APLTCL025 SGD L-01 - Azerfrema · PDF fileMODULE INTRODUCTION APLTCL025 © Caterpillar of...

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Caterpillar Service Technician ModuleAPLTCL025 HYDRAULIC FUNDAMENTALS

Published by Asia Pacific Learning1 Caterpillar DriveTullamarine Victoria Australia 3043

Version 3.2, 2003

Copyright © 2003 Caterpillar of Australia Pty Ltd Melbourne, Australia.

All rights reserved. Reproduction of any part of this work without the permission of the copy-right owner is unlawful. Requests for permission or further information must be addressed to the Manager, Asia Pacific Learning, Australia.

This subject materials is issued by Caterpillar of Australia Pty Ltd on the understanding that:

1. Caterpillar of Australia Pty Ltd, its officials, author(s), or any other persons involvedin the preparation of this publication expressly disclaim all or any contractual,tortious, or other form of liability to any person (purchaser of this publication or not)in respect of the publication and any consequence arising from its use, includingany omission made by any person in reliance upon the whole or any part of thecontents of this publication.

2. Caterpillar of Australia Pty Ltd expressly disclaims all and any liability to any personin respect of anything and of the consequences of anything done or omitted to bedone by any such person in reliance, whether whole or partial, upon the whole orany part of the contents of this subject material.

Acknowledgements

A special thanks to the Caterpillar Family for their contribution in reviewing the curricula for this program, in particular:

Caterpillar engineers and instructors

Dealer engineers and instructors

Caterpillar Institutes.

MODULE INTRODUCTION

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© Caterpillar of Australia Pty Ltd 1

Module TitleHydraulic Fundamentals.

Module DescriptionThis module covers the knowledge and skills of Hydraulic Fundamentals. Upon satisfactory completionof this module students will be able to competently service and repair basic hydraulic components.

Pre-RequisitesThe following modules must be completed prior to delivery of this module:

Occupational Health and Safety

Mechanical Principles.

Learning & DevelopmentDelivery of this facilitated module requires access to the Hydraulic Fundamentals Activity Workbook.

The successful completion of the curriculum provides the knowledge for competency assessment,on further learning outcomes, by an Accredited Workplace Assessor.

Suggested ReferencesNo references recommended.

Assessment Methods

Classroom and WorkshopTo satisfactorily complete this module, students must demonstrate competence in all learning outcomes.Consequently, activities and assessments will measure all the necessary module requirements.

For this module, students are required to participate in classroom and practical workshop activitiesand satisfactorily complete the following:

Activity Workbook

Knowledge Assessments

Practical Activities.

WorkplaceTo demonstrate competence in this module students are required to satisfactorily complete theWorkplace Assessment(s).

HYDRAULIC FUNDAMENTALS MODULE INTRODUCTION

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KNOWLEDGE AND SKILLS ASSESSMENT

HYDRAULIC FUNDAMENTALS

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Learning Outcome 1: Explain hydraulic principles.

Assessment Criteria1.1. Describe hydrodynamics and hydrostatics in relation to

hydraulic systems

1.1.1 Energy principles applicable

1.2. State the physical aspects of a gas

1.2.1 Gas compresses

1.3. State the properties of a fluid

1.3.1 Liquids virtually incompressible

1.3.2 Liquids conform to the shape of the container

1.3.3 Liquids apply pressure in all directions

1.3.4 Purposes of fluids in a hydraulic system

1.4. State Pascal’s Law and how hydraulic force can be used to create a mechanical advantage

1.4.1 Pascal’s Law

1.4.2 Fluid power

1.4.2.1 Force transmission

1.4.2.2 Force transmitted through a liquid

1.5. Explain the basic principles of work, flow and pressure, energy transfer and power

1.5.1 Fluid weight

1.5.2 Atmospheric pressure

1.5.3 Barometric pressure

1.5.4 Work

1.5.5 Flow

1.5.5.1 What is flow

1.5.5.2 Laminar flow

1.5.5.3 Turbulent flow

1.5.5.4 Flow across an orifice

1.5.6 Energy transfer

1.5.6.1 Bernoulli’s Law

1.5.7 Pressure

1.5.8 Power

1.5.9 Fluid power advantages

1.5.10Key hydraulic principles

1.6. Describe series and parallel hydraulic circuits

1.6.1 Series circuit

1.6.1.1 Pressure drop across a series circuit

1.6.2 Parallel circuit

1.6.2.1 Pressure drop across a parallel circuit

HYDRAULIC FUNDAMENTALS KNOWLEDGE AND SKILLS ASSESSMENT

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1.7. Calculate force, pressure and area in a hydraulic system

1.7.1 Formula

1.7.2 Force

1.7.3 Pressure

1.7.4 Area

1.7.5 Imperial/Metric conversion factors

1.7.5.1 Length

1.7.5.2 Area

1.7.5.3 Volume

1.7.5.4 Mass

1.7.5.5 Velocity

1.7.5.6 Force

1.7.5.7 Pressure

1.7.5.8 Torque

1.7.5.9 Temperature

1.7.5.10Power

1.7.6 Prefixes commonly linked to base units.

Learning Outcome 2: Identify and explain the purpose and operation of basic hydrauliccircuits and components.

Assessment Criteria2.1. Demonstrate knowledge of the basic layout of a hydraulic

circuit

2.1.1 Tank

2.1.2 Pump

2.1.3 Main Pressure Relief Valve (PRV)

2.1.4 Lines

2.1.5 Flow control valves

2.1.6 Actuators

2.1.7 Filters

2.1.8 Motor

2.1.9 Graphic symbols

2.2. Identify and describe the function of hydraulic tanks

2.2.1 Vented

2.2.2 Closed

2.3. Identify, describe the function and explain the operation of hydraulic pumps

2.3.1 Positive and non positive displacement

2.3.2 Gear

2.3.2.1 Construction

2.3.2.2 Operation

2.3.3 Vane


2.3.3.2 Operation

2.3.4 Piston


2.3.4.2 Operation



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2.4. Identify, describe the function and explain the operation of hydraulic control valves

2.4.1 Directional

2.4.2 Pressure

2.4.3 Flow-volume

2.4.4 Pilot

2.4.5 Open centred

2.4.6 Closed centred


2.5. Identify, describe the function and explain the operation of hydraulic actuators

2.5.1 Linear

2.5.1.1 Single acting

– Construction

– Cap end head

– Body

– Rod end head

– Piston

– Piston rods

– Seals

– ‘O’ ring with backup ring

– Lip seal

– Lip seal with garter spring

– Lip seal with rod wiper

– ‘U’ packing

– ‘V’ packing

2.5.1.2 Operation

– Integrated counterbalance cartridge

– Cushion plunger

– Stroke limiting stop tube

– Adjustable stop valve

– Thermal relief valve

– Operating pressures

2.5.1.3 Double acting

– Construction

– Operation


2.5.1.4 Rams

– Construction

– Operation


2.5.1.5 Telescopic

– Construction

– Operation


2.5.2 Rotary (hydraulic motors)

2.5.2.1 Gear

– Construction

– Operation


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2.5.2.2 Vane

– Construction

– Operation

2.5.2.3 Piston

– Construction

– Operation

2.5.2.4 Graphic symbols

2.6. Explain the purpose and function of hydraulic oil, filters and coolers

2.6.1 Oil

2.6.1.1 Lubrication

2.6.1.2 Friction

2.6.1.3 Viscosity effects

– To a system

– By temperature

– By pressure

– On lubrication

– Hydrodynamic lubrication

– Clearance flow

2.6.1.4 Water in hydraulic oil

2.6.1.5 Foaming

2.6.1.6 Dirt

2.6.1.7 Cavitation

2.6.2 Filters

2.6.2.1 Reservoir strainer

2.6.2.2 Suction filters

– Location

– Type

2.6.2.3 Pressure filters

– Location

– Type

2.6.2.4 Return line filter

– Location

– Type

2.6.2.5 Full flow filters (including by-pass)

2.6.2.6 Wire mesh filters

– Cleaning

2.6.3 Coolers

2.6.3.1 Air coolers

2.6.3.2 Water coolers

2.6.4 Graphic symbols.

TABLE OF CONTENTS


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TOPIC 1: Principles of HydraulicsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Why Are Hydraulic Systems Used? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Hydraulic Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Fluid Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Fluid Power Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Flow (Q) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Pressure Drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Hydraulics Doing Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

TOPIC 2: Hydraulic Circuit & ComponentsGraphic Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Hydraulic lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Hydraulic Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Graphic Symbol - Hydraulic Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Hydraulic Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Gear Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Vane Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Piston Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Graphic Symbol - Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Linear Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Telescopic Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Modified Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Cylinder Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Graphic Symbol - Hydraulic Cylinder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Rotary Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Graphics Symbol - Rotary Actuator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Directional Control Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Pressure Control Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Flow Control Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Graphic Symbols - Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Pressure Control Valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Hydraulic Fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Filters & Strainers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Coolers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Graphic Symbols - Fluid Conditioner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

Contamination Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

HYDRAULIC FUNDAMENTALS TABLE OF CONTENTS

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TOPIC 1Principles of Hydraulics

INTRODUCTIONHydraulic systems are extremely important to the operation of heavy equipment.Hydraulic principles are used when designing hydraulic implement systems, steeringsystems, brake systems, power assisted steering, power train systems and automatictransmissions. An understanding of the basic hydraulic principles must beaccomplished before continuing into machine systems.

