OBJECTIVESAfter completing this chapter, you will:

■ Calculate unknown values in given fluid power applications.

■ Draw graphic diagrams of hydraulic circuits.

■ Make graphic diagrams of hydraulic circuits based onsequence of operations functional descriptions.

■ Prepare pictorial diagrams of hydraulic circuits.

■ Make graphic diagrams of pneumatic circuits.

■ Develop fluid power systems from engineering sketches,component lists, or from sketches of revised units.

1

WEB CHAPTERFluid Power

1

T H E E N G I N E E R I N G DESIGN A P P L I C AT I O N

When a draftsperson begins a fluid power drafting proj-ect, she or he works from one of several starting points.The drafter may receive an engineer’s sketch and a list ofcomponents for a new unit, or a sketch for an addition toan existing unit. The drafter may need to show the fin-ished product with a Component List (or Bill of Materials)and/or include a Sequence of Operations List.

In Example 1, the engineer has given the draftspersona rough sketch and bill of materials for a hydraulic cir-cuit. (See Figure 1.) The drafter used a CADD system toproduce the final product shown in Figure 2. Figure 3shows the actual top-mounted hydraulic power unitdescribed in Figure 2. A top-mounted unit means thatthe pump and motor are mounted on top of the tank. In

FIGURE 1 ■ Example 1—engineer’s sketch. Courtesy Fluid-Air Components, Inc.(Continued)

14 ■ Fluid Power

P A

T

POSITION 1

POSITION 2

P A

T

FIGURE 40 ■ Three-way, two-position valve.

FIGURE 41 ■ Hydraulic circuit.

SYMBOL RULESANSI These symbol rules apply to both hydraulics andpneumatics.

1. Symbols show connections, flow paths, and functionsof components represented. They can indicate condi-tions occurring during transition from one flow patharrangement to another. Symbols do not indicateconstruction, nor do they indicate values, such aspressure, flow rate, and other component settings.

2. Symbols do not indicate location of ports, direction ofshifting of spools, or positions of controls on actualcomponents.

3. Symbols may be rotated or reversed without alter-ing their meaning except in cases of (a) lines toreservoir, (b) accumulator.

4. Line width does not alter meaning of symbols.

5. Basic symbols may be shown in any suitable size. Sizemay be varied for emphasis or clarity. Relative sizesshould be maintained.

6. Letter combinations used as parts of graphic sym-bols are not necessarily abbreviations.

7. In multiple envelope symbols, the flow conditionshown nearest an actuator symbol takes place whenthat control is caused or permitted to actuate.

8. Each symbol is drawn to show normal, at-rest, orneutral condition of component unless multiple dia-grams are furnished showing various phases of cir-cuit operation.

9. An arrow through a symbol at approximately 45° indi-cates that the component can be adjusted or varied.

10. External ports are located where flow lines connectto basic symbols, except where the componentenclosure symbol is used.

11. External ports are located at intersections of flowlines and component enclosure symbols whenenclosure is used.

Fluid Power ■ 15

FIGURE 42 ■ Three-way, three-position valve.

FIGURE 43 ■ Four-way, three-position valve.

PNEUMATICSIn review, a fluid is defined as something that can flow and isable to move and change shape without separating when underpressure. Fluid power includes both liquids and gases. Pneu-matics is the science that pertains to gaseous pressure and flow.

Pneumatic devices include any tool or instrument that uti-lizes compressed air, such as riveters, paint sprayers, atomizers,and rock drills. Using compressed-air power is economical andsafe. Pneumatic devices have no spark hazard and can be usedunder wet conditions without electric shock hazard. Otheradvantages are that pneumatic systems have relatively few mov-ing parts, and devices can be easily exchanged with one anotherby pipe, tubing, or flexible hose.

Pascal’s principle applies to pneumatics as well as hydraulics.It states that if a pressure is exerted at one portion of fluid thatis at rest in a closed container, then that pressure is transmittedequally in all directions without loss through the rest of the fluidand to the walls of the container. (See Figure 6.)

Another basic physical law pertaining to pneumatics isBoyle’s Law, which states: the absolute pressure of a fixed massof gas varies inversely to the volume, provided the temperatureremains constant. Note that this law is in terms of absolute pres-sure (psia or pounds per square inch absolute), not gauge pres-

sure (psig or pounds per square inch gauge). At sea level, theweight of the earth’s atmosphere is 14.7 psi. This is the pressurethat is actually being exerted on the gauge, even though thegauge reading is zero. To find absolute pressure, 14.7 psi mustbe added to the gauge pressure. Figure 45 demonstrates Boyle’s

16 ■ Fluid Power

CA

DD

APPLIC

ATION

S

APPLICATIONS FOR FLUID POWER

Fluid power graphic diagrams lend themselves well toCADD applications because of their simplicity and stan-dardization. Several companies offer a Fluid Power Sym-bols Library that can be made compatible with mostmicro-, mini-, and mainframe CADD systems. A symbols

library usually includes a full spectrum of graphic sym-bols from basic flow and pressure control valves to com-plex hydrostatic transmissions. Figure 44 shows a samplepage from one company’s symbol library.

INSERT POINT

INSERT POINT

V420

INSERT POINT

V680

MAN-1

M1740

INSERT POINT

INSERT POINT

20CU. FT.

30PSIA

(a)

10CU. FT.

60PSIA

(b)

5CU. FT.

120PSIA

(c)

FIGURE 44 ■ CADD system fluid power symbols. Courtesy PriceEngineering Company Inc.

FIGURE 45 ■ Boyle’s Law with gauges reading absolute pressure.

2 ■ Fluid Power

T H E E N G I N E E R I N G DESIGN A P P L I C AT I O N (continued)

a skid-mounted unit, they are mounted on the bottomof the tank.

