber endof the sphere, and the liquid port to the hydraulicsystem is located on the opposite end of thesphere. This accumulator operates in a mannersimilar to that of the bladder-type accumulator.

FILTRATION

You have learned that maintaining hydraulicfluids within allowable limits is crucial tothe care and protection of hydraulic equipment.While every effort must be made to preventcontaminants from entering the system, con-taminants which do find their way into the systemmust be removed. Filtration devices are installedat key points in fluid power systems to removethe contaminants that enter the system alongwith those that are generated during normaloperations.

Filtration devices for hydraulic systems differsomewhat from those of pneumatic systems.Therefore, they will be discussed separately.

The filtering devices used in hydraulic systemsare commonly referred to as strainers and filters.Since they share a common function, the termsstrainer and filter are often used interchangeably.As a general rule, devices used to remove largeparticles of foreign matter from hydraulic fluidsare referred to as strainers, while those used toremove the smallest particles are referred to asfilters.

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STRAINERS

Strainers are used primarily to catch only verylarge particles and will be found in applicationswhere this type of protection is required. Mosthydraulic systems have a strainer in the reservoirat the inlet to the suction line of the pump. Astrainer is used in lieu of a filter to reduce itschance of being clogged and starving the pump.However, since this strainer is located in thereservoir, its maintenance is frequently neglected.When heavy dirt and sludge accumulate on thesuction strainer, the pump soon begins to cavitate.Pump failure follows quickly.

FILTERS

The most common device installed inhydraulic systems to prevent foreign particles andcontamination from remaining in the system arereferred to as filters. They may be located in thereservoir, in the return line, in the pressure line,or in any other location in the system where thedesigner of the system decides they are needed tosafeguard the system against impurities.

Filters are classified as full flow andproportional or partial flow. In the full-flow typeof filter, all the fluid that enters the unit passesthrough the filtering element, while in theproportional-flow type, only a portion of the fluidpasses through the element.

Full-Flow Filter

The full-flow filter provides a positive filteringaction; however, it offers resistance to flow,particularly when the element becomes dirty.Hydraulic fluid enters the filter through the inletport in the body and flows around the filterelement inside the filter bowl. Filtering takes placeas the fluid passes through the filtering elementand into the hollow core, leaving the dirt andimpurities on the outside of the filter element.The filtered fluid then flows from the hollowcore through the outlet port and into the system(fig. 9-10).

Some full-flow filters are equipped with acontamination indicator (fig. 9-11). Theseindicators, also known as differential pressureindicators, are available in three types—gaugeindicators, mechanical pop-up indicators, andelectrical with mechanical pop-up indicators. Ascontaminating particles collect on the filterelement, the differential pressure across theelement increases. In some installations using

Figure 9-10.—Full-flow hydraulic filter.

gauges as indicators, the differential pressure mustbe obtained by subtracting the readings of twogauges located somewhere along the filter inletand outlet piping. For pop-up indicators, whenthe increase in pressure reaches a specific value,an indicator (usually in the filter head) pops out,signifying that the filter must be cleaned orreplaced. A low-temperature lockout feature isinstalled in most pop-up types of contaminationindicators to eliminate the possibility of falseindications due to cold weather because thepressure differential may be much higher with acold fluid due to increased viscosity.

Filter elements used in filters that have acontamination indicator are not normallyremoved or replaced until the indicator isactuated. This decreases the possibility of systemcontamination from outside sources due tounnecessary handling.

The use of the nonbypassing type of filtereliminates the possibility of contaminated fluidbypassing the filter element and contaminating theentire system. This type of filter will minimize thenecessity for flushing the entire system and lessenthe possibility of failure of pumps and othercomponents in the system.

A bypass relief valve is installed in some filters.The bypass relief valve allows the fluid to bypassthe filter element and pass directly through theoutlet port in the event that the filter elementbecomes clogged. These filters may or may notbe equipped with the contamination indicator.Figure 9-11 shows a full-flow bypass-type

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Figure 9-11.—Full-flow bypass-type hydraulic filter (with contamination indicator).

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hydraulic filter with a contamination indicator.Figure 9-12 shows a full-flow bypass-typehydraulic filter without a contamination indicator.

A filter bypass indicator provides a positiveindication, when activated, that fluid is bypassingthe filter element by flowing through the bypassrelief valve. This indicator should not be confusedwith the pop-up differential pressure indicatorpreviously discussed which simply monitors thepressure across the element. With the bypassindicator, a similar pop-up button is often usedto signal that maintenance is needed. However,the bypass indicators further signal that, as aresult of the high differential pressures across theelement, an internal bypass relief valve has liftedand some of the fluid is bypassing the element.

Identification of the type of installed indicatorcan be obtained from filter manifold drawings orrelated equipment manuals. Both a fluid bypassindicator and a differential pressure indicator orgauge may be installed on the same filterassembly.

As with differential pressure indicators, bypassrelief indicators can be activated by pressuresurges, such as may develop during cold starts orrapid system pressurization. On some reliefindicators, the pop-up button, or whatever signaldevice is used, will return to a normal positionwhen the surge passes and pressure is reduced.Other relief indicators may continue to indicatea bypass condition until they are manually reset.

Figure 9-12.—Full-flow bypass-type hydraulic filter.

Before corrective action is taken based onindicator readings, the bypass condition shouldbe verified at normal operating temperature andflow conditions by attempting to reset theindicator.

Proportional-Flow Filter

This type of filter operates on the venturiprinciple. (See glossary.) As the fluid passesthrough the venturi throat a drop in pressure iscreated at the narrowest point. See figure 9-13.A portion of the fluid flowing toward and awayfrom the throat of the venturi flows through thepassages into the body of the filter. A fluidpassage connects the hollow core of the filter withthe throat of the venturi. Thus, the low-pressurearea at the throat of the venturi causes the fluidunder pressure in the body of the filter to flowthrough the filter element, through the hollowcore, into the low-pressure area, and then returnto the system. Although only a portion of the fluidis filtered during each cycle, constant recirculationthrough the system will eventually cause all thefluid to pass through the filter element.

Figure 9-13.Proportional-flow filter.

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Filter Rating

Filters are rated in several ways—absolute,mean, and nominal. The absolute filtration ratingis the diameter in microns of the largest sphericalparticle that will pass through the filter under acertain test condition. This rating is an indicationof the largest opening in the filter element. Themean filtration rating is the measurement of theaverage size of the openings in the filter element.The nominal filtration rating is usually interpretedto mean the size of the smallest particles of which90 percent will be trapped in the filter at each passthrough the filter.

Filter Elements

Filter elements generally may be divided intotwo classes—surface and depth. Surface filters aremade of closely woven fabric or treated paper witha uniform pore size. Fluid flows through the poresof the filter material and contaminants arestopped on the filter’s surface. This type ofelement is designed to prevent the passage of ahigh percentage of solids of a specific size. Depthfilters, on the other hand, are composed of layersof fabric or fibers which provide many tortuouspaths for the fluid to flow through. The pores orpassages must be larger than the rated size of thefilter if particles are to be retained in the depthof the media rather than on the surface.Consequently, there is a statistical probabilitythat a rather large particle may pass through adepth-type filter.

Filter elements may be of the 5-micron, wovenmesh, micronic, porous metal, or magnetic type.The micronic and 5-micron elements havenoncleanable filter media and are disposed ofwhen they are removed. Porous metal, wovenmesh, and magnetic filter elements are usuallydesigned to be cleaned and reused.

5-MICRON NONCLEANABLE FILTERELEMENTS.— The most common 5-micron filtermedium is composed of organic and inorganicfibers integrally bonded by epoxy resin and facedwith a metallic mesh upstream and downstreamfor protection and added mechanical strength.Filters of this type are not to be cleaned underany circumstances and will be marked Disposableor Noncleanable.

Another 5-micron filter medium uses layersof very fine stainless-steel fibers drawn into arandom but controlled matrix. Filter elements

Figure 9-14.—Cross-section of a stainless steel hydraulic filterelement.

of this material may be either cleanable ornoncleanable, depending upon their construction.

WOVEN WIRE-MESH FILTER ELE-MENTS.— Filters of this type are made ofstainless steel and are generally rated as 15 or 25micron (absolute). Figure 9-14 shows a magnifiedcross section of a woven wire-mesh filter element.This type of filter is reusable.

MICRONIC HYDRAULIC FILTER ELE-MENT.— The term micronic is derived from theword micron. It could be used to describe anyfilter element; however, through usage, this termhas become associated with a specific filter witha filtering element made of a specially treatedcellulose paper (fig. 9-15). The filter shown infigure 9-10 is a typical micronic hydraulic filter.This filter is designed to remove 99 percent of allparticles 10 to 20 microns in diameter or larger.

Figure 9-15.—Micronic filter element.

9-11 &

The replaceable element is made of speciallytreated convolutions (wrinkles) to increase itsdirt-holding capacity. The element is noncleanableand should be replaced with a new filter elementduring maintenance inspections.

MAGNETIC FILTERS.— Some hydraulicsystems have magnetic filters installed at strategicpoints. Filters of this type are designed primarilyto trap any ferrous particles that may be in thesystem.

PNEUMATIC GASES

Clean, dry gas is required for the efficientoperation of pneumatic systems. Due to thenormal conditions of the atmosphere, free airseldom satisfies these requirements adequately.The atmosphere contains both dust and impuritiesin various amounts and a substantial amount ofmoisture in vapor form.

Solids, such as dust, rust, or pipe scale inpneumatic systems, may lead to excessive wearand failure of components and, in some cases,may prevent the pneumatic devices from operating.Moisture is also very harmful to the system. Itwashes lubrication from moving parts, therebyaiding corrosion and causing excessive wear ofcomponents. Moisture will also settle in low spotsin the system and freeze during cold weather,causing a stoppage of the system or ruptured lines.

An ideal filter would remove all dirt andmoisture from a pneumatic system withoutcausing a pressure drop in the process. Obviously,such a condition can only be approached; itcannot be attained.

Removal of Solids

The removal of solids from the gas ofpneumatic systems is generally done by screening(filtering), centrifugal force, or a combination ofthe two. In some cases, the removal of moistureis done in conjunction with the removal of solids.

Some types of air filters are similar in designand operation to the hydraulic filters discussedearlier. Some materials used in the constructionof elements for air filters are woven screen wire,steel wool, fiber glass, and felt fabrics. Elementsmade of these materials are often used in the unitthat filters the air as it enters the compressor.

Porous metal and ceramic elements arecommonly used in filters that are installed in thecompressed air supply lines. These filters also usea controlled air path to provide some filtration.Internal design causes the air to flow in a circularpath within the bowl (fig. 9-16). Heavy particlesand water droplets are thrown out of the airstream

and drop to the bottom of the bowl. The air thenflows through the filter element, which filters outmost of the smaller particles. This type of filteris designed with a drain valve at the bottom ofthe bowl. When the valve is opened with airpressure in the system, the accumulation of solidsand water will be blown out of the bowl.

An air filter that uses moving mechanicaldevices as an element is illustrated in figure 9-17.As compressed air passes through the filter theforce revolves a number of multi-blade rotors athigh speed. Moisture and dirt are caught on theblades of the rotors. The whirling blades hurl theimpurities by centrifugal force to the outer rimsof the rotors and to the inner walls of the filterhousing. Here, contaminating matter is out of theairstream and falls to the bottom of the bowlwhere it must be drained at periodic intervals.

Removal of Moisture

The removal of moisture from compressed airis important for a compressed air system. If airat atmospheric pressure, even at a very low relativehumidity, is compressed to 3000 or 4500 psi, itbecomes saturated with water vapor. Somemoisture is removed by the intercoolers andaftercoolers (see glossary). Also, air flasks,receivers, and banks are provided with low pointdrains to allow periodic draining of any collectedmoisture. However, many uses of air require airwith an even smaller moisture content than canbe obtained through these methods. Moisture in

Figure 9-16.—Air filter.

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Figure 9-17.—Air filter using rotating blades as element.

air lines can create problems which are potentiallyhazardous, such as the freezing of valves andcontrols. This can occur, for example, if very highpressure air is throttled to a very low pressure ata high flow rate. The venturi effect of the throttledair produces very low temperatures which willcause any moisture in the air to freeze into ice.This makes the valve (especially an automaticvalve) either very difficult or impossible tooperate. Also, droplets of water can cause seriouswater hammer in an air system which has highpressure and a high flow rate and can causecorrosion, rust, and dilution of lubricants withinthe system. For these reasons, air driers

(dehydrator, air purifier, and desiccator are allterms used by different manufacturers to identifythese components) are used to dry the compressedair. Some water removal devices are similar indesign and operation to the filters shown in figures9-16 and 9-17. Two basic types of air dehydratorsare the refrigerated-type and the desiccant-type.

REFRIGERATED-TYPE DEHYDRATORS.–In refrigerated-type dehydrators, compressed airis passed over a set of refrigerated cooling coils. Oiland moisture vapors condense from the air and canbe collected and removed via a low point drain.

DESICCANT-TYPE DEHYDRATORS.– Adesiccant is a chemical substance with a highcapacity to absorb water or moisture. It also hasthe capacity to give off that moisture so that thedesiccant can be reused.

Some compressed air system dehydrators usea pair of desiccant towers (flasks full of desiccant).One is kept in service dehydrating the compressedair, while the other one is being reactivated. Adesiccant tower is normally reactivated by passingdry, heated air through it in the direction oppositethe normal dehydration airflow.

Another type of chemical drier is shown infigure 9-18. This unit consists of the housing, acartridge containing a chemical agent, a filter(sintered bronze), and a spring. Various types ofabsorbent chemicals are used by the differentmanufacturers in the construction of thecartridges. To ensure proper filtering, the air mustpass through the drier in the proper direction. Thecorrect direction of flow is indicated by an arrowand the word FLOW printed on the side of thecartridge.

Figure 9-18.–Chemical drier.

9-13

CHAPTER 10

ACTUATORS

One of the outstanding features of fluid powersystems is that force, generated by the powersupply, controlled and directed by suitablevalving, and transported by lines, can be con-verted with ease to almost any kind of mechanicalmotion desired at the very place it is needed.Either linear (straight line) or rotary motion canbe obtained by using a suitable actuating device.

An actuator is a device that converts fluidpower into mechanical force and motion.Cylinders, motors, and turbines are the mostcommon types of actuating devices used in fluidpower systems.

This chapter describes various types ofactuating cylinders and their applications,different types of fluid motors, and turbines usedin fluid power systems.

CYLINDERS

An actuating cylinder is a device that convertsfluid power to linear, or straight line, force andmotion. Since linear motion is a back-and-forthmotion along a straight line, this type of actuatoris sometimes referred to as a reciprocating, orlinear, motor. The cylinder consists of a ram orpiston operating within a cylindrical bore. Actuat-ing cylinders may be installed so that the cylinderis anchored to a stationary structure and the ramor piston is attached to the mechanism to beoperated, or the piston or ram may be anchoredto the stationary structure and the cylinderattached to the mechanism to be operated.

Actuating cylinders for pneumatic andhydraulic systems are similar in design andoperation. Some of the variations of ram- andpiston-type actuating cylinders are described inthe following paragraphs.

RAM-TYPE CYLINDERS

The terms ram and piston are often usedinterchangeably. However, a ram-type cylinder is

usually considered one in which the cross-sectionalarea of the piston rod is more than one-half thecross-sectional area of the movable element. Inmost actuating cylinders of this type, the rod andthe movable element have equal areas. This typeof movable element is frequently referred to asa plunger.

The ram-type actuator is used primarily topush rather than to pull. Some applicationsrequire simply a flat surface on the external partof the ram for pushing or lifting the unit tobe operated. Other applications require somemechanical means of attachment, such as a clevisor eyebolt. The design of ram-type cylinders variesin many other respects to satisfy the requirementsof different applications.

Single-Acting Ram

The single-acting ram (fig. 10-1) applies forcein only one direction. The fluid that is directedinto the cylinder displaces the ram and forces itoutward, lifting the object placed on it. Since there

Figure 10-1.—Single-acting ram-type actuating cylinder.

10-1

is no provision for retracting the ram by fluidpower, when fluid pressure is released, either theweight of the object or some mechanical means,such as a spring, forces the ram back into thecylinder. This forces the fluid back to thereservoir.

The single-acting ram-type actuating cylinderis often used in the hydraulic jack. The elevatorsused to move aircraft to and from the flight deckand hangar deck on aircraft carriers also usecylinders of this type. In these elevators, thecylinders are installed horizontally and operate theelevator through a series of cables and sheaves.Fluid pressure forces the ram outward and liftsthe elevator. When fluid pressure is released fromthe ram, the weight of the elevator forces the ramback into the cylinder. This, in turn, forces thefluid back into the reservoir.

Double-Acting Ram

A double-acting ram-type cylinder is illustratedin figure 10-2. In this cylinder, both strokes ofthe ram are produced by pressurized fluid. Thereare two fluid ports, one at or near each end ofthe cylinder. Fluid under pressure is directed tothe closed end of the cylinder to extend the ramand apply force. To retract the ram and reducethe force, fluid is directed to the opposite end ofthe cylinder.

A four-way directional control valve isnormally used to control the double-acting ram.When the valve is positioned to extend the ram,pressurized fluid enters port A (fig. 10-2), acts on

Figure 10-2.—Double-acting ram-type actuating cylinder.

the bottom surface of the ram, and forces the ramoutward. Fluid above the ram lip is free to flowout of port B, through the control valve, and tothe return line in hydraulic systems or to theatmosphere in pneumatic systems.

Normally, the pressure of the fluid is the samefor either stroke of the ram. Recall from chapter2 that force is equal to pressure times area(F= PA). Notice the difference of the areas uponwhich the pressure acts in figure 10-2. Thepressure acts against the large surface area on thebottom of the ram during the extension stroke,during which time the ram applies force. Sincethe ram does not require a large force during theretraction stroke, pressure acting on the small areaon the top surface of the ram lip provides thenecessary force to retract the ram.

Telescoping Rams

Figure 10-3 shows a telescoping ram-typeactuating cylinder. A series of rams is nested inthe telescoping assembly. With the exception ofthe smallest ram, each ram is hollow and servesas the cylinder housing for the next smaller ram.The ram assembly is contained in the maincylinder assembly, which also provides the fluidports. Although the assembly requires a smallspace with all the rams retracted, the telescopingaction of the assembly provides a relatively longstroke when the rams are extended.

An excellent example of the application of thistype of cylinder is in the dump truck. It is usedto lift the forward end of the truck bed and dumpthe load. During the lifting operation, the greatestforce is required for the initial lifting of the load.

Figure 10-3.—Telescoping ram-type actuating cylinder.

10-2

As the load is lifted and begins to dump, therequired force becomes less and less until the loadis completely dumped. During the raise cycle,pressurized fluid enters the cylinder through portA (fig. 10-3) and acts on the bottom surface ofall three rams. Ram 1 has a larger surface areaand, therefore, provides the greater force for theinitial load, As ram 1 reaches the end of its strokeand the required force is decreased, ram 2 moves,providing the smaller force needed to continueraising the load. When ram 2 completes its stroke,a still smaller force is required. Ram 3 then movesoutward to finish raising and dumping the load.

Some telescoping ram-type cylinders are of thesingle-acting type. Like the single-acting ramdiscussed previously, these telescoping ram-typecylinders are retracted by gravity or mechanicalforce. Some hydraulic jacks are equipped withtelescoping rams. Such jacks are used to liftvehicles with low clearances to the required height.

Other types of telescoping cylinders, like theone illustrated in figure 10-3, are of the double-acting type. In this type, fluid pressure is used forboth the extension and retraction strokes. A four-way directional control valve is commonly usedto control the operation of the double-acting type.Note the small passages in the walls of rams 1 and2. They provide a path for fluid to flow to andfrom the chambers above the lips of rams 2 and3. During the extension stroke, return fluid flowsthrough these passages and out of the cylinderthrough port B. It then flows through thedirectional control valve to the return line orreservoir.

To retract the rams, fluid under pressure isdirected into the cylinder through port B and actsagainst the top surface areas of all three ram lips.This forces the rams to the retracted position. Thedisplaced fluid from the opposite side of the ramsflows out of the cylinder through port A, throughthe directional control valve to the return line orreservoir.

Dual Rams

A dual ram assembly consists of a single ramwith a cylinder at either end (fig. 10-4). Fluid canbe directed to either cylinder, forcing the ram tomove in the opposite direction. The ram isconnected through mechanical linkage to the unitto be operated. A four-way directional controlvalve is commonly used to operate the dual ram.When the control valve is positioned to direct fluidunder pressure to one of the cylinders (let’s saythe left one), the ram is forced to the right. This

Figure 10-4.-Dual ram actuating assembly.

action displaces the fluid in the opposite cylinder.The displaced fluid flows back through thedirectional control valve to the return line orreservoir in hydraulic systems or to theatmosphere in pneumatic systems.

Dual ram actuating assemblies are used insteering systems of most ships. In some systems,one assembly is used to actuate the rudder in eitherdirection; while in other systems, two assembliesare used for the same purpose.

PISTON-TYPE CYLINDERS

An actuating cylinder in which the cross-sectional area of the piston is less than one-halfthe cross-sectional area of the movable elementis referred to as a piston-type cylinder. This typeof cylinder is normally used for applications thatrequire both push and pull functions. The piston-type cylinder is the most common type used influid power systems.

The essential parts of a piston-type cylinderare a cylindrical barrel, a piston and rod, end caps,and suitable seals. The end caps are attached tothe ends of the barrel. These end caps usuallycontain the fluid ports. The end cap on the rodend contains a hole for the piston rod to passthrough. Suitable seals are used between the holeand the piston rod to keep fluid from leaking outand to keep dirt and other contaminants fromentering the barrel. The opposite end cap of mostcylinders is provided with a fitting for securingthe actuating cylinder to some structure. This endcap is referred to as the anchor end cap.

The piston rod may extend through either orboth ends of the cylinder. The extended end ofthe rod is normally threaded so that some typeof mechanical connector, such as an eyebolt ora clevis, and a locknut can be attached. Thisthreaded connection of the rod and mechanicalconnector provides for adjustment between therod and the unit to be actuated. After the correct

10-3

adjustment is made, the locknut is tightenedagainst the connector to prevent the connectorfrom turning. The other end of the connector isattached, either directly or through additionalmechanical linkage, to the unit to be actuated.

In order to satisfy the many requirements offluid power systems, piston-type cylinders areavailable in various designs.

Single-Acting Cylinder

The single-acting piston-type cylinder is similarin design and operation to the single-actingram-type cylinder. The single-acting piston-typecylinder uses fluid pressure to provide the forcein one direction, and spring tension, gravity,compressed air, or nitrogen is used to provide theforce in the opposite direction. Figure 10-5 showsa single-acting, spring-loaded, piston-typeactuating cylinder. In this cylinder the spring islocated on the rod side of the piston. In somespring-loaded cylinders the spring is located onthe blank side, and the fluid port is on the rodside of the cylinder.

A three-way directional control valve isnormally used to control the operation of thesingle-acting piston-type cylinder. To extend thepiston rod, fluid under pressure is directedthrough the port into the cylinder (fig. 10-5). Thispressure acts on the surface area of the blank sideof the piston and forces the piston to the right.This action moves the rod to the right, throughthe end of the cylinder, thus moving the actuatedunit in one direction. During this action, thespring is compressed between the rod side of thepiston and the end of the cylinder. The length ofthe stroke depends upon the physical limits withinthe cylinder and the required movement of theactuated unit.

To retract the piston rod, the directionalcontrol valve is moved to the opposite workingposition, which releases the pressure in the

Figure 10-5.—Single-acting, spring-loaded, piston-typeactuating cylinder.

cylinder. The spring tension forces the piston tothe left, retracting the piston rod and moving theactuated unit in the opposite direction. The fluidis free to flow from the cylinder through the port,back through the control valve to the return linein hydraulic systems or to the atmosphere inpneumatic systems.

The end of the cylinder opposite the fluid portis vented to the atmosphere. This prevents airfrom being trapped in this area. Any trapped airwould compress during the extension stroke,creating excess pressure on the rod side of thepiston. This would cause sluggish movement ofthe piston and could eventually cause a completelock, preventing the fluid pressure from movingthe piston.

The spring-loaded cylinder is used in arrestinggear systems on some models of carrier aircraft.To raise (retract) the arresting hook, fluid pressureis directed through the arresting hook controlvalve to the rod side of the cylinder. This forcemoves the piston, which, through the rod andmechanical linkage, retracts the arresting hook.The arresting hook extends when fluid pressureis released from the rod side of the cylinder,allowing the spring to expand.

Leakage between the cylinder wall and pistonis prevented by adequate seals. The piston infigure 10-5 contains V-ring seals.

Double-Acting Cylinder

Most piston-type actuating cylinders aredouble-acting, which means that fluid underpressure can be applied to either side of the pistonto apply force and provide movement.

One design of the double-acting cylinder isshown in figure 10-6. This cylinder contains onepiston and piston rod assembly. The stroke of thepiston and piston rod assembly in either directionis produced by fluid pressure. The two fluid ports,one near each end of the cylinder, alternate as inletand outlet ports, depending on the direction of

Figure 10-6.-Doub1e-acting piston-type actuating cylinder.