Hydraulics play a major role in mining, construction, agricultural and materialshandling equipment.

Hydraulics are used to operate implements to lift, push and move materials. It wasn’tuntil the 1950s that hydraulics were widely used on earthmoving equipment. Sincethen, this form of power has become standard to the operation of machinery.

In hydraulic systems, forces that are applied by the liquid are transmitted to amechanical mechanism. To understand how hydraulic systems operate, it is necessaryto understand the principles of hydraulics. Hydraulics is the study of liquids in motionand pressure in pipes and cylinders.

WHY ARE HYDRAULIC SYSTEMS USED?There are many reasons. Some of these are that hydraulic systems are versatile,efficient and simple for the transmission of power. This is the hydraulic system’s job,as it changes power from one form to another.

The science of hydraulics can be divided into two sciences:

Hydrodynamics

Hydrostatics.

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HydrodynamicsThis describes the science of moving liquids.

Figure 1 - a & b

Applications of hydrodynamics:

water wheel or turbine; the energy that is used is that created by the water’smotion (Figure 1a)

torque converter (Figure 1b).

Hydrostatics This describes the science of liquids under pressure.

Applications of hydrostatics:

hydraulic jack or hydraulic press

hydraulic cylinder actuation.

In hydrostatic devices, pushing on a liquid that is trapped (confined) transfers power.If the liquid moves or flows in a system then movement in that system will happen. Forexample, when jacking up a car with a hydraulic jack, the liquid is moved so that thejack will rise, lifting the car. Most hydraulic machines or equipment in use todayoperate hydrostatically.

HYDRAULIC PRINCIPLESThere are several advantages for using a liquid:

1. Liquids conform to the shape of the container.

2. Liquids are practically incompressible.

3. Liquids apply pressure in all directions.


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Liquids Conform to Shape

Figure 2

Liquids will conform to the shape of any container. Liquids will also flow in anydirection through lines and hoses of various sizes and shapes.

We have three oddly shaped containers shown in Figure 2, all connected together andfilled to the same level with liquid. The liquid has conformed to the shape of the containers.

A Liquid is Practically Incompressible

Figure 3

Hydraulic oil compresses approximately 1 - 1.5% at a pressure of 3000 psi (20,685kPa). For machine hydraulic applications, hydraulic oil is considered as ideal anddoesn’t compress at all.

When a substance is compressed, it takes up less space. A liquid occupies the sameamount of space or volume even when under pressure.

Gas would be unsuitable for use in hydraulic systems because gas compresses andtakes up less space.

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Liquids Apply Pressure in All Directions

Figure 4

There is equal distribution of pressure in a liquid. The pressure measured at any pointin a hydraulic cylinder or line will be the same wherever it is measured (Figure 4).

Figure 5

When a pipe connects two cylinders of the same size (Figure 5), a change in volumein one cylinder will transmit the same volume to the other. The space or volume thatany substance occupies is called ‘displacement’. Liquids are useful for transmittingpower through pipes, for small or large distances, and around corners and up anddown. The force applied at one end of a pipe will immediately be transferred with thesame force to the other end of the pipe.

Most hydraulic systems use oil, because it cannot be compressed andit lubricates the system.


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Water would be unsuitable because:

1. it freezes at cold temperatures and boils at 100oC

2. it causes corrosion and rusting and furnishes little lubrication.

Purpose of the FluidMany types of fluids are used in hydraulic systems for many reasons, depending onthe task and the working environment, but all perform basic functions:

First, the fluid is used to transmit forces and power through conduits (or lines) to anactuator where work can be done.

Second, the fluid is a lubricating medium for the hydraulic components used in the circuit.

Third, the fluid is a cooling medium, carrying heat away from the “hot spots” in thehydraulic circuit or components and discharging it elsewhere.

And fourth, the fluid seals clearances between the moving parts of components toincrease efficiencies and reduce the heat created by excess leakage.

FLUID POWERIn the seventeenth century, a French Philosopher and Mathematician named BlaisePascal, formulated the fundamental law which forms the basis for hydraulics.

Pascal’s Law states:

“Pressure applied to a confined liquid is transmitted undiminished in all directions, andacts with equal force on all equal areas, and at right angles to those areas.”

Figure 6 - Applying pressure to a liquid

This principle, also referred to as the laws of confined fluids, is best demonstrated byconsidering the result of driving a stopper into a full glass bottle (Figure 6).

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Figure 7 - Container bursting due to pressure

Because liquid is essentially incompressible, and forces are transmitted undiminishedthroughout the liquid and act equally on equal areas of the bottle, and the area of thebody of the bottle is much greater than the neck, the body will break with a relativelylight force on the stopper. Figure 7 illustrates this phenomenon.

Figure 8 - Pressure, area, force relationship

Figure 8 illustrates the relationship of areas that causes a greater force on the body ofthe bottle than is applied to the neck. In this illustration, the neck of the bottle has across sectional area of .001m2. When the pressure created by this force istransmitted throughout the fluid, it influences all adjacent areas with equal magnitude.It stands to reason that a larger area (a greater number of square inches) will besubjected to a higher combined force.

The bottom of the bottle in Figure 8 has a total area of .02m2 as shown, and the forceapplied by the liquid is 50N. Therefore, the combined force over the entire bottomarea is the sum of 50N acting on each of the .001m2 areas. Because there are20 areas of .001m2 to make up 0.02m2 and 50N on each, the combined force at thebottom of the bottle is 1000N.


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This relationship is represented by the following formula:

Force = Pressure x Area.

This formula allows the Force to be determined and the Pressure and the Area whentwo of the three are known.

Figure 9

P = Pressure = Force per unit of area.

The unit of measurement of pressure is the Pascal (Pa).

F = Force - which is the push or pull acting upon a body. Force is equal to the pressuretimes the area (F = P x A).

Force is measured in Newtons (N).

A = Area - which is the extent of a surface. Sometimes the surface area is referred toas effective area. The effective area is the total surface that is used to create a force inthe desired direction.

Area is measured in square metres (m2).

The surface area of a circle (as in a piston) is calculated with the formula:

Area = Pi (3.14) times radius-squared.

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Figure 10 - Pressure created by weight

The same relationship is used to determine the pressure in a fluid resulting froma force applied to it. Figure 10 shows a weight being supported by fluid over a.01m2 area. By rearranging the above formula, the fluid pressure of 100,000Pacan be determined by:

Pressure = Force ÷ Area

Figure 11 - Transmitting force by fluid

Pascal demonstrated the practical use of his laws with illustrations such as thatshown in Figure 11. This diagram shows how, by applying the same principledescribed above, a small input force applied against a small area can result in a largeforce by enlarging the output area.

This pressure, applied to the larger output area, will produce a larger force asdetermined by the formula on the previous page. Thus, a method of multiplying force,much the same as with a pry-bar or lever, is accomplished using fluid as the medium.


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FLUID POWER ADVANTAGESMultiplying forces is only one advantage of using fluid to transmit power. As thediagram in Figure 11 shows, the forces do not have to be transmitted in a straight line(linearly). Force can be transmitted around corners or in any other non-linear fashionwhile being amplified. Fluid power is truly a flexible power transmission concept.

Actually, fluid power is the transmission of power from an essentially stationary, rotarysource (an electric motor or an internal combustion engine) to a remotely positionedrotary (circular) or linear (straight line) force amplifying device called an actuator. Fluidpower can also be looked upon as part of the transformation process of converting abenign form of potential energy (electricity or fuel) to an active mechanical form (linearor rotary force and power).

Once the basic energy is converted to fluid power, other advantages exist:

1. Forces can be easily altered by changing their direction or reversing them.

2. Protective devices can be added that will allow the load operating equipment tostall, but prevent the prime mover (motor or engine) from being overloaded andthe equipment components from being excessively stressed.

3. The speed of different components on a machine, such as the boom and winch ofa crane, can be controlled independently of each other, as well as independentlyof the prime mover speed.

Figure 12 - Simplified Hydraulic Circuit

A complete hydraulic system consists of a reservoir of fluid, a hydraulic pump driven byan internal combustion (IC) engine or an electric motor, a system of valves to control anddirect the output flow of the pump, and actuators that apply the forces to conduct thework being performed. Figure 12 is a simplified illustration of these major components.

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PRESSURE

Figure 13 - Pressure at reservoir outlet

The system fluid is forced out of the reservoir into the inlet side of a pump by the sumof several pressures that act on the fluid (Figure 13). The first pressure is the onecaused by the weight of the fluid; the second is caused by the weight of theatmosphere; a third may be present if a pressurised reservoir is employed.

Fluid Weight

Figure 14 - Pressure caused by weight of water

A cubic meter of water weighs approximately 1000kg. This weight acts downward dueto the force of gravity, and causes pressure at the bottom of the fluid. Figure 14 showshow this weight is distributed across the entire bottom of the water volume. In thisexample, the entire weight is supported by an area measuring one metre by onemetre or 1m2.

The pressure of acting at the bottom of 1 cubic metre of water is 9810kPa.

A two metre tall column of water would develop twice as much pressure if spread overthe same area (i.e. 19620 Pa).