In Example 2, the drafter has received the followingsequence of operations and functional description todescribe what takes place in the hydraulic circuit. Fig-ure 4 shows the resulting hydraulic graphic diagram.Note the use and function of a sequence valve (7) inthis example.

Sequence of Operations

1. With motor (3) running, lever on valve (6) is raisedmanually. Delivery of pump (4) is directed into headend of cylinder (8) for pressing phase.

2. When pressure reaches setting of valve (7), flow issequenced through (7) to drive motor (9).

FIGURE 2 ■ Example 1—final product. Courtesy Fluid-Air Components, Inc.

FIGURE 3 ■ Top-mounted hydraulic power unit. Courtesy Fluid-Air Components, Inc. (Continued)

Fluid Power ■ 3

T H E E N G I N E E R I N G DESIGN A P P L I C AT I O N (continued)

COMPONENT LISTKey Quantity Name, Model Number, and Manufacturer(1) 1 Reservoir, oil, 30 gal. cap., Co. A(2) 1 Strainer, ST104, Co. A(3) 1 Motor, Electric, 5 HP, 1800 rpm,

NO-13-53, Co. JKS(4) 1 Pump, 7 1/2 GPH at 1800 rpm, FE-22-52,

Co. VCS(5) 2 Valve, relief, FE-12-76, Co. EECS(6) 1 Valve, directional control, DE-04-77, Co. QJS(7) 1 Valve, sequence, DE-18-49, Co. CS(8) 1 Cylinder, differential, AU-21-43, Co. CPM(9) 1 Axial motor, fixed displacement, MO-1313,

torque 15 in. lb./100 psi, displacement .96 cu. in./rev., Co. E

Functional Description

Valve (5-A) provides overload protection. Valve (7) causeswork to be pressed with cylinder (8) before motor (9) startsto rotate, and ensures minimum pressure during operationof (9). It also controls maximum thrust of cylinder (8). Valve(5-B) limits maximum torque of motor (9). Valve (6) con-trols direction of motion of cylinder (8) and the running andstopping of (9).

In Example 3, the same sequence of operations andfunctional description are used for a pneumatic circuit.The resulting pneumatic graphic diagram is shown inFigure 5.

FIGURE 4 ■ Example 2—hydraulic graphic diagram. FIGURE 5 ■ Example 3—pneumatic graphic diagram.

3. Manually depressing lever shifts (6) to return cylin-der (8) and permit pump (9) to stop.

4. When piston of (8) returns, valve (6) is mechanicallycentered, unloading delivery of (4) to tank throughvalve (6), but maintaining sufficient pressure drop tohold work head in raised position.

For thousands of years, water has been controlled for varioususes by utilizing dams, water wheels, and tanks. The practicalapplication of this fluid in motion was the beginning of the sci-ence of hydraulics.

Today, hydraulics and pneumatics are referred to as fluidpower or fluidics. A fluid is defined as something that can flowand is able to move and change shape without separating whenunder pressure. Fluid power, therefore, includes both liquidsand gases. The science of hydraulics refers to liquids and thescience of pneumatics refers to gases.

The fluid power technology of using the flow characteristicsof a liquid or gas to perform work is used extensively in theoperation and control of automobile and aircraft systems,machine tools, earth moving equipment, ships, and spacecraft.To understand the concept of fluid power systems, it is neces-sary to first look at the basic principles of force, pressure, work,and power.

FORCE, PRESSURE,WORK, POWERForce can be described as any action that produces motion oralters the position of an object. In the U.S. Customary System,force is usually expressed in pounds, whereas in the Interna-tional System it is expressed in newtons.

Pressure is the force per unit area exerted on an object. It isshown mathematically as: Pressure = Force/Unit Area. Whenusing force expressed in pounds, the common unit area is asquare inch. The pressure is described as pounds per squareinch, or psi. When using force expressed in newtons, the unitarea is one square meter. However, in this case, instead of thepressure being described as one newton per square meter, it iscalled a pascal (Pa). Since this measurement is inconvenientlysmall for most engineering work, the kilopascal (kPa), which is1,000 newtons per square meter, is more commonly used. Onepsi equals 6.895 kPa.

Work is the measurement of force applied to an object mul-tiplied by the distance the object is moved. Therefore, no workis done unless the object is moved or displaced. Work isdescribed mathematically as: Work = Force × Distance. In theU.S. Customary System, distance is expressed in either inchesor feet. Work, therefore, is measured in in.-lb or ft-lb. Forexample, if a force of 800 pounds displaces an object 3 feet, theamount of work done is 2,400 ft-lb. In the International Systemthe distance is expressed in meters. If a force of 300 newtonsdisplaces an object 40 meters, the amount of work done is12,000 newtonmeters, or joules (J). The joule is the amount ofwork done when one newton is displaced a distance of onemeter. A kilojoule (kJ), which is 1,000 joules, is more com-monly used than a joule. Therefore, 12,000 joules would beshown as 12 kJ. A joule equals .7377 ft-lb.

Power is described as the work accomplished per unit oftime. In other words, an equal amount of work can be done bya high-powered motor in a short time or by a low-poweredmotor in a long time. The mathematical expression is Power =Work/Time. Units of power are expressed in foot-pounds per

minute, or in joules per second. The more common expres-sions are horsepower, which equals 33,000 ft-lb per minute,and watt, which equals one joule of work per second.

For example, imagine an assembly line motor pushing anobject from one conveyor belt to another belt 8 feet away with aforce of 1,000 lb in 5 seconds. The work being done is 8,000 ft-lb. The amount of time is 1/12th of a minute, or .08333 minute.In one minute, the amount of work done is 8,000 ft-lb + 1/12 =96,000 ft-lb/minute. The motor, therefore, has a horsepower of96,000/33,000 or 2.9.