10-4

flow from the directional control valve. Thisactuator (fig. 10-6) is referred to as an unbalancedactuating cylinder because there is a difference inthe effective working areas on the two sides ofthe piston. Therefore, this type of cylinder isnormally installed so that the blank side of thepiston carries the greater load; that is, the cylindercarries the greater load during the piston rodextension stroke.

A four-way directional control valve isnormally used to control the operation of this typeof cylinder. The valve can be positioned to directfluid under pressure to either end of the cylinderand allow the displaced fluid to flow from theopposite end of the cylinder through the controlvalve to the return line in hydraulic systems orto the atmosphere in pneumatic systems.

There are applications where it is necessary tomove two mechanisms at the same time. In thiscase, double-acting piston-type actuating cylindersof different designs are required. See figures 10-7and 10-8.

Figure 10-7 shows a three-port, double-actingpiston-type actuating cylinder. This actuatorcontains two pistons and piston rod assemblies.Fluid is directed through port A by a four-waydirectional control valve and moves the pistonsoutward, thus moving the mechanisms attachedto the pistons’ rods. The fluid on the rod side ofeach piston is forced out of the cylinder throughports B and C, which are connected by a commonline to the directional control valve. The displacedfluid then flows through the control valve to thereturn line or to the atmosphere.

When fluid under pressure is directed into thecylinder through ports B and C, the two pistonsmove inward, also moving the mechanismsattached to them. Fluid between the two pistonsis free to flow from the cylinder through port Aand through the control valve to the return lineor to the atmosphere.

The actuating cylinder shown in figure 10-8is a double-acting balanced type. The piston rodextends through the piston and out through bothends of the cylinder. One or both ends of the

Figure 10-7.—Three-port, double-acting actuating cylinder.

Figure 10-8.-Balanced, double-acting piston-type actuatingcylinder.

piston rod may be attached to a mechanism tobe operated. In either case, the cylinder providesequal areas on each side of the piston. Therefore,the same amount of fluid and force is used tomove the piston a certain distance in eitherdirection.

Tandem Cylinders

A tandem actuating cylinder consists of twoor more cylinders arranged one behind the otherbut designed as a single unit (fig. 10-9). This typeof actuating cylinder is used in applications thatrequire two or more independent systems; forexample, power-operated flight control systemsin naval aircraft.

The flow of fluid to and from the twochambers of the tandem actuating cylinder isprovided from two independent hydraulic systemsand is controlled by two sliding spool directionalcontrol valves. In some applications, the controlvalves and the actuating cylinder are two separateunits. In some units, the pistons (lands) of the twosliding spools are machined on one common shaft.In other applications, the valves and the actuatorare directly connected in one compact unit.Although the two control valves are hydraulicallyindependent, they are interconnected mechanically.In other units, the two sliding spools are connectedthrough mechanical linkages with a synchronizingrod. In either case, the movement of the twosliding spools is synchronized, thus equalizing the

Figure 10-9.—Tandem actuating cylinder.

10-5

flow of fluid to and from the two chambers ofthe actuating cylinder.

Since the two control valves operateindependently of each other as far as hydraulicpressure is concerned, failure of either hydraulicsystem does not render the actuator inoperative.Failure of one system does reduce the output forceby one-half; however, this force is sufficient topermit operation of the actuator.

RACK-AND-PINION PISTON-TYPEROTARY ACTUATORS

The rack-and-pinion-type actuators, alsoreferred to as limited rotation cylinders, of thesingle or multiple, bidirectional piston are usedfor turning, positioning, steering, opening andclosing, swinging, or any other mechanicalfunction involving restricted rotation. Figure10-10 shows a typical rack-and-pinion double-piston actuator.

The actuator consists of a body and tworeciprocating pistons with an integral rack forrotating the shaft mounted in roller or journalbearings. The shaft and bearings are located ina central position and are enclosed with a bearingcap. The pistons, one on each side of the rack,are enclosed in cylinders machined or sleeved intothe body. The body is enclosed with end caps andstatic seals to prevent external leakage ofpressurized fluid.

Only a few of the many applications ofactuating cylinders were discussed in the precedingparagraphs. Figure 10-11 shows additional typesof force and motion applications.

In addition to its versatility, the cylinder-typeactuator is probably the most trouble-freecomponent of fluid power systems. However, itis very important that the cylinder, mechanicallinkage, and actuating unit are correctly aligned.Any misalignment will cause excessive wear of thepiston, piston rod, and seals. Also, properadjustment between the piston rod and theactuating unit must be maintained.

Figure 10-10.—Rack-and-pinion double-piston rotary actuator.

10-6

Figure 10-11.—Applications of actuating cylinders.

10-7

MOTORS controlled by either a four-way directional controlvalve or a variable-displacement pump.

A fluid power motor is a device that convertsfluid power energy to rotary motion and force.The function of a motor is opposite that of apump. However, the design and operation offluid power motors are very similar to pumps.Therefore, a thorough knowledge of the pumpsdescribed in chapter 4 will help you understandthe operation of fluid power motors.

Motors have many uses in fluid powersystems. In hydraulic power drives, pumps andmotors are combined with suitable lines and valvesto form hydraulic transmissions. The pump,commonly referred to as the A-end, is driven bysome outside source, such as an electric motor.The pump delivers fluid to the motor. The motor,referred to as the B-end, is actuated by this flow,and through mechanical linkage conveys rotarymotion and force to the work. This type of powerdrive is used to operate (train and elevate) manyof the Navy’s guns and rocket launchers.Hydraulic motors are commonly used to operatethe wing flaps, radomes, and radar equipment inaircraft. Air motors are used to drive pneumatictools. Air motors are also used in missiles toconvert the kinetic energy of compressed gas intoelectrical power, or to drive the pump of ahydraulic system.

Fluid motors may be either fixed or variabledisplacement. Fixed-displacement motors provideconstant torque and variable speed. The speed isvaried by controlling the amount of input flow.Variable-displacement motors are constructed sothat the working relationship of the internal partscan be varied to change displacement. Themajority of the motors used in fluid powersystems are the fixed-displacement type.

Although most fluid power motors are capableof providing rotary motion in either direction,some applications require rotation in only onedirection. In these applications, one port of themotor is connnected to the system pressure line andthe other port to the return line or exhausted tothe atmosphere. The flow of fluid to the motoris controlled by a flow control valve, a two-waydirectional control valve, or by starting andstopping the power supply. The speed of themotor may be controlled by varying the rate offluid flow to it.

In most fluid power systems, the motor isrequired to provide actuation power in eitherdirection. In these applications the ports arereferred to as working ports, alternating as inletand outlet ports. The flow to the motor is usually

Fluid motors are usually classified accordingto the type of internal element, which is directlyactuated by the flow. The most common types ofelements are the gear, the vane, and the piston,AU three of these types are adaptable for hydraulicsystems, while only the vane type is used inpneumatic systems.

GEAR-TYPE MOTORS

The spur, helical, and herringbone designgears are used in gear-type motors. The motorsuse external-type gears, as discussed in chapter 4.

The operation of a gear-type motor is shownin figure 10-12. Both gears are driven gears;however, only one is connected to the outputshaft. As fluid under pressure enters chamber A,it takes the path of least resistance and flowsaround the inside surface of the housing, forcingthe gears to rotate as indicated. The flowcontinues through the outlet port to the return.This rotary motion of the gears is transmittedthrough the attached shaft to the work unit.

The motor shown in figure 10-12 is operatingin one direction; however, the gear-type motor iscapable of providing rotary motion in eitherdirection. To reverse the direction of rotation, theports may be alternated as inlet and outlet. Whenfluid is directed through the outlet port (fig. 10-12)into chamber B, the gears rotate in the oppositedirection.

Figure 10-12.—Gear-type motor.

10-8

VANE-TYPE MOTORS

A typical vane-type air motor is shown infigure 10-13. This particular motor providesrotation in only one direction. The rotatingelement is a slotted rotor which is mounted ona drive shaft. Each slot of the rotor is fitted witha freely sliding rectangular vane. The rotor andvanes are enclosed in the housing, the innersurface of which is offset from the drive shaft axis.When the rotor is in motion, the vanes tend toslide outward due to centrifugal force. Thedistance the vanes slide is limited by the shape ofthe rotor housing.

This motor operates on the principle ofdifferential areas. When compressed air is directedinto the inlet port, its pressure is exerted equallyin all directions. Since area A (fig. 10-13) is greaterthan area B, the rotor will turn counterclockwise.Each vane, in turn, assumes the No. 1 and No.2 positions and the rotor turns continuously. Thepotential energy of the compressed air is thusconverted into kinetic energy in the form of rotarymotion and force. The air at reduced pressure isexhausted to the atmosphere. The shaft of themotor is connected to the unit to be actuated.

Many vane-type motors are capable ofproviding rotation in either direction. A motorof this design is shown in figure 10-14. This motoroperates on the same principle as the vane motorshown in figure 10-13. The two ports may bealternately used as inlet and outlet, thus providingrotation in either direction. Note the springs inthe slots of the rotor. Their purpose is to hold thevanes against the housing during the initial

Figure 10-13.—Vane-type air motor.

Figure 10-14.—Vane-type motor.

starting of the motor, since centrifugal force doesnot exist until the rotor begins to rotate.

PISTON-TYPE MOTORS

Piston-type motors are the most commonlyused in hydraulic systems. They are basically thesame as hydraulic pumps except they are used toconvert hydraulic energy into mechanical (rotary)energy.

The most commonly used hydraulic motor isthe fixed-displacement piston type. Someequipment uses a variable-displacement pistonmotor where very wide speed ranges are desired.

Although some piston-type motors arecontrolled by directional control valves, theyare often used in combination with variable-displacement pumps. This pump-motor combina-tion is used to provide a transfer of power betweena driving element and a driven element. Someapplications for which hydraulic transmissionsmay be used are speed reducers, variable speeddrives, constant speed or constant torque drives,and torque converters. Some advantages ofhydraulic transmission of power over mechanicaltransmission of power are as follows:

1.

2.3.4.

Quick, easy speed adjustment over a widerange while the power source is operatingat a constant (most efficient) speed. Rapid,smooth acceleration or deceleration.Control over maximum torque and power.Cushioning effect to reduce shock loads.Smoother reversal of motion.

10-9

Radial-Piston Motor continues as long as fluid under pressure entersthe cylinders.

The radial-piston motor operates in reverse ofthe radial-piston pump. In the radial-piston pump,as the cylinder block rotates, the pistons pressagainst the rotor and are forced in and out of thecylinders, thereby receiving fluid and pushing itout into the system. In the radial motor, fluid isforced into the cylinders and drives the pistonsoutward. The pistons pushing against the rotorcause the cylinder block to rotate.

The operation of a radial-piston motor isshown in figure 10-15. This motor is shown withthree pistons for simplicity. Normally it containsseven or nine pistons. When liquid is forced intothe cylinder bore containing piston 1, the pistonmoves outward since the liquid cannot becompressed. This causes the cylinder to rotate ina clockwise direction. As the force acting onpiston 1 causes the cylinder block to rotate, piston2 starts to rotate and approach the position ofpiston 3. (Note that the distance between thecylinder block and the reaction ring of the rotorgets progressively shorter on the top and right halfof the rotor.)

As piston 2 rotates, it is forced inward and,in turn, forces the fluid out of the cylinder. Sincethere is little or no pressure on this side of thepintle valve, the piston is easily moved in by itscontact with the reaction ring of the rotor. Thefluid is easily forced out of the cylinder and backto the reservoir or to the inlet side of the pump.As the piston moves past the midpoint, or pastthe shortest distance between the cylinder blockand the rotor, it enters the pressure side of thepintle valve and fluid is forced into the cylinder.Piston 3 then becomes the pushing piston and inturn rotates the cylinder block. This action

Figure 10-15.—Operation of a radial-piston motor.

The direction of rotation of the motor (fig.10-15) is changed by reversing the flow of fluidto it. Admitting fluid under pressure on the topside of the pintle valve forces piston 3 out of thecylinder block. This causes the cylinder to rotatein the counterclockwise direction.

Axial-Piston Motor

The variable-stroke axial-piston pump is oftenused as a part of variable speed gear, such aselectrohydraulic anchor windlasses, cranes,winches, and the power transmitting unit inelectrohydraulic steering engines. In those cases,the tilting box is arranged so that it maybe tiltedin either direction. Thus it maybe used to transmitbidirectional power hydraulically to pistons orrams, or it may be used to drive a hydraulicmotor. In the latter use, the pump is the A-endof the variable speed gear and the hydraulic motoris the B-end.

The B-end of the hydraulic unit of thehydraulic speed gear is exactly the same as theA-end of the variable-stroke pump mentionedpreviously. However, it generally does not havea variable-stroke feature. The tilting box isinstalled at a permanently fixed angle. Thus, theB-end becomes a fixed-stroke axial-piston motor.Figure 10-16 illustrates an axial-piston hydraulicspeed gear with the A-end and B-end as a singleunit. It is used in turrets for train and elevationdriving units. For electrohydraulic winches andcranes, the A-end and B-end are in separatehousings connected by hydraulic piping.

Hydraulic fluid introduced under pressure toa cylinder (B-end) tries to push the piston out ofthe cylinder. In being pushed out, the piston,through its piston rod, will seek the point ofgreatest distance between the top of the cylinderand the socket ring. The resultant pressure of thepiston against the socket ring will cause thecylinder barrel and the socket ring to rotate. Thisaction occurs during the half revolution while thepiston is passing the intake port of the motor,which is connected to the pressure port of thepump. After the piston of the motor has takenall the hydraulic fluid it can from the pump, thepiston passes the valve plate land and starts todischarge oil through the outlet ports of the motor

10-10

Figure 10-16.—Exploded view of a axial-piston hydraulic speed gear.

to the suction pistons of the pump. The pump isconstantly putting pressure on one side of themotor and receiving hydraulic fluid from the otherside. The fluid is merely circulated from pumpto motor and back again.

Both of the axial-piston motors described inthis section may be operated in either direction.The direction of rotation is controlled by thedirection of fluid flow to the valve plate. Thedirection of flow may be instantly reversedwithout damage to the motor.

TURBINES

Turbines are used in pneumatic systems toconvert kinetic energy of gases to mechanicalenergy. Turbines are used to drive electricgenerators, to convert mechanical energy intoelectrical energy, and to drive pumps to supplyfluid flow in hydraulic systems.

The basic parts of a turbine are the rotor,which has blades projecting radially from itsperiphery; and nozzles, through which the gas isexpanded and directed. The conversion of kineticenergy to mechanical energy occurs on the blades.

The basic distinction between types of turbinesis the manner in which the gas causes the turbinerotor to move. When the rotor is moved by adirect push or “impulse” from the gas impingingupon the blades, the turbine is said to be animpulse turbine. When the rotor is moved by forceof reaction, the turbine is said to be a reactionturbine.

Although the distinction between impulseturbines and reaction turbines is a useful one,it should not be considered as an absolutedistinction in real turbines. An impulse turbineuses both the impulse of the gas jet and,to a lesser extent, the reactive force that resultswhen the curved blades cause the gas to changedirection. A reaction turbine is moved primarilyby reactive force, but some motion of the rotoris caused by the impact of the gas against theblades.

IMPULSE TURBINE

The impulse turbine consists essentially of arotor mounted on a shaft that is free to rotate ina set of bearings. The outer rim of the rotor carriesa set of curved blades, and the whole assemblyis enclosed in an airtight case. Nozzles direct the

10-11

rapidly moving fluid against the blades and turnthe rotor (fig. 10-17).

REACTION TURBINE

The reaction turbine, as the name implies, isturned by reactive force rather than by a directpush or impulse. In reaction turbines, there areno nozzles as such. Instead, the blades that projectradially from the periphery of the rotor areformed and mounted so that the spaces betweenthe blades have, in cross section, the shape ofnozzles. Since these blades are mounted on therevolving rotor, they are called moving blades.

Fixed or stationary blades of the same shapeas the moving blades (fig. 10-18) are fastened to

the stator (casing) in which the rotor revolves. Thefixed blades guide the gas into the moving bladesystem and, since they are also shaped andmounted to provide nozzle-shaped spaces betweenthe blades, the freed blades also act as nozzles.

A reaction turbine is moved by three mainforces: (1) the reactive force produced on themoving blades as the gas increases in velocity asit expands through the nozzle-shaped spacesbetween the blades; (2) the reactive force producedon the moving blades when the gas changesdirection; and (3) the push or impulse of the gasimpinging upon the blades. Thus, as previouslynoted, a reaction turbine is moved primarily byreactive force but also to some extent by directimpulse.

Impulse and reaction blades can be combinedto form an impulse-reaction turbine. This turbinecombines the rotational forces of the previouslydescribed turbines; that is, it derives its rotationfrom both the impulse of the gas striking theturbine blades and the reactive force of the gaschanging direction.

Figure 10-17 .—Impulse turbine. Figure 10-18.—Reaction turbine blading.

10-12

CHAPTER 11

PNEUMATICS

The word pneumatics is a derivative of theGreek word pneuma, which means air, wind, orbreath. It can be defined as that branch ofengineering science that pertains to gaseouspressure and flow. As used in this manual,pneumatics is the portion of fluid power in whichcompressed air, or other gas, is used to transmitand control power to actuating mechanisms.

This chapter discusses the origin of pneu-matics. It discusses the characteristics of gases andcompares them with those of liquids. It alsoexplains factors which affect the properties ofgases, identifies and explains the gas laws, andidentifies gases commonly used in pneumatics andtheir pressure ranges. It also discusses hazards ofpneumatic gases, methods of controlling contami-nation, and safety precautions associated withcompressed gases.

DEVELOPMENT OF PNEUMATICS

There is no record of man’s first uses of airto do work. Probably the earliest uses were toseparate chaff from grain and to move ships. Oneof the first pneumatic devices was the blow gunused by primitive man. In the latter part of theeighteenth century, heated air was used to carrythe first balloon aloft. The heated air, beinglighter than the surrounding air, caused theballoon to rise.

Every age of man has witnessed the develop-ment of devices which used air to do work.However, man used air to do work long beforehe understood it.

Many of the principles of hydraulics apply topneumatics. For example, Pascal’s law applies togases as well as liquids. Also, like hydraulics, thedevelopment of pneumatics depended on closelyfitted parts and the development of gaskets andpackings. Since the invention of the air com-pressor, pneumatics has become a very reliableway to transmit power.

Probably one of the most common uses ofpneumatic power is in the operation of pneumatictools. However, you should understand thatpneumatics is also of great importance in largeand complex systems such as the controls of vitalpropulsion and weapon systems.

CHARACTERISTICS OF GASES

Recall from chapter 1 that gas is one of thethree states of matter. It has characteristics similarto those of liquids in that it has no definite shapebut conforms to the shape of its container andreadily transmits pressure.

Gases differ from liquids in that they have nodefinite volume. That is, regardless of the size orshape of the containing vessel, a gas willcompletely fill it. Gases are highly compressible,while liquids are only slightly so. Also, gases arelighter than equal volumes of liquids, makinggases less dense than liquids.

DENSITY

Early experiments were conducted concerningthe behavior of air and similar gases. Theseexperiments were conducted by scientists such asBoyle and Charles (discussed later in this chapter).The results of their experiments indicated that thegases’ behavior follows the law known as theideal-gas law. It states as follows: For a givenweight of any gas, the product of the absolutepressure and the volume occupied, divided by theabsolute temperature, is constant. In equationform, it is expressed as follows:

Equation 11-1

For 1 pound of gas,

Equation 11-2

11-1

The specific volume (v) is expressed in cubic feetper pound.

For any weight of a gas this equation maybemodified as follows:

W = weight of the gas in pounds,

V = volume of W pounds of the gas in cubic feet.

The volume of 1 pound would then be V/W.If we substitute this for v in equation 11-3, it thenbecomes

Solving equation 11-4 for pressure,

In chapter 2 we defined density as the massper unit volume. In equation 11-5,

represents density. (Notice that this is the reverseof the specific volume.) We can now say thatpressure is equal to the density of the gas timesthe gas constant times the absolute temperatureof the gas. (The gas constant varies for differentgases.) From this equation we can show howdensity varies with changes in pressure andtemperature. Decreasing the volume, with theweight of the gas and the temperature heldconstant, causes the pressure to increase.

NOTE: During the compression of the gas,the temperature will actually increase; however,the explanation is beyond the scope of this text.

a decrease in volume with the weight held constantwill cause density to increase.

TEMPERATURE

As indicated previously, temperature is adominant factor affecting the physical propertiesof gases. It is of particular concern in calculatingchanges in the states of gases.

Three temperature scales are used extensivelyin gas calculations. They are the Celsius (C), theFahrenheit (F), and the Kelvin (K) scales. TheCelsius (or centigrade) scale is constructed byidentifying the freezing and boiling points ofwater, under standard conditions, as fixed pointsof 0° and 100°, respectively, with 100 equaldivisions between. The Fahrenheit scale identifies32° as the freezing point of water and 212° as theboiling point, and has 180 equal divisionsbetween. The Kelvin scale has its zero point equalto –273°C, or –460°F.

Absolute zero, one of the fundamentalconstants of physics, is commonly used in thestudy of gases. It is usually expressed in terms ofthe Celsius scale. If the heat energy of a gassample could be progressively reduced, sometemperature should be reached at which themotion of the molecules would cease entirely. Ifaccurately determined, this temperature couldthen be taken as a natural reference, or as a trueabsolute zero value.

Experiments with hydrogen indicated that ifa gas were cooled to –273.16°C (–273° for mostcalculations), all molecular motion would ceaseand no additional heat could be extracted. Sincethis is the coldest temperature to which an idealgas can be cooled, it is considered to be absolutezero. Absolute zero may be expressed as 0°K,–273°C, or –459.69°F (–460°F for mostcalculations).

When you work with temperatures, always besure which system of measurement is being usedand how to convert from one to another. Theconversion formulas are shown in figure 11-1. Forpurposes of calculations, the Rankine (R) scaleillustrated in figure 11-1 is commonly used to

11-2

Figure 11-1.-Comparison of Kelvin, Celsius, Fahrenheit, and Rankine temperature.

convert Fahrenheit to absolute. For Fahrenheitreadings above zero, 460° is added. Thus, 72°Fequals 460° plus 72°, or 532° absolute (532°R).If the Fahrenheit reading is below zero, it issubtracted from 460°. Thus, -40°F equals 460°minus 40°, or 420° absolute (420°R).

The Kelvin and Celsius scales are usedinternationally in scientific measurements; there-fore, some technical manuals may use these scalesin directions and operating instructions. TheFahrenheit scale is commonly used in the UnitedStates; therefore, it is used in most areas of thismanual.

PRESSURE

We defined pressure in chapter 2 as force perunit area. Remember, liquids exert pressure onall surfaces with which they come in contact.Gases, because of their ability to completely fillcontainers, exert pressure on all sides of acontainer.

In practice, we maybe interested in either oftwo pressure readings. We may desire either thegauge pressure or the absolute pressure.

Absolute pressure is measured from absolutezero pressure rather than from normal oratmospheric pressure (approximately 14.7 psi).Gauge pressure is used on all ordinary gauges, andindicates pressure in excess of atmosphericpressure. Therefore, absolute pressure is equal toatmospheric pressure plus gauge pressure. Forexample, 100 psi gauge pressure (psig) equals 100psi plus 14.7 psi or 114.7 psi absolute pressure(psia). Whenever gas laws are applied, absolutepressures

Gases

are required.

COMPRESSIBILITY ANDEXPANSION OF GASES

can be readily compressed and areassumed to be perfectly elastic. This combinationof properties gives a gas the ability to yield to a

11-3

force and return promptly to its original conditionwhen the force is removed. These are theproperties of air that is used in pneumatic tires,tennis balls and other deformable objects whoseshapes are maintained by compressed air.

KINETIC THEORY OF GASES

In an attempt to explain the compressibilityof gases, Bernoulli proposed the hypothesis thatis accepted as the kinetic theory of gases.According to this theory, the pressure exerted bya gas on the walls of a closed container is causedby continual bombardment of the walls bymolecules of the gas.

Consider the container shown in figure 11-2as containing a gas. At any given time, somemolecules are moving in one direction, some aretraveling in other directions; some are travelingfast, some slow, and some may even be in a stateof rest. The average effect of the moleculesbombarding each container wall corresponds tothe pressure of the gas.

As more gas is pumped into the container,more molecules are available to bombard thewalls; thus the pressure in the container increases.

The gas pressure in a container can also beincreased by increasing the speed with which themolecules hit the walls. If the temperature of thegas is raised, the molecules move faster causingan increase in pressure. This can be shown byconsidering the automobile tire. When you takea long drive on a hot day, the pressure in the tiresincreases and a tire which appeared to besomewhat “soft” in cool morning temperaturemay appear normal at a higher midday tempera-ture.

BOYLE’S LAW

When the automobile tire is initially inflated,air which normally occupies a specific volume iscompressed into a smaller volume inside the tire.This increases the pressure on the inside of the tire.