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This is the same pressure felt on eardrums when swimming under water, andexperience says that the pressure increases with depth. The pressure can beexpressed as follows:

Pressure (Pa) = water depth (m) x 9810 Pa per metre of depth.

Other fluids behave the same as water, the difference being relative to the differencein weight of the fluids. The difference is usually defined by the Specific Gravity of thefluid (SG), which is the ratio of the fluid’s weight to the weight of water.

SG = Weight of fluid ÷ Weight of water

A typical specific gravity for oil used in hydraulic systems is approximately 0.92,meaning the weight of the oil is 92% of the weight of water. The relationship of the firstformula then becomes:

Pressure (Pa) = Fluid Depth (m) x 9810 Pa/m water x SG.

Figure 15 - Pressure caused by the weight of oil

Pure water weighs 1000kg per cubic metre at 4oC, the temperature at which it is mostdense. The weight will be slightly less at higher temperatures, but the difference isgenerally ignored in hydraulic calculations.

Typical hydraulic oil in a reservoir creates a pressure of 9200 Pa per metre of height,as illustrated in Figure 15. This pressure at the bottom of a reservoir helps to push thefluid out of the reservoir and into the inlet of a hydraulic pump, if the pump inlet isbelow the fluid level.

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Atmospheric Pressure

Figure 16 - Weight of air causes atmospheric pressure

Generally air is considered as not having weight. Any reasonable quantity of it is so lightthat the weight is usually ignored. A column of air measuring one metre by one metreacross (1 square metre of area), and extending from the earth’s surface at sea level tothe extreme of the atmosphere, would actually have a significant weight. This weight, onan average day is approximately 10,000kg, as illustrated in Figure 16. Therefore thepressure that continuously exists at sea level due to the weight of the air above, is100,000Pa. This is referred to as a standard atmosphere, or the atmospheric pressureon a typical day at sea level which is also known as 1 bar or 1000 millibars.

This pressure, acting on the reservoir fluid, also helps to push fluid out of thereservoir and into the inlet of a pump.

People are so accustomed to this pressure, and because it exists all the time thepressure under these conditions is considered to be ‘zero’. Pressure gauges alsoread “zero” under these conditions, so the standard atmospheric pressure is referredto as a gauge reading. It is, of course, possible to obtain pressures below this level byremoving some of the atmospheric pressure, and this is called a vacuum.


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Figure 17 - Gauge and absolute pressure

By removing all of the atmospheric pressure, a “new” zero is derived, and this is called“absolute zero”. Absolute zero is 100 kPa below gauge zero, and is considered aperfect vacuum (Figure 17). There is no pressure below absolute zero.

To differentiate between the two pressures, gauges which read absolute values arelabelled as such. This means that the zero for this pressure is absolute zero, and allpositive pressure readings start from this level. If the pressure starts at atmosphericpressure as the “zero”, then it is designated gauge pressure. Gauges which read thisway are not normally labelled.

Barometric PressureOne can see now that as we move above sea level, such as up a mountain, the column ofair above us becomes shorter, and thus the weight of the air above us becomes less. Theatmospheric pressure is then reduced, and the air is not compressed as much. Werecognise this as “thin” air at higher altitudes, and we feel a shortness of breath; the reasonbeing that we get less air into our lungs each time we inhale.

It is important to recognise this phenomenon; at higher altitudes, the atmosphericpressure available to help push fluid out of the bottom of a hydraulic reservoir and intothe inlet of a pump is less than at lower altitudes.

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Figure 18 - Barometer principle

Atmospheric pressure is measured by use of a barometer, and this is illustrated inFigure 18. A tube full of mercury is inverted in a pool of mercury as shown. Themercury will fall out of the tube until it reaches a specific height. The space above themercury in the tube will become a perfect vacuum of 0kPa. The height of the mercuryin the tube will correspond to atmospheric pressure, because it is atmosphericpressure that is preventing the mercury from falling the rest of the way out of the tube.

At standard atmospheric pressure of 100 kPa mercury will fall in the tube until itreaches a height of 760mm above the pool. As the atmospheric pressure changes(due to climate or altitude change), the height of the mercury will change accordingly.

FLOW (Q)Flow is simply the movement of a quantity of fluid during a period of time. Fluids areconfined in hydraulics, such as in hoses, tubes, reservoirs and components, so flow isthe movement of a fluid through these confining elements.

Flow is normally designated by the letter “Q”, and is usually expressed in litres-per-minute, or LPM, but may also be expressed in cubic-centimetres-per-minute (cm3/min) or per-second (cm3/sec).

In using the above formula the correct units must be used so that they are equal onboth sides of the equation. For example, if area is in sq cm, then velocity must be incm per second or cm per minute. The flow will then be cubic centimetres (cc) persecond or minute.

Flow is basically the velocity of a quantity of fluid past a given point. To visualize this,consider a cross-sectional area of fluid inside a tube. If this cross-sectional “slice” offluid moved at the rate of one metre in one second, then it would “push” one metre offluid ahead of it every second. The volume of that fluid is the cross sectional areatimes the length. The time, in this case, is one second. This gives rise to the basicformula for flow in hydraulics:

Flow = Area x Velocity, or Q = A x V.


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Laminar Flow

Figure 19 - Laminar flow

We would like to think of flow in a hydraulic system as a smooth transition of fluid fromone point to another; all particles of the fluid would be moving parallel to all otherparticles, and there would be no turmoil within the fluid. This we would call laminarflow (Figure 19), and it is very desirable.

Turbulent Flow

Figure 20 - Turbulent flow

In fact, hydraulic system flow often experiences more turmoil than is desirable.Although the fluid generally move in the direction which is required, it also travelsthrough small conduits, across sharp-edged restrictions, through small orifices,around sharp bends, in fact, through all the places that have a tendency to causeanything but a nice, smooth transition.

Particles of the fluid are travelling helter-skelter among each other (see Figure 20),causing friction and inefficient movement. This type of flow, called turbulent flow, isundesirable and wasteful. Unfortunately, the economic and practical aspects of mobilefluid power result in most flow being in the turbulent variety.

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PRESSURE DROP

Figure 21 - Flow past an orifice creates a pressure drop

When fluid flows across an orifice, as in Figure 21, it loses some of its energy. This isreflected in a lower pressure at the downstream side of the orifice, as illustrated by thetwo gauges. The difference between the upstream and downstream pressure is called apressure drop; it is the drop in pressure caused by the flow and the restriction (orifice).

The magnitude of the pressure drop will vary, depending upon:

The rate of flow passing across the orifice

The size of the orifice

The ease with which the fluid will flow (viscosity).

The downstream flow must be the same as the upstream flow in Figure 21, becausethere is nowhere for the fluid to escape. However, if the pressure in the fluid is lower,then the energy in the fluid is less. A law of physics states that energy cannot bedestroyed, therefore the difference in energy must be given off in the form of heat.

Figure 22 - If there is no flow across an orifice, there is no pressure drop

If the magnitude of the pressure drop is dependent on the amount of flow passing therestriction, then it stands to reason that if there is no flow, there will be no pressuredrop. This is demonstrated by Figure 22; there being no flow across the orifice willresult in equal pressure on both sides. With no flow and no pressure drop, there willbe no heat rejected due to a drop in energy.


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This direct relationship between flow and pressure drop is an important considerationin hydraulics; if there is no flow between point A and point B, there will be no pressuredrop. Conversely, if there is no difference in pressure between points A and B, there isno fluid flow between these two points.

Bernoulli’s Principle

Figure 23

Bernoulli’s Principle tells us that the sums of pressure and kinetic energy at variouspoints in a system must be constant, if flow is constant. When a fluid flows throughareas of different diameters as shown in Figure 23, there must be correspondingchanges in velocity. At the left, the section is large so velocity is low. In the centre,velocity must be increased because the area is smaller. Again, at the right, the areaincreases to the original size and the velocity again decreases.

Bernoulli proved that the pressure component at C must be less than at A and Bbecause velocity is greater. An increase in velocity at C means an increase in kineticenergy. Kinetic energy can only increase if pressure decrease. At B, the extra kineticenergy has been converted back to pressure and flow decreases. If there is nofrictional loss, the pressure at B is equal to the pressure at A.

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Figure 24

Figure 24 shows the combined effects of friction and velocity changes. Pressuredrops from a maximum at C to zero at B. At D, velocity is increased, so the pressurehead decreases. At E, the head increases as most of the kinetic energy is given up topressure energy because velocity is decreased. Again, at F, the head drops asvelocity increases.

Put simply, Bernoulli’s Principle is indicating that:

As flow increases, pressure decreases

As flow decreases, pressure increases.

Summary for some key Hydraulic principlesHydraulic “work done” is a combination of pressure, and flow, over time.

Pressure without flow results in no action.

Flow without pressure results in no action.

Hydraulic pressure is a result of resistance to flow and in force:

Increase in flow, decrease in pressure

Decrease in flow, increase in pressure.

Hydraulic flow is movement.


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Series & Parallel Circuits

Figure 25

Most machines require multiple components that can be connected in series orparallel (Figure 25).

When components are connected in series (1), fluids flow from one component to thenext, before returning to the tank. When components are connected in parallel (2),fluid flows through each component simultaneously.

Restrictions in Series

Figure 26

In Figure 26, a pressure of 620 kPa (90 psi) is required to send 4 litres per minute(lpm) through either circuit.