HYDRAULICSThe science of hydraulics had its beginning in about 1650 whena French mathematician and physicist named Blaise Pascal firstobserved the law that became known as Pascal’s principle. Itstates that if a pressure is exerted at one portion of fluid that isat rest in a closed container, then that pressure is transmittedequally in all directions without loss through the rest of the fluidand to the walls of the container. (See Figure 6.)

What this means practically in a hydraulic circuit is thatpressure applied to one part of the system (a piston, for exam-ple) will affect another part of the circuit (another piston) withthe same pressure. The amount of force produced on the sec-ond piston depends, of course, on the area of that piston. Sim-ilarly, the amount of work done depends on the distance thatthe second piston was moved.

Figure 7 shows an example of this principle. In this simplehydraulic circuit, the surface of piston A is 10 square inches.When a force of 50 lb is applied, the pressure exerted in the fluid,on all walls, and against the surface of piston B is 5 lb per squareinch (psi). The force of piston B is, therefore, 5 psi × 100 squareinches, or 500 lb. If piston A moves a distance of 20 feet, theamount of work done is the force × distance, or 50 lb × 20 feet,

4 ■ Fluid Power

FIGURE 6 ■ Pascal’s principle.

FIGURE 7 ■ Pascal’s principle in a hydraulic circuit.

or 1,000 ft-lb. The same work will be done at piston B. Since theforce at piston B is 500 lb, the work will be 1,000 ft-lb divided by500 lb (work divided by force equals distance), and the pistonwill move 2 feet. Another way to conceptualize this is to imaginethe fluid at piston A being displaced to piston B. In other words,since the surface area of piston A is 10 square inches and pistonA moves 20 feet (240 inches), the total amount of fluid displacedto piston B is 2,400 cubic inches. Since piston B has a surface areaof 100 square inches, the piston will move 24 inches, or 2 feet.

HYDRAULIC SYSTEMSAND EQUIPMENTHydraulic systems perform work by transmitting energy from apower source through pressurized fluid to actuators (in the pre-vious example, the actuator was piston B). In most cases thepressurized fluid is a water-soluble oil or water-glycol mixture,with oil being the fluid used most frequently.

In all hydraulic circuits, there are five basic elements,regardless of the work performed or the complexity of the sys-tem. These five elements are: a reservoir, a driver, a pump,valves, and an actuator.

Reservoir

The reservoir, similar to the drawing in Figure 8, is the holdingtank for the hydraulic fluid. It can also help in separating airand contaminants from the fluid, as well as dissipating some ofthe heat that is produced within the system.

Driver

The driver may be an electric motor or an internal combustionengine which drives the pump.

Hydraulic Pump

The hydraulic pump is used to pressurize the liquid in thehydraulic system. The pump brings in air at its inlet by creatinga partial vacuum, thereby creating the atmospheric pressure thatforces the hydraulic liquid through the rest of the system.Pumps such as this, in which the liquid is displaced mechani-cally, are called positive displacement pumps. Most pumps usedin hydraulic systems are of this type. These pumps are dividedinto two types: reciprocating and rotary. A reciprocating pumppressurizes the liquid by using a back and forth, straight-linemotion such as that produced by a piston, plunger, ordiaphragm. A rotary pump uses a circular motion such as thatproduced by gears, vanes, or cams. (See Figure 9.) Rememberthat a hydraulic pump only pressurizes the liquid, thereby pro-ducing the flow. It does not pump pressure. A piston pump canbe seen in Figure 10.

Fluid Power ■ 5

FIGURE 8 ■ Hydraulic reservoir.

(c) VANE PUMP

(b) GEAR PUMP

(a) RECIPROCATING PISTON PUMP

FIGURE 9 ■ Positive displacement pumps.

Valves

Valves are devices that control the pressure, direction, and flow ofliquids in the hydraulic system. They accomplish this by opening,closing, or partially obstructing passageways throughout the sys-tem. A variety of hydraulic valves is shown in Figure 11. The dis-cussion of valves in the following section is divided into three cat-egories: pressure control valves, directional control valves, andflow control valves.

Pressure Control Valves

These are used to maintain a particular pressure within the system.The relief valve is the most common of this type. It remains closeduntil a predetermined pressure is reached, at which time it opensautomatically, allowing the fluid to pass through the valve toreturn to the reservoir. Figures 12 and 13 show how this is accom-plished. (The type of valve illustrated is a spool valve, so namedbecause of the movable portion inside the casting.) In Figure 12,we see the valve in the closed position. The spring, which forcesthe spool to the far left, has a particular pressure setting. When theinlet pressure of the hydraulic fluid exceeds the spring setting, asin Figure 13, the spool is forced against the spring, thereby allow-ing the fluid to pass through the outlet holes to the reservoir ortank. (Both the inlet and outlet holes are called “ports.”) Thesevalves provide protection to other parts of the system from thedamage that can be caused by pressure that is too high.

A sequence valve operates on basically the same principle asthat of the relief valve. The difference is that instead of the fluidbeing returned to the reservoir, it is routed to another part ofthe system to perform more work. This is necessary in systemsthat must provide work in the proper sequence.

Pressure-reducing valves, unlike relief valves and sequencevalves, are normally open. One of their functions in hydraulicsystems is to allow a secondary circuit to operate at a lowerpressure than the primary circuit. See Figure 14 for an exampleof a pressure control valve.

6 ■ Fluid Power

FIGURE 10 ■ Piston pump. Courtesy Parker Hannifin Corporation.

FIGURE 11 ■ Hydraulic valves. Courtesy Parker Hannifin Corporation.

OUT

OUT

IN

FIGURE 12 ■ Closed relief valve.

FIGURE 13 ■ Open relief valve.

FIGURE 14 ■ Pressure control valve. Courtesy Parker Hannifin Corporation.