Charles Boyle, an English scientist, was amongthe first to experiment with the pressure-volumerelationship of gas. During an experiment whenhe compressed a volume of air he found that thevolume decreased as the pressure increased, andby doubling the force exerted on the air he coulddecrease the volume of the air by half. See figure11-3. Recall from the example of the automobiletire that changes in temperature of a gas alsochange the pressure and volume. Therefore, theexperiment must be performed at a constanttemperature. The relationship between pressureand volume is known as Boyle’s law. It states:When the temperature of a gas is kept constant,the volume of an enclosed gas varies inversely withits pressure.

In equation form, this relationship may beexpressed as either

or Equation 11-6

where V1 and P1 are the original volume andpressure, and V2 and P2 are the final volumeand pressure (P1 and P2 are absolute pressures).

Figure 11-3.-Gas compressed to half its original volume byFigure 11-2.—Molecular bombardment creating pressure.

11-4

a doubled force.

Example of Boyle’s law: 4 cubic feet ofnitrogen are under a pressure of 100 psi (gauge).The nitrogen is allowed to expand to a volumeof 6 cubic feet. What is the new gauge pressure?Remember to convert gauge pressure to absolutepressure by adding 14.7.

Using equation 11-6, V1P1 = V2P2, where V1 is4 ft3, V2 is 6 ft, and P1 is 100 psig:

CHARLES’S LAW

Boyle’s law assumes conditions of constanttemperature. In actual situations this is rarely thecase. Temperature changes continually and affectsthe volume of a given mass of gas.

Jacques Charles, a French physicist, providedmuch of the foundation for the modern kinetictheory of gases. Through experiments, he foundthat all gases expand and contract proportionallyto the change in the absolute temperature,providing the pressure remains constant. Therelationship between volume and temperature isknown as Charles’s law. It states: The volume ofa gas is proportional to its absolute temperature,if constant pressure is maintained. In equationform, this relationship may be expressed as

Equation 11-7

where V1 and V2 are the original and finalvolumes, and T1 and T2 are the original and finalabsolute temperatures.

Since an increase in the temperature of a gascauses it to expand if the pressure is kept constant,it is reasonable to expect that if a given sampleis heated within a closed container and its volumeremains constant, the pressure of the gas willincrease. Experiments have proven this to be true.In equation form, this becomes

P 1T 2 = P2T 1Equation 11-8

or

This equation states that for a constant volume,the absolute pressure of a gas varies directly withthe absolute temperature.

Example: A cylinder of gas under a pressureof 1800 psig at 70°F is left out in the sun in thetropics and heats up to a temperature of 130°F.What is the new pressure within the cylinder?(Remember that both pressure and temperaturemust be converted to absolute pressure andabsolute temperature.)

Converting absolute pressure to gauge pressure:

11-5

GENERAL GAS LAW

We have learned that Boyle’s law pertains tosituations in which the temperature remainsconstant (fig. 11-4), and that Charles’s lawpertains to situations in which pressure remainsconstant (fig. 11-4). It is usually not possible tocontrol pressure or temperature in tanks or bottlesof gas subject to the weather and shipboarddemands. Boyle’s and Charles’s laws are com-bined to form the general gas law. This law states:The product of the initial pressure, initial volume,and new temperature (absolute scale) of anenclosed gas is equal to the product of the newpressure, new volume, and initial temperature. Itis a mathematical statement which allows manygas problems to be solved by using the principlesof Boyle’s law and/or Charles’s law. The equationis expressed as

or

(P and T represent absolute pressure and absolutetemperature, respectively.)

You can see by examining figure 11-4 that thethree equations are special cases of the generalequation. Thus, if the temperature remainsconstant, T1 equals T2 and both can be eliminatedfrom the general formula, which then reduces tothe form shown in part A. When the volumeremains constant, V1 equals V2, thereby reducing

Figure 11-4.—The general gas law.

the general equation to the form given in part B.Similarly, P1 is equated to P2 for constantpressure, and the equation then takes the formgiven in part C.

The general gas law applies with exactness onlyto “ideal” gases in which the molecules areassumed to be perfectly elastic. However, itdescribes the behavior of actual gases withsufficient accuracy for most practical purposes.

Two examples of the general equation follow:

1. Two cubic feet of a gas at 75 psig and 80°Fare compressed to a volume of 1 cubic foot andthen heated to a temperature of 300°F. What isthe new gauge pressure?

Using equation 11-9, P1V 1T2 = P2V 2T1, whereV 1 is 2 ft3, P1 is 75 psig, T1 is 80°F, V2 is 1 ft3

and T2 is 300°F:

Solution:

Substituting:

Converting absolute pressure to gauge pressure:

2. Four cubic feet of a gas at 75 psig and 80°Fare compressed to 237.8 psig and heated to atemperature of 300°F. What is the volume of thegas resulting from these changes? Using equation11-9, P1V 1T2 = P2V 2T1, where V1 is 4 ft3, P2 i s

11-6

75 psig, T1 is 800, P1 is 237.8 psig, and T2 i s300°F:

Solution:

Substituting:

PNEUMATIC GASES

In chapter 1, you learned that many factorsare considered in determining whether to usehydraulics or pneumatics as a power source in afluid power system. Once it is determined thatpneumatics will be used as the source of power,some of the same factors are considered inselecting the pneumatic gas.

QUALITIES

The ideal fluid medium for a pneumaticsystem is a readily available gas that isnonpoisonous (nontoxic), chemically stable, freefrom any acids that cause corrosion of systemcomponents, and nonflammable. It also will notsupport combustion of other elements.

Gases that have these desired qualities may nothave the required lubricating power. Therefore,lubrication of the components of some pneumaticsystems must be arranged by other means. Forexample, some air compressors are provided witha lubricating system, some components arelubricated upon installation or, in some cases,lubrication is introduced into the air supply line.

Two gases meeting these qualities and mostcommonly used in pneumatic systems are com-pressed air and nitrogen.

COMPRESSED AIR

Compressed air is a mixture of all gasescontained in the atmosphere. In this manual,

compressed air is referred to as a gas when it isused as a fluid medium.

The unlimited supply of air and the ease ofcompression make compressed air the most widelyused fluid for pneumatic systems. Althoughmoisture and solid particles must be removedfrom the air, it does not require the extensivedistillation or separation process required in theproduction of other gases.

Compressed air has most of the desiredproperties and characteristics of a gas forpneumatic systems. It is nonpoisonous andnonflammable but does contain oxygen, whichsupports combustion. One of the most undesirablequalities of compressed air as a fluid medium forpneumatic systems is moisture content. Theatmosphere contains varying amounts of moisturein vapor form. Changes in the temperature ofcompressed air will cause condensation ofmoisture in the pneumatic system. This condensedmoisture can be very harmful to the system, asit increases corrosion, dilutes lubricants, and mayfreeze in lines and components during coldweather. Moisture separators and air driers(dehydrators) are installed in the compressed airlines to minimize or eliminate moisture insystems where moisture would deteriorate systemperformance.

The supply of compressed air at the requiredvolume and pressure is provided by an aircompressor. (For information on air compressors,refer to Naval Ships’ Technical Manual, chapter551.) In most systems the compressor is part ofthe system with distribution lines leading from thecompressor to the devices to be operated. In thesesystems a receiver is installed in-line between thecompressor and the device to be operated to helpeliminate pulsations in the compressor dischargeline, to act as a storage tank during intervals whenthe demand for air exceeds the compressor’scapacity, and to enable the compressor to shutdown during periods of light load. Other systemsreceive their supply from cylinders which must befilled at a centrally located air compressor andthen connected to the system.

Compressed air systems are categorized bytheir operating pressures as follows: high-pressure(HP) air, medium-pressure (MP) air, and low-pressure (LP) air.

High-Pressure Air Systems

HP air systems provide compressed air at anominal operating pressure of 3000 psi or 5000psi and are installed whenever pressure in excess

11-7

of 1000 psi is required. HP compressed air plantssupport functions which require high pressuresand high flow rates of compressed air by theaddition of HP storage flasks to the system. Anexample of such a system is one that provides airfor starting diesel and gas turbine engines.Reduction in pressure, if required, is doneby using specially designed pressure-reducingstations.

Medium-Pressure Air

MP air systems provide compressed air at anominal operating pressure of 151 psi to 1000 psi.These pressures are provided either by an MP aircompressor or by the HP air system supplying airthrough an air bank and pressure-reducingstations.

Low-Pressure Air

LP air systems provide compressed air at anominal operating pressure of 150 psi and below.The LP air system is supplied with LP air by LPair compressors or by the HP air system supplyingair through an air bank and pressure-reducingstations. LP air is the most extensive and variedair system used in the Navy,

In addition to being used for variouspneumatic applications, LP and HP compressedair are used in the production of nitrogen.

NITROGEN

For all practical purposes, nitrogen isconsidered to be an inert gas. It is nonflammable,does not form explosive mixtures with air oroxygen, and does not cause rust or decay. Dueto these qualities, its use is preferred overcompressed air in many pneumatic systems,especially aircraft and missile systems, andwherever an inert gas blanket is required.

Nitrogen is obtained by the fractionaldistillation of air. Oxygen/nitrogen-producingplants expand compressed air until its temperaturedecreases to –196°C (–320°F), the boiling pointof nitrogen at atmospheric pressure. The liquidnitrogen is then directed to a storage tank. Aliquid nitrogen pump pumps the low-pressureliquid nitrogen from the storage tank anddischarges it as a high-pressure (5000 psi) liquidto the vaporizer where it is converted to a gas at5000 psi. Oxygen/nitrogen-producing plants arelocated at many naval installations and onsubmarine tenders and aircraft carriers.

CONTAMINATION CONTROL

As in hydraulic systems, fluid contaminationis also a leading cause of malfunctions inpneumatic systems. In addition to the solidparticles of foreign matter which find a way toenter the system, there is also the problem ofmoisture. Most systems are equipped with one ormore devices to remove this contamination. Theseinclude filters, water separators, air dehydrators,and chemical driers, which are discussed inchapter 9 of this manual. In addition, mostsystems contain drain valves at critical low pointsin the system. These valves are opened periodicallyto allow the escaping gas to purge a largepercentage of the contaminants, both solids andmoisture, from the system. In some systems thesevalves are opened and closed automatically, whilein others they must be operated manually.

Complete purging is done by removing linesfrom various components throughout the systemand then attempting to pressurize the system,causing a high rate of airflow through the system.The airflow will cause the foreign matter to bedislodged and blown from the system.

NOTE: If an excessive amount of foreignmatter, particularly oil, is blown from any onesystem, the lines and components should beremoved and cleaned or replaced.

In addition to monitoring the devices installedto remove contamination, it is your responsibilityas a maintenance person or supervisor to controlthe contamination. You can do this by using thefollowing maintenance practices:

1. Keep all tools and the work area in a clean,dirt-free condition.

2. Cap or plug all lines and fittingsimmediately after disconnecting them.

3. Replace all packing and gaskets duringassembly procedures.

4. Connect all parts with care to avoidstripping metal slivers from threaded areas. Installand torque all fittings and lines according toapplicable technical instructions.

5. Complete preventive maintenance asspecified by MRCs.

Also, you must take care to ensure that theproper cylinders are connected to systems beingsupplied from cylinders.

Cylinders for compressed air are paintedblack. Cylinders containing oil-pumped air have

11-8

two green stripes painted around the top of thecylinder, while cylinders containing water-pumpedair have one green stripe. Oil-pumped air indicatesthat the air or nitrogen is compressed by anoil-lubricated compressor. Air or nitrogen com-pressed by a water-lubricated (or nonlubricated)compressor is referred to as water pumped.Oil-pumped nitrogen can be very dangerous incertain situations. For example, nitrogen iscommonly used to purge oxygen systems. Oxygenwill not burn, but it supports and acceleratescombustion and will cause oil to burn easily andwith great intensity. Therefore, oil-pumpednitrogen must never be used to purge oxygensystems. When the small amount of oil remainingin the nitrogen comes in contact with the oxygen,an explosion may result. In all situations, useonly the gas specified by the manufacturer orrecommended by the Navy. Nitrogen cylinders arepainted gray. One black stripe identifies cylindersfor oil-pumped nitrogen, and two black stripesidentify cylinders for water-pumped nitrogen. Inaddition to these color codes, the exact identi-fication of the contents is printed in two locationsdiametrically opposite one another along thelongitudinal axis of the cylinder. For compressedair and nitrogen cylinders, the lettering is white.

POTENTIAL HAZARDS

All compressed gases are hazardous. Com-pressed air and nitrogen are neither poisonous norflammable, but should not be handled carelessly.Some pneumatic systems operate at pressuresexceeding 3000 psi. Lines and fittings haveexploded, injuring personnel and property.Literally thousands of careless workers haveblown dust or harmful particles into their eyes bythe careless handling of compressed air outlets.

Nitrogen gas will not support life, and whenit is released in a confined space, it will causeasphyxia (the loss of consciousness as a result oftoo little oxygen and too much carbon dioxide in

the blood). Although compressed air and nitrogenseem so safe in comparison with other gases, donot let overconfidence lead to personal injury.

SAFETY PRECAUTIONS

To minimize personal injury and equipmentdamage when using compressed gases, observe allpractical operating safety precautions, includingthe following:

1. Do not use compressed air to clean partsof your body or clothing, or to perform generalspace cleanup in lieu of vacuuming or sweeping.

2. Never attempt to stop or repair a leak whilethe leaking portion is still under pressure. Alwaysisolate, repressurize and danger tag out theportion of the system to be repaired. For pressuresof 1000 psi or greater, double valve protection isrequired to prevent injury if one of the valvesshould fail.

3. Avoid the application of heat to the airpiping system or components, and avoid strikinga sharp or heavy blow on any pressurized part ofthe piping system.

4. Avoid rapid operation of manual valves.The heat of compression caused by a sudden high--pressure flow into an empty line or vessel cancause an explosion if oil is present. Valves shouldbe slowly cracked open until airflow is noted andshould be kept in this position until pressures onboth sides of the valve have equalized. The rateof pressure rise should be kept under 200 psiper second, if possible. Valves may then be openedfully.

5. Do not discharge large quantities ofnitrogen into closed compartments unlessadequate ventilation is provided.

6. Do not subject compressed gas cylindersto temperatures greater than 130°F.

Remember, any pressurized system can behazardous to your health if it is not maintainedand operated carefully and safely.

11-9

CHAPTER 12

BASIC DIAGRAMS AND SYSTEMS

In the preceding chapters, you learned abouthydraulic and pneumatic fluids and componentsof fluid power systems. While having a knowledgeof system components is essential, it is difficultto understand the interrelationship of thesecomponents by simply watching the systemoperate. The knowledge of system interrelationis required to effectively troubleshoot andmaintain a fluid power system. Diagrams pro-vided in applicable technical publications ordrawings are a valuable aid in understanding theoperation of the system and in diagnosing thecauses of malfunctions.

This chapter explains the different types ofdiagrams used to illustrate fluid power circuits,including some of the symbols that depict fluidpower components. Included in this chapterare descriptions and illustrations denoting thedifferences between open-center and closed-centerfluid power systems. The last part of the chapterdescribes and illustrates some applications of basicfluid power systems.

DIAGRAMS

As mentioned earlier in this chapter, totroubleshoot fluid power systems intelligently, amechanic or technician must be familiar with thesystem on which he or she is working. Themechanic must know the function of eachcomponent in the system and have a mentalpicture of its location in relation to othercomponents. This can best be done by studyingthe diagrams of the system.

A diagram may be defined as a graphicrepresentation of an assembly or system thatindicates the various parts and expresses themethods or principles of operations. The abilityto read diagrams is a basic requirement forunderstanding the operation of fluid powersystems. Understanding the diagrams of a systemrequires having a knowledge of the symbols usedin the schematic diagrams.

SYMBOLS

The Navy uses two military standards thatlist mechanical symbols that must be used inpreparing drawings that will contain symbolicrepresentation. These standards are as follows:

1. Military Standard, Mechanical Symbols(Other than Aeronautical, Aerospacecraft, andSpacecraft Use), Part 1, MIL-STD-17B-1.

2. Military Standard, Mechanical Symbols forAeronautical, Aerospacecraft, and SpacecraftUse, Part 2, MIL-STD-17B-2.

Some of the symbols frequently used in fluidpower systems have been selected from thesetwo standards and are shown in Appendixes IIand III. Appendix II contains symbols fromMIL-STD-17B-1. Appendix III contains symbolsfrom MIL-STD-17B-2.

While the symbols shown in the appendixesare not all encompassing, they do provide a basisfor an individual working with fluid powersystems to build upon. Some rules applicable tographical symbols for fluid diagrams are asfollows:

1. Symbols show connections, flow paths,and the function of the component representedonly. They do not indicate conditions occurringduring transition from one flow path to another;nor do they indicate component construction orvalues, such as pressure or flow rate.

2. Symbols do not indicate the location ofports, direction of shifting of spools, or positionof control elements on actual components.

3. Symbols may be rotated or reversedwithout altering their meaning except in cases oflines to reservoirs and vented manifolds.

4. Symbols may be drawn in any size.5. Each symbol is drawn to show the normal

or neutral condition of each component unlessmultiple circuit diagrams are furnished showingvarious phases of circuit operation.

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For more detailed information concerning thesymbols used in fluid power diagrams, consult theabove-mentioned military standards. Additionalinformation concerning symbols and the readingof diagrams is contained in BIueprint Reading andSketching, NAVEDTRA 10077-F1.

TYPES OF DIAGRAMS

There are many types of diagrams. Those thatare most pertinent to fluid power systems arediscussed in this text.

Pictorial Diagrams

Pictorial diagrams (fig. 12-1) show thegeneral location and actual appearance of each

component, all interconnecting piping, and thegeneral piping arrangement. This type of diagramis sometimes referred to as an installationdiagram. Diagrams of this type are invaluable tomaintenance personnel in identifying and locatingcomponents of a system.

Cutaway Diagrams

Cutaway diagrams (fig. 12-2) show the internalworking parts of all fluid power components ina system. This includes controls and actuatingmechanisms and all interconnecting piping.Cutaway diagrams do not normally use symbols.

Figure 12-1.—Hydraulic system pictorial diagram.

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Figure 12-2.—Cutaway diagram—pneumatic.

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Graphic Diagrams

The primary purpose of a graphic (schematic)diagram is to enable the maintenance person totrace the flow of fluid from component tocomponent within the system. This type ofdiagram uses standard symbols to show eachcomponent and includes all interconnecting

piping. Additionally, the diagram contains acomponent list, pipe size, data on the sequenceof operation, and other pertinent information.The graphic diagram (fig. 12-3) does not indi-cate the physical location of the various com-ponents, but it does show the relation of eachcomponent to the other components within thesystem.

,

Figure 12-3.—Graphic diagram of LST 1182 class hydraulic steering gear.

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Notice that figure 12-3 does not indicate thephysical location of the individual componentswith respect to each other in the system. Forexample, the 3/4-inch, solenoid-operated, 4-wayvalve (10) is not necessarily located directly abovethe relief valve (26). The diagram does indicate,however, that the 4-way valve is located in theworking line, between the variable-displacementpump and the 1-inch rotary selector valve, andthat the valve directs fluid to and from the rotaryactuator.

Combination Diagrams

A combination drawing uses a combinationof graphic, cutaway, and pictorial symbols. Thisdrawing also includes all interconnecting piping.

FLUID POWER SYSTEMS

A fluid power system in which the fluid in thesystem remains pressurized from the pump (orregulator) to the directional control valve whilethe pump is operating is referred to as a closed-center system. In this type of system, any numberof subsystems may be incorporated, with aseparate directional control valve for eachsubsystem. The directional control valves arearranged in parallel so that system pressure actsequally on all control valves.

Another type of system that is sometimes usedin hydraulically operated equipment is the open-center system. An open-center system has fluidflow but no internal pressure when the actuatingmechanisms are idle. The pump circulates the fluidfrom the reservoir, through the directional controlvalves, and back to the reservoir. (See fig. 12-4,view A.) Like the closed-center system, the open-center system may have any number of subsystems,with a directional control valve for each subsystem.Unlike the closed-center system, the directionalcontrol valves of an open-center system are alwaysconnected in series with each other, an arrange-ment in which the system pressure line goesthrough each directional control valve. Fluid isalways allowed free passage through each controlvalve and back to the reservoir until one of the con-trol valves is positioned to operate a mechanism.

When one of the directional control valves ispositioned to operate an actuating device, asshown in view B of figure 12-4, fluid is directedfrom the pump through one of the working linesto the actuator. With the control valve in thisposition, the flow of fluid through the valve tothe reservoir is blocked. Thus, the pressure buildsup in the system and moves the piston of the

Figure 12-4.—Open-center hydraulic system.

actuating cylinder. The fluid from the other endof the actuator returns to the control valvethrough the opposite working line and flows backto the reservoir.

Several different types of directional controlvalves are used in the open-center system. Onetype is the manually engaged and manuallydisengaged. After this type of valve is manuallymoved to the operating position and the actuatingmechanism reaches the end of its operating cycle,pump output continues until the system reliefvalve setting is reached. The relief valve thenunseats and allows the fluid to flow back to thereservoir. The system pressure remains at thepressure setting of the relief valve until thedirectional control valve is manually returned to

the neutral position. This action reopens theopen-center flow and allows the system pressureto drop to line resistance pressure.

Another type of open-center directionalcontrol valve is manually engaged and pressuredisengaged. This type of valve is similar to thevalve discussed in the preceding paragraph;however, when the actuating mechanism reachesthe end of its cycle and the pressure continues to

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rise to a predetermined pressure, the valveautomatically returns to the neutral position and,consequently, to open-center flow.

One of the advantages of the open-centersystem is that the continuous pressurization of thesystem is eliminated. Since the pressure isgradually built up after the directional controlvalve is moved to an operating position, there isvery little shock from pressure surges. Thisprovides a smooth operation of the actuatingmechanisms; however, the operation is slowerthan the closed-center system in which the pressureis available the moment the directional controlvalve is positioned. Since most applicationsrequire instantaneous operation, closed-centersystems are the most widely used.

HYDRAULIC POWER DRIVE SYSTEM

The hydraulic power drive has been usedin the Navy for many years. Proof of itseffectiveness is that it has been used to train andelevate nearly all caliber guns, from the 40-mmgun mount to the 16-inch turret. In addition togun mounts and turrets, hydraulic power drivesare used to position rocket launchers andmissile launchers, and to drive and control suchequipment as windlasses, capstans, and winches.

In its simplest form, the hydraulic power driveconsists of the following:

1. The prime mover, which is the outsidesource of power used to drive the hydraulic pump

2. A variable-displacement hydraulic pump3. A hydraulic motor4. A means of introducing a signal to the

hydraulic pump to control its output5. Mechanical shafting and gearing that

transmits the output of the hydraulic motor to theequipment being operated

Hydraulic power drives differ in somerespects, such as size, method of control, and soforth. However, the fundamental operatingprinciples are similar. The unit used in thefollowing discussion of fundamental operatingprinciples is representative of the hydraulic powerdrives used to operate the 5"/38 twin mounts.

Figure 12-5 shows the basic components ofthe train power drive. The electric motor isconstructed with drive shafts at both ends. Theforward shaft drives the A-end pump throughreduction gears, and the after shaft drives theauxiliary pumps through the auxiliary reductiongears. The reduction gears are installed because

Figure 12-5.-Train power drive—components.

the pumps are designed to operate at a speed muchslower than that of the motor.

The replenishing pump is a spur gear pump.Its purpose is to replenish fluid to the activesystem of the power drive. It receives its supplyof fluid from the reservoir and discharges it tothe B-end valve plate. This discharge of fluid fromthe pump is held at a constant pressure by theaction of a pressure relief valve. (Because thecapacity of the pump exceeds replenishingdemands, the relief valve is continuously allowingsome of the fluid to flow back to the reservoir.)

The sump pump and oscillator has a twofoldpurpose. It pumps leakage, which collects in thesump of the indicator regulator, to the expansiontank. Additionally, it transmits a pulsating effectto the fluid in the response pressure system.Oscillations in the hydraulic response system helpeliminate static friction of valves, allowinghydraulic control to respond faster.

The control pressure pump supplies high-pressure fluid for the hydraulic control system,brake pistons, lock piston, and the hand-controlled clutch operating piston. The controlpressure pump is a fixed-displacement, axial-piston type. An adjustable relief valve is used tolimit the operating pressure at the outlet of thepump.

Control

For the purpose of this text, control constitutesthe relationship between the stroke control shaftand the tilting box. The stroke control shaft is oneof the piston rods of a double-acting piston-typeactuating cylinder. This actuating cylinder and itsdirect means of control are referred to as the maincylinder assembly (fig. 12-6). It is the link betweenthe hydraulic followup system and the power driveitself.

In hand control, the tilting box is mechanicallypositioned by gearing from the handwheelthrough the A-end control unit. In local andautomatic control, the tilting box is positioned bythe stroke control shaft. As shown in figure 12-6,the extended end of the control shaft is connectedto the tilting box. Movement of the shaft will pivotthe tilting box one way or the other; which, inturn, controls the output of the A-end of thetransmission. The other end of the shaft isattached to the main piston. A shorter shaft isattached to the opposite side of the piston. Thisshaft is also smaller in diameter. Thus the workingarea of the left side of the piston is twice that of thearea of the right side, as it appears in figure 12-6.