Orifices or relief valves in series in a hydraulic circuit offer a resistance that is similarto resistors in series in an electrical circuit, in that the oil must flow through eachresistance. The total resistance equals the sum of each individual resistance.

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Restriction in Parallel

Figure 27

In a system with parallel circuits, pump oil follows the path of least resistance. InFigure 27, the pump supplies oil to three parallel circuits. Circuit three has the highestresistance and therefore would have the lowest priority. Circuit one has the lowestresistance and therefore would have the highest priority.

When the pump oil flow fills the passage from the pump to the valves, pump oilpressure increases to 207 kPa (30 psi). The pressure created by the restriction of oilflow, opens the valve to circuit one and oil flows into the circuit. Circuit pressure willnot increase until circuit one if full. When circuit one fills, fluid pressure will increaseto 414 kPa (60 psi) and opens the valve in circuit two. Again, circuit pressure will notincrease until circuit two is full. Pump oil pressure must exceed 620 kPa (90 psi) toopen the valve in circuit three.

There must be a system relief valve in one of the circuits or at the pump to limit themaximum pressure in the system.


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HYDRAULICS DOING WORK

Figure 28

In order to perform useful work, a hydraulic system must convert and control energy asit flows from one component to the next.

Figure 28 above represents the key conversion and control points in the system.

The hydraulic system receives input energy from a source, normally from an engine orrotating gear train. The hydraulic pump converts the energy into hydraulic energy inthe form of flow and pressure. Valves control the transfer of hydraulic energy throughthe system by controlling fluid flow and direction. The actuator (which can be either acylinder or a hydraulic motor) converts hydraulic energy into mechanical energy in theform of linear or rotary motion, which is used to perform work.

To perform hydraulic work, both flow and pressure are required. Hydraulicpressure is force and flow provides movement.

Imperial/Metric Conversion Factors

Length 0.03937 inches (ins) = 1 millimetre (mm)

0.3937 inches (ins) = 1 centimetres (cm))

39.37 inches (ins)=1 metro (m)

1 inch (in or “) = 25.4 millimetres (mm)

1 foot (ft or ‘) = 0.3048 metres.

Area0.00155 ins2 = 1 mm2

0.155 ins2 = 1 cm2

1 square inch (in2) = 6.452 square centimetres (cm2).

Volume0.061 in3 = 1 cm3

61.02 in3 = 1 litre (L)

0.22 Imperial gallon = 1 litre (L)

0.2642 U.S. Gallon = 1 litre (L)

1 cubic inch (in3) = 16.39 cubic centimeters (cm3 or cc)

1 imperial gallon (imp gal)= 4.546 litres (lt)

1 US gallon (US gal) = 3.785 litres (lt).

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Mass2.205 pounds (lb) = 1 kilogram (kg)

0.9844 tons (t) = 1 tonne (t)

1 pound (lb) = 0.4536 kilograms (kg).

Velocity196.8 feet per minute (ft/min) = 1 meter per second (m/s).

Force0.2248 pounds force (lb.force) = 1 Newton (N)

0.1004 tons force (t.force) = 1 KiloNewton (kN).

Pressure0.145 pounds per square inch (psi) = 1 Kilopascal (kPa) Note: 101.325 kPa = 1 Atmosphere (atms)

1 kg/sq.cm = 14.22 psi or 0.9678 atms or 100 kPa.

Torque 0.7376 pound foot (lb.ft) = 1 Newton Meter (Nm)

7.23 (lb.ft) = 1 Kg/m.

Temperature Degrees Fahrenheit (°F) = °C x 1.8 + 32 (Degree Celsius°C).

Power1 kilowatt (kW) = 1.341 Horsepower (hp) Note: 1 watt (w) = 1 Nm/s.

Prefixes Commonly Linked to Base Units Micro = 0.000 00 One Millionth

Milli = 0.001 One Thousandth

Centi = 0.01 One Hundredth

Deci = 0.1 One Tenth

1.0 = One

Deca = 10.0 Ten

Hecto = 100.0 One Hundred

Kilo = 1000.0 One Thousand

Mega = 1000000.0 One Millionth.


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TOPIC 2Hydraulic Circuit & Components

Figure 29

Mobile earthmoving machines are designed with hydraulic systems which use avariety of hydraulic components to provide efficient operation.

Hydraulic cylinders of various types are used to operate implements, e.g. buckets,blades, rippers, backhoes and truck beds.

Hydraulic motors drive tracks, wheels, car bodies and conveyors.

Brakes, steering, transmissions, suspensions and other vehicle systems rely onhydraulics for power and control.

Figure 29 depicts a basic hydraulic system. For a basic hydraulic system to operate(e.g. cylinder extend and retract), it must contain the following components:

fluid (A)

reservoir (B)

filter (C)

pump (D)

directional control valve (E)

actuator or hydraulic cylinder (F)

lines (G)

pressure control valve (H)

cooler (I).

Most manufacturers use graphic symbol circuits to identify the circuit components, andto illustrate the circuit function and operation.

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Figure 30

GRAPHIC SYMBOLSGraphic symbols for fluid power diagrams (Figure 30) were originally developed bythe American National Standards Institute (ANSI) and are presently adopted by theInternational Standards Organisation (ISO). They provide communication standardsthat serve industry and education. The standards simplify design, fabrication,analysis, and servicing of fluid power systems.

The symbols describe component function rather than construction. In addition, theyshow how some of the fluid power components operate pneumatically, hydraulically,electrically, manually, and the like.

In order to use the graphic symbol system to its maximum potential, the followingrules must be understood and followed:

1. Symbols show connections, flow paths, and functions of components. They donot indicate conditions occurring during transition from one flow arrangement toanother. Further, they do not indicate construction or values, such as pressure,flow rate, and other component settings.

2. Symbols do not indicate location of ports, shifting of spools, or position of controlelements on an actual component.


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3. The position or size of a symbol can be altered for component emphasis in acircuit without changing its meaning.

4. Each symbol is drawn to show normal or neutral condition of a component unlessmultiple circuit diagrams are furnished showing various phases of circuit operation.

5. Arrows used within a symbol envelope show direction of flow in a component asused in the application represented. Double-end arrows indicate reverse flow.

The graphic symbols utilize elementary geometrical forms to depict components andcircuits. These forms include: circles, squares, rectangles, triangles, arcs, arrows,lines, dots, and crosses.

Elemental Symbols

Figure 31 - Supplemental Component Symbols

The first elemental symbols are energy triangles (Figure 31). A triangle is used torepresent a conversion point of energy and its direction of flow. A shaded or darkenedtriangle as seen to the left indicates the energy medium is a fluid, such as hydraulicoil. The clear triangle on the right denotes the energy medium is gaseous. Orientationof the triangle will indicate the direction of energy flow into or out of a component. Adark triangle pointing out of a component envelope would indicate the component isgenerating energy (such as a pump) and that the medium is a fluid. Another examplemight be a clear triangle pointing into the component, meaning the energy medium ispneumatic and the component is absorbing or using this energy to do work (such as apneumatic motor). We will see examples of these later.

Rotating shafts are shown by a short solid line connected to the component outline.An arrow is used to indicate the direction of rotation. The arrow is always assumed tobe on the near side of the shaft, and may denote either uni or bidirectional rotation.

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Basic Components

Figure 32 - Basic Component Symbols

The basic component symbols used are the outlines of circles, squares,diamonds, rectangles and a few other geometrical features (Figure 32). Theseraw component symbols or envelopes will have supplemental component symbolsor elements added to them in order to create or, illustrate a specific type ofcomponent such as a valve, pump, or motor.

Size of the component's outline may be varied to emphasize certain components orindicate a difference between a main and auxiliary component. Otherwise, variationsin size are not an indication of the components' physical sizes.

Tubes, Hoses and Internal PassagesThree types of lines are used in graphic symbol illustrations to represent tubes, hosesand internal fluid passages that connect the hydraulic components.

1. Working Line:

A solid line is used to show a hydraulic working line. The working line carries themajor flow of oil in a hydraulic system.

2. Pilot Line:

A dashed line is used to show a hydraulic pilot line. The pilot line carries a smallvolume of oil used as an auxiliary flow to actuate or control a hydrauliccomponent. The length of the dash will be drawn at least ten times its width.

3. Drain Line:

A dashed line is used to show a drain line that carries leakage oil back to thereservoir. The drain line will be illustrated as a dashed line with the length of thedash less than five times its width.


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Some other symbols are:

1. Enclosure Line:

The enclosure line is used to illustrate the boundary of the area on a machine, inwhich hydraulic components are contained. e.g. an operation compartment, or thefront half of an articulated machine.

2. Instrument Line:

The instrument line is used to connect an instrument to its sensor.

Crossing Lines

Figure 33 Techniques for crossing and joining lines

Here are two techniques used for representing both lines crossing and joining(Figure 33). It is important to note that graphic symbols may be drawn several differentways to represent the same thing.

First let's take a look at the techniques, where A & B are used to represent linescrossing. In method “A” a small half circular line is used to jump or cross another line.The alternative method “B” merely shows the lines crossing each other.

On the right two methods are used for joining lines. The standard and most desirableway to show lines which join is to use a solid dot at the point of junction as seen in “C.”An alternative method is seen at “D” in which the lines join without the connectingdots. This may appear confusing at first, since this is also a way in which linescrossing may be shown as in “B.” The way to determine the type of junction you havewill depend on which of the other techniques is used throughout the rest of the circuitto represent line crossing or junctions.