Directional Control Valves

These are used to control the direction that the fluid flows in thesystem. The simplest of directional control valves is the checkvalve. (See Figure 15.) This ball check valve allows the fluid toflow in only one direction. As long as the inlet pressure is greaterthan the pressure of the internal spring, the fluid will flow throughthe valve and to the rest of the system. If the flow begins to reverseor if the pressure drops below the pressure of the spring, the springpressure will seat the ball and the flow will be stopped.

Multiple-way valves provide for the opening or closing ofdifferent flow paths. They usually contain a spool. These valvesare classified by both the number of ports they contain and thenumber of spool positions. For example, two different two-way(referring to two ports), two-position valves are illustrated inFigure 16. Valve A in position 1 is normally closed, or in itsunactuated position. When the push button is pressed and thevalve is actuated (position 2), the spool slides to the left and thefluid is allowed to flow through the valve from port P (pres-sure) to port T (tank). With valve B, the unactuated position isopen (position 1). When the push button is pressed and thevalve is actuated, it then becomes closed as in position 2.

Figure 17 shows a three-way, two-position valve. In position A,the fluid flows from port P through the valve and out port A. PortT is blocked. In position B, port P is blocked and the fluid flowsfrom port A to port T.

Two types of directional control valves are shown in Figure 18.

Fluid Power ■ 7

(1) NORMALLY CLOSED, ORUNACTUATED

VALVE A

P

T

(2) ACTUATED

P

T

(1) NORMALLY OPEN, ORUNACTUATED

VALVE B

P

T

(2) ACTUATED

P

T

POSITION A

A

T P

POSITION B

A

T P

FIGURE 16 ■ Two-way, two-position directional control valves.

FIGURE 15 ■ Check valve.

FIGURE 17 ■ Three-way, two-position valve.

Flow Control Valves

These valves control the rate of flow through the hydraulic sys-tem. One type of flow control valve, the throttle valve, is illus-trated in Figure 19. Figure 20 shows a flow control valve witha fixed output. In this type of valve, the rate of flow is notaffected by variations in the inlet pressure. Two types of flowcontrol valves are shown in Figure 21.

Actuators

An actuator in a hydraulic system is the device that convertsthe fluid power to mechanical energy for the purpose of per-forming work. Actuators are either linear or rotary.

Linear actuators are most often a cylinder or ram. The single-acting cylinder is the simplest of this type. (See Figure 22.) In this

cylinder, the fluid force is applied to only one surface of the pis-ton, which is the head end of the cylinder. The piston is retractedby an external force, such as a spring or the force of gravity.

In a double-acting cylinder, such as the one illustrated in Fig-ure 23, the fluid force can be applied to either surface of the pis-ton. This allows the movement of the piston to be controlledhydraulically in two directions. This double-acting cylinder witha single piston rod is a differential type because there is a differ-ence in the piston surface area between the right and left. Sincethe area at the left is larger, the force applied to that surface isgreater, and the work stroke will be slower and more powerfulthan the opposite work stroke. The nondifferential type of dou-ble-acting cylinder shown in Figure 24 has a double-ended pistonrod that extends through both ends of the cylinder. The surface

8 ■ Fluid Power

FIGURE 18 ■ Directional control valves. Courtesy Parker HannifinCorporation.

FIGURE 19 ■ Throttle valve.

CONTROLORIFICE

FIXEDORIFICE

INLET OUTLET

FIGURE 20 ■ Flow control valve.

FIGURE 21 ■ Flow control valves. Courtesy Parker Hannifin Corporation.

FIGURE 22 ■ Single-acting cylinder. Courtesy InternationalStandards Organization (ISO).

FIGURE 23 ■ Double-acting cylinder. Courtesy InternationalStandards Organization (ISO).

areas of both sides of the piston are equal, so the forces in bothdirections will also be equal. Several types of cylinders are shownin Figure 25.

Rotary actuators can be of the gear, vane, or piston type(refer to Figure 9).

Filters and strainers are also a necessary part of the hydraulicsystem to ensure long life of the components. They keep thehydraulic fluid clean by removing foreign particles. See Figure 26for a wide variety of filters.

FLUID POWER DIAGRAMSTypes of Diagrams

Four types of diagrams are used when representing fluid powersystems. They are: graphic, pictorial, cutaway, and combinationdiagrams. Each type emphasizes a different aspect of the system.

A graphic diagram emphasizes the function of the circuitand of each component. The components consist of simplegeometric shapes that are linked together with interconnectinglines. (See Figure 27.) This type of diagram is most frequentlyused for designing and troubleshooting fluid power circuits.

A pictorial diagram, Figure 28, is used to show the pipingbetween components. The drawings of the components them-selves are pictorial and do not attempt to show the function ormethod of operation.

Fluid Power ■ 9

FIGURE 24 ■ Double-acting, nondifferential cylinder. CourtesyInternational Standards Organization (ISO).

FIGURE 26 ■ Filters. Courtesy Parker Hannifin Corporation.

FIGURE 25 ■ Cylinders. Courtesy Parker Hannifin Corporation. FIGURE 27 ■ Graphic diagram.

FIGURE 28 ■ Pictorial diagram.

The purpose of a cutaway diagram is to show the principalinternal working parts and the function of each component.Sometimes several cutaway drawings are used to show the dif-ferent flow paths that are possible depending upon the positionof the various moving parts. (See Figure 29.)

A combination diagram uses graphic, pictorial, and cutawaysymbols with interconnecting lines. (See Figure 30.) This typeof diagram provides a way to emphasize function, piping, orflow paths for each component as needed.