Figure 12–6.–Main cylinder assembly.

Intermediate high-pressure fluid (IHP) istransmitted to the left side of the piston, whilehigh-pressure hydraulic fluid (HPC) is transmittedto the right side. The HPC is held constant at 1000psi. Since the area of the piston upon which HPCacts is exactly one-half the area upon which IHPacts, the main piston is maintained in a fixedposition when IHP is one-half HPC (500 psi).Whenever IHP varies from its normal value of500 psi, the main piston will move, thus movingthe tilting box.

Operation

Assume that a right train order signal isreceived. This will cause the pilot valve to bepulled upward. The fluid in the upper chamberof the amplifier piston can now flow through thelower land chamber of the fine pilot to exhaust.This will cause the amplifier piston to moveupward, and the fluid in the right-hand chamberof the main control valve can flow into the lowerchamber of the amplifier valve.

The main control valve will now move to theright, IHP will drop below 500 psi, and the strokepiston will move to the left. Movement of the

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stroke piston will cause tilt to be put on the tiltplate, and the A-end will cause the mount to trainright.

Figure 12-7 is a simplified block diagramshowing the main element of the hydraulic powerdrive system under automatic control forclockwise and counterclockwise rotation.

There are two principal problems in posi-tioning a gun to fire. One is to get an accurategun-order signal. This problem is solved by thedirector-computer combination. The otherproblem is to transmit the director signal promptlyto the gun so that the position and movementsof the gun will be synchronized with the signalsfrom the director.

The problem of transforming gun-ordersignals to mount movements is solved by thepower drive and its control—the indicatorregulator. The indicator regulator controls thepower drive, and this, in turn, controls themovement of the gun.

The indicator regulator receives an initialelectrical gun-order from the director-computer,compares it to the existing mount position, andsends an error signal to the hydraulic controlmechanism in the regulator. The hydraulic controlmechanism controls the flow to the stroke controlshaft, which positions the tilting box in the A-endof the transmission. Its tilt controls the volumeand direction of fluid pumped to the B-end and,therefore, the speed and direction of the driveshaft of the B-end. Through mechanical linkage,the B-end output shaft moves the gun in the

direction determined by the signal. At the sametime, B-end response is transmitted to theindicator regulator and continuously combineswith incoming gun-order signals to give theerror between the two. This error is modifiedhydraulically, according to the system ofmechanical linkages and valves in the regulator.When the gun is lagging behind the signal, itsmovement is accelerated; and when it begins tocatch up, its movement is slowed down so thatit will not overrun excessively.

LANDING GEAR EMERGENCYSYSTEM

If the landing gear in a naval aircraft fails toextend to the down and locked position, theaircraft has an emergency method to extend thelanding gear. This text will cover the nitrogensystem.

The nitrogen storage bottle system is aone-shot system powered by nitrogen pressurestored in four compressed nitrogen bottles(fig. 12-8). When the landing gear control handleis used to actuate the emergency landing gearsystem, a cable between the control and themanually operated nitrogen bottle opens theemergency gear down release valve on the bottle.Nitrogen from this bottle actuates the releasevalves on the other three bottles so that theydischarge. Nitrogen flows from the manuallyoperated bottle, actuates the dump valves, andcauses the shuttles within the shuttle valves on the

Figure 12-7.—Operation of the hydraulic power drive.

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aft doors’ cylinders and the shuttle valve on thenose gear cylinder to close off the normal portand operate these cylinders. The nose gear cylinderextends; this unlocks the uplock and extends thenose gear. The nitrogen flowing into the aft doorcylinders opens the aft doors. Fluid on the closeside of the door cylinder is vented to returnthrough the actuated dump valves. Nitrogen fromanother bottle actuates the shuttle valves on theuplock cylinders. Nitrogen flows into the uplockcylinders and causes them to disengage theuplocks. As soon as the uplocks are disengaged,the main gear extends by the force of gravity.Fluid on the up side of the main gear cylindersis vented to return through the actuated dumpvalves, preventing a fluid lock.

JET BLAST DEFLECTORS

Jet blast deflectors (JBD) onboard aircraftcarriers are raised and lowered by hydrauliccylinders through mechanical linkage. Two

hydraulic cylinders are attached to each JBD panelshaft by crank assemblies. (See fig. 12-9.) Theshaft is rotated by the push and pull operationof the hydraulic cylinders. Shaft rotation extendsor retracts the linkage to raise or lower the JBDpanels. This operation is designed so that in theevent of a failure of one of the hydraulic cylinders,the other one will raise or lower the panels.

Figure 12-10 is a diagram of the hydrauliccontrol system of a JBD during the raise cycle.Hydraulic fluid from the catapult hydraulic supplysystem is supplied to the JBD hydraulic systemthrough an isolation valve and a filter to the 4-waycontrol valve assembly. (The 4-way control valveassembly consists of a pilot-operated controlvalve, a direct- or solenoid-operated control valve,and a sequence valve, which is not shown.)

To raise the JBD, solenoid B of the 4-waycontrol valve assembly is energized. The spoolsof the 4-way valve assembly shift, allowingmedium-pressure hydraulic fluid to flow into portA of the hydraulic cylinder. The cylinders extend,

Figure 12-9.—Operating gear assembly (panels raised).

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I

II

I

II

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pushing the crank assembly aft and rotating theshaft. The rotation of the shaft extends theoperating gear linkage and raises the panelassemblies. Fluid from port B of the piston isdirected through the 4-way valve assembly andback to the gravity tank.

To lower the JBD (fig. 12-11), solenoid A ofthe 4-way control valve assembly is energized. Thespools of the 4-way valve assembly shift, allowmedium-pressure hydraulic fluid to flow into portB of the hydraulic cylinder. The cylinders retract,pulling the crank assembly forward and rotatingthe shaft. The rotation of the shaft retracts theoperating gear linkage and lowers the panel

assemblies. Fluid from port A of the piston isdirected through the 4-way valve assembly andback to the gravity tank.

To lower the JBD in the event of hydrauliccontrol failure, each JBD panel is equipped witha manual bypass valve, which allows bypassingthe 4-way control valve. This allows venting thehydraulic pressure from the “raise” side of thecylinder back to the gravity tank.

The three lines to port A of the hydrauliccylinders have orifice assemblies in them. Theseorifice assemblies control the flow of hydraulicfluid in both the raise and lower operations.

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APPENDIX I

GLOSSARY

A part of this glossary has been extracted fromthe American Standard Glossary of Terms forFluid Power (ASA B93.2-1965) with permissionof the publisher, The National Fluid PowerAssociation.

ABSOLUTE TEMPERATURE—The tempera-ture measured using absolute zero as a reference.Absolute zero is –273.16°C or –459.69°F.

ACCELERATION—Time rate of change ofvelocity.

ACCUMULATOR—A device for storingliquid under pressure. It usually consists of achamber separated into a gas compartment anda liquid compartment by a piston or diaphragm.An accumulator also serves to smooth outpressure surges in a hydraulic system.

ACTUATOR—A device that converts fluidpower into mechanical force and motion.

ADDITIVE—A chemical compound orcompounds added to a fluid to change itsproperties.

AIR, COMPRESSED—Air at any pressuregreater than atmospheric pressure.

AMBIENT—Surrounding, such as ambientair, meaning surrounding air.

BAROMETER—An instrument that mea-sures atmospheric pressure.

BERNOULLI’S PRINCIPLE—If a fluidflowing through a tube reaches a constriction, ornarrowing of the tube, the velocity of the fluidflowing through the constriction increases and thepressure decreases.

BLEEDER, AIR—A bleeder for the removalof air.

BOYLE’S LAW—The absolute pressure of afixed mass of gas varies inversely as the volume,provided the temperature remains constant.

CAVITATION—A loca l ized gaseouscondition within a liquid stream that occurs wherethe pressure is reduced to the vapor pressure.

CELSIUS—The temperature scale using thefreezing point of water as zero and the boilingpoint as 100, with 100 equal divisions between,called degrees. This scale was formerly known asthe centigrade scale.

CENTIGRADE—(See Celsius.)

CENTRIFUGAL FORCE—A force exertedon a rotating object in a direction outward fromthe center of rotation.

CHARLES’S LAW—If the pressure isconstant, the volume of dry gas varies directlywith the absolute temperature.

CHEMICAL CHANGE—A change thatalters the composition of the molecules of asubstance.

CIRCUIT—An arrangement of intercon-nected component parts.

COMPRESSIBILITY—The change in volumeof a unit volume of a fluid when it is subjectedto a unit change of pressure.

COMPRESSOR—A device that convertsmechanical force and motion into pneumatic fluidpower.

COMPUTER—A device capable of acceptinginformation, applying prescribed processes to theinformation, and supplying the results of theseprocesses.

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CONDENSATION—The change from agaseous (or vapor) state to a liquid state.

CONTAMINANT—Detrimental matter in afluid.

CONTINUITY EQUATION—The mass rateof fluid flow into any fixed space is equal to themass flow rate out. Hence, the mass flow rate offluid past all cross sections of a conduit is equal.

CONTROL—A device used to regulate thefunction of a component or system.

CONTROL, CYLINDER—A control inwhich a fluid cylinder is the actuating device.

CONTROL, ELECTRIC—A control actuatedelectrically.

CONTROL, HYDRAULIC—A contro lactuated by a liquid.

CONTROL, MANUAL—A control actuatedby the operator.

CONTROL, MECHANICAL—A controlactuated by linkages, gears, screws, cams, or othermechanical elements.

CONTROL, PNEUMATIC—A contro lactuated by air or other gas pressure.

CONTROL, SERVO—A control actuated bya feedback system that compares the output withthe reference signal and makes corrections toreduce the difference.

CONTROLS, PUMP—Controls applied topositive-displacement variable delivery pumps toadjust their volumetric output or direction offlow.

CONVERGENT—That which inclines andapproaches nearer together, as the inner walls ofa tube that is constricted.

COOLER—A heat exchanger, which removesheat from a fluid.

COOLER, AFTERCOOLER—A device thatcools a gas after it has been compressed.

COOLER, INTERCOOLER—A device thatcools a gas between the compressive steps of amultiple stage compressor.

COOLER, PRECOOLER—A device thatcools a gas before it is compressed.

CORROSION—The slow destruction ofmaterials by chemical agents and electromechanicalreactions.

CYCLE—A single complete operationconsisting of progressive phases starting andending at the neutral position.

CYLINDER—A device that converts fluidpower into linear mechanical force and motion.It usually consists of a movable element, such asa piston and piston rod, plunger, or ram,operating within a cylindrical bore.

CYLINDER, CUSHIONED—A cylinder witha piston-assembly deceleration device at one ofboth ends of the stroke.

CYLINDER, DOUBLE-ACTING—Acylinder in which fluid force can be applied to themovable element in either direction.

CYLINDER, DOUBLE-ROD—A cylinderwith a single piston and a piston rod extendingfrom each end.

CYLINDER, DUAL-STROKE—A cylindercombination that provides two working strokes.

CYLINDER, PISTON—A cylinder in whichthe movable element has a greater cross-sectionalarea than the piston rod.

CYLINDER, PLUNGER—A cylinder inwhich the movable element has the same cross-sectional area as the piston rod.

CYLINDER, SINGLE-ACTING—A cylinderin which the fluid force can be applied to themovable element in only one direction.

CYLINDER, SINGLE-ROD—A cylinderwith a piston rod extending from one end.

CYLINDER, SPRING-RETURN—A cylin-der in which a spring returns the piston assembly.

CYLINDER, TANDEM—Two or morecylinders with interconnected piston assemblies.

CYLINDER, TELESCOPING—A cylinderwith nested multiple tubular rod segments whichprovide a long working stroke in a short retractedenvelope.

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DENSITY—The weight per unit volume of asubstance.

DIAGRAM, COMBINATION—A drawingusing a combination of graphical, cutaway, andpictorial symbols.

DIAGRAM, CUTAWAY—A drawing show-ing principal internal parts of all components,controls, and actuating mechanisms, all inter-connecting lines and functions of individualcomponents.

DIAGRAM, GRAPHICAL—A drawing ordrawings showing each piece of apparatusincluding all interconnecting lines by approvedstandard symbols.

DIAGRAM, PICTORIAL—A drawing show-ing each component in its actual shape accordingto the manufacturer’s installation.

DIAGRAM, SCHEMATIC—(See Diagram,graphical.)

DIAPHRAGM—A dividing membrane orthin partition.

DIFFUSER—A duct of varying cross sectiondesigned to convert a high-speed gas flow intolow-speed at an increased pressure.

DISPLACEMENT—The volume of fluid thatcan pass through a pump, motor, or cylinder ina single revolution or stroke.

DIVERGENT—Moving away from eachother, as the inner wall of a tube that flaresoutward.

EFFICIENCY—The ratio of the outputpower to the input power, generally expressed asa percentage.

ENERGY—The ability or capacity to dowork.

EQUILIBRIUM—A state of balance betweenopposing forces or actions.

FAHRENHEIT—The temperature scale usingthe freezing point of water as 32 and the boilingpoint as 212, with 180 equal divisions between,called degrees.

FEEDBACK—A transfer of energy from theoutput of a device to its input.

FILTER—A device whose primary functionis the retention by a porous media of insolublecontaminants from a fluid.

FILTER ELEMENT—The porous device thatperforms the actual process of filtration.

FILTER MEDIA—The porous materials thatperform the actual process of filtration.

FILTER MEDIA, SURFACE—Porousmaterials that primarily retain contaminants onthe influent face.

FLASH POINT—The temperature to whicha liquid must be heated under specified conditionsof the test method to give off sufficient vapor toform a mixture with air that can be ignitedmomentarily by a specified flame.

FLOW, LAMINAR—A flow situation inwhich fluid moves in parallel layers (also referredto as streamline flow).

FLOW, METERED—Flow at a controlledrate.

FLOW, TURBULENT—A flow situation inwhich the fluid particles move in a randommanner.

FLOW RATE—The volume, mass, or weightof a fluid passing through any conductor per unitof time.

FLOWMETER—An instrument used tomeasure quantity or the flow rate of a fluidmotion.

FLUID—A liquid or a gas.

FLUID FLOW—The stream or movement ofa fluid, or the rate of its movement.

FLUID FRICTION—Friction due to theviscosity of fluids.

FLUID, FIRE-RESISTANT—A f lu iddifficult to ignite, which shows little tendency topropagate flame.

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FLUID, HYDRAULIC—A fluid suitable foruse in a hydraulic system.

FLUID, PETROLEUM—A fluid composedof petroleum oil. It may contain additives.

FLUID, PHOSPHATE ESTER BASE—Afluid that contains a phosphate ester as one of themajor components.

FLUID, SILICONE—A fluid composed ofsilicones. It may contain additives.

FLUID, WATER-GLYCOL—A fluid whosemajor constituents are water and one or moreglycols or polyglycols.

FLUID STABILITY—Resistance of a fluid topermanent change in properties.

FLUID POWER—Energy transmitted andcontrolled through the use of fluids underpressure.

FLUID POWER SYSTEM—A system thattransmits and controls power through use of apressurized fluid within an enclosed circuit.

FOOT-POUND—The amount of workaccomplished when a force of 1 pound producesa displacement of 1 foot.

FORCE—The action of one body on anothertending to change the state of motion of the bodyacted upon.

FREE FLOW—Flow that encounters negli-gible resistance.

FRICTION—The action of one body orsubstance rubbing against another, such as fluidflowing against the walls of pipe; the resistanceto motion caused by this rubbing.

FRICTION PRESSURE DROP—The decreasein the pressure of a fluid flowing through apassage attributable to the friction between thefluid and the passage walls.

GAS—The form of matter that has neither adefinite shape nor a definite volume.

GASKET—A class of seals that provides a sealbetween two stationary parts.

GAUGE—An instrument or device for

characteristic.measuring, indicating, or comparing a physical

GAUGE PRESSURE—Pressure aboveatmospheric pressure.

GAUGE SNUBBER—A device installed inthe line to the pressure gauge used to dampenpressure surges and thus provide a steady readingand a protection for the gauge.

GAUGE, BELLOWS—A gauge in which thesensing element is a convoluted closed cylinder.A pressure differential between the outside andthe inside causes the cylinder to expand or contractaxially.

GAUGE, BOURDON TUBE—A pressuregauge in which the sensing element is a curvedtube that tends to straighten out when subjectedto internal fluid pressure.

GAUGE, DIAPHRAGM—A gauge in whichthe sensing element is relatively thin and its innerportion is free to deflect with respect to itsperiphery.

GAUGE, PRESSURE—A gauge thatindicates the pressure in the system to which itis connected.

GAUGE, VACUUM—A pressure gauge forpressures less than atmospheric.

GRAVITY—The force that tends to draw allbodies toward the center of the earth. The weightof a body is the resultant of gravitational forceacting on the body.

HEAD—The height of a column or body offluid above a given point expressed in linear units.Head is often used to indicate gauge pressure.Pressure is equal to the height times the densityof the fluid.

HEAD, FRICTION—The head required toovercome the friction at the interior surface ofa conductor and between fluid particles in motion.It varies with flow, size, type, and condition ofconductors and fittings, and fluid characteristics,

HEAD, STATIC—The height of a column orbody of fluid above a given point.

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HEAD, VELOCITY—The equivalent headthrough which the liquid would have to fall toattain a given velocity. Mathematically it is equalto the square of the velocity (in feet) divided by64.4 feet per second square.

HEAT EXCHANGER—A device thattransfers heat through a conducting wall from onefluid to another.

HYDRAULICS—Engineering science pertain-ing to liquid pressure and flow.

HYDROMETER—An instrument for deter-mining the specific gravities of liquids.

HYDROPNEUMATICS—Pertaining to thecombination of hydraulic and pneumatic fluidpower.

HYDROSTATICS—Engineering sciencepertaining to the energy of liquids at rest.

IMPACT PRESSURE—The pressure of amoving fluid brought to rest that is in excess ofthe pressure the fluid has when it does not flow;that is, total pressure less static pressure. Impactpressure is equal to dynamic pressure in incom-pressible flow; but in compressible flow, impactpressure includes the pressure change owing to thecompressibility effect.

IMPINGEMENT—The striking or dashingupon with a clash or sharp collision, as airimpinging upon the rotor of a turbine or motor.

IMPULSE TURBINE—A turbine driven bya fluid at high velocity under relatively lowpressure.

INERTIA—The tendency of a body at rest toremain at rest, and a body in motion to continueto move at a constant speed along a straight line,unless the body is acted upon in either case by anunbalanced force.

INHIBITOR—Any substance which slows orprevents chemical reactions such as corrosion oroxidation.

INVERSE PROPORTION—The relation thatexists between two quantities when an increase inone of them produces a corresponding decreasein the other.

KELVIN SCALE—The temperature scaleusing absolute zero as the zero point and divisionsthat are the same size as centigrade degrees.

KINETIC ENERGY—The energy that asubstance has while it is in motion.

KINETIC THEORY—A theory of matter thatassumes that the molecules of matter are inconstant motion.

LINE—A tube, pipe, or hose that is used asa conductor of fluid.

LIQUID—A form of matter that has adefinite volume but takes the shape of itscontainer.

LOAD—The power that is being delivered byany power-producing device. The equipment thatuses the power from the power-producing device.

LUBRICATOR—A device that addscontrolled or metered amounts of lubricant intoa fluid power system.

MANIFOLD—A type of fluid conductor thatprovides multiple connections ports.

MANOMETER—A differential pressuregauge in which pressure is indicated by the heightof a liquid column of known density. Pressure isequal to the difference in vertical height betweentwo connected columns multiplied by the densityof the manometer liquid. Some forms ofmanometers are U tube, inclined tube, well, andbell types.

MATTER—Any substance that occupiesspace and has weight.

MECHANICAL ADVANTAGE—The ratioof the resisting weight to the acting force. Theratio of the distance through which the force isexerted divided by the distance the weight israised.

METER-IN—To regulate the amount of fluidinto a system or an actuator.

METER-OUT—To regulate the flow of fluidfrom a system or actuator.

MICRON—A millionth of a meter or about0.00004 inch.

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MOLECULE—A small natural particle ofmatter composed of two or more atoms.

MOTOR—A device that converts fluid powerinto mechanical force and motion. It usuallyprovides rotary mechanical motion.

MOTOR, FIXED-DISPLACEMENT—Amotor in which the displacement per unit ofoutput motion cannot be varied.

MOTOR, LINEAR—(See Cylinder.)

MOTOR, ROTARY—A motor capable ofcontinuous rotary motion.

MOTOR, ROTARY LIMITED—A rotarymotor having limited motion.

MOTOR, VARIABLE-DISPLACEMENT—A motor in which the displacement per unit ofoutput motion can be varied.

NEOPRENE—A synthetic rubber highlyresistant to oil, light, heat, and oxidation.

NEUTRALIZATION NUMBER—A mea-sure of the total acidity or basicity of an oil; thisincludes organic or inorganic acids or bases or acombination of them.

OXIDATION—The process by which oxygenunites with some other substance, causing rust orcorrosion.

PACKING—A class of seal that is used toprovide a seal between two parts of a unit whichmove in relation to each other.

PASCAL’S LAW—A pressure applied to aconfined fluid at rest is transmitted with equalintensity throughout the fluid.

PERIPHERY—The outside surface, espe-cially that of a rounded object or body.

PIPE—A type of fluid line whose dimensionsare designated by nominal (approximate) insidediameter and wall thickness.

PNEUMATICS—Engineering science per-taining to gaseous pressure and flow.

PORT—An internal or external terminus ofa passage in a component.

POTENTIAL ENERGY—The energy a sub-stance has because of its position, its condition,or its chemical composition.

POUR POINT—The lowest temperature atwhich a liquid will flow under specified con-ditions.

POWER UNIT—A combination of pump,pump drive, reservoir, controls, and conditioningcomponents which may be required for itsapplication.

POWER—The rate of doing work or the rateof expanding energy.

PRESSURE—The amount of force distrib-uted over each unit of area, usually expressed inpounds per square inch.

PRESSURE, ABSOLUTE—The sum ofatmospheric and gauge pressures.

PRESSURE, ATMOSPHERIC—Pressureexerted by the atmosphere at any specific location.

PRESSURE, BACK—The pressure encoun-tered on the return side of a system.

PRESSURE, DIFFERENTIAL—The dif-ference in pressure between any two points of asystem or a component.

PRESSURE, HEAD—The pressure due to theheight of a column or body of fluid. It is usuallyexpressed in feet.

PRESSURE, OPERATING—The pressure atwhich a system operates.

PRESSURE, PRECHARGE—The pressureof compressed gas in an accumulator prior to theadmission of a liquid.

PRESSURE, PROOF—The nondestructivetest pressure in excess of the maximum ratedoperating pressure.

PRESSURE, STATIC—The pressure in afluid at rest.

PRESSURE SWITCH—An electrical switchoperated by the increase or decrease of fluidpressure.

AI-6

PRIME MOVER—The source of mechanicalpower used to drive the pump or compressor.

PUMP—A device that converts mechanicalforce and motion into hydraulic fluid power.

PUMP, AXIAL PISTON—A pump havingmultiple pistons disposed with their axes parallel.

PUMP, CENTRIFUGAL—A pump thatproduces fluid velocity and converts it to pressurehead.

PUMP, FIXED-DISPLACEMENT—Apump in which the displacement per cycle cannotbe varied.

PUMP, RADIAL PISTON—A pump havingmultiple pistons disposed radially actuated by aneccentric element.

PUMP, VARIABLE-DISPLACEMENT—Apump in which the volume of fluid per cycle canbe varied.

RANKINE SCALE—A thermometer scalebased on absolute zero of the Fahrenheit scale,in which the freezing point of water isapproximately 492°R.

RATIO—The value obtained by dividing onenumber by another, indicating their relativeproportions.

RECEIVER—A container in which gas isstored under pressure as a supply source forpneumatic power.

RECIPROCATING—Moving back andforth, as a piston reciprocating in a cylinde.,

RESERVOIR—A container for storage ofliquid in a fluid power system.

RESPONSE TIME—The time lag between asignal input and the resulting change of output.

RESTRICTOR—A device that reduces thecross-sectional flow area.

RESTRICTOR, ORIFICE—A restrictor, thelength of which is relatively small with respect toits cross-sectional area. The orifice may be fixedor variable. Variable types are noncompensated,pressure compensated, or pressure and tempera-ture compensated.

RETURN LINE—A line used for returningfluid back into the reservoir or atmosphere.

SEPARATOR—A device whose primaryfunction is to isolate undesirable fluids and orcontaminants by physical properties other thansize.

SERVO—A device used to convert a smallmovement into a greater movement of force.

SOLID—The form of matter that has adefinite shape and a definite volume.

SPECIFIC GRAVITY—The ratio of theweight of a given volume of a substance to theweight of an equal volume of some standardsubstance.

STEADY FLOW—A flow in which thevelocity, pressure, and temperature at any pointin the fluid do not vary with time.

STRAINER—A coarse filter.

STOKE—The standard unit of kinematicviscosity in the cgs system. It is expressed in squarecentimeters per second; 1 centistoke equals 0.01stoke.