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Figure 34 - Junctions and Crossings

The key to determining the type of line junction, seen in Figure 34 at point (1) in views“B” or “C”, will be dependent on the other technique used throughout the circuit. If asmall half circular line is used to depict lines crossing, then lines joining may be drawnwith or without a connecting dot, as seen in views “A” or “B”. Both methods arecorrect. Whichever method is chosen, it must be used consistently throughout theentire circuit. In a similar fashion when a connecting dot is used to indicate linesjoining throughout the circuit, then lines crossing can be shown with the half circularline as in “A” or without it as shown in “C”. The important point to remember is that thetechnique chosen will be used consistently throughout the circuit.

View “D” shows what would happen if the same method was used to illustrate bothline crossings and junctions in a circuit. This is incorrect because no one would beable to differentiate between the two.


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HYDRAULIC LINESHydraulic lines are used to connect the various components together to allowtransmission of fluid in the circuit. The lines can be either tubes or hoses.

Figure 35

A tube (Figure 35) is a rigid hydraulic line, usually made of steel. Tubes are used toconnect components that do not move in relation to each other. Tubes also generallyrequire less space than hoses and can be firmly attached to the machine, resulting inbetter protection to the lines and a better overall machine appearance.

Figure 36

Hydraulic hoses (Figure 36) are used whenever flexibility is needed, such as whencomponents move in relation to each other. Hoses absorb vibration and resistpressure variations.

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HYDRAULIC TANK

Figure 37

When construction machines and equipment are in the design stage, considerablethought is given to the type, size and location of the hydraulic oil tank (Figure 37).Once the machine or equipment is in operation, the hydraulic tank functions as astorage place for the hydraulic oil, a device to remove heat from the oil, a separator toremove air from the oil and allows particles to settle out of the oil.

The hydraulic oil tank main function is to store oil and ensure there is enough oil forany requirements of the system. Tanks must have sufficient strength, adequatecapacity and keep dirt out. Hydraulic tanks are usually but not always sealed.

Tanks are mounted in any convenient location, sometimes as part of a majorcomponent housing.

The components of the tank are:

Fill cap

Sight glass

Supply and return line

Drain.

Fill Cap

Figure 38

The fill cap keeps contaminants out of the opening that’s used to fill and add oil to thetank and seals pressurized tanks (Figure 38).


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Sight Glass

Figure 39

The sight glass (Figure 39) is used to check the oil level according to the operationand maintenance manual. The oil level is usually correct when the oil is in the middleof the sight glass. The oil level should be checked when the oil is cold. Refer tomanufacturer’s specifications for the correct procedures for reading oil level.

Breather

Figure 40

The breather (Figure 40) is fitted to unpressurised tanks and allows atmosphericpressure to flow in and out of the tank.

DrainLocated at the lowest point in the tank, the drain is used to remove old oil from thetank. The drain also allows for the removal of water and sediment from the oil

Drain plugs often contain a strong magnet to capture particles at the bottom of the tank.

Supply and Return LinesThe supply line allows oil to flow from the tank to the system. The return line allows oilto flow from the system to the tank.

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Pressurised Tank

Figure 41

The pressurised tank is completely sealed. Atmospheric pressure does not affect thepressure in the tank. When the oil circulates through the system, it absorbs heat andexpands. The expanding heated oil compresses the air in the tank and creates apositive pressure in the system.

Pressurised tanks prevent the entry of dirt and moisture and help force oil into thehydraulic pump. Air pressurised reservoirs receive air from the vehicles compressedair system although these are not used often.

Never open the reservoir when it is pressurised.

Thermally pressurised tanks rely on the expansion of the oil as it is heated to exert aslight pressure on the surface of the oil.

The pressure relief valve controls the pressure in the tank and the vacuum valveprevents negative pressures when the system cools.

Vacuum/Relief Valve The vacuum/relief valve serves two purposes. It prevents a vacuum and limits themaximum pressure in the tank.

The vacuum relief valve prevents a vacuum by opening and allowing air to enter thetank when the tank pressure drops to 3.45 kPa (0.5 psi). When pressure in the tankreaches the vacuum relief valve pressure setting, the valve opens and ventscompressed air to the atmosphere.

The vacuum relief valve pressure setting may vary from 70 kPa (10 psi) to 207 kPa (30 psi).

Filler ScreenPrevents large contaminants from entering the tank.

Filler TubeAllows the tank to be filled to the correct level, but not over-filled.


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BafflesPrevents the return oil from flowing directly to the tank outlet, allowing time for bubbles torise to the top. It prevents the oil from sloshing which helps reduce foaming of the oil.

Ecology DrainUsed to prevent accidental spills when removing water and sediment from the tank.

Return LineReturns oil from the hydraulic circuit/s to the tank.

Return screenPrevents larger particles from entering the tank, but does not provide fine filtering.

Pump Pick-up LineThe pump pick-up line directs oil to the inlet side of the pump. The line does notnormally touch the bottom of the tank. This prevents sediment at the bottom of thetank being directed to the pump.

Vented Tank

Figure 42 - Vented tank and ISO Symbol

The vented tank is the most common type of tank. It has a breather that allows air toenter and exit freely. Therefore, pressure inside the tank is atmospheric. An air cleaningelement or screen is usually fitted in the vent to reduce the entry of airborne dust.

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GRAPHIC SYMBOL - HYDRAULIC TANK

Figure 43

The vented reservoir symbols (Figure 43) are shown to the left and are merely anopen-topped box or rectangle. A pressurised reservoir is drawn as a completelyclosed box. Return line connections can be either above or below the fluid level.Examples of both types are shown. The reservoir symbol is one symbol which can bedrawn as many times as necessary on a circuit schematic to reduce the number oflines which must be drawn. Although the symbol may be drawn several times, it onlyrepresents one tank, unless otherwise specified.

Fluid Filters

Figure 44 - Basic Symbol

The general symbol used for a fluid filter is an empty diamond as seen in Figure 44above. Several different types of fluid conditioner symbols can be made by makingslight changes or additions to this basic symbol.

Figure 45

First is a filter or strainer, shown in Figure 45 above, with a dashed vertical line inside thebasic symbol. This represents the filtering media which the fluid must flow through.


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Figure 46

A line drawn horizontally across the bottom portion of the symbol, as shown in Figure 46,represents the separated matter that has settled to the bottom, such as water.

Figure 47

A short line extending from the bottom of the symbol, shown in Figure 47, represents amanual drain. If the short line is not on the symbol, it must be assumed to be a manualdrain. Nothing on the symbol will indicate

Figure 48

Automatic drains will be represented by a small ‘V’ placed below the horizontalseparation line as shown in Figure 48.

HYDRAULIC PUMPSThe hydraulic pump transfers mechanical energy into hydraulic energy. It is a devicethat takes energy from one source (i.e. engine, electric motor, etc.) and transfers thatenergy into a hydraulic form.

The function of the pump is to supply the hydraulic system with a sufficient flow of oilto enable the circuits to operate at the correct speed.

Pumps can generally be classified into two types:

non-positive displacement

positive displacement.

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Figure 49

Figure 49 is showing a gear type pump. The gears are in mesh and rotated by apower source. The pump takes oil from a storage container (i.e. tank) and pushes itinto a hydraulic system.

All pumps produce oil flow in the same way. A partial vacuum is created at the pumpinlet and outside pressure (tank pressure and/or atmospheric pressure) forces the oilto the inlet passage and into the pump inlet chambers. The pump idler gears carry theoil to the pump outlet chamber.

With each element of fluid that is discharged from a hydraulic pump, an equal amountmust be available at the inlet side to replace it. The availability of the fluid at the inletis entirely dependent upon the reservoir pressures that force the fluid into the pump.

The larger the pump, or the faster the pump runs, the more fluid is needed to replacethe amount that is discharged. This will depend upon there being adequate pressurein the reservoir to force fluid into the pump. Without sufficient pressure, “starvation” ofthe pump will occur, and this will cause severe damage to the pump components, andultimately cause pump failure.

There are many factors that can hinder the flow of fluid between the reservoir and the pump:

A fluid line that is too small for the volume of fluid going through it.

A clogged outlet on the reservoir.

A pump that is located too far away from the reservoir, or too far above it.

A fluid that is too viscous to flow easily.

When one or more of these conditions exist to the point that "starvation" of the pumpbegins to occur, they must be corrected immediately.

Pumps DO NOT produce or cause ‘pressure’. The resistance to the flow causespressure. Resistance can be caused by flow through hoses, orifices, fittings,cylinders, motors, or anything in the system that hinders free flow to the tank. Pumpscreate flow only.


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Positive Displacement PumpA positive displacement pump will discharge a specified amount of fluid during eachrevolution or stroke, almost regardless of the restriction on the outlet side. Because ofthis characteristic, positive displacement pumps are nearly always the pump of choicein hydraulic systems.

Figure 50

The hand pump illustrated in Figure 50 provides an example of the operation of apositive displacement pump.

Positive displacement hydraulic pumps are designated by their volume ofdisplacement, such as gallons per minute, litres per minute, cubic inches or cubiccentimeters per revolution. This designation is usually a theoretical displacement, anddoes not allow for any losses that may occur within the pump due to internal leakage.