Symbols

ANSI Symbols are arranged in the diagram to facilitatethe use of direct and straight interconnecting lines.Where components have definite mechanical, func-tional, or otherwise important relationships to oneanother, their symbols are so placed in the diagram. Sin-gle lines are used in graphic diagrams. Double lines areused in cutaway diagrams. Pictorial and combinationdiagrams may use single or double lines or both. (“FluidPower Diagrams,” ANSI/(NFPA)T3.28.9R1—1989.)

Graphic diagrams and symbols are best suited to interna-tional use and standardization because of their simplicity. Theremainder of this section shows graphic symbols that wereapproved by the International Organization for Standardization(ISO) in 1976. Figure 31 shows several graphic symbols com-monly used.

Control valves except for nonreturn valves are usuallyshown in single or multiple squares known as envelopes, withports shown on the active envelope. (See Figure 32.)

ISO Single envelopes indicate pressure or flow controlvalves in which there are an infinite number of positionspossible. This allows the system to operate at a constantpredetermined pressure or flow.

Pressure control valve symbols are shown in Figure 33. Thesymbol for the pressure relief valve is the one that would beused for the valve shown in Figures 12 and 13.

The sequence valve differs from the pressure relief valveonly in that the fluid flows to other parts of the circuit to per-form more work instead of returning to the reservoir.

Flow control valve symbols are shown in Figure 34. Thethrottle valve symbol would be used for the valve in Figure 19,with the arrow indicating that the valve is adjustable. The sym-bol for a flow control valve with variable output would be usedfor the valve in Figure 20. Directional control valves are shownin multiple envelopes with each envelope indicating a distinctoperating position. Several possible flow paths for these andother valves are shown in Figure 35. Figure 36 shows examplesof valves with the ports open, and Figure 37 shows examples ofvalves with ports closed or blocked. Figure 38 shows the sym-bols used for various methods of actuating (or controlling)valves.

Figure 39 shows how these graphic symbols correlate withunactuated and actuated directional control valves. In position 1,this unactuated two-way, two-position valve is normally closed.In the graphic symbol shown above it, the active ports areblocked. In position 2, the valve is actuated and flow through thevalve occurs. Its graphic symbol shows the active ports on theenvelope with the flow path.

ANSI Note that when possible, the envelope nearestthe control symbol (in this case, a push-button control)represents the condition that occurs when the valve isactuated.

10 ■ Fluid Power

FIGURE 29 ■ Cutaway diagram.

FIGURE 30 ■ Combination diagram.

A three-way, two-position directional control valve is shown inFigure 40 along with its graphic symbol. Since the valve is con-trolled by an unspecified pressure in both directions, it is not pos-sible to tell from the graphic symbol which position is actuated.In position 1, the flow from the pressure port P exits port A. Inposition 2, port P is blocked, and the flow from port A goes to thereservoir via port T.

This valve could be used in a hydraulic circuit with a single-acting cylinder as shown in Figure 41. When the valve is in

position 1, the pressurized flow from the pump flows throughthe valve, through the adjustable flow control valve, into thecylinder, and forces the piston up. When the valve is in posi-tion 2, the fluid pressure from port P is blocked and is divertedback to the reservoir through the pressure relief valve. Gravityfrom the piston forces the fluid in the cylinder back throughthe check valves and through port A to the reservoir.

ANSI Note that the graphic symbol for reservoir can beused in one graphic diagram as often as necessary.

Fluid Power ■ 11

WORKING LINE

JOINING LINES FIXED CAPACITY HYDRAULIC PUMPWITH ONE DIRECTION OF FLOW

CROSSING LINES

ONE DIRECTION

TWO DIRECTIONS

RESERVOIR OPEN TO ATMOSPHERE

RETURNEDBYUNSPECIFIEDFORCE

RETURNEDBYSPRING

SINGLE-ACTINGCYLINDERS

RESERVOIR WITH INLET PIPE ANDDRAIN LINE ABOVE FLUID LEVEL

RESERVOIR WITH INLET PIPE ANDDRAIN LINE BELOW FLUID LEVEL

PRESSURIZED RESERVOIR

ELECTRIC MOTORM

(a) FLOW LINES

(b) FLOW

(c) RESERVOIRS

(d) ENERGY SOURCES

(f) PUMPS

(g) CYLINDERS

PILOT LINE(FOR CONTROL)

DRAIN LINEFIXED CAPACITY HYDRAULIC PUMPWITH TWO DIRECTIONS OF FLOW

VARIABLE CAPACITY HYDRAULIC PUMPWITH ONE DIRECTION OF FLOW

HEAT ENGINE

(e) MISCELLANEOUS

CHECK VALVE (SEE FIGURE 15)

SHUT-OFF VALVE

FILTER OR STRAINER

ACCUMULATOR

M

DOUBLE-ACTINGDIFFERENTIAL CYLINDERS

DOUBLE-ACTINGNONDIFFERENTIAL CYLINDERS

FIGURE 31 ■ Graphic symbols.

A three-way, three-position valve is shown in Figure 42 withits corresponding graphic symbol. This is an example of adirectional control valve with an intermediate position inwhich all ports are blocked.

ISO The dashed lines between the envelopes indicatethat the center position is not a distinct position; it rep-resents a transitory intermediate condition.

A four-way, three-position valve can be used with a double-acting cylinder as shown in Figure 43. In the unactuated posi-tion, the only flow that occurs is the flow from the pumpthrough the valve to the reservoir. When the valve is actuatedright, the pressurized flow from the pump goes through thedirectional control valve and through the adjustable flow controlvalve to the lower chamber of the cylinder, pushing the piston

12 ■ Fluid Power

ONE THROTTLING ORIFICENORMALLY CLOSED

PRESSURE RELIEF VALVE

SEQUENCE VALVE

PRESSURE REGULATOR ORREDUCING VALVE

THROTTLE VALVE

FLOW CONTROL VALVEWITH FIXED OUTPUT

FLOW CONTROL VALVEWITH VARIABLE OUTPUT

FIGURE 33 ■ Pressure control valve symbols.