STUFFING BOX—A cavity and closure withmanual adjustment for a sealing device.

SUPPLY LINE—A line that conveys fluidfrom the reservoir to the pump.

SURGE—A momentary rise of pressure in acircuit.

SYNCHRONIZE—To make two or moreevents or operations occur at the proper time withrespect to each other.

SYNTHETIC MATERIAL—A complexchemical compound that is artificially formed bythe combining of two or more simpler compoundsor elements.

TANK—A container for the storage of fluidin a fluid power system.

THEORY—A scientific explanation, tested byobservations and experiments.

THERMAL EXPANSION—The increase involume of a substance due to temperature change.

A4-7

TORQUE—A force or combination of forcesthat produces or tends to produce a twisting orrotary motion.

TUBING—A type of fluid line whosedimensions are designated by actual measuredoutside diameter and by actual measured wallthickness.

TURBINE—A rotary motor actuated bythe reaction, impulse, or both, of a flow ofpressurized fluid.

VALVE—A device that controls fluid flowdirection, pressure, or flow rate.

VALVE, CHECK—A directional controlvalve that permits flow of fluid in only onedirection.

VALVE, COUNTERBALANCE—A pressurecontrol valve that maintains back pressure toprevent a load from falling.

VALVE, DIRECTIONAL CONTROL—Avalve whose primary function is to direct orprevent flow through selected passages.

VALVE, FLOW CONTROL—A valve whoseprimary function is to control flow rate.

VALVE, HYDRAULIC—A valve for con-trolling liquid.

VALVE, PILOT—A valve used to operateanother valve or control.

VALVE, PNEUMATIC—A valve for con-trolling gas.

VALVE, PRESSURE REDUCING—Apressure control valve whose primary function isto limit outlet pressure.

VALVE, PRIORITY—A valve that directsflow to one operating circuit at a fixed rate anddirects excess flow to another operating circuit.

VALVE, RELIEF—A pressure control valvewhose primary function is to limit systempressure.

VALVE, SELECTOR—A directional controlvalve whose primary function is to selectivelyinterconnect two or more ports.

VALVE, SEQUENCE—A valve whoseprimary function is to direct flow in a pre-determined sequence.

VALVE, SERVO—A directional control valvethat modulates flow or pressure as a function ofits input signal.

VALVE, SHUTOFF—A valve that operatesfully open or fully closed.

VALVE, UNLOADING—A pressure controlvalve whose primary function is to permit a pumpor compressor to operate at minimum load.

VELOCITY—The rate of motion in aparticular direction. The velocity of fluids isusually expressed in feet per second.

VENTURI—A tube having a narrowingthroat or constriction to increase the velocity offluid flowing through it. The flow through theventuri causes a pressure drop in the smallestsection, the amount being a function of thevelocity of flow.

VISCOSITY—A measure of the internalfriction or resistance of a fluid to flow.

VISCOSITY INDEX—A measure of theviscosity-temperature characteristics of a fluid asreferred to that of two arbitrary reference fluids.

VISCOSITY, SAYBOLT UNIVERSALSECONDS (SUS)—The time in seconds for 60milliliters of oil to flow through a standard orificeat a given temperature.

VISCOSITY, KINEMATIC—The absoluteviscosity divided by the density of the fluid. It isusually expressed in centistokes.

VOLUME OF FLOW—The quantity of fluidthat passes a certain point in a unit of time. Thevolume of flow is usually expressed in gallons perminute for liquids and cubic feet per minute forgases.

WORK—The transference of energy from onebody or system to another. That which isaccomplished by a force acting through a distance.

AI-8

APPENDIX II

MECHANICAL SYMBOLS OTHER THANAERONAUTICAL FOR FLUID

POWER DIAGRAMS

AII-1

AII-2

AII-3

AII-4

APPENDIX III

AERONAUTICAL MECHANICAL SYMBOLSFOR FLUID POWER DIAGRAMS

AIII-1

AIII-2

INDEX

A

Accumulators, 9-3 to 9-7Actuators, 10-1 to 10-12

cylinders, 10-1 to 10-7piston-type cylinders, 10-3 to 10-6

double-acting cylinder, 10-4 to10-5

single-acting cylinder, 10-4tandem cylinders, 10-5 to 10-6

rack-and-pinion piston-type rotaryactuators, 10-6 to 10-7

ram-type cylinders, 10-1 to 10-3double-acting ram, 10-2dual rams, 10-3single-acting ram, 10-1 to 10-2telescoping rams, 10-2 to 10-3

motors, 10-8 to 10-11gear-type motors, 10-8piston-type motors, 10-9 to 10-11

axial-piston motor, 10-10 to10-11

radial-piston motor, 10-10vane-type motors, 10-9

turbines, 10-11 to 10-12impulse turbine, 10-11 to 10-12reaction turbine, 10-12

Aeronautical mechanical symbols for fluidpower diagrams, AIII-1 to AIII-2

Air-pressurized reservoirs, 9-2 to 9-3Atmospheric pressure, 2-2 to 2-3Axial piston pumps, 4-12 to 4-15Axial-piston motor, 10-10 to 10-11

B

Backup rings, 7-12 to 7-15Ball valves, 6-1 to 6-2

Basic diagrams and systems, 12-1 to 12-13diagrams, 12-1 to 12-5

symbols, 12-1 to 12-2types of diagrams, 12-2 to 12-5

combination diagrams, 12-5cutaway diagrams, 12-2 to 12-3graphic diagrams, 12-4 to 12-5pictorial diagrams, 12-2

fluid power systems, 12-5 to 12-13hydraulic power drive system, 12-6 to 12-8

control, 12-7operation, 12-7 to 12-8

jet blast deflectors, 12-10 to 12-13landing gear emergency system, 12-8

to 12-10Bellows elastic elements, 8-3 to 8-5Bernoulli’s principle, 2-14Bimetallic expansion thermometer, 8-7Bladder-type accumulators, 9-6Bourdon tube gauges, 8-1 to 8-3Boyle’s law, 11-4 to 11-5Brazed connectors, 5-13

C

C-shaped bourdon tube, 8-2 to 8-3Centered internal gear pump, 4-6Charles’s law, 11-5Check valve, 6-16 to 6-18Combination diagrams, 12-5Compressed air, 11-7 to 11-8Compressibility and expansion of gases, 11-3

to 11-7Connectors for flexible hose, 5-17 to 5-19Cork, 7-2Cork and rubber, 7-2Counterbalance valve, 6-14 to 6-15Cup packings, 7-16Cutaway diagrams, 12-2 to 12-3Cylinders, 10-1 to 10-7

piston-type cylinders, 10-3 to 10-6rack-and-pinion piston-type rotary

actuators, 10-6 to 10-7ram-type cylinders, 10-1 to 10-3

INDEX-1

D

Diagrams, 12-1 to 12-5Diaphragm accumulators, 9-7Direct-contact gas-to-fluid accumulators, 9-6

to 9-7Directional control valves, 6-15 to 6-25

check valve, 6-16 to 6-18classification, 6-15 to 6-16four-way valves, 6-20 to 6-25shuttle valve, 6-18three-way valves, 6-19 to 6-20two-way valves, 6-18 to 6-19

Dirt exclusion seals (wipers and scrapers), 7-17Distant-reading thermometers, 8-7 to 8-8Dual bellows indicators, 8-4 to 8-5

F

Filtration, 9-7 to 9-13filters, 9-8 to 9-12pneumatic gases, 9-12 to 9-13strainers, 9-8

Flange connectors, 5-12Flange packings, 7-16 to 7-17Flared connectors, 5-13 to 5-14Flareless-tube connectors, 5-15 to 5-17Flexible hose, 5-8 to 5-12Flow control valves, 6-1 to 6-6

ball valves, 6-1 to 6-2gate valves, 6-3globe valves, 6-3 to 6-5hydraulic and pneumatic globe valves, 6-5

to 6-6needle valves, 6-5

Fluid lines and fittings, 5-1 to 5-21flexible hose, 5-8 to 5-12

application, 5-9 to 5-10fabrication and testing, 5-10identification, 5-10installation, 5-11 to 5-12PFTE, 5-9synthetic rubber hose, 5-8 to 5-9

cure date, 5-8 to 5-9sizing, 5-8

pipes and tubing, 5-1 to 5-8preparation of pipes and tubing, 5-3

to 5-8tube bending, 5-5 to 5-7tube cutting and deburring, 5-4

to 5-5tube flaring, 5-7 to 5-8

Fluid lines and fittings—Continuedpipes and tubing—Continued

selection of pipes and tubing, 5-1 to5-3

materials, 5-2 to 5-3sizing of pipes and tubing, 5-1

to 5-2precautionary measures, 5-20 to 5-21types of fittings and connectors, 5-12 to

5-20brazed connectors, 5-13connectors for flexible hose, 5-17 to

5-19hose connection side of hose

fitting, 5-18 to 5-19piping connection side of hose

fitting, 5-18flange connectors, 5-12flared connectors, 5-13 to 5-14flareless-tube connectors, 5-15 to

5-17final assembly, 5-17inspection, 5-16 to 5-17presetting, 5-15 to 5-16

manifolds, 5-19 to 5-20quick-disconnect couplings, 5-19threaded connectors, 5-12welded connectors, 5-12 to 5-13

types of lines, 5-1Fluid power, introduction to, 1-1 to 1-4Fluid power systems, 12-5 to 12-13

hydraulic power drive system, 12-6 to12-8

jet blast deflectors, 12-10 to 12-13landing gear emergency system, 12-8 to

12-10Fluid-pressurized reservoir, 9-2Forces in liquids, 2-1 to 2-17

liquids at rest, 2-1 to 2-9pressure and force, 2-1 to 2-3

atmospheric pressure, 2-2 to 2-3computing force, pressure, and

area, 2-1 to 2-2transmission of forces through

liquids, 2-3 to 2-9density and specific gravity, 2-4Pascal’s law, 2-5 to 2-6pressure and force in fluid

power systems, 2-6 to 2-9liquids in motion, 2-9 to 2-15

Bernoulli’s principle, 2-14factors involved in flow, 2-11 to 2-13

inertia and force, 2-11 to 2-12kinetic energy, 2-12 to 2-13

INDEX-2

Forces in liquids—Continuedliquids in motion—Continued

minimizing friction, 2-14 to 2-15relationship of force, pressure, and

head, 2-13static and dynamic factors, 2-13 to

2-14streamline and turbulent flow, 2-10

to 2-11volume and velocity of flow, 2-9 to

2-10volume of flow and speed, 2-10

operation of hydraulic components, 2-15to 2-17

hydraulic brakes, 2-16 to 2-17hydraulic jack, 2-15 to 2-16

Four-way valves, 6-20 to 6-25

Gate valves, 6-3Gauge snubbers,

G

8-8 to 8-9Gear pumps, 4-2 to 4-6Gear-type motors, 10-8General gas law, 11-6 to 11-7Globe valves, 6-3 to 6-5Glossary, AI-1 to AI-8Graphic diagrams, 12-4 to 12-5

H

Hand pumps, 4-9Helical gear pump, 4-5Herringbone gear pump, 4-4Hydraulic and pneumatic globe

6-6Hydraulic brakes, 2-16 to 2-17Hydraulic fluids, 3-1 to 3-11

contamination, 3-6 to 3-10

valves, 6-5 to

classification, 3-7 to 3-8fluid contamination, 3-7 to 3-8particulate contamination, 3-7

contamination control, 3-9 to 3-10origin of contamination, 3-8 to 3-9

hydraulic fluid sampling, 3-10 to 3-11properties, 3-1 to 3-5

chemical stability, 3-3 to 3-4cleanliness, 3-5density and compressibility, 3-4fire point, 3-4flashpoint, 3-4foaming tendencies, 3-4 to 3-5freedom from acidity, 3-4

Hydraulic fluids-Continuedproperties—Continued

lubricating power, 3-3minimum toxicity, 3-4viscosity, 3-1 to 3-3

measurement of viscosity, 3-1 to3-3

viscosity index, 3-3types of hydraulic fluids, 3-5 to 3-6

petroleum-based fluids, 3-5synthetic fire-resistant fluids, 3-5 to

3-6lightweight synthetic fire-

resistant fluids, 3-6phosphate ester fire-resistant

fluid, 3-5 to 3-6silicone synthetic fire-resistant

fluids, 3-6water-based fire-resistant fluids, 3-6

Hydraulic jack, 2-15 to 2-16Hydraulic power drive system, 12-6 to 12-8Hydraulics, 1-2 to 1-3

I

Impulse turbine, 10-11 to 10-12Introduction to fluid power, 1-1 to 1-4

advantages of fluid power, 1-2hydraulics, 1-2 to 1-3

development of hydraulics, 1-2 to 1-3use of hydraulics, 1-3

special problems, 1-2states of matter, 1-3 to 1-4

J

Jet blast deflectors, 12-10 to 12-13

K

Kinetic energy, 2-12 to 2-13Kinetic theory of gases, 11-4

L

Landing gear emergency system, 12-8 to 12-10Leather, 7-2Lightweight synthetic fire-resistant fluids, 3-6Liquids in motion, 2-9 to 2-15Lobe pump, 4-6 to 4-7

INDEX-3

M

Manifolds, 5-19 to 5-20Matter, states of, 1-3 to 1-4Measurement and pressure control devices, 8-1

to 8-9gauge snubbers, 8-8 to 8-9pressure gauges, 8-1 to 8-5

bellows elastic elements, 8-3 to 8-5dual bellows indicators, 8-4 to

8-5simple bellows elements, 8-4

bourdon tube gauges, 8-1 to 8-3C-shaped bourdon tube, 8-2 to

8-3spiral and helical bourdon tubes,

8-3pressure switches, 8-5 to 8-6temperature switches, 8-8temperature-measuring instruments, 8-6 to

8-8bimetallic expansion thermometer,

8-7distant-reading thermometers, 8-7 to

8-8Mechanical symbols other than aeronautical

for fluid power diagrams, AII-1 to AII-4Metal, 7-2 to 7-3Motors, 10-8 to 10-11

gear-type motors, 10-8piston-type motors, 10-9 to 10-11vane-type motors, 10-9

N

Needle valves, 6-5Nitrogen, 11-8Nonpressurized reservoirs, 9-1 to 9-2

O

Off-centered internal gear pump, 4-6O-rings, 7-6 to 7-12

P

Pascal’s law, 2-5 to 2-6Petrolium-based fluids, 3-5PFTE hose, 5-9Phosphate ester fire-resistant fluid, 3-5 to 3-6Pictorial diagrams, 12-2Pipes and tubing, 5-1 to 5-8

Piston pumps, 4-9 to 4-15Piston-type accumulators, 9-5 to 9-6Piston-type cylinders, 10-3 to 10-6Piston-type motors, 10-9 to 10-11Pneumatic gases, 9-12 to 9-13Pneumatics, 11-1 to 11-9

characteristics of gases, 11-1 to 11-3density, 11-1 to 11-2pressure, 11-3temperature, 11-2 to 11-3

compressibility and expansion of gases,11-3 to 11-7

Boyle’s law, 11-4 to 11-5Charles’s law, 11-5general gas law, 11-6 to 11-7kinetic theory of gases, 11-4

contamination control, 11-8 to 11-9development of pneumatics, 11-1pneumatic gases, 11-7 to 11-8

compressed air, 11-7 to 11-8high-pressure air systems, 11-7

to 11-8low-pressure air, 11-8medium-pressure air, 11-8

nitrogen, 11-8qualities, 11-7

potential hazards, 11-9safety precautions, 11-9

Pressure control valves, 6-6 to 6-15counterbalance valve, 6-14 to 6-15pressure regulators, 6-9 to 6-10pressure-reducing valves, 6-12 to 6-14relief valves, 6-6 to 6-9sequence valves, 6-11 to 6-12

Pressure gauges, 8-1 to 8-5bellows elastic elements, 8-3 to 8-5bourdon tube gauges, 8-1 to 8-3

Pressure switches, 8-5 to 8-6Pressurized reservoirs, 9-2 to 9-3Proportional-flow filter, 9-10Pumps, 4-1 to 4-15

classification of pumps, 4-1 to 4-2operation, 4-1performance, 4-1purpose, 4-1reciprocating pumps, 4-8 to 4-15

hand pumps, 4-9piston pumps, 4-9 to 4-15

axial piston pumps, 4-12 to 4-15radial piston pumps, 4-10 to

4-11

INDEX-4

Pumps—Continuedrotary pumps, 4-2 to 4-8

gear pumps, 4-2 to 4-6centered internal gear pump, 4-6helical gear pump, 4-5herringbone gear pump, 4-4off-centered internal gear pump,

4-5spur gear pump, 4-3 to 4-4

lobe pump, 4-6 to 4-7screw pump, 4-7 to 4-8vane pump, 4-8

Q

seals, 7-15Quad-Rings, 7-15Quick-disconnect couplings, 5-19

R

Rack-and-pinion piston-type rotary actuators,10-6 to 10-7

Radial-piston motor, 10-10Radial piston pumps, 4-10 to 4-11Ram-type cylinders, 10-1 to 10-3Reaction turbine, 10-12Reciprocating pumps, 4-8 to 4-15

hand pumps, 4-9piston pumps, 4-9 to 4-15

Relief valves, 6-6 to 6-9Reservoirs, strainers, filters, and

accumulators, 9-1 to 9-13accumulators, 9-3 to 9-7

bladder-type accumulators, 9-6diaphragm accumulators, 9-7direct-contact gas-to-fluid

accumulators, 9-6 to 9-7piston-type accumulators, 9-5 to 9-6

filtration, 9-7 to 9-13filters, 9-8 to 9-12

filter elements, 9-11 to 9-12filter rating, 9-11full-flow filter, 9-8 to 9-10proportional-flow filter, 9-10

pneumatic gases, 9-12 to 9-13removal of moisture, 9-12 to

9-13removal of solids, 9-12

strainers, 9-8

Reservoirs, strainers, filters, andaccumulators—Continued

reservoirs, 9-1 to 9-3nonpressurized reservoirs, 9-1 to 9-2pressurized reservoirs, 9-2 to 9-3

air-pressurized reservoirs, 9-2 to9-3

fluid-pressurized reservoir, 9-2Rotary pumps, 4-2 to 4-8

gear pumps, 4-2 to 4-6lobe pump, 4-6 to 4-7screw pump, 4-7 to 4-8vane pump, 4-8

Rubber, 7-3

S

Screw pump, 4-7 to 4-8Sealing devices and materials, 7-1 to 7-18

seal materials, 7-1 to 7-3cork, 7-2cork and rubber, 7-2leather, 7-2metal, 7-2 to 7-3rubber, 7-3

types of seals, 7-3 to 7-18backup rings, 7-12 to 7-15

installation, 7-12 to 7-15packaging and storing, 7-12

cup packings, 7-16dirt exclusion seals (wipers and

scrapers), 7-17flange packings, 7-16 to 7-17O-rings, 7-6 to 7-12

cure date, 7-8dimensions, 7-8identification, 7-7replacement, 7-9 to 7-12shelf life and expiration date,

7-8sizes, 7-8specifications, 7-8

seals, 7-15Quad-Rings, 7-15storage of seals, 7-17 to 7-18T-seals, 7-3 to 7-5U-cups and U-packings, 7-16

leather U-packings, 7-16U-cups, 7-16

V-rings, 7-5 to 7-6Sequence valves, 6-11 to 6-12Shuttle valve, 6-18Silicone synthetic fire-resistant fluids, 3-6Spiral and helical bourdon tubes, 8-3

INDEX-5

Spur gear pump, 4-3 to 4-4Synthetic fire-resistant fluids,Synthetic rubber hose, 5-8 to

T

T-seals, 7-3 to 7-5Temperature switches. 8-8

Valves—Continued3-5 to 3-6 directional control valves—Continued5-9 shuttle valve, 6-18

three-way valves, 6-19 to 6-20cam-operated three-way valves,

6-19 to 6-20pilot-operated three-way valves,

6-20Temperature-measuring instruments, 8-6 to two-way valves, 6-18 to 6-19

8-8 flow control valves, 6-1 to 6-6bimetallic expansion thermometer, 8-7distant-reading thermometers, 8-7 to 8-8

ball valves, 6-1 to 6-2

Threaded connectors, 5-12 gate valves, 6-3

Three-way valves, 6-19 to 6-20 globe valves, 6-3 to 6-5Tube bending, 5-5 to 5-7 hydraulic and pneumatic globeTube cutting and deburring, 5-4 to 5-5 valves, 6-5 to 6-6 -

Tube flaring, 5-7 to 5-8Turbines, 10-11 to 10-12Two-way valves, 6-18 to 6-19

U

U-cups and U-packings, 7-16

V

V-rings, 7-5 to 7-6Valves, 6-1 to 6-25

classifications, 6-1directional control valves, 6-15 to 6-25 pressure-controlled sequence

check valve, 6-16 to 6-18 valve, 6-11 to 6-12

classification, 6-15 to 6-16 Vane pump, 4-8poppet, 6-15 to 6-16 Vane-type motors, 10-9 to 10-11rotary spool, 6-16sliding spool, 6-16

four-way valves, 6-20 to 6-25poppet-type four-way valves, W

6-20 to 6-22rotary spool valve, 6-22 Water-based fire-resistant fluids, 3-6sliding spool valve, 6-22 to 6-25 Welded connectors, 5-12 to 5-13

needle valves, 6-5pressure control valves, 6-6 to 6-15

counterbalance valve, 6-14 to 6-15pressure regulators, 6-9 to 6-10pressure-reducing valves, 6-12 to 6-14

pilot-controlled pressure-reducingvalve, 6-13 to 6-14

spring-loaded reducer, 6-13relief valves, 6-6 to 6-9sequence valves, 6-11 to 6-12

mechanically operated sequencevalve, 6-12

INDEX-6

Assignment Questions

Information: The text pages that you are to study areprovided at the beginning of the assignment questions.

Assignment 1

Textbook Assignment: “Fluid Power,” chapter 1; “Forces in Liquids,” chapter 2;“Hydraulic Fluids, ” chapter 3, pages 3-1 through 3-6.

1-1.

1-2.

1-3.

Learning Objective: Recognizethe scope of the text and thebreadth of the topic, FluidPower, including pertinentdefinitions, applications andfundamental concepts.

The term “fluid power” includeshydraulics and pneumatics, and ispower that is applied throughliquids or gases pumped orcompressed to provide force andmotion to mechanisms.

1. True2. False

The purpose of your textbook,Fluid Power, is to provide youwith

1. a basic guide for use inmaintaining hydraulicequipment

2. a basic reference concerningfundamentals of fluid power

3. information on fluid powerapplication for specificequipment

4. a reference concerningadvanced concepts of fluidpower

Which of the following is afavorable characteristic of afluid power system?

1. Very large forces can becontrolled by much smallerones

2. Different parts of the systemcan be located at widelyseparated points

3. Motion can be transmittedwithout the slack inherent inthe use of solid machineparts

4. Each of the above

IN ANSWERING QUESTIONS 1-4 THROUGH 1-6,SELECT FROM COLUMN B THE SYSTEM THATMEETS THE PRESSURE AND CONTROLREQUIREMENTS LISTED IN COLUMN A.

A. Requirements B. Systems

1-4.

1-5.

1-6.

A medium amount 1.of pressure andfairly accurate 2.control

A medium amount 3.of pressure andmore accuratecontrol

A great amount ofpressure and/orextremely accuratecontrol

Hydraulic

Pneumatic

Combinationhydraulicandpneumatic

1-7. Which of the following is aspecial problem of fluid powersystems?

1. Loss in efficiency as theforce of the fluid isconveyed up and down oraround corners

2. Loss of force as the fluid istransmitted over considerabledistances

3. Leaks4. Each of the above

1-8. The study of hydraulics wasoriginally confined to the studyof the physical behavior of waterat rest and in motion. The term“hydraulics” now includes thephysical behavior of all

1. liquids2. gases3. liquids and gases4. liquids, gases, and solids

1

1-9. Pascal’s law pertains to the

1. construction of aqueducts2. use of water wheels for doing

work3. differences of floating and

submerged bodies4. transmission of force in

confined fluids

IN QUESTIONS 1-10 THROUGH 1-12, SELECTFROM COLUMN B THE TYPE OF POWER USED INEACH ITEM OF EQUIPMENT OR SYSTEM LISTEDIN COLUMN A.

_ EQUIPMENTA B. POWER TYPES

1-10. Dental Chair 1. Hydraulic

1-11. Anchor Windlass 2. Hydro-pneumatic

1-12. Service stationlift 3. Pneumatic

1-13.

1-14.

1-15.

Learning Objective: Identify thestates of matter and the factorsaffecting them.

All matter is classifiedaccording to its state as asolid, a liquid, or a gas.

1. True2. False

The critical factors affectingthe state of matter are

1.2.3.4.

temperature and weightpressure and densitydensity and specific gravitypressure and temperature

Learning Objective: Recognizethe pressure characteristics ofliquids, including how pressureis caused by the weight of theatmosphere, and identify howpressures are measured.

Pressure can be measured in termsof force per unit area.

1. True2. False

1-16. Mark each of the followingstatements, concerning the atmosphereand atmospheric pressure, true or false;then select the alternative below thatlists

1-17.