Positive displacement pumps have small clearances between components. Thisreduces leakage and provides a much higher efficiency when used in a high-pressurehydraulic system. The output flow in a positive displacement pump is basically thesame for each pump revolution. Both the control of their output flow and theconstruction of the pump classify positive displacement pumps.

Positive displacement pumps are rated in two ways. One is by the maximum systempressure (21,000 kPa or 3000 psi) at which the pump is designed to operate. Thesecond is by the specific output delivered either per revolution or at a given speedagainst a specified pressure. As an example a pump maybe rated in lpm @ rpm @kPa (i.e.380 lpm @ 2000 rpm @ 690 kPa).

When expressed in output per revolution, the flow rate can be easily converted bymultiplying by the speed, in rpm, (i.e. 2000 rpm) and dividing by a constant. Forexample, we will calculate the flow of a pump that rotates 2000 rpm and has a flow of11.55 in3 /rev or 190 cc/rev.

GPM = in3 /rev X rpm/231 LPM = cc/rev X rpm/1000

GPM = 11.55 X 2000/231 LPM = 190 X 2000/1000

GPM = 100 LPM = 380.

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Non Positive Displacement Pump

Figure 51

The outlet flow of a non-positive displacement pump is dependent on the inlet andoutlet restrictions. The greater the restriction on the outlet side, the less flow thepump will discharge. An example of a non-positive displacement pump is the waterpump rod on an engine (Figure 51).

The centrifugal impeller is an example of a non-positive displacement pump andconsists of two basic parts; the impeller (2) that is mounted on an input shaft (4) andthe housing (3). The impeller has a solid disc back with curved blades (1) moulded onthe input side.

Fluid enters the centre of the housing (5) near the input shaft and flows into theimpeller. The curved impeller blades propel the fluid outward against the housing. Thehousing is shaped to direct the oil to the outlet port.


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GEAR PUMPS

Figure 52

Gear pumps (Figure 52) are positive displacement pumps. They deliver the sameamount of oil for each revolution of the input shaft. Changing the speed of rotationcontrols the pump’s output. The maximum operating pressure for gear pumps islimited to 27,579 kPa (4000 psi). This pressure limitation is due to the hydraulicimbalance that is inherent in the design. The hydraulic imbalance produces a side loadon the shafts that is resisted by the bearings and the gear teeth to housing contact.The gear pump maintains a ‘volumetric efficiency’ above 90% when pressure is keptwithin the designed operating pressure range.

Figure 53

Figure 53 shows the components of the gear pump: seal retainers (1), seals (2), sealback- ups (3), isolation plates (4), spacers (5), a drive gear (6), an idler gear (7), ahousing (8), a mounting flange (9), a flange seal (10) and pressure balance plates (11)on either side of the gears.

Bearings are mounted in the housing and mounting flange on the sides of the gears tosupport the gear shafts during rotation.

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Pressure Balance Plates

Figure 54

There are two different types of pressure balance plates used in gear pumps(Figure 54). The earlier type (1) has a flat back. This type uses an isolation plate, aback up for the seal, a seal shaped like a 3 and a seal retainer. The later type (2) hasa groove shaped like a 3 cut into the back and is thicker than the earlier type. Twodifferent types of seals are used with the later type of pressure balance plates.

Gear Pump Flow The output flow of the gear pump is determined by the tooth depth and gear width.Most manufacturers standardised on a tooth depth and profile determined by thecentreline distance (1.6", 2.0", 2.5", 3.0", etc.) between gear shafts. Withstandardised tooth depths and profiles, the tooth width totally determines flowdifferences within each centreline classification.

Figure 55

As the pump rotates (Figure 55), the gear teeth carry the oil from the inlet to the outletside of the pump. The direction of rotation of the drive gear shaft is determined by thelocation of the inlet and outlet ports and drive gear will always move the oil around theoutside of the gears from inlet to outlet port. This is true on both gear pumps and gearmotors. On most gear pumps the inlet port is larger in diameter than the outlet porttoo ensure that there is always an ample supply of oil for the demand of the systemand to ensure pump starvation does not occur. On bi-directional pumps and motors,the inlet port and outlet port will be the same size.


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Gear Pump Forces The outlet flow from a gear pump is created by pushing the oil out of the gear teeth asthey come into mesh on the outlet side. The resistance to oil flow creates the outletpressure. The imbalance of the gear pump is due to outlet port pressure being higherthan inlet port pressure. The higher pressure oil pushes the gears toward the inlet portside of the housing.

The shaft bearings carry the majority of the side load to prevent excessive wearbetween the tooth tips and the housing. On the higher pressure pumps, the gearshafts are slightly tapered from the outboard end of the bearings to the gear. Thisallows full contact between the shaft and bearing as the shaft bends slightly under theunbalanced pressure.

The pressurised oil is also directed between the sealed area of the pressure balanceplates and the housing and mounting flange to seal the ends of the gear teeth. Thesize of the sealed area between the pressure balanced plates and the housing is whatlimits the amount of force that pushes the plates against the ends of the gears.

VANE PUMP

Figure 56

As shown in Figure 56, vane pumps are positive displacement pumps. The pumpoutput can be either fixed or variable.

Both the fixed and variable vane pumps use common part nomenclature. Eachpump consists of the housing (1), cartridge (2), mounting plate (3), mounting plateseal (4), cartridge seals (5), cartridge backup rings (6), snap ring (7), and inputshaft and bearing (8).

The cartridge consists of the support plates (9) displacement ring (10), flex plates(11), slotted rotor (12) and the vanes (13).

The input shaft turns the slotted rotor. The vanes move in and out of the slots in therotor and seal on the outer tips against the cam ring.

The inside of the fixed pump displacement ring is elliptical in shape.

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The inside of the variable pump displacement ring is round in shape.

The flex plates seal the sides of the rotor and the ends of the vanes. In some lowerpressure designs, the support plates and housing seal the sides of the rotating rotorand the ends of the vanes.

The support plates are used to direct the oil into the proper passages in the housing.The housing, in addition to providing support for the other parts of the vane pump,directs the flow in and out of the vane pump.

Vanes

Figure 57

The vanes (Figure 57) are initially held against the displacement ring by centrifugalforce created by the rotation of the rotor. As flow increases, the resultant pressurethat builds from the resistance to that flow is directed into passages in the rotorbeneath the vanes (1).

This pressurised oil beneath the vanes keep the vane tips pushed against thedisplacement ring to form a seal. To prevent the vanes from being pushed too hardagainst the displacement ring, the vanes are bevelled back (arrow) to permit abalancing pressure across the outer end.

Flex Plates

Figure 58

The same pressurised oil is also directed between the flex plates and the supportplates to seal the sides of the rotor and the end of the vanes (Figure 58). The size ofthe seal area between the flex plate and the support plates is what controls the forcethat pushes the flex plates against the sides of the rotor and the end of the vanes. Thekidney shaped seals must be installed in the support plates with the rounded o-ringside into the pocket and the flat plastic side against the flex plate.


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Vane Pump Operation

Figure 59

When the rotor rotates around the inside of the displacement ring Figure 59, the vanesslide in and out of the rotor slots to maintain the seal against the displacement ring. Asthe vanes move out of the slotted rotor, the volume between the vanes changes. Anincrease in the distance between the displacement ring and the rotor causes anincrease in the volume. The increase in volume creates a slight vacuum that allows theinlet oil to be pushed into the space between the vanes by atmospheric or tankpressure. As the rotor continues to rotate, a decrease in the distance between thedisplacement ring and the rotor causes a decrease in the volume. The oil is pushedout of that segment of the rotor into the outlet passage of the pump.

The vane pump just described is known as the “unbalanced” vane pump.

Figure 60

Figure 60 shows the balanced design principle. This design has opposing sets of inletand outlet ports. Since the ports are positioned exactly opposite each other, the highforces generated at the outlet ports cancel each other out. This prevents side-loadingof the pump shaft and bearings and means that the shaft and bearings only have tocarry the torque load and external loads. Since there are two lobes to the cam ring perrevolution, the displacement of the pump is equal to twice the amount of fluid which ispumped by the vanes moving from one inlet to its corresponding outlet.

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PISTON PUMP

Figure 61

Most piston pumps and motors have similar or common parts and use the samenomenclature. The pump parts in the Figure 61 are the head (1), housing (2), shaft(3), pistons (4), port plate (5), barrel (6) and the swashplate (7).

The two designs of piston pumps are the axial piston pump and the radial pistonpump. Both pumps are highly efficient, positive displacement pumps. However, theoutput of some pumps is fixed and the output of some pumps is variable.

Axial Piston Pumps

Figure 62 - Fixed & Variable Displacement

The fixed displacement axial piston pumps and motors are built in a straight housing orin an angled housing. The basic operation of piston pumps and motors are the same.

Straight Housing Axial Piston Pumps and Motors Figure 62 shows an illustration of the positive displacement fixed output axial pistonpump and the positive displacement variable output axial piston pump. Mostpublications assume that the fact that both pumps are positive displacement and referto the pumps as fixed displacement pumps and variable displacement pumps.

In the fixed displacement axial piston pumps, the pistons move backward and forwardin a line that is near parallel to the centreline of the shaft.