FIGURE 37 ■ Valves with ports closed or blocked.

FIGURE 36 ■ Valves with ports open.

FIGURE 35 ■ Flow paths.

FIGURE 34 ■ Flow control valve symbols.

PORTS

FIGURE 32 ■ Envelopes.

up. At the same time, the fluid in the upper chamber flowsthrough the directional control valve to the reservoir. When thevalve is actuated left, the pressurized flow is directed to the upperchamber, and the fluid in the lower chamber flows through thecheck valves and through the directional control valve to the

reservoir. If at any time the pressure in the system exceeds a cer-tain preset amount, such as when the piston is actuated all theway to the top, then the fluid from the pump flows through thepressure control valve (relief valve) to the reservoir.

Fluid Power ■ 13

FIGURE 38 ■ Symbols for methods for actuating valves.

POSITION 1

P

T

POSITION 2

P

T

FIGURE 39 ■ Two-way, two-position valve.

Law with gauges reading absolute pressure. As the volume isdecreased by one-half, the pressure doubles. Figure 46 showsthe adjustments that needs to be made when the gauge readingat sea level is zero. In position a, 14.7 psi is added to the gaugepressure of 30 psi to get an absolute pressure of 44.7 psia. Thisfigure is then doubled in position b to get 89.4 psia, and 14.7 psiis subtracted to get 74.7 psig. The same procedure applies toposition c.

Charles’s Law also applies to pneumatics. It states that thevolume of a fixed mass of gas varies directly with absolute tem-perature, provided the pressure remains constant. This law hasmany implications for pneumatic equipment, as will be seenlater.

Another law is that air flow will occur only when there is adifference in pressure. The flow will be from high pressure tolow pressure.

PNEUMATIC SYSTEMSAND EQUIPMENTThere are eight elements involved in a complete pneumatic cir-cuit: a driver, an air compressor, an air receiver, a filter, a pres-sure regulator, an air lubricator, valves, and pneumatic devices.A driver can be an electric motor or some other power sourcethat drives the air compressor.

An air compressor is a machine that forces air into a smallerspace than it normally occupies. Two things happen with theair at this point: (1) the air pressure increases (Boyle’s Law),and (2) the air temperature increases due to the increased pres-sure (Charles’s Law). The amount of pressurized air availablefor useful work from the compressor is expressed in cubic feetper minute (or standard cu. ft. per min. or SCFM). This mea-surement of air is known as free air delivered, or FAD. The aircompressor is often mounted on the top of the air receiver, asshown in Figure 47.

An air receiver is the storage tank for the compressed air.Because the temperature of the compressed air has increased,

Fluid Power ■ 17

(a) 44.7 PSIA

164.1PSIG

(c) 178.8 PSIA

5CU. FT.

20CU. FT.

30PSIG

10CU. FT.

74.7PSIG

FIGURE 46 ■ Boyle’s Law with difference between gauge pressureand absolute pressure.

FIGURE 47 ■ Tank-mounted air compressor. Courtesy DaytonElectric Manufacturing Company.

FIGURE 48 ■ Air filter.

the amount of water vapor has increased also. This water needsto be removed. A drain is usually provided in the air receiver toremove any precipitation that takes place.

An air filter removes water vapor and dirt. (See Figure 48.)The air enters the filter and is quickly forced into a rotarymotion. The centrifugal force spins out the moisture and dirt,which then collects at the bottom and is drained by an auto-matic or manual valve.

A pressure regulator is necessary in pneumatic systems toconsistently supply the correct pressure to the pneumatic tools.(See Figure 49.) The tools usually operate with compressed airat about 90 psig.

An air lubricator (pneumatic lubricator) adds measuredamounts of lubricant to the air supply for the purpose of lubri-cating the equipment receiving the air. In an oil-fog-type lubri-cator, the oil in the container enters the metering chamber.Because there is a difference in pressure at that point, the oil isthen sprayed into the pipeline as fog. An air lubricator shouldalways be downstream of a pressure regulator because some oilscan react with the regulator diaphragm and contaminate the air.

Valves are devices that control the direction of compressedair in the pneumatic system. They are actuated manually, elec-trically, or by air. Pneumatic valves operate on the same basicprinciple as hydraulic directional control valves. For example,

a two-way, two-position valve is similar in function to thehydraulic valve shown in Figure 16. A variety of pneumaticvalves is shown in Figure 50.

Pneumatic devices are the elements that utilize compressed airto perform work. One type of element is the cylinder, which canbe either single-acting or double-acting. (Refer to Figures 22 and23.) Another type is referred to as an air motor. Air motors aredivided into two groups based on their type of driving method(either reciprocating piston or rotor). The two main types of airmotors with reciprocating pistons are the axial piston (Figure 51)and the radial piston (Figure 52). Figure 53 shows a rotary vane

air motor. A tool such as a grinding wheel may use a rotor type ofair motor, whereas a riveting hammer may use a reciprocating pis-ton air motor.

In a hydraulic circuit the fluid is returned to the reservoirafter going through the system. In a pneumatic circuit the com-pressed air is returned to the atmosphere after being utilized.

18 ■ Fluid Power

CONTROL SPRING

PISTON

HOUSING

POPPET

FIGURE 49 ■ Pressure regulator. Courtesy Parker HannifinCorporation.

FIGURE 50 ■ Pneumatic valves. Courtesy Parker HannifinCorporation.

FIGURE 51 ■ Axial piston.

FIGURE 52 ■ Radial piston.

FIGURE 53 ■ Rotary vane air motor. Courtesy Dayton ElectricManufacturing Company.