1-18.

1-19.

the statements that are true.

A. The troposphere is thatpart of the atmospheretouching the earth’s surface

B. The atmosphere has weight.c. Atmospheric pressure

decreases as altitudedecreases.

D. Atmospheric pressure at points below sea level is

less than at sea level.

1. A and B2. B and C3. C and D4. A, B, C, and D

The reference standard used as anindicator of atmospheric pressureis a column of mercury that atsea level is

1. 76 inches high at 0°C2. 76 centimeters high at 4°C3. 76 centimeters high at 0°C4. 29.92 inches high at 4°C

The side of a thin-walled chamberpartially evacuated of air is thesource of movement for the

1. hydrometer2. aneroid barometer3. mercury thermometer4. Fahrenheit thermometer

Learning Objective: Identifyterms and facts applicable to thephysics of fluids and use thesefacts with related formulas tosolve problems pertaining todensity and specific gravity.

In the metric system the densityof a substance is its weight in

1. grams per cubic foot2. pounds per cubic foot3. grams per cubic centimeter4. pounds per cubic centimeter

2

1-20. What change, if any, will occurin the volume and weight of asubstance if its temperaturechanges?

1. Both its volume and weightwill change

2. Both its volume and weightwill be unaffected

3. Its volume will change, butits weight will remainconstant

4. Its weight will change, butits volume will remainconstant

1-21. Which statement about specificgravity is false?

1. The density of a solid can bedetermined by multiplying itsspecific gravity times thedensity of water

2. Specific gravity can also bedescribed as specific weightor specific density

3. Specific gravity of asubstance should be measuredat a standardized temperatureand pressure

4. Specific gravity will varywith the size of the samplebeing tested

1-22. How can the specific gravity of aliquid or solid be expressed?

1. As a ratio between the weightof the substance and thedensity of a volume of water

2. As a ratio between the weightof the substance and theweight of an equal volume ofwater

3. As the number that shows thedensity of the substance inthe metric system

4. As in 2 and 3 above

1-23. What is the specific gravity of aliquid which weighs 44 pounds percubic foot at 4°C?

1. 0.4402. 0.6243. 0.7054. 0.789

1-24.

1-25.

1-26.

1-27.

1-28.

What is the density of a solidthat has a specific gravity of2.5?

1. 156 pounds per cubic foot2. 250 pounds per cubic foot3. 312 pounds per cubic foot4. 482 pounds per cubic foot

What is the specific gravity of asolid object which weighs 49.92pounds per cubic foot?

1. 0.7892. 0.83. 2.74. 0.9

A device used for measuring thespecific gravity of a liquid isknown as a

1. hydrography2. hydrometer3. hydrostat4. hydroscope

Learning Objective: Recognizethe principles and equationsinvolved with the transmission offorces, and solve relatedproblems.

The pressure of force exerted onthe end of a rigid metal bar isapplied equally and undiminishedto all surfaces of the bar.

1. True2. False

The head, or pressure due to theweight of a fluid, depends on thedensity of the fluid and the

1. area of the bottom surface ofthe container

2. total volume of the fluid3. vertical height of the fluid4. geometric shape of the

container

3

REFER TO FIGURE 2-11 OF YOUR TEXTBOOK INANSWERING QUESTIONS 1-29 AND 1-30, WHICHDEAL WITH THE MULTIPLICATION OF FORCESIN POWER SYSTEMS.

1-29. Assume that the input piston hasan area of 3 square inches with aforce of 45 pounds. What is thepressure in the system?

1. 5 psi2. 10 psi3. 15 psi4. 20 psi

1-30. Assume that the output piston hasa diameter of 6 inches and issubject to a pressure of 10pounds per square inch. What isthe force exerted on the outputpiston?

1. 28.26 pounds2. 31.4 pounds3. 282.6 pounds4. 314.0 pounds

Refer to figure 1A in answeringquestions 1-31 and 1-32. The ruleapplying to the action of the pistonstates that the force acting on thepiston surface area from chamber C isproportional to the pressure in chamberC times the area of the piston head.The force acting on the piston fromchamber D is proportional to thepressure in chamber D times theeffective area of the piston head (whichis the cross-sectional area of thepiston minus the cross-sectional area ofthe piston shaft.) The piston surfacein chamber C is 25 square inches, andthe effective area in chamber D is 20square inches.

1-31. The pressure in line A is 200psi. No force is exerted onshaft S. How much pressure willbe required in line B to preventthe piston from moving?

1. 160 psi2. 200 psi3. 250 psi4. 500 psi

1-32. Lines A and B are pressurized to50 psi. How much force isapplied to each surface and whichway will the piston move?

1. C = 1250 pounds, D = 1000pounds, piston will move tothe right

2. C = 1250 pounds, D = 1000pounds, piston will move tothe left

3. C = 1000 pounds, D = 1250pounds, piston will move tothe right

4. C = 1000 pounds, D = 1250pounds, piston will move tothe left

1-33. For two pistons in the same fluidpower system, the distances movedare inversely proportional to the

1. pressure of the fluid2. volume of fluid moved3. expansion of the fluid4. areas of the pistons

Learning Objective: Recognizethe characteristics and behaviorof fluids in motion, includingmethods for measuring volume andvelocity, and relate the dynamicand static factors involved withfluid flow.

1-34. In fluid power syetems usingliquids, the measurement of thevolume of fluid flow is made inunits of

1. cubic inches per minute2. gallons per minute3. cubic feet per minute4. cubic yards per minute

Figure 1A

4

1-35.

1-36.

1-37.

1-38.

1-39.

Water flows through a pipe of 5square-inch cross section at thevelocity of 3 feet per second(fps). At what velocity does itflow through a constriction inthe pipe with a cross section of3 square inches?

1. 1.8 fps2. 3.0 fps3. 3.6 fps4. 5.0 fps

Two pistons with different cross-sectional areas will travel atthe same speed as long as therate of fluid flow into theircylinders is identical.

1. True2. False

In streamline flow, each particleof fluid moves in what manner?

1. In uniform helical swirls2. In parallel layers3. At a velocity proportional to

the cross-sectional area ofthe pipe

4. At the same velocity in thecenter of the pipe as alongthe walls

Losses due to friction increasewith velocity at a higher rate inturbulent flow than in streamlineflow.

1. True2. False

What is inertia of fluids in apower system?

1. The resistance of the fluidto movement or change of rateof movement

2. The force required tomaintain the fluid atconstant velocity

3. The capacity to move andchange rate of flow

4. The force required toovercome friction

1-40. Neglecting friction, how muchforce is required to accelerate 3pounds of fluid from rest to avelocity of 322 feet per secondin 2 seconds?

1. 1.5 pounds2. 3.0 pounds3. 15 pounds4. 30 pounds

ANSWER QUESTIONS 1-41 THROUGH 1-45 ASTRUE OR FALSE BASED ON THE RELATIONSHIPOF FORCE, PRESSURE, AND HEAD.

1-41.

1-42.

1-43.

1-44.

1-45.

1-46.

Head is a statement of force perunit area.

1. True2. False

Velocity headenergy caused

1. True2. False

is the loss ofby inertia.

Gravity head depends on whichportions of the system areexposed to open air.

1. True2. False

Friction head cannot existwithout velocity head.

1. True2. False

There can be no static head ifthe fluid is in motion.

1. True2. False

Which factors affecting fluidaction are classified as staticfactors?

1. Applied forces, inertia, andfriction

2. Atmospheric pressure, appliedforces , and inertia

3. Gravity, applied forces, andfriction

4. Gravity, atmosphericpressure, and applied forces

5

1-47. Refer to figure 2-18 in yourtextbook. If this were apractical situation, the pressurein chamber A would be greaterthan that in chamber B by theamount of pressure required to

1. absorb inertia2. prevent the fluid from moving3. overcome friction4. raise the pressure at an

intermediate point

Learning Objective: Recognizesimilarities and differencesbetween pneumatic and hydraulicfluid power systems, and indicateoperating characteristics andcomponent functions of basicfluid power systems.

1-48. The similarity between hydraulicand pneumatic fluid power systemsis correctly indicated by whichof the following statements?

1. The basic components of thesystems are essentially thesame

2. Both systems depend uponinternal lubrication by thesystem fluid

3. Both 1 and 2 above correctlyindicate the similarity

4. The basic components of thesystems are identical andinterchangeable

1-49. Which component of a hydraulicfluid power system performs thesame function as the receiver ina pneumatic fluid power system?

1. Reservoir2. Compressor3. Actuator4. Selector valve

Learning Objective: Identify thecharacteristic of liquid thatmakes it desirable for use inhydraulic systems and propertiesand characteristics that must beconsidered in selecting ahydraulic liquid for a particularsystem, including related data.

1-50. Liquids rather than gases areused in hydraulic systems becauseliquids are

1. more compressible2. less compressible3. more expensive4. less corrosive to system

components

1-51. A liquid that is satisfactory foruse in a hydraulic systemprovides

1. a low viscosity index, goodsealing quality, andlubricity

2. a high viscosity index, goodsealing quality, and a lowflashpoint

3. good lubrication and sealingqualities, and a viscositythat does not result in anincrease in flow resistancein” system piping

4. good lubrication and aviscosity that decreases astemperature increases

1-52. The viscosity reading of a liquidis expressed as Saybolt universalseconds (SUS), which representsthe time, in seconds, it takesfor 60 cubic centimeters of theliquid at a specified temperatureto pass through an orifice ofgiven diameter.

1. True2. False

1-53. A low V.I. indicates that aliquid will

1. maintain a constant viscosityover a wide temperature range

2. vary greatly in viscositywith changes in temperature

3. vary only slightly inviscosity with changes intemperature

4. have a response totemperature changes very muchlike the response ofparaffinic oil

6

1-54. Which of the following statementsis NOT a true statement of fluidviscosity?

1. An ideal fluid viscosityremains constant throughouttemperature changes

2. The average hydraulic fluidhas a relatively lowviscosity

3. There is a large choice ofliquids available for theviscosity range required

4. Liquids derived from the samesource have equal resistanceto heat

1-55. The film strength and lubricatingqualities of a liquid aredirectly related to the liquid’sphysical properties.

1. True2. False

1-56. Which statement about a hydraulicliquid that is continuouslysubjected to high temperatureconditions is true?

1. It accumulates moisture2. It changes unfavorably in

composition3. Its life is unaffected by the

hours of use4. The carbon and sludge formed

in it are of little concernif the reservoir temperatureremains normal

IN QUESTIONS 1-57 THROUGH 1-59, SELECTFROM COLUMN B THE DEFINITION OF EACHPROPERTY OF LIQUIDS LISTED IN COLUMN A.

A. Properties B. Definitions

1-57. Fluidity 1.

1-58. Viscosity

1-59. Chemicalstability

2.

3.

4.

The internalresistancethat tendsto preventliquids from.flowing

The quality,state, ordegree ofliquidsbeingpoisonous

The physicalpropertythat enablesliquids toflow

The abilityof liquidsto resistoxidationanddeteriora-tion forlong periods

1-60. The desirable flashpoint of ahydraulic liquid is one whichprovides a

1. low degree of evaporation andgood resistance to combustion

2. high degree of evaporationand poor resistance tocombustion

3. low degree of evaporation andlow resistance to combustion

4. high degree of evaporationand high resistance tocombustion

1-61. Hydraulic liquid must possesswhich of the followingproperties?

1. Chemical stability andfreedom from acidity

2. Lubricating ability andproper viscosity

3. Minimum toxicity and highflashpoint

4. All of the above

7

1-62. Although manufacturers strive toproduce hydraulic liquids thatcontain no toxic chemicals, someliquids contain chemicals thatare harmful. How do thesepoisonous chemicals enter thebody?

1. Absorption through the skin2. Through the eyes or mouth3. Through inhalation4. All of the above

Learning Objective: Recognizevarious types of hydraulicliquids and their particularcharacteristics and uses.

1-63. The bases of the most commontypes of hydraulic liquids areclassified as

1. synthetic, water, orvegetable

2. water , petroleum, orsynthetic

3. water , petroleum, orvegetable

4. petroleum, vegetable, orsynthetic

1-64. What is the moat widely usedmedium for hydraulic systems?

1. Petroleum-based liquid2. Synthetic-based liquid3. Vegetable-based liquid4. Water-based liquid

1-65. Which of the following propertiesof a hydraulic liquid can beimproved by additives?

1. viscosity2. Chemical stability3. Lubricating power4. All of the above

1-66. The fluid currently being used ina hydraulic system that requiresa nonflammable liquid willprobably be a

1-67. Which of the following statementsis/are true concerning synthetic-based fluids?

1. They will not burn2. They are compatible with most

commonly used packing andgasket materials

3. They may contain toxicchemicals

4. All of the above

1-68. You have accidentally gotten asynthetic hydraulic fluid in youreyes . You should flush your eyesfor at LEAST 15 minutes and seekimmediate medical attention.

1. True2. False

1-69. You are required to dispose ofcontaminated synthetic fluidwhile deployed. HOW should youdispose of the fluid?

1. Pump it to the collecting,holding, and transfer (CHT)tank

2. Place it in drums fordisposal ashore

3. Pump it over the side4. Dilute it with soapy water

and pump it over the side

1-70. Water-based fluids’ resistance tofire depends on the vaporizationand smothering effect of steamgenerated from water.

1. True2. False

1. synthetic-based liquid2. blend of water and oil3. petroleum-based liquid4. blend of petroleum and

vegetable oil

8

Assignment 2

Textbook Assignment: “Hydraulic Fluids,” chapter 3, pages 3-6 through 3-11;“Pumps,” chapter 4; and “Fluid Lines and Fittings,” chapter 5,pages 5-1 through 5-11.

2 - 5. Compatibility of hydraulic liquidLearning Objective: Identifytypes, characteristic, origin,control, and checks for varioushydraulic system contaminants.

2-1. Trouble develops in a hydraulicsystem when the fluid becomescontaminated as the result of

1. system componentdeterioration

2. friction at hotspots3. abrasive wear4. any action that places

foreign matter in the fluid

2-2. By which of the following ways 2-6.may air enter into a hydraulicsystem?

1. Through improper maintenance2. Past leaky seals in gas-

pressurized accumulators3. Past actuator piston rod

seals 2-7.4. Each of the above

2-3. Water contamination of ahydraulic system is NOT a majorconcern since its presence aidsin reducing the flammability ofthe fluid.

1. True2. False 2-8.

2-4. Chemical contamination ofhydraulic liquid by oxidation isindicated when the liquidcontains which of the followingmaterials?

1. Sludge2. Asphaitine particles3. Organic acids

with the seals and hoses in asystem prevents which of thefollowing problems fromoccurring?

1. Gum formation around theseals and within the hoses

2. Deposits of contaminants onthe seals and within thehoses

3. Condensation of moisturewithin the system

4. Chemical reaction between theliquid acid the seal or hosematerial and consequentbreakdown of these parts

All of the following contaminantsare abrasive EXCEPT

1. lint2. rust3. sludge4. sand particles

Whenever drained or usedhydraulic fluid is returned to asystem, straining is necessaryonly if the cleanliness of thestorage container isquestionable.

1. True2. False

Which of the following agentsshould parts of a hydrauliccomponent be cleaned with priorto being assembled?

1. An approved dry-cleaningsolvent

2. Trichlorotrifluoroethane3. Chlorinated solvents4. Trichlorofluoromethane

4. Each cf the above

9

2-9. Which of the following agents, ifcombined with minute amounts ofwater found in operatinghydraulic systems, does NOTchange into hydrochloric acid?

1. An approved dry-cleaningsolvent

2. Trichlorotrifluoroethane3. Chlorinated solvents4. Trichlorofluoromethane

2-10. When you analyze operatinghydraulic fluids, changes inwhich of the following areas maybe of particular interest to you?

1. Chemical properties2. physical properties3. particulate contamination4. Any of the above

2-11. From which of the followinglocations can fluid samples betaken?

1. Filter bowls2. Tops of tanks3. Pipe drains after sufficient

fluid has drained4. Each of the above

Learning Objective: Indicatefunctions, operatingcharacteristics, and related datapertinent to hydraulic pumps.

2-12. Which of the following is thefunction of a hydraulic pump?

1. To provide flow to thehydraulic system

2. To create the pressurerequired in a hydraulicsystem

3. To control the pressurerequired in a hydraulicsystem

4. To compensate for atmosphericpressure at varying altitudes

2-13. If a hydraulic pump is locatedbelow the reservoir, fluid issupplied to its inlet port bywhich of the following forces?

1. Fluid head2. Gravity3. Atmospheric pressure4. A combination of all of the

above

2-14. The ratings of most hydraulicpumps are determined by their

1. efficiency2. output per unit time3. volumetric output at a given

pressure4. amount of internal slippage

2-15. Pump performance can be expressedin which of the following terms?

1. Gallons per minute2. Cubic inches per revolution3. Both 1 and 2 above4. Cubic feet per minute

2-16. In contrast to a nonpositive-displacement pump that canoperate with its discharge outletcompletely restricted, apositive-displacement pump cannotdo so and must be used with apressure regulator.

1. True2. False

Learning Objective: Identifyoperating principles andconstruction features of rotarypumps

2-17. Slippage is the term given to theamount of fluid that can returnfrom the discharge side to thesuction side of a rotary pumpthrough the space or clearancesbetween the stationary and movingparts.

1. True2. False

2-18. Which of the following isgenerally the basis for rotarypump classification?

1. Type of drive2. Shaft position3. Service application4. Type of rotating element

2-19. What type of gears is illustratedin figure 4-1 of your textbook?

1. Spur2. Helical3. Crescent4. Herringbone

10

2-20.

2-21.

2-22.

2-23.

Which type of gear-type rotarypumps discharges the smoothestfluid flow?

1. Spur2. Helical3. Herringbone4. Crescent

Why are helical gear pumpsclassified as external gearpumps?

1. Both sets of teeth projectinward toward the center ofthe gears

2. Both sets of teeth projectoutward from the center ofthe gears

3. The teeth of the interiorgear project inward towardthe center of the gears, andthe teeth of the exteriorgear project outward from thecenter of the gears

4. The teeth of the interiorgear project outward from thecenter of the gears, and theteeth of the exterior gearproject inward toward thecenter of the gears

Refer to figure 4-2, view B, inyour textbook, What determinesthe volume delivery of this pump?

1. The size of the crescent2. The size of the internal gear3. The speed of rotation of the

crescent4. The speed of rotation of the

drive gear

Refer to figure 4-7 In yourtextbook. The vanes of the lobepump are used for which of thefollowing purposes?

1. To reduce wear of the pumpcaused by surface to surfacecontact

2. To provide a good sealbetween the lobes and thepoint of lobe junction in thecenter of the pump

3. To provide a good sealbetween the lobes and thechamber

4. To do both 2 and 3 above

2-24.

2-25.

The pump illustrated In figure4.9 of your textbook isdesignated as unbalanced becausethe pumping action is done by oneside of the shaft and rotor.

1. True2. False

Which, if any, of the followingstatements is true of a screwpump ?

1. Its performance is based onthe fluid’ s viscosity

2. It is very efficient3. The idler rotors are

connected by gears4. None of the above

Learning Objective: Recognizefunctions, principles ofoperation, and constructionfeatures of various types ofreciprocating pumps.

REFER TO FIGURE 4-10 IN YOUR,TEXTBOOK INANSWERING QUESTIONS 2-26 AND 2-27.

2-26. This type of pump is used in someaircraft hydraulic systems toprovide a source of hydraulicpower for what purpose(s)?

1. Emergencies2. Testing certain subsystems

during preventive maintenance3. Determining the causes of

malfunctions in certainsubsystems

4. All of the above

11

2-27. Why is liquid discharged throughthe outlet port when the pistonis moved to the right?

1. The piston rod makes theinlet chamber smaller thanthe outlet chamber

2. Check valve B opens,admitting liquid to the inletport and outlet port throughcheck valve A

3. Check valve A opens, causingthe liquid confined in theinlet chamber to flow to thesmaller outlet chamber andout the outlet port

4. Check valve A closes, causingthe liquid confined in theinlet chamber to flow to theoutlet chamber and out theoutlet port

REFER TO FIGURE 4-11 IN YOUR TEXTBOOK INANSWERING QUESTIONS 2-28 THROUGH 2-30.

2-28. Which of the following componentswill revolve during the operationof this pump?

1. Cylinder block2. Slide block3. Both 1 and 2 above4. Pintle

2-29. The pumping action of this pumpis obtained by which of thefollowing actions?

1. Rotating the pintle at thecenter of the cylinder block

2. Moving the cylinder block offcenter from the axis of thepintle

3. Positioning the sliding blockto provide unequal travel ofthe pistons in the cylinderblock

4. Moving the rotor and reactionring to provide unequalpiston travel radially aroundthe cylinder block

2-30. In which of the following pistonpositions will the cylinder havetaken on a full charge of liquid?

1. Position 1, view D2. Position 2, view A3. Position 3, view C4. Position 4, view B

2-31. Pulsations of fluid flow from aradial-piston pump are muchgreater if the pump has an evennumber of pistons than if it hasan odd number.

1. True2 . False

2-32. Which of the following componentsof a radial-piston pump isconnected to the cylinder block?

1. Rotor2. Pintle3. Piston4. Drive shaft

REFER TO FIGURE 4-15 IN YOUR TEXTBOOK INANSWERING QUESTIONS 2-33 AND 2-34,

2-33. The rocker arm will beperpendicular to the shaft whenthe shaft has been rotated howfar?

1. One-quarter of a turn only2. One-half of a turn 3. Three-quarters of a turn only4. Either one-quarter or three-

quarters of a turn

2-34. Starting from the position of theshaft as indicated in figure4-15, view G, how many times willrod A be pushed out and pulled inthrough the wheel during eachshaft revolution?

1. Once2. Twice3. Four times4. Eight times

2-35. The output of the axial-pistonpump is determined by which ofthe following factors?

1. Number of pistons2. Length of the piston rods3. Length of the drive shaft4. Angle given to the tilting

plane

2-36. What component of a Stratopowerpump holds the pistons inconstant contact with themechanical drive mechanism?

1. Wobble plate2. Creep plate3. Check spring4. Piston return spring

12

2-37. Automatic variation of the volumeoutput of a variable-displacementStratopower pump is controlled bywhich of the following factors?

1. Atmospheric pressure2. Reciprocating action of the

pistons3. The position of the rocker

arm on the shaft4. The pressure in the hydraulic

system

2-38. During nonflow operation of avariable-displacement Stratopowerpump, what provides itslubrication?

1. Compensator spring2. Compensator piston3. Bypass system4. Drive cam

Learning Objective: Indicatebasic requirements for fluidpower system lines andconnectors, and recognizepertinent facts concerningidentification, sizing, uses, andconstruction of pipe and tubing.

2-39. You must consider which of thefollowing factors when selectingthe types of fluid lines for aparticular fluid power system?

1. The required pressure of thesystem

2. The type of fluid medium3. The location of the system4. All of the above

2-40. You must give primaryconsideration to all but which ofthe following factors inselecting the lines for aparticular fluid power system?

1. The type of material2. The material’s wall thickness3. The material’s inside

diameter4. The material’s outside

diameter

2-41. Replacement of a piece of tubingwith one having a smaller insidediameter will result in which ofthe following conditions?

1. Fluid heating2. Turbulent fluid flow3. System power loss4. All of the above

2-42. Which, if any, of the followingstatements is true for pipes ofthe same nominal size?

1. As the pipe schedule sizeincreases, the ID remains thesame and the wall thicknessand OD increase

2. As the pipe schedule sizeincreases, the ID increases ,the wall thickness decreases,and the OD remains the same

3. As the pipe schedule sizeincreases, the ID decreases,the wall thickness increases,and the OD remains the same

4. None of the above

REFER TO TABLE 5-1 IN YOUR TEXTBOOK INANSWERING QUESTIONS 2-43 AND 2-44.

2-43. The nominal size of pipe whoseoutside diameter is 1.900 inchesis

1. 1 1/22. 1 3/43. 24. 2 1/4

2-44. What is the schedule 40 wallthickness of pipe with a nominalpipe size of 2 inches?

1. 0.154 In.2. 0.218 in.3. 0.308 in.4. 0.436 in.

2-45. What is the size of No. 4 rigidtubing , and where is themeasurement taken?

1. 0.004 inch, wall thickness2. 0.040 inch, wall thickness3. 4/16 inch, inside diameter4. 1/4 inch, outside diameter

13

2-46. Which statement about therelative bursting pressure forvarious sizes of tubing made ofthe same material is true?

1. It is different for each wallthickness regardless of size

2. It is the same for all sizeshaving the same wallthickness

3. It is lower for small tubingthan for larger tubing of thesame wall thickness

4. It is higher for small tubingthan for larger tubing of thesame wall thickness

2-47. Which of the following metals maybe used to provide a strong,inexpensive pipe or tubingcapable of withstanding highpressures and temperatures?

1. Steel2. Copper3. Stainless steel4. Aluminum

2-48. Which of the following basicrequirements must be consideredin designing the lines andconnectors of a fluid powersystem?