In the straight housing piston pump shown in Figure 62, the pistons are held against afixed, wedge-shaped swashplate.


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The angle of the swashplate controls the distance the pistons move in and out of thebarrel chambers. The larger the angle of the wedge-shaped swashplate, the greaterthe distance of piston movement and the greater the pump output per revolution.

In the variable displacement axial piston pump, either the swashplate or the barrel andport plate may pivot back and forth to change its angle to the shaft. The changingangle causes the output flow to vary between the minimum and maximum settingsalthough the shaft speed is held constant.

On either pump, when a piston moves backward, oil flows through the intake anddisplaces the piston. As the pump rotates, the piston moves forward, the oil is pushedout through the exhaust creating flow into the system.

Most piston pumps used on mobile equipment are axial piston pumps.

Angled Housing Axial Piston Pump

Figure 63

In the angled housing piston pump in Figure 63, the pistons are connected to the inputshaft by piston links or spherical piston ends that fit into sockets in a plate. The plate isan integral part of the shaft. The angle of the housing to the shaft centreline controlsthe distance the pistons move in and out of the barrel chambers.

The larger the angle of the housing, the greater the pump output per revolution.

The output flow of a fixed displacement piston pump can only be changed by changingthe input shaft speed.

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GRAPHIC SYMBOL - PUMP

Figure 64 - Fixed Displacement

Pump ISO symbols are distinguished by a dark triangle in a circle, with the point ofthe triangle pointing toward the edge of the circle.

Figure 65 - Variable Displacement Non-Compensated

LINEAR ACTUATORS

Figure 66

Actuator is the general term used for the output device of hydraulic systems. Twobroad categories are rotary actuators, that deliver their power in a rotating or circularmotion, and linear actuators (Figure 66) that deliver their power in a straight line.

Hydraulic cylinder is the most common term for linear actuators, although other termssuch as "ram", "jack", or "stroker" are frequently used. These other terms often haveapplication specific meanings, so cylinder or hydraulic cylinder will be used todescribe the majority of linear actuators.

As discussed, power in a hydraulic system is generated initially from a rotating device,such as an IC engine, and converted to fluid flow by a pump. The flow is directedthrough the system to the actuators, where it is converted to rotary power by motors,or into linear power by cylinders. It can be said that without actuators, there is noreason for hydraulic systems to exist as the actuator actually does the work. Forceand motion are produced in a straight line to operate machine implements, eg.blades,buckets, rippers etc.


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Construction

Figure 67

SealsMost cylinders have two locations where fluid must be sealed: across the piston andaround the rod. Cylinders must also be sealed between the body and the two heads.There are many different types of seals for each of the these purposes depending onthe fluid being used, the integrity of the sealing required and the desired services life.

Figure 68 - Seal Construction

Rod seals are a flexible material that is held against the rod surface by a combinationof initial compression (the seal inside diameter is slightly smaller than the rod outsidediameter) and hydraulic pressure acting against it. A simple “O” ring seal with back-uprings may be used, a lip-type seal is a popular design although the “U” cup or “V” cuppacking are most commonly used. Typical seal designs are shown in Figure 68.

The lip seal is moulded material, usually moulded onto a metal or hard plastic frame. A coilspring may be inserted over the lip to provide initial contact of the lip to the sliding orrotating surface. As with the U-cup or V-cup, the concave side of the seal faces thepressure, and the lip is forced against the sealing surface by pressure to create a tight seal.

Materials used for seals are usually synthetic rubber, although rubber compounds andplastic compounds are also used. The main criteria for material selection is compatibilitywith the fluid being used, wear resistance and temperature conformance.

Seal wear depends a great deal on factors other than the material used; lubricationqualities and cleanliness of the fluid in contact with the seal is by far the most important.Also, for the seal to be lubricated properly, it must be kept “wet” by the fluid.

A “perfect” seal would be one that prevents all leakage. In practice, however, a minuteamount of lubrication film must be present for the seal to slide easily over the matingsurfaces. In most applications, a seal is considered effective if there is no obviouslydetected quantity of fluid passing it.

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There are two basic types of cylinder.

1. Single acting

2. Double acting.

Single acting cylinders

Figure 69

Figure 69 is a schematic view of a single acting cylinder.

The dark area represents oil under pressure and lighter coloured area is oil attank pressure.

Single acting cylinders use pressure oil from one end of the cylinder and provide forcein one direction only.

They are retracted using the weight of the load or spring force.

Single acting cylinders are rarely used in mobile equipment.


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Figure 70

The simplest single acting cylinder is the hydraulic ram (Figure 70). It has only onefluid chamber and exerts force in only one direction. Most are mounted or usedvertically and retract by the force of gravity. Practical for long strokes, rams can befound in “bottle” jacks and automobile hoists.

Single acting cylinders apply a force in one direction, relying on gravity or a counter-force to retract. The primary difference between a single-acting cylinder and a ram isthe single acting cylinder uses a piston, and leakage flow past the piston is ported tothe reservoir to minimise external leakage. Single-acting cylinders re typically used fortruck hoists and crane booms.

Double acting cylinders

Figure 71

Figure 71 shows a double acting cylinder. Darker shaded oil is oil under pressure andlighter shaded oil is at tank pressure. This is the most common hydraulic actuator usedtoday on mobile equipment. It is used on the implement, the steering and othersystems where the cylinder is required to do work in both directions.

Double acting means that the cylinder will provide force and movement in each direction.

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Figure 72

Double-acting cylinder (Figure 72): Perhaps the most popular type of cylinder onmobile equipment, the double-acting cylinder exerts force in both directions,extending and retracting. To extend, fluid is ported into the cap end of the cylinder andthe rod end port is vented to the reservoir. During retraction, fluid is ported into therod end of the cylinder, and the cap end port is vented to the reservoir. Double-actingcylinders are also called differential cylinders because the effective area, andtherefore volume, of each end is different by virtue of the space taken up by the rodarea and volume. This differential area and volume causes a different force andvelocity during extension and retraction.

Figure 73

A variation of the double-acting cylinder is the double-rod cylinder. In this version, thecylinder rod extends through both end caps (Figure 73), thus equalising the area andvolume between both ends of the cylinder. This equalises the forces and velocitiesduring extension and retraction. A typical use for double-rod cylinders is in powersteering applications.


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TELESCOPIC CYLINDERS

Figure 74

Most telescopic cylinders (Figure 74) are single-acting. Telescopic cylinders consist ofa series of nested tubular rod segments called sleeves. Each sleeve extendsindividually during extension. There may be two, perhaps three, and possibly up to fivesleeves in a cylinder. A long working stroke and a short collapsed length result,making them ideal for applications such as industrial lift trucks and large tilt bed ordump trucks. An inherent feature of telescopic cylinders, due to the sequencingsmaller diameters of sleeves, is a reduction of force capability and an increase ofvelocity during each succeeding stage.

Figure 75

Telescopic cylinders may also be double-acting, see Figure 75, although these are nottoo common. Because of the very small areas involved in the retracting phase,retracting forces are quite low. Double-acting telescopic cylinders are typically used inrefuse haulers as high compaction forces and a long stroke are required, but retractionforce requirements are very small.

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MODIFIED CYLINDERSThere are many special configurations of linear actuators that customise them forparticular applications. Some of these are proprietary to specific machinemanufacturers, and are not typical beyond those applications. Others, though, aremore generally available and may be considered standard options.

Counterbalance valve

Figure 76

It frequently can be advantageous to incorporate some specific valve requirementswithin the cylinder, usually into the cap end head. The popularity of screw-in cartridgevalves has made some of these features relatively easy and inexpensive toaccomplish. Although the counterbalance valve (Figure 76) is probably the mostpopular type of integral valve, other types such as directional control, flow control andsequence valves can be found.

Cushions

Figure 77

Frequent and abrupt end-of-stroke stops can damage a cylinder; cylinders that extendand/or retract at high speed can fail catastrophically in just a few strokes. Cylindercushions are a fairly common feature on mobile equipment that helps to slow thepiston down near the end of its stroke and reduce the impact. Cushions may be foundon one or both ends of a cylinder to act as hydraulic brakes during cycling. Figure 77shows one type of cushion used on the retraction stroke.


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Stroke limiting

Figure 78

Although a simple stop tube (Figure 78) can be used to limit the travel of a cylinder, itis frequently desirable to externally adjust the stroke travel.

Figure 79

This is done with a stroke control valve similar to that shown in Figure 79. The strokeis adjusted by locating the stop flange on the cylinder rod to activate the stop valve atdifferent retraction positions. Reversing the flow direction will compress the valvespring and allow the cylinder to extend.

Thermal relief valves

Figure 80

Cylinders that have cooled down, and are then subjected to heat from the sun (orother source) may be damaged by the resulting fluid expansion. Extremely highpressures can be developed as the fluid expands, unless there is a way to relieve thepressure and drain off a small amount of the fluid. A small integral relief valve orcartridge (Figure 80), set much higher than system pressure, will accomplish this taskand prevent damage to the cylinder.

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CYLINDER RATINGSThe ratings of a cylinder include its size and pressure capability. Principle sizefeatures are bore (piston diameter or inside body diameter), piston rod diameter andstroke length. The pressure rating is based on the size, design and materials used,and is established by the manufacturer. Refer to the cylinder nameplate or themanufacturer’s catalogue for this information.