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PNEUMATIC DIAGRAMSGraphic symbols for pneumatic systems are identical to those forhydraulic systems with the exception of those shown in Figure 54.Pictorial and cutaway diagrams for pneumatic systems are also thesame as for hydraulic systems. Figure 55 shows a simple pneu-matic circuit with a nondifferential double-acting cylinder.

FIGURE 54 ■ Pneumatic graphic symbols.

FIGURE 55 ■ Simple pneumatic circuit.

FIGURE 56 ■ Conditioning unit.

Sometimes a conditioning unit is used in place of a filter,pressure regulator and gauge, and lubricator. A detailed symboland simplified symbol for this unit is shown in Figure 56. Notethe style of line used around the detailed symbol. This is some-times used to represent an enclosure around several compo-nents that are combined in one unit.

P R O F E S S I O N A LPERSPECTIVE

From a professional point of view, drafting is not simply amatter of tracing symbols on paper or of shifting symbolsaround on a CADD terminal. Drafting also involves inquis-itiveness and innovation. It sometimes takes a great deal ofcreative thought to put together a complicated pneumaticcircuit or add two extra pumps to an existing hydraulic cir-cuit. Although you may learn many things from experi-enced draftspeople and qualified salespeople, your abilityto think for yourself and question situations that do notmake sense to you will remain your greatest asset.

20 ■ Fluid Power

Fluid Power TestDIRECTIONSAnswer the questions with short complete statements or drawings as needed.

1WEB

CHAPTER

QUESTIONS1. Why does fluidics refer to both hydraulics and

pneumatics?2. What is the difference between hydraulics and

pneumatics?3. What is the common unit of measurement of

work in the U.S. Customary System? In the Inter-national System?

4. Describe the meaning of ft-lb.5. What is a “Pa” and what does it describe?6. Describe the elements in a hydraulic circuit.7. Define a positive displacement pump and describe

the two categories of this kind of pump. Do theypump pressure?

8. A relief valve is the most common pressure con-trol valve. Describe how it works.

9. What is the difference between a relief valve and asequence valve?

10. Compare a relief valve to a check valve.11. What is a port in a valve?12. Describe the purpose of a directional control

valve in a fluid power system.

13. What does “two-way, two-position directionalcontrol valve” mean?

14. Describe the difference between a differential dou-ble-acting cylinder and a nondifferential double-acting cylinder.

15. What is the difference between a single-actingcylinder and a double-acting cylinder?

16. What is an actuator?17. Which type of fluid power diagram would nor-

mally be used to show the function of each com-ponent? Which would be used for troubleshoot-ing? Which type is internationally standardized?

18. What does it mean when a valve is actuated?19. In a graphic diagram, what is the name given to

the squares used in directional control valves?20. In a graphic diagram, how would you know

which envelope of a two-way, two-position direc-tional control valve is unactuated?

21. If a graphic symbol for a directional control valveis drawn upside down, is the meaning of the cir-cuit changed?

22. What symbol indicates when a component can beadjusted or varied?

VOLUME CALCULATIONS

Problem: The company wants to transfer old hydraulicfluid that fills a cylindrical tank to a recycling firm using55 gallon drums. You have been asked to order enoughdrums to do the job. How many should you order? Thetank is 72 inches in diameter and 11 feet long. There are7.48 gallons to the cubic foot.

Solution: Fluid power problems often involve calculatingvolumes. The geometry section of the Math Appendixhas formulas for the volumes of some common shapes.Also, Table 4 of Appendix B has useful conversion

factors. For this application we need to first calculate thevolume of a cylinder from the formula V = πR2h. It is bestto have all dimensions in the same units, so we will use R = 3 feet (because the diameter of the tank is 6 feet) and h = 11 feet. Then substituting into the formula: V = 3.14)(3 feet)2(11 feet) = 311 feet3. Next we multiply by7.48 to obtain the volume in gallons. That gives 2,325 gal-lons. Finally, since each drum holds 55 gallons, we divideby 55, giving 42.3 drums. So you should order 43 drumsto do the job.

MA

TH

APPLIC

ATION

23. Can a person build a hydraulic unit from a graphic dia-gram only? Why or why not?

24. Describe the elements in a pneumatic circuit.25. With an air compressor, air is compressed from 30 cubic

feet to 15 cubic feet, and the pressure increases from 40 psi to 80 psi. Is this an example of Pascal’s principle,Boyle’s Law, or Charles’s Law?

26. In a container of compressed air, the gauge reading is 20 psi. Assuming that the temperature remains constant,what will the gauge read when the air is compressed intoone-half the original space? What will the gauge read whenthe air is compressed into one-fourth the original space?Which law describes this phenomenon?

27. Should a pressure regulator be upstream or downstreamof an air lubricator? Why?

28. What is FAD?29. Is there any difference in the basic function between a

hydraulic directional control valve and a pneumatic direc-tional control valve?

30. If only one pneumatic diagram is shown, are the valvesshown in actuated or unactuated phases?

31. What is a pneumatic device? What are the two types?32. What special considerations are necessary for pneumatics

that do not apply to hydraulics?33. What special considerations are necessary for hydraulics

that do not apply to pneumatics?

Fluid Power ■ 21

Fluid Power Problems1WEB

CHAPTER

PROBLEM 1 What is the pressure exerted on a 10 in. ×24 in. panel when a force of 1,000 pounds isapplied to it?

PROBLEM 2 What is the pressure exerted on a brickwall that is 4 meters high and 28 meters longwhen a force of 50,000 newtons is applied to it?

PROBLEM 3 A 2,350 newton force is applied to anobject with a surface area of 72 square meters.What is the pressure? What is the pressure in psi?

PROBLEM 4 A hydraulic piston pushes an objectfrom one conveyor belt to another conveyor belt,which is 4'–6'' away. The amount of work done is1,800 ft-lb. How much force is being used to pushthe object? How many kilojoules are needed?