1. Inside surfaces that do notcreate turbulent fluid flow

2. Sizes sufficient to deliveradequate quantities of fluidto all components

3. Strength to withstandpressure surges that exceedthe system’ s working pressure

4. All of the above

2-49. Bends in piping serve to absorbvibration and to compensate forthermal expansion andcontraction.

1. True2. False

2-50. The determining factor for theradius of the bend to be made ina pipe is the pipe’s

1. length2. wall thickness3. inside diameter4. outside diameter

2-51. Coarse-toothed hacksaw blades arepreferred for cutting tubingbecause they cut faster and areless liable to choke up with thechips.

1. True2. False

2-52. Which of the following proceduresshould you follow when cutting atube with a tube cutter?

1. Apply continual lightpressure to the cutting wheel

2. Remove all burrs on theinside and outside of thetube

3. Remove all foreign particlesfrom the tube

4. All of the above

2-53. Which of the following statementsis NOT correct for cutting tubingwith a hacksaw?

1. A fine-tooth hacksaw of 48teeth per inch could be used

2. When you clamp the tubing ina vice, tighten the viceuntil the tubing is juststarting to hold withoutcollapsing

3. All hacksaw marks must beremoved by filing

2-54. What parts of the hand tubebender are used to obtain thecorrect bend radius and thedesired bend angle on tubing?

1. The clip and the slide bar2. The radius block and the

slide bar3. The radius block and the clip4. The forming bar and the slide

bar

2-55. Which of the following statementsis NOT true concerning theflaring of a tube?

1. The flare must be largeenough to seat properlyagainst the fitting

2. The correct diameter of theflare is obtained by ensuringthat the tube is flush withthe top face of the die block

3. The flare must be smallenough to allow the threadsof the flare nut to slideover it

14

Learning Objective: Recognizecharacteristics, uses,construction features, andinstallation procedures offlexible hose.

2-56. Flexible hose should be used inlocations where it will besubjected to

1. intense heat2. severe vibration3. excessive abrasion4. an oily environment

2-57. Which of the followinginformation is found along thelayline of synthetic rubber hoseshaving a rubber cover?

1. Hose size2. Cure date3. Federal supply code4. All of the above

2-58. The size of flexible hose isdesignated In what incrementsmeasured at what place?

1. Thousandths of an inch,”outside diameter,

2. Thousandths of an inch,inside diameter

3. Sixteenths-inch, outside,diameter

4. Sixteenths-inch, Insidediameter

2-59. The flexible hose that is inertto all fluids presently used andthat does not absorb water iscomposed of what material?

1. PTFE2. Natural rubber3. Synthetic rubber4. Rubber impregnated cotton or

nylon

2-60. You have completed fabrication ofa flexible hose assembly. Which,if any, of the following stepsmust you NOT perform?

1. Proof test the assembly2. Ensure that the hose is

compatible with system fluid3. Flush and dry the hose and

cap its ends4. None of the above

2-61. Mark each of the followingstatements about the correctinstallation and use of flexiblehose as true or false, thenselect the alternative below thatlists the true statements.

A. Sharp bends may reduce thebursting pressure of thehose

B. Supports are never requiredwhen the hose is used.

C. The hose should be stretchedtightly between connecetions.

D. The hose should be wrappedwhere necessary forprotection against chafing.

1. A and D2. A and C3. B and D4. B and C

2-62. A characteristic of flexible hoseis that under pressure it will

1.

2.

3.

4.

expand in both diameter andlengthretain its manufactureddimensionsexpand in diameter andcontract in lengthcontract in diameter andexpand in length

15

Assignment 3

Textbook Assignment: “Fluid Lines and Fittings,” chapter 5, pages 5-11 through5-21; “Valves,” chapter 6; and Sealing Devices andMaterials,” chapter 7.

Learning Objective: Recognizeuses, construction features,operational characteristics andprocedures, functions, andprecautionary measures associatedwith fluid power systemconnectors.

QUESTIONS 3-1 THROUGH 3-4 CONCERN THEUSE OF THREADED CONNECTORS IN FLUIDPOWER CIRCULATORY SYSTEMS.

3-1.

3-2.

3-3.

3-4.

The threads of newly threadedpipe do not corrode if thefittings cover all of the exposedthreading.

1. True2. False

Pipe compounds prevent corrosionand assist in the disassembly ofthreaded joints.

1. True2. False

Excess pipe compound that mayooze inside lines does notpresent problems if the compoundis compatible with the fluid inthe system.

1. True2. False

The use of threaded connectors isgenerally limited to low-pressuresystems.

1. True2. False

IN ANSWERING QUESTIONS 3-5 THROUGH 3-7,SELECT FROM COLUMN B THE TYPE OFCONNECTOR TO WHICH EACH STATEMENT INCOLUMN A APPLIES. NOT EVERY CONNECTORIN COLUMN B IS USED.

3-5.

3-6.

3-7.

A. STATEMENTS B. CONNECTORS

This connector is 1. Brazedattached to thepiping by welding, 2. Flaredbrazing, taperedthread , or rolling 3. Weldedand bending4. Flange

This connectorconnects sub-assemblies in somefluid power systems,especially in high--pressure systems thatuse pipe for the fluidlines

This connector iscommonly used forjoining nonferrouspiping in the pressureand temperature rangewhere its use ispractical

3-8. The fitting of a flared connectorshould be made of material havinggreater strength than that of itssleeve and nut and of the piping.

1. True2. False

16

3-9. A universal fitting is one thatcan be

1. positioned to the anglerequired for the installation

2. adapted to operate with anysize tubing

3. positioned to any angle inany plane

4. routed through a bulkhead

IN QUESTIONS 3-10 THROUGH 3-13, SELECTFROM COLUMN B THE CONNECTOR TO WHICHEACH STATEMENT CONCERNING TIGHTENINGDATA IN COLUMN A APPLIES.

A. TIGHTENING DATA

3-10. This connectortightened 1/6turn past thespecified torque

3-11. This connectormay not betightened pastthe specifiedtorque

3-12. This connectormust be presetprior to beingtightened

3-13. This connectormust be turnedwith a wrench1/6 turn pasthandtight

3-14.

3-15.

B. CONNECTORS

1. Flarelesstype

2. Aluminumalloyflared type

3. Steelflared type

Quick-disconnect couplings areprovided with an automaticshutoff feature which preventsloss of fluid from the system orentrance of foreign matter intothe system when they aredisconnected.

1. True2. False

Manifolds are used in thepressure supply and/or returnlines of fluid power systems toperform which of the followingfunctions?

1. Conserve space2. Reduce joints3. Eliminate piping4. All of the above

3-16. In long pieces of tubing orpieces bent to a complex shape,rust and scale can be removed bywhat process?

1. Degaussing2. Pickling3. Scraping4. Sandblasting

Learning Objective: Identifyfunctions of valves in a fluidpower system; also recognizefunctions, operatingcharacteristics, and constructionfeatures of various types of flowcontrol valves.

3-17. Valves are used to control whichof the following in fluid powersystems?

1. Direction of fluid flow2. Fluid pressure3. Fluid flow4. All of the above

IN ANSWERING QUESTIONS 3-18 THROUGH3-20, SELECT FROM COLUMN B THE TYPE OFFLOW CONTROL VALVE MOST CLOSELYIDENTIFIED WITH EACH STATEMENT IN COLUMNA.

3-18.

3-19.

3-20.

A. STATEMENTS

Its flow is con-trolled byraising or low-ering discs orwedges

Flow or no-flowthrough it iscontrolled byturning the valveshaft one-quarterturn

Certain types areused as variablerestrictors

B. TYPES

1. Ball

2. Gate

3. Globe

4. Needle

3-21. Gate valves are suitable for useas throttling valves because theyclose in small increments.

1. True2. False

17

3-22.

3-23.

3-24.

3-25.

3-26.

The globe valve gets its namefrom the globular shape of itsbody, a shape that is unique tothis valve.

1. True2. False

Approximately how far must thehandwheel of a globe valve beturned toward the closed positionafter the valve has been fullyopened?

1. 1/4 turn2. 1/2 turn3. 3/4 turn4. 7/8 turn

What type of flow control valvemakes the most suitable throttlevalve?

1. Gate2. Plug3. Globe4. Needle

Learning Objective: Relate theoperation, functions,requirements, and constructioncharacteristics of pressurecontrol devices to fluid powersystems.

Relief valves are used for whichof the following functions?

1. To maintain pressures above apredetermined level

2. To maintain fluid flow belowa predetermined rate

3. To prevent pressure fromrising above a predeterminedlevel

4. To prevent thermal expansionof the fluids

If a fluid power system uses twoor more relief valves, they mustall be the same size.

1. True2. False

3-27. Chatter in a relief valve is theresult of

1. rapid opening and closing ofthe valve as it ‘hunts - aboveand below a set pressure

2. too much difference betweenopening and closing pressuresof the valve

3. concurrent operation of thesmall relief valve and themain relief valve

4. improper seating of the valveelement

REFER TO FIGURE 6-13 IN YOUR TEXTBOOK INANSWERING QUESTIONS 3-28 AND 3-29CONCERNING THE OPERATION OF A COMPOUNDRELIEF VALVE.

3-28. When the system pressureincreases above the pressure towhich the valve is set, the mainvalve opens

1. independently of the pilotvalve

2. only after the systempressure increases to morethan can be relieved by thepilot valve

3. concurrently with the pilotvalve

4. every time the pilot valveopens but at a predeterminedtime interval afterward

3-29. After the main valve has relievedthe system and when pressurereturns to normal, what doespilot valve do?

1.

2.

3.

4.

It remains open until afterthe main valve closesIt closes simultaneously the main valveIt closes first and allowspressure to equalize aboveand below the main pistonIt closes first and causespressure above the mainpiston to force the mainvalve closed

the

with

18

3-30. A hydraulic pressure regulatordoes which of the following?

1. Maintains the system pressurebetween two predeterminedlevels

2. Regulates the quantity offluid flow in the system

3. Maintains the system pressureabove a predeterminedpressure level

4. Maintains the system pressurebelow a predeterminedpressure level

3-31. Chatter of a pressure regulatormay be prevented by

1. using a constant displacementpump

2. installing a snubber in thefluid supply line

3. maintaining a very smalldifferential pressure

4. making cutout (closing)pressure higher than cutin(opening) pressure

REFER TO FIGURE 6-14 IN YOUR TEXTBOOK INANSWERING QUESTIONS 3-32 AND 3-33.

3-32. What is the operational state ofthe regulator when the systempressure is less than thatrequired to operate one of theactivating units in the system?

1. The pilot valve is seated,the check valve is unseated,and fluid is flowing into thesystem

2. The pilot valve is unseated,the check valve is seated,and fluid is flowing into thesystem

3. The check valve is unseated,the pilot valve is seated,and fluid is flowing into thereturn line

4. The check valve is unseated,the pilot valve is unseated,and fluid is flowing into thesystem and into the returnline

3-33.

3-34.

3-35.

For the pressure-controlledsequence valve to operateproperly, the tension of thespring must be sufficient to holdthe piston in the closed positionagainst pressure required tooperate the primary unit.

1. True2. False

Refer to figure 6–17 in yourtextbook. Under what conditiondoes the valve operate as aconventional check valve?

1. Any time pressure in port Ais greater than the pressurein port B

2. Any time the pressures inport A and port B are equal

3. Only when the plunger isdepressed

4. Only when the plunger isreleased

Refer to figure 6-18 in yourtextbook. The valve decreasesfluid flow when which of thefollowing conditions exist(s)?

1. The pressure in the outletport exceeds the adjustingspring pressure

2. The pressure in the inletport exceeds the pressuredesired in the outlet port

3. The pressure on the valvediaphragm moves the valvestem up to close the valve

4. All of the above

REFER TO FIGURE 6-19 IN YOUR TEXTBOOK INANSWERING QUESTIONS 3-36 AND 3-37.

3-36. If the input pressure of theinlet port is less than thesetting of the pressure reducingvalve, what should be therespective positions of thepoppet valve and the spool valve?

1. Open, open2. Open, closed3. Closed, closed4. Closed, open

3-37. A restriction in the drain wouldcause the outlet port pressure to

1. pulsate2. increase3. decrease4. remain the same

19

3-38. The following statements concernthe operation of thecounterbalance valve shown infigure 6-20 of your textbook.Mark each statement true offalse, then select thealternative below that liststhose that are true.

A. The main valve has equalsurfaces which are the inner

areas of the spool. The activation of the valveB.

results from the appliedpressure opening the checkvalve, allowing the fluid tobypass the main valve.

C. Reverse action of the valve is controlled by the

pressure required toovercome the spring tensionof a check valve.

D. The weight supported by thevalve depends upon thespring tension on the spool.

1. A, B, C2. A, C, D3. B, C, D4. A, D

Learning Objective: Recognizeconstruction features, operatingcharacteristics, and uses ofvarious types of directionalcontrol valves.

3-39. A poppet is used as the valvingelement for which of thefollowing fluid power valveapplications?

1. Flow control2. Pressure control3. Directional control4. All of the above

3-40. What type of valving element ismost commonly used in directionalcontrol applications?

3-41. Check valves usually contain whattypes of valving elements?

1. Ball and cone2. Ball and poppet3. Sleeve and poppet4. Rotary spool and sliding

spool

3-42. What type of check valve permitsfree flow of fluid in onedirection and a limited flow offluid in the opposite direction?

1. Orifice2. Vertical3. Swing4. Ball

3-43. Refer to figure 6-25 in yourtextbook. Force caused by whichof the following plays no part inthe opening and closing of thisvalve?

1. Gravity2. Spring action3. Backflow of fluid4. Forward flow of fluid

3-44. Refer to figure 6-27 in yourtextbook. If normal system inletpressure is lost, when thealternate system is activated,its pressure will cause theshuttle to move sufficiently to

1. close the outlet port toprevent reverse flow from theoutlet port to the normalsystem inlet

2. close the outlet port andconnect the normal systeminlet to the alternate systeminlet

3. apply the alternate systempressure to both the outletport and the normal system

4. close the normal system inletto prevent loss of alternatesystem pressure

1. Ball2. Poppet3. Rotary spool4. Sliding spool

20

3-45. Refer to figure 6-28 in yourtextbook. Which statementrelative to the operation of thisvalve is false?

1. The upper poppet iscontrolled by the inside cam

2. Fluid flow to the return lineis controlled by the lowerpoppet

3. Fluid flow from the pressureline is controlled by theupper poppet

4. The lower poppet is unseatedby the outside cam to allowthe fluid to flow into thecylinder and actuate thepiston

3-46. When the pilot chamber of thethree-way, poppet-type, normallyclosed directional control valveis pressurized, fluid flows fromthe actuating cylinder throughthe valve and out the exhaustport .

1. True2. False

3-47. Which four-way valves areactuated by cams?

1. Rotary spool2. Poppet3. Sliding spool4. All of the above

3-48. Which type of valve is consideredmost trouble free of all four-wayvalves?

1. Poppet2. Rotary spool3. Sliding spool4. Cam operated

3-49. Which of the following representsthe flow of fluid as illustratedin figure 6-34, view B in yourtextbook?

1.

2.

3.

4.

Learning Objective: Recognizerequired characteristics,functions, types, and materialsof sealing devices used in fluidpower systems.

3-50. Suitable packing devices forfluid power systems are made frommaterials that possess which ofthe following characteristics?

1. Compatibility with fluidsused in the systems

2. Effective sealing ability3. Durability4. All of the above

3-51. The term “sealing devices” is aclassification applicable topacking materials used to providean effective seal between whichof the following parts?

1. Two moving parts2. Two stationary parts3. A moving part and a

stationary part4. All of the above parts

combinations

3-52. No internal leakage should beallowed to occur within ahydraulic power system because ofthe resulting loss in systemefficiency.

1. True2. False

3-53. Which of the following factorsis/are used in determining thematerial used as a sealing devicefor a particular application?

1. Location of the seal2. Storage of the seal3. Both 1 and 2 above4. Type of motion

3-54. Cork is suitable for use asgaskets because of which of thefollowing characteristics?

1. Its resiliency2. Its flexibility3. Its compressibility4. All of the above

21

3-55. You are reassembling a vitalcomponent which uses a coppersealing ring and discover thereis not a new replacement ring.Which, if any, of the followingsteps should you take?

1. Reinstall the old ring afterinspecting it for damage

2. Install an O-ring that iscompatible with the fluidused in the system

3. Reinstall the old ring afterit has been annealed

4. None of the above

3-56. Although it has many of thecharacteristics required in aneffective seal , which of thefollowing materials is not usedas packing material in a systemin which petroleum-base fluid isused?

1. Cork2. Asbestos3. Natural rubber4. Synthetic rubber

3-59.

3-60.

3-61.

3-62.

Learning Objective: Recognizefunctions, identificationprocedures, inspection andinstallation techniques, andcharacteristics of various typesof seals.

3-63.

3-57. Which of the following statementsis NOT true of T-seals?

1. T-seals provide a positiveseal at low pressure

2. There is no military standardpart numbering system toidentify T-seals

3. The dash (-) numbers used toidentify the size of T-sealsare part of a preliminarynumbering system

4. The Navy has created anumbering system to identifyT-seals for hydraulicactuators

3-50. To obtain the correct squeeze orclearance on V-ring packing,shims or spacers are used toadjust the packing gland depth.

1. True2. False

3-64.

Regardless of its condition, anO-ring must be discarded if itcannot be positively identified.

1. True2. False

Which of the following items canbe used to identify replacementO-rings?

1. Allowance parts lists (APLs)2. Technical manuals3. System drawings4. All of the above

What is the basis for computingthe age of an O-ring’?

1. Service life2. The cure date3. Replacement schedule4. Operational conditions

What is the expiration date of anO-ring which was cured on 13 July1990 and has a 4-year shelf life?

1. 30 September 19942. 31 August 19943. 31 July 19944. 13 July 1994

Which of the following materialsshould NOT be used to fabricatetools for use in removing andinstalling O-ring and backuprings?

1. Wood2. Steel3. Brass4. Phenolic rod

Why are O-rings sometimes rolledon a cone or dowel?

1. To expose the manufacturer’sidentification code

2. To expose and stretch theinner diameter surface forinspection

3. To determine their breakingpoint

4. To condition them beforeinstallation

22

3-65. What is the first step inreplacing an O-ring in adisassembled fluid power systemcomponent?

1. Identify the ring’s size andmaterial

2. Inspect the ring for cuts,nicks, and flaws

3. Install felt washers on bothsides of the ring

4. Lubricate the O-ring grooveand all surfaces over whichthe ring must slide

3-66. What is/are used when the O-ringinstallation requires spanning orinserting through sharp threadedareas, ridges, slots, and edges?

1. O-ring expanders2. O-ring entering sleeves3. A rolling motion of the

O-ring4. A light coating of the

threads with MIL-S-8802

3-67. What device is used to prevent

3-68.

3-69.

3-70.

O-ring seal extrusion underpressure?

1. Backup ring2. Cup packing3. Flange packing4. Gasket

Backup rings made from which ofthe following materials are themost widely used?

1. Cork2. Leather3. Tetrafluorethylene (TFE)4. Bakelite

What is the age of deteriorationof TFE backup rings?

1. 1 year2. 3 years3. 5 years4. TFE does not deteriorate

When the packing in a fluid powersystem component is beingreplaced, the backup washersshould be inspected for which ofthe following conditions?

1. Fray2. cuts3. Evidence of compression

damage4. All of the above

3-71. Which of the following statementsabout a Quad-Ring is false?

1. It can be used at extremelyhigh pressures

2. It provides a seal in onlyone direction

3. It eliminates the spiraltwist sometimes encounteredwith O-rings

4. It can be used as a staticseal as well as a packing forreciprocating or rotarymotion

3-72. Which of the following statementsis incorrect concerning U-cupsand U-packings?

1. They are usually made ofdifferent materials

2. They both seal on the OD andthe ID

3. They are interchangeable4. They have cross sections

resembling the letter U

3-73. What type of seal is leastdesirable and is used only wherethere is not sufficient space fora U-ring packing or a V-ringpacking?

1. Cup2. Flange3. O-ring4. Quad-Ring

3-74. How are O-rings stored?

1. They are hung from pegs2. They are kept under tension3. They are kept in their

original envelopes4. They are kept in a light,

moist atmosphere with astrong draft

3-75. A torn O-ring package is properlysecured with which of thefollowing materials?

1. Staples2. Moistureproof glue3. Outer covering of

moistureproof paper4. Pressure-sensitive,

moistureproof tape

23

Assignment 4

Textbook Assignment: “Measurement and Pressure Control Devices,” Chapter 8:“Reservoirs, Strainers, Filters, and Accumulators,” chapter 9;and “Actuators,” chapter 10.

4-1.

4-2.

Learning Objective: Recognizethe construction, operationalcharacteristics, and uses ofdifferent types of fluid pressureindicators, thermometers, andcontrol switches.

The pressure sensing elements ofBourdon-tube gauges are commonlymade in which of the followingshapes?

1. The letter C2. Helical3. Spiral4. All of the above

Which, if any, of the followingstatements correctly explains theaction of a C-shaped Bourdontube?

1.

2.

3.

4.

Centrifugal force of fluidflowing through the curvedtube causes it to straightenoutPressure applied to the tubecauses its cross section tobecome more circular, causingIt to straighten outPressure applied to the tubecauses its cross section tobecome more circular, causingit to contractNone of the above

4-3. A duplex Bourdon gauge iscomposed of

1. one indicator dependent uponboth of two separatemechanisms

2. two separate and independentmechanisms and indicators

3. one mechanism with oneindicator showing currentpressure and a secondindicator showing the maximumpressure reached

4. one mechanism with oneindicator showing pressure inpounds per square inch (psi)and a second indicatorshowing the load on a ram intons

4-4. A Bourdon-tube differentialpressure gauge is composed of

1. one indicator dependent uponboth of two separatemechanisms

2. two separate and independentmechanisms and indicators

3. one mechanism with oneindicator showing currentpressure and the secondindicator showing the maximumpressure reached

4. one mechanism with oneindicator which can registerpressure either above orbelow atmospheric pressure

4-5. Which of the following gauges canbe used to measure thedifferential pressure across astrainer?

1. Duplex gauge2. Differential pressure gauge3. Both 1 and 2 above4. Compound gauge

24

4-6. Which of the following statementsdescribes hydraulic pressuregauges?

1. The tube is designed forhydraulic fluids only

2. The gauge is designed tooperate at higher pressures

3. Some gauges are designed witha special type ofspring-loaded linkage toprevent damage

4. All of the above

4-7. Gauges having bellows elementsare used only for pressureindicating.

1. True2. False

4-8. Which of the following is NOT afunction of pressure switches?

1. Indicating pressure2. Energizing an auxiliary

control system3. De-energizing an auxiliary

control system4. Signaling a visual warning or

audible alarm when a presetpressure is reached

4-9. The pressure switch sensingelement operates on the sameprinciple as the Bourdon-tubepressure gauge.

1. True2. False

4-10. A change in which of thefollowing properties is the basisof operation of the bimetallicthermometer?

1. Chemical2. Electrical3. Physical4. All of the above

4-11. What is the maximum length, infeet, of the capillary tube ofdistant-reading thermometers?

4-12. Distant-reading thermometersoperate similarly to Bourdon-tube pressure gauges.

1. True2. False

4-13. In the operation of pressuregauges within a hydraulic system,what does a gauge snubber do?

1. Dampens out system pressuresurges and oscillations tothe gauge, thereby preventinginternal damage

2. Prevents hydraulic pressureindicators from oscillating,thereby ensuring an accuratesystem pressure reading

3. Both 1 and 2 above4. Meters the flow of

pressurized hydraulic fluidfrom the gauge ortransmitter, therebypreventing internal damage

Learning Objective: Recognizefunctions, operating requirementsand characteristics, andconstruction features ofhydraulic reservoirs and thefunctions of related components.

4-14. The reservoir serves the primaryfunction of storing the hydraulicfluid required by the system,Which of the following secondaryfunctions does it also serve?

1. Separates air from the system2. Dissipates heat3. Traps foreign matter4. All of the above

4-15. The baffles In a reservoir servewhich of the following functions?

1. Dissipate heat2. Trap foreign matter3. Separate air form the system4. All of the above

1. 502. 753. 1004. 125

25

4-16. Which of the following factorsmust be considered In determiningthe reservoir capacity of ahydraulic system?

1. The thermal expansion of thefluid

2. Whether the system is fixedor mobile

3. The volume of fluid requiredby the system

4. All of the above

4-17. Why must the reservoir of anaircraft designed for high-altitude operations bepressurized?

1. To maintain a net positivesuction head to the pump

2. To use atmospheric pressureto assist fluid flow

3. To prevent the fluid fromcongealing at high altitudes

4. To vent the system duringperiods of high fluid demand

4-18. A pressurized reservoir may beInstead at a level below the pumpsuction and still maintain apositive flow of fluid to thepump.

1. True2. False

Learning Objective: Identifyoperating principles andapplications of accumulators.

4-19. Hydraulic systems are equippedwith one or more accumulatorsthat serve to perform which ofthe following functions?