PROBLEM 5 The hydraulic piston in the previousexample accomplishes its task in five seconds.How much horsepower is the piston performing?How many watts of power is this?

PROBLEM 6 One piston in a hydraulic circuitexerts a 28 psi pressure in the hydraulic fluid. Asecond piston in the circuit is 12 ft. away fromthe first piston and is directly affected by the firstpiston. It has an area of 15 sq. in., which is one-half the area of the first piston. What is the pres-sure exerted on the second piston? With whatforce does the second piston move? With whatforce does the first piston move? If the first pis-ton moves 10 in., how far does the second pistonmove?

PROBLEM 7 The gauge reading of a container ofcompressed air at sea level is 80 psi. The volumeof that air is double. What is the psia? What is thepsig?

Whenever possible, draw the following diagrams usinga CADD terminal and Fluid Power Symbols Library.

PROBLEM 8 Make a graphic diagram of a two-way,two-position directional control valve with a sole-noid actuator.

PROBLEM 9 Make a graphic diagram of a direc-tional control valve that has four ports and anintermediate position.

PROBLEM 10 Make a graphic diagram of a pneu-matic circuit that will lift a load with a differentialsingle-acting cylinder. Use a conditioning unit anda two-way, two-position directional control valve.

PROBLEM 11 Make a graphic diagram of a hydrauliccircuit. Use a nondifferential double-acting cylin-der, a check valve, a relief valve, and a three-way,two-position valve.

PROBLEM 12 Make a graphic diagram of the picto-rial hydraulic circuit shown in the following fig-ure. Label each component.

PROBLEM 13 Make a pictorial diagram of the graphichydraulic circuit shown in the following figure. Label eachcomponent.

PROBLEM 14 Make a graphic diagram of a hydraulic circuitbased on the following sequence of operations and func-tional description. Also make a component list specifyingthe key number, quantity, and name of component.

SEQUENCE OF OPERATIONS:1. With valve (6) in the neutral position, delivery of flow from

pump (4) unloads freely through valve (6) to reservoir (1).2. With motor (3) running, valve (6) is manually actuated right,

directing flow from pump (4) to extend clamp cylinder (7-A).3. When pressure reaches setting of valve (8-A), flow is

sequenced through (8-A) to extend nondifferential workcylinder (9) right.

4. When pressure reaches setting of valve (8-B), flow issequenced through (8-B) to retract clamp cylinder (7-B).

5. Manually actuating valve (6) left, flow is directed frompump (4) to extend clamp cylinder (7-B).

6. When pressure reaches setting of valve (8-C), flow issequenced through (8-C) to extend work cylinder (9) left.

7. When pressure reaches setting of valve (8-D), flow issequenced through (8-D) to retract clamp cylinder (7-A).

FUNCTIONAL DESCRIPTION:Valve (5) provides overload protection. Valve (6) controls

direction of motion of (7-A), (7-B), and (9). Valve (8-A) causeswork to be clamped by (7-A) before cylinder (9) performswork. Valve (8-B) causes nondifferential work cylinder (9) tobe retracted before cylinder (7-B) is retracted. Valve (8-C)causes work to be clamped by (7-B) before cylinder (9) per-forms work. Valve (8-D) causes nondifferential work cylinder(9) to be retracted before cylinder (7-A) is retracted.

NOTE: Item (2) is the strainer. Work cylinder (9) performswork in two directions, and works in conjunction with cylin-ders (7-A) and (7-B).

PROBLEM 15 Make a graphic diagram of the pneumatic cir-cuit shown.

PROBLEM 16 The hydraulic sketch in the following figure isreceived from the engineer. Make an appropriate graphicdrawing, label the components, and make a componentlist.

PROBLEM 17 Make a pneumatic graphic drawing using theengineer’s sketch in the previous example. Make the appro-priate changes needed to depict a pneumatic circuit. Donot use a conditioning unit.

22 ■ Fluid Power

MATH PROBLEMSPROBLEM 18 Find the volume, in cubic feet, of a cylindrical

tank 100 inches in diameter and 18 feet in length.

PROBLEM 19 Find the volume, in liters, of a hydraulic reser-voir in the shape of a box measuring 20'' by 18'' by 42''.

PROBLEM 20 Find the volume, in cubic centimeters, of aright circular cone having a radius of 2.54 cm and a heightof 8.5 cm.

PROBLEM 21 A storage tank at a paper mill is in the form ofa large soccer ball (sphere) having a diameter of 15 feet.What is its volume in cubic feet?

PROBLEM 22 The piston of a single-acting cylindrical lin-ear actuator has a diameter of 3.5 cm. If the piston moves8.5 cm, what volume of fluid, in cm3, must have enteredthe actuator?

PROBLEM 23 The piston of a single-acting cylindrical linearactuator has a diameter of 5 cm. If a fluid of volume 137.4 cm3

enters the actuator, how far does the piston move?

PROBLEM 24 The piston of a reciprocating piston pump has adiameter of 7'' and in one stroke moves 12''. How much fluid,in cubic feet, is pumped if the pump makes 1,000 strokes?

PROBLEM 25 What is the displacement, in liters, of a 225 in3

gasoline engine? (See Table 4 of Appendix B.)

PROBLEM 26 The fluid in a rectangular reservoir 5' wide by12' long is 36'' in depth. If it is transferred to a tank in theshape of a cylinder with a circular base 3' in diameter, whatwill be the depth of the fluid?

PROBLEM 27 Two tanks are sitting in a storage yard. One isin the shape of a cylinder with a circular base and the otheris in the shape of a right circular cone. The tanks have thesame height and top radius. Which tank has the greatercapacity? By how much?

Fluid Power ■ 23