1. To provide pressure foremergency operation of thesystem in the event of systemfailure

2. To act as a buffer and absorbsurges and shock pressuresthat might damage pipes andother components of thesystem

3. To equalize and readjust forany pressure losses in thesystem due to small leaks andthermal reaction of the fluid

4. All of the above

4-20. Which of the following statementsbest describe(s) the advantage avented tailrod accumulator hasover a floating pistonaccumulator?

1.

2.

3.4.

The tailrod allows theaccumulator to be used as ahydraulic actuator, thuseliminating the number ofsystem components requiringmaintenanceThe vented tailrodaccumulator has the spacebetween the piston sealsvented to the atmosphere,causing air or oil leakagepast the seals to be apparentBoth 1 and 2 aboveThe vented tailrodaccumulator has a gauge thatprovides a quick indicationof the amount of fluid in theaccumulator

4-21 Why does a bladder-type, air-operated accumulator have a veryhigh volumetric efficiency?

1. The bladder is largerbottom and the rubberthinner at the top

2. The bladder is largertop and the rubber isat the bottom’

3. The bladder is largertop and the rubber isat the top

4. The bladder is largerbottom and the rubberthinner at the bottom

at theis

at thethinner

at thethinner

at theis

4-22. Which of the following statementsdescribe(s) how an excessiveamount of gas is prevented frombeing entrained In direct-contact accumulators?

1. Safety fluids are used inthis type of accumulator

2. The fluid port is located atthe bottom of the accumulator

3. These accumulators aregenerally not used forpressures over 1200 psi

4. All of the above

4-23. Both the bladder-type accumulatorand the diaphragm accumulatoroperate in a similar manner.

1. True2. False

26

Learning Objective: Recognizethe effects of foreign matter onfiltration in a hydraulic powersystem Recall the functions,construction features, andoperating characteristic offilters, strainers, anddehydrators.

4-24. A filter should be used to removelarge particles of foreign matterfrom the fluid in a hydraulicpower system.

1. True2. False

4-25. To prevent the higherdifferential pressure that isgenerated at cold temperatures byhigh fluid viscosity from causinga false indication of a loadedfilter element, what device isinstalled in the button-typepressure differential indicator?

1. Thermal lockout2. Viscosity sensor3. Collapsible filter element4. Pressure-operated bypass

valve

4-26. Nonbypassing filters are used ina hydraulic system to serve whichof the following functions?

1. Decrease the frequency offlushing the system

2. Reduce the probability of thefailure of other systemcomponents

3. Reduce the circulation ofcontaminated fluid in thesystem

4. All of the above

4-27. How is the bypass valve, locatedwithin the head assembly of somefilters, operated?

1. Manually2. Pressure3. Electrically4. Magnetically

4-28. When you find a filterdifferential pressure indicatorbutton extended, what is thefirst action you should take?

1. Replace the indicator2. Replace the filter el3. Replace the filter assembly4. Verify that the releace of

the button is due to a loadedfilter element

4-29. The recirculation of fluidthrough a proportional-flowfilter over a period of time willeventually accomplish the samepurpose as passage of the fluidonce through a full flow filter.

1. True2. False

4-30. The diameter, in microns, of thelargest spherical particle thatwill pass through a filter undera certain test condition defineswhat filtration rating?

1. Mean2. Nominal3. Absolute4. Adequate

4-31. Which of the following types offilter elements would most likelybe found in the air intake of acompressor?

1. Ceramic2 . Porous metal3. Woven screen wire4. Moving mechanical device

4-32. Some pneumatic systems usechemical driers to remove anymoisture that might collect inthe lines beyond the waterseparators. The driers removethis moisture by what process?

1. Absorption2. Condensation3. Evaporation4. Precipitation

4-33. The chemical driers referred toin the preceding question may beidentified by which of thefollowing terms?

1. Air driers2. Desiccators3. Dehumidifiers4. Each of the above

27

4-34.

4-35.

4-36.

4-37.

Learning Objective: Recognizethe types of fluid poweractuating devices and identifyconstruction features, uses, andoperating characteristics ofvarious types of actuatingcylinders.

What component of a fluid powersystem converts fluid power intomechanical force and motion?

1. Pump2. Valve3. Actuator4. Solenoid

What actuating devices arecommonly used in fluid powersystems?

1. Turbines2. Motors3. Cylinders4. All of the above

A cylinder is identified as a ramtype if its

1. piston rod diameter is lessthan one-half of the diameterof the piston

2. piston rod area is less thanOn-half the area of it

3. area is more than one-half ofthe area of the piston rod

4. piston rod cross-sectionalarea exceeds one-half of thecross-sectional area of thepiston

Ram-type sinqle-acting cylindersare designed for which type offunctions?

1. Push functions where springsassist the functions

2. Pull functions where springsassist the functions

3. Push functions where returnaction depends on springs orgravity

4. Pull functions where returnaction depends on springs orgravity

4-38.

4-39.

Four-way control valves arenormally used to control theactions of the

1. single–acting ram2. double-acting ram3. sinqle-acting ram through two

ports4. double–acting ram using equal

pressure on all valvesurfaces

Refer to figure 10-2 of yourtextbook. Why does the extensionstroke exert a greater force thanthe retraction stroke?

1. The pressure is much greaterfor the extension stroke

2. The bottom of the ram has alarger surface area than thelip

3. Both pressure and surfacearea are greater for theextension stroke

4. The extension stroke isusually assisted by gravity

IN QUESTIONS 4-40 THROUGH 4-42 SELECTFROM COLUMN B AN APPLICATON OF EACHTYPE OF ACTUATING CYLINDER LISTED INCOLUMN A.

A. CYLINDER TYPES B. APPLICATIONS

4-40. Sinqle–acting. 1. Dump trucksspring-loadedpiston 2. Ships’ steer.

ing systems4-41. Telescoping ram

3. Anchor wind-4-42. Dual ram lass

4. Carrier air-craft arrest-ing hooks

4-43. The piston-type cylinder has across-sectional area thatmeasures more than twice thecross-sectional area of itspiston rod.

1. True2. False

28

4-44. Refer to figure 10-5 in your

4-45.

4-46.

4-47.

4-48.

textbook. Which statementrelative to the operation of thiscylinder is correct?

1. Fluid pressure extends andreturns the rod

2. Fluid pressure extends therod and gravity returns it

3. Mechanical force extends therod and fluid pressurereturns it

4. Fluid pressure extends therod and mechanical forcereturns it

What type of directional controlvalve is normally used to controla single-acting, spring-loaded,piston-type actuating cylinder?

1. Shuttle2. Transfer3. Three-way4. Four-way

Refer to figure 10-6 of yourtextbook. This type of cylinderis normally installed so that thegreater load is carried as thepiston travels in whichdirection?

1. To the right2. To the left3. To either the right or left:

it does not matter since thesame pressure is applied toboth sides of the piston

Refer to figures 10-6 and 10-8 inyour textbook. A double-actingunbalanced cylinder differs froma double-acting balanced cylinderin that the balanced cylinder has

1. equal, opposing pistonsurfaces

2. unequal piston rod areas3. unequal piston surface areas4. springs to equalize pressures

on the piston

Rotary actuation of fluid powerequipment can be done only withthe use of fluid power motors.

1. True2. False

4-49. Although pumps and fluid powermotors are similar in design andconstruction, the function ofeach is the direct opposite tothat of the other.

1. True2. False

4-50. Which of the following

4-51.

4-52.

4-53.

operational conditions areprovided by a fixed-displacementfluid motor?

1. Variable torque and constantspeed

2. Constant torque and constantspeed

3. Constant torque and variablespeed

4. Variable torque and variablespeed

In a system requiring rotation ofa motor in one direction, fluidflow to the motor can becontrolled by which of thefollowing components?

1. A flow control valve2. A variable-displacement pump3. A two-way directional cantrol

valve4. Each of the above

Although hydraulic systems useall of the following types offluid power motors, pneumaticsystems are limited to usingwhich type?

1. Vane2. Gear3. Radial piston4. Axial piston

Refer to figure 10-12 in yourtextbook. Which statement aboutthe gears is true?

1. Both 1 and 2 are drivinggears

2. Both 1 and 2 are driven gears3. 1 is the driven gear and 2 is

the driving gear4. 1 is the driving gear and 2

is the driven gear

29

4-54. Which of the following statements 4-57.concerning the operation of thevane-type motor illustrated infigure 10-13 of your textbook isfalse?

1. The rotor turns because areaA is greater than area B

2. The pressure of the drivingforce is equal in alldirections

3. When the rotor turnsclockwise, the vanes tend tobend backward due tocentrifugal force

4. The potential energy of thedriving force is convertedinto kinetic energy in theform of rotary motion andforce

4-55. Piston-type motors and variable-displacement pumps are oftencombined to form a hydraulictransmission. The advantages ofsuch a transmission over amechanical transmission includewhich of the following? 4-58.

1. Smooth acceleration anddeceleration

2. Shock load effect reduction3. Smooth operating action4. All of the above

REFER TO FIGURE 10-16 IN YOUR TEXTBOOKIN ANSWERING QUESTIONS 4-56 THROUGH4-58.

4-56. The direction of the hydraulicmotor is controlled by which ofthe

1.2.3.4.

Which of the following statementsconcerning the design of thehydraulic transmissionillustrated in figure 10-16 ofyour textbook is true?

1. The A-end is a variable-displacement axial-pistonmotor, and the B-end is afixed-displacement axial-piston pump

2. The A-end is a fixed-displacement axial-pistonpump , and the B-end is avariable-displacement axial-piston motor

3. The A-end is a variable-displacement axial-pistonpump, and the B-end is afixed-displacement axial-piston motor

4. The A-end is a fixed-displacement axial-pistonmotor, and the B-end is avariable displacement axial-piston pump

The B-end of the speed gear is afixed-displacement motor whosepistons make a full stroke forevery revolution of the outputshaft

1. True2. False

Learning Objective: Identifyfunctions, operatingcharacteristics, and constructionfeatures of various types of

following components? turbines.

Electric motorHydraulic pump 4-59. Which of the following is NOT aPrime mover use of turbines?B-end

1. Convert kinetic energy of gasto mechanical energy

2. Supply fluid flow inhydraulic systems

3. Drive electric generators4. Drive pumps

4-60. Which of the following turbineparts convert(s) kinetic energyto mechanical energy?

1. Blade2. Nozzle3. Both 1 and 2 above4. Rotor

30

4-61. Which of the following forcescauses the reaction turbine torotate?

1. Reactive force produced onthe moving blades as the gasincreases in velocity

2. Reactive force produced onthe moving blades as the gaschanges direction

3. The impulse of the gasimpinging upon the movingblades

4. Each of the above

4-62. The nozzles of a reaction turbineare mounted between the blades.

1. True2. False

31

Assignment 5

Textbook Assignment: “Pneumatics,” chapter 11; “Basic Diagrams and Systems,”chapter 12; chapters 9 and 10.

Learning Objective: Recall factspertaining to the development ofgases and the characteristics ofgases.

5-1.

5-2

Pneumatic power is most commonlyused in complex systems.

1. True2. False

Which of the followingcharacteristics is/are true forgases?

1. They have no definite volume2. They have no definite shape3. Gases are lighter than equal

volumes of liquids4. All of the above

Learning Objective: Relate thecommon temperature scales byconverting temperature readingsbetween them.

5-7. Which of the follownig statementsis true concerning absolute zero?

1.

2.

3.

4.

It is the temperature atwhich no heat remains in agas but not the lowesttemperature obtainableIt was attained only once, atwhich time the absolute zeropoint of -273.16°C wasdeterminedIt is the temperature atwhich all molecular activityin a substance ceasesIt is the temperature towhich liquids, solids, andgases can be reduced and atwhich most molecular activityceases

IN ANSWERING QUESTIONS 1-8 THROUGH 5-12REFER TO FIGURE 11-1 IN YOUR TEXTBOOK.

5-8.

IN ANSWERING QUESTIONS 5-3 THROUGH 5-6,SELECT FROM COLUMN B THE TEMPERATURETHAT CORRESPONDS TO THE ABSOLUTE ZEROTEMPERATURE FOR EACH OF THE SCALES INCOLUMN A.

A. SCALES B. TEMPERATURES

5-3. Celsius 1. -460°

2. -273°5-4. Fahrenheit

5-5. Kelvin 3. 0°

5-6. Rankine

5-9.

5-10.

What is the Celsius scaleequivalent of 68°F?

1. 5.7°C20.O°C2.

3. 37.7°C4. 52.0°C

What is the Kelvin scaleequivalent of 68°F?

1. 253°K273°K2.

3. 293°K4. 341°K

What is the Rankine scaleequivalent of 68°F?

1. 341°R2. 441°R3. 460°R4. 528°R

32

5-11. What Is the Celsius scaleequivalent of 263°K?

1. 90°C2.3. O°c4. -10°C

10°C

5-12. What is the Fahrenheit scaleequivalent of 263°K?

1. -l8°F2. -14°F3. 14°F4. 18°F

Learning Objective: Recognizethe pressure characteristics ofgases and liquids, including howpressure is caused by the weightof the atmosphere, and identifyhow pressures are measured.

5-13. Gases exert equal pressure on allsurface areas of theircontainers.

1. True2. False

5-14. When a reading is taken of thepressure in an automobile tire,what does the gauge readingrepresent?

1. Local atmospheric pressureplus the absolute pressure

2. Absolute pressure minus thelocal atmospheric pressure

3. Local atmospheric pressureminus the absolute pressure

4. Absolute pressure

5-15. What is the absolute pressure(psia) in a cylinder that has agauge reading of 1990 psig?

1. 18432. 1975.33. 2004.74. 2137

5-16. What is the gauge pressure (psig)of a container that has aninternal pressure of 113 psia?

1. 98.32. 99.73. 125.34. 127.7

5-17 Whenever you apply the gas laws,you must use absolute pressure.

1. True2. False

5-18

5-19.

5-20.

5-21.

Learning Objective: Identifyvarious theories, laws, andproperties of gases, correlatethese with applicable formulas,and solve related problems.

When you observe that thepressure of gas in a sealedcontainer has increased, you canassume that

1. heat has been absorbed by thegas

2. heat has been removed fromthe gas

3. the kinetic energy of the gashas decreased

4. molecules of the gas gainedenergy from each other whilecolliding

Four cubic feet of nitrogen areunder a pressure of 50 psig. Ifthe nitrogen is compressed to 2cubic feet, what is the new gaugepressure?

1. 104 psig2. 114.7 psig3. 124 psig4. 134 psig

A cylinder of gas at 75°F has apressure of 900 psig, To whatmaximum temperature may it beheated without exceeding 1000psig?

1. 211.9°F2. 174.9°F3. 158.4°F4. 133.4°F

The general gas equation used inthe study of gases is acombination of the gas laws of

1. Charles and Boyle2. Charles and Kelvin3. Boyle and Fahrenheit4. Boyle, Charles, and Kelvin

33

5-22. Four cubic feet of a gas at 40°Fhas a gauge pressure of 100 psig.If the volume of the gas isexpanded to 6 cubic feet and thegas heated to a temperature of90°F, what will the new gaugepressure be?

1. 67.9 psig2. 69.4 psig3. 71.5 psig4. 73.6 psig

Learning Objective: Recognizecharacteristics of gases used inpneumatic systems, safetyprecautions for handlingcompressed gas, and color codesof compressed gas cylinders.

5-23. In addition to being nonpoisonousand free from any acids thatmight cause system corrosion, thegas used as the fluid medium fora pneumatic system must possesswhich of the followingcharacteristics?

1. Nonflammability2. Chemical stability3. Ready availability4. All of the above

5-24. The gases used in Navy pneumaticsystems are similar to theliquids used in hydraulicsystems, except that the gasesare not

1. acid free2. nontoxic3. good lubricants4. chemically stable

5-25. What characteristic of compressedair makes it undesirable as amedium for pneumatic systems?

1. Its toxicity2. Its flammability3. Its moisture content4. Its lubricating qualities

5-26. In all compressed air systems,the compressor, due to theunlimited supply of air, isinstalled in the distributionlines leading to the device to beoperated.

5-27. Which of the following statementsis NOT true of LP air systems?

1. The LP air system is suppliedwith LP air by LP aircompressors

2. The LP air system is suppliedwith air by the HP air systemsupplying air through apressure-reducing station

3. The LP air system is suppliedwith air by the MP air systemsupplying air through apressure-reducing station

4. LP compressed air is used inthe production of nitrogen

5-28. Why is the use of nitrogenpreferred over the use ofcompressed air in many aircraftand missile pneumatic systems?

1. Nitrogen cannot supportliving organisms

2. Nitrogen cannot supportcombustion and fire

3. Nitrogen does not cause rustor decay of the surfaces withwhich it comes in contact

4. All of the above

5-29. Which of the following steps cana maintenance person take tocontrol contamination ofpneumatic systems?

1. Install an air filter in thesupply line

2. Keep all tools and the workair clean and dirt free

3. Cap or plug all lines andfittings immediately afterdisconnecting them

4. Both 2 and 3 above

5-30. You must NEVER use the contentsof a cylinder identified by whichof the following color codes forpurging an oxygen system?

1. Gray2. Black3. One black stripe around its

top4. One green stripe around its

top

1. True2. False

34

5-31.

5-32.

5-33.

5-34.

Inasmuch as compressed air isneither toxic nor flammable, theordinary precautions for handlingcompressed gases do not apply tohandling it.

1. True2. False

Inasmuch as nitrogen is nontoxic,the usual ventilation precautionsneed not be observed whennitrogen is used in confinedspaces.

1. True2. False

Which, if any, of the followingoperations is an acceptablepractice during the use ofcompressed gases?

1. Perform general space cleanup2. Tighten leaking portions of

compressed gas systems whilethey are pressurized to

ensure that you stop the leak3. Pressurize empty lines and

vessels rapidly4. None of the above

REFER TO APPENDIX II OF YOUR TEXTBOOK INANSWERING QUESTIONS 5-35 THROUGH 5-3B.

FOR QUESTIONS 5-35 THROUGH 5-38, SELECTFROM COLUMN B THE MECHANICAL SYMBOL FOREACH HYDRAULIC SYSTEM COMPONENT LISTEDIN COLUMN A.

A C O M P O N E N T S _- B SYMBOLS

5-35. Sequence valve 1.

5-36. Variable displace-ment pump

5-37. Check Valve2.

5-38. Pressure gauge

3.

4.

REFER TO APPENDIX III OF YOURTEXTBOOK IN ANSWERING QUESTIONS5-39 through 5-41.

Learning Objective: Recognizethe importance of diagrams andsymbols, identify symbols used indiagrams, and types of diagrams.

For a mechanic or technician.which of the following aidsis/are provided by diagrams?

1. Location of components withina system

2. Location of generalcomponents

3. Understanding of how a systemoperates

4. All of the above

FOR QUESTIONS 5-35 THROUGH 5-41, SELECTFROM COLUMN B THE AERONAUTICALMECHANICAL SYMBOL FOR EACH HYDRAULICSYSTEM COMPONENT LISTED IN COLUMN.

A. COMPONENTS B. SYMBOLS

5-39. Power-driven 1.pump

5-40. Actuatingcylinder

5-41. Automatic 2.check valve

3.

4.

35

FOR. QUESTIONS 5-42 THROUGH 5-45, SELECTFROM COLUMN B THE DIAGRAM THAT ISDEFINED IN COLUMN A.

A. DEFINITIONS B. DIAGRAMS

5-42. Shows the intern- 1. Combina-al parts of the tioncomponents

2. Pictorial5-43. Shows the general

location of 3. Graphiccomponents

4. Cutaway5-44. Uses symbols,

shows actualappearance, andshows internalworking part

5-45. Uses symbols toshow components

5-46. Which of the following diagramsincludes the interconnectingsystem piping?

1. Combination2. Pictorial3. Graphic4. Each of the above

5-47. Which, if any, of the followingdiagrams contains pipe sizes anddata on the sequence of systemoperation?

1. Combination2. Pictorial3. Graphic4. None of the above

5-48. A schematic diagram of ahydraulic system enables amechanic to accomplish which ofthe following tasks?

1. Understand the operation ofthe system

2. Identify components of thesystem

3. Trace the flow of fluidthrough the system

4. All of the above

5-49. Which of the followingstatements about an oper-centerhydraulic system is false?

1. The directional controlvalves are connected inparallel

2. There is no pressure in thesystem when the actuatorsare idle

3. The system may have anynumber of subsystems with adirectional control valves,for each

4. The pump circulates fluidfrom the reservoir, throughthe directional controlvalves, and back to thereservoir

5-50. Why are closed-center hydraulicsystems the most widely usedsystems.?

1. They provide smoothoperation of their actuators

2. They eliminate continuoussystem pressurization

3. They operate very rapidly4. They do all of the above

Learning Objective: RecognizeNavy applications, componentfunctions. constructionfeatures, and operatingcharacteristics of hydraulicpower drive systems.

5-51. Hydraulic power drives are usedin the Navy to perform which ofthe following functions?

1. Drive and control winches,capstans , and windlasses

2. Train and elevate nearly allcalibers of guns

3. Position rocket and missilelaunchers

4. All of the above

36

QUESTIONS 5-52 THROUGH 5-55, SELECT FROMCOLUMN B THE HYDRAULIC POWER DRIVESYSTEM COMPONENT TO WHICH EACH STATEMENTIN COLUMN A APPLIES.

A. STATEMENTS B. COMPONENTS

5-52. It can be an 1. A- endelectric motor

2. B-end5-53. It is a hydraulic

motor mover 3. Prime

5-54. It is a hydraulicpump

5-55. It can be a gaso-line enigine

REFER TO FIGURE 12-5 IN YOUR TEXTBOOK INANSWERING QUESTIONS 5-56 THROUGH 5-62.

5-56. The forward shaft of the primemover drives which of thefollowing components?

1. The hydraulic pump2. The hydraulic motor3. The auxiliary pumps4. All of the above

5-57. What type of pump is the A-endpump of this power drive?

1. Axial-flow variable-displacement

2. Radial-flow variable-displacement

3. Axial-flow constant-displacement

4. Radial-flow constant-displacement

5-58. Which of the following statementsis true concerning the operationof the A-end?

1. Its output is variablebecause it is driven at avariable speed

2. Its output is constantbecause it is driven at aconstant speed

3. Its output is variable eventhough it is driven at aconstant speed

4. Its output is constant eventhough it is driven at avariable speed

IN QUESTIONS 5-59 THROUGH 5-62, SELECTFROM COLUMN B THE AUXILIARY PUMP THATPERFORMS EACH FUNCTION LISTED IN COLUMNA.

A. FUNCTIONS B. PUMPS

5-59. Transmits a puls- 1. Replen-ing effect to the ishingfluid in the res-ponse pressure 2. sump

pump5-60. Replaces fluid in and

the active systems oscil-of the power drive lator

5-61. Supplies high- 3. Controlpressure fluid to presthe various pistons surein the system

5-62. Pumps leakage to theexpansion tank

5-63. What function(s) does thereservoir provide?

1. A method of cleansing andstoring fluid

2. A reserve supply of fluid3. A cooling surface for the

fluid4. Both 2 and 3 above

REFER TO FIGURE 12-6 IN YOUR TEXTBOOK INAWSWERING QUESTIONS 5-64 AND 5-65.

5-64. How is the tilting boxpositioned?

1. Locally by the strokecontrol shaft

2. Automatically by the strokecontrol shaft

3. Mechanically by hand control4. By each of the above means

5-65. The tilting box will not moveunder which of the conditionslisted below?

1. IHP = 385 psi, HPC = 900psi

2. IHP = 500 psi, HPC = 1000psi

3. IHP = 750 psi, HPC = 750psi

4. IHP = 800 psi, HPC = 1000psi

37

5-66. The direction and speed of thehydraulic motor are controlled bythe

1. electric motor2. hydraulic pump3. prime mover4. B-end

REFER TO FIGURE 12-8 IN YOUR TEXTBOOK INANSWERING QUESTIONS 5-67 THROUGH 5-71.

5-67. Which of the following componentsis/are NOT operated by nitrogenfrom the manually operatednitrogen bottle?

1. Dump valves2. Nose gear cylinder3. Main gear unlock cylinders4. Aft door cylinders

5-68. What provides the force toreposition the shuttle valves foremergency operation?

1. Hydraulic fluid2. Gravity3. Springs4. Nitrogen

5-69. When the emergency system isactuated, what force extends themain gear after the unlock hooksare released?

1. Gravity2. Hydraulic pressure3. Nitrogen pressure4. A combination of gravity and

nitrogen pressure

5-70. When the emergency system isactuated, what component is usedin the system to prevent a fluidlock in the landing gear?

1. Dump valve2. Timer valve3. Relief valve4. Shuttle valve

REFER TO FIGURE 12-10 IN YOUR TEXTBOOKIN ANSWERING QUESTIONS 5-71 AND 5-72.

5-71. How is the main valve in the4-way valve assembly normallyoperated?

1. Electrically2. Hydraulically3. Manually

5-72. What is the function of theorifice plate installed in thelines to port A of the hydrauliccylinders?

1. To control the flow ofhydraulic fluid to thecylinder for raisingoperations

2. To control the flow ofhydraulic fluid to thecylinder for lowringoperations

3. Both 1 and 2 above4. To allow for changes in the

viscosity of the hydraulicflluid as its temperaturechanges

38