GRAPHIC SYMBOL - HYDRAULIC CYLINDER

Figure 81

The symbols for a few basic types of hydraulic and pneumatic cylinders are shown inFigure 81. The rod size and length of stroke is not reflected by the symbol's dimensions.

At the top two examples of single acting cylinders, or cylinders which can only bepressure activated in one direction. In the centre is a common example of a doubleacting cylinder while at the bottom an example of a double rod end cylinder similar to thesteering cylinder used on the backhoe loaders and wheeled tractor scrapers is shown.

Figure 82

The basic symbols for cylinders can also be modified to show other features andfunctions. Figure 82 has two examples of modified cylinders.

When the rod to bore diameter is important to the function of the circuit, the leftsymbol shown should be used which depicts a large diameter rod. The right symbolrepresents a pressure intensifying cylinder used for pressure multiplication.


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Figure 83

The two symbols in Figure 83 represent fixed and adjustable cushions or snubbers.These are used to slow piston and rod travel near the end of the cylinder stroke. Thetop symbol has a fixed cushion in both directions of travel. The bottom symbol is anadjustable cushion on the extension side only, denoted by the variability arrow drawnthrough the cushion block. Note the cushion block symbol is only on the rod side of thecylinder. To the left we see a cutaway drawing of a cylinder with fixed cushions on boththe extension and retraction sides. Another example of how a cylinder may becushioned is seen on the next slide.

ROTARY ACTUATORSLinear actuators convert fluid power to linear motion, rotary actuators convert fluidpower to rotary motion. Fluid is pushed into the inlet of the rotary actuator and causesthe output shaft to rotate. Resistance to rotation by an external load creates pressurein the hydraulic circuit and in the inlet of the motor.

Gear Type

Figure 84

A cross sectional view of an external gear hydraulic motor is shown in Figure 84. This designis referred to as “external gear” design because the gear teeth are machined on the outsideof the gears. One of the gears will be connected to an output shaft, and the other will be anidler gear. Not shown in the illustration are the side plates, which create a sealing wearsurface on the sides of the gear set (and are similar to these used in a gear type pump).

Gear motors operate because of a pressure differential, or DP, between the motorinlet and outlet. This pressure differential acts across the gear teeth, creating aforce that rotates the gear.

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Vane Motors

Figure 85

A cross sectional view of a balanced vane rotating group is shown in Figure 85. Theelements shown in the view are the cam (cam ring or displacement ring), rotor andvanes. The output shaft of the motor is connected to the centre of the rotor. The vanesslide in and out of the slots in the rotor so as to make contact with the cam surface.

Figure 86

A form of spring, either a spring clip or a small coil spring, is placed under the vane tocause it to stay against the cam surface (Figure 86). In addition, inlet fluid is alsoported under the vanes so as to balance the pressure between the top and bottomand prevent pressure from pushing the vane back into the slot.

Fluid entering the motor will pressure two opposite sides of the rotor assembly, andreturn fluid will exit two opposite sides. This way, equal pressures are always oppositeeach other, balancing forces across the rotor. This relieves any loading on the driveshaft and bearings caused by internal pressures and forces.


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Figure 87

Figure 87 shows how the differential pressure across a vane will create a force on thevane. The amount of vane that is exposed to pressure will determine the magnitude ofthe force (force equals pressure times area), and the distance from the centre of theexposed vane area to the centre of the drive shaft will determine the torque that isgenerated. Therefore, the torque output of a vane motor is dependent upon pressure,size of the vane (height extending above the rotor and width) and radius of the rotor(distance from the centreline of the drive shaft).

In-line Piston Motors

Figure 88

A cutaway view of an in-line piston motor is shown in Figure 88. The components that makeup a piston motor rotating group are a cylinder block, pistons and shoes, shoe hold-downplate, swash plate, valve plate and drive shaft. The drive shaft is connected by splines to thecylinder block, and the show are held down to the swash plate by a hold-down plate.

As fluid is forced through the valve plate into the cylinder block, pistons are forced outof the cylinder block, causing them to slide along the angled swash plate. This causesthe cylinder block to rotate along with the pistons, turning the drive shaft. As pistonsare forced back into the cylinder block by the swash plate, fluid is forced out throughthe valve plate and back to the reservoir.

The amount of torque that the motor will deliver is based on the force of the piston(pressure times area), the radius of the piston circle (force times distance) and theangle of the swash plate. The higher the swash plate angle, the greater the torqueoutput for any given pressure.

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Bent Axis Piston Motors

Figure 89

A cross sectional view of a bent axis piston motor is shown in Figure 89. Themain elements are a cylinder block, pistons and shoes, drive shaft and flange, auniversal link and a valve plate. The piston shoes are lodged in the drive shaftflange, and the universal link maintains alignment between the cylinder block andthe drive shaft so that they turn together.

As fluid is forced through the valve plate into the cylinder block, pistons are forced out ofthe cylinder block, forcing the drive shaft flange to rotate. This causes the drive shaft torotate along with the cylinder block and pistons. Pistons are forced back into the cylinderblock by the drive shaft flange, and fluid is forced out through the valve plate and back tothe reservoir. The entire operation is much the same as the in-line piston motor, exceptthat the cylinder block and piston assembly is angled instead of a swash plate.

The amount of torque that a motor will deliver is based on the force of the piston(pressure times the piston cross sectional area), the radius of the drive shaft flange(force times distance) and the angle of the cylinder block. The higher the cylinderblock angle, the greater the torque output for any given pressure and piston size.


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GRAPHICS SYMBOL - ROTARY ACTUATOR

Figure 90 - Fixed Displacement

The symbols used for motors are very similar to those used for pumps. Rather thancreating flow or converting mechanical energy into fluid power though, motors do justthe opposite and convert fluid power into mechanical energy. This is shown by usingthe same basic component symbol as a pump, but with the energy triangle reversed,pointing inward. This indicates that fluid power is taken into the component as thetriangle's orientation shows. All other supplemental component elements used formotors are the same as those used for pumps.

It is important to remember that graphic symbols such as these for pumps and motors donot indicate the kind of pump or motor? (gear, vane or piston type) being represented. Thesymbols only, designate the type of pump displacement and method of operation.

DIRECTIONAL CONTROL VALVES

Figure 91

Directional Control Valves (DCV) are used to direct oil into separate circuits of ahydraulic system (Figure 91). The maximum flow capacity and the pressure dropthrough the valve are the first considerations.

Directional control valves may be interfaced with manual, hydraulic, pneumatic andelectronic controls. These factors are mostly determined during the initial systemdesign. Directional control valves direct the flow of oil in the hydraulic system. This is ameans by which the operator controls the machine. The directional control valvedirects the supply oil to the actuator in a hydraulic system. The valve body is drilled,honed and sometimes the bore is heat-treated.

The inlet and outlet ports are drilled and threaded. The valve spool is machined fromhigh-grade steel. Some valve spools are heat- treated, ground to size and polished.Other valve spools are chrome plated, ground to size and polished. The valve bodyand valve spool are then mated in assembly to the design specifications. Whenassembled, the valve spool is the only part that moves.

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Simple Spool DCV

Figure 92

The spool shown in the Figure 92, operates a double acting cylinder, by directing flowto either end of the cylinder. Ports A and B are the ports to the cylinder Port P ispressure oil from the pump. Port T is drain oil to the tank.

Valve CentredOil to the cylinder is blocked by the position of the spool.

Valve shifted leftOil can now flow from the P port to the cylinder A port and oil can also flow from thenon-active side of a double acting cylinder through B to tank (T).

Valve shifted rightOil can now flow from the P port to the cylinder B port. Oil can also flow from the non-activeside of a double acting cylinder through port A, through the drillings in a valve to tank (T).


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Open Centre Directional Control Valve

Figure 93 DCV in Hold Position

Open centre valves have a passage designed in the valve body casting that allows allinlet flow, when the spool is in neutral or centre position, to pass through a bypass area.

This flow either exits the valve back to the tank or is available for another valveconnected in series to the first valve.

The advantage of an open centre valve is that there is little work done by the pump.When the valve is in neutral, minimal pressure is built up.

The main disadvantage is that there is a small time lag when the valve opens aspressure needs to build up in all of the circuit.

Figure 93 shows a cutaway diagram of a typical open centre directional control valvein the HOLD position.

In the HOLD position, the pump oil flows into the valve body, around the valve spooland returns to the tank. The valve spool also blocks the oil in the line to the rod endand the head end of the cylinder.

The above example of a directional control valve also contains a load check valve. Inthe hold position, the load check valve has spring tension behind it, to keep the valveclosed and prevent drift from the implements attached to the cylinder.

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Figure 94 - Raise Position

Figure 94 shows the valve spool at the instant the spool is moved to the RAISE position.

When the valve spool is moved to the RAISE position, the valve spool blocks thepump oil flow to the tank. However, pump oil flow is open to the load check valve. Thevalve spool also connects the cylinder head end to the oil behind the load check valveand the cylinder rod end to the tank passage. The load check valve prevents the oil inthe head end of the cylinder from flowing into the pump oil passage. The blockedpump oil flow causes an increase in the oil pressure.

This action prevents drift of the implement until pump pressure builds up.

Figure 95

The increase in pump oil pressure overcomes the pressure behind the load checkvalve and unseats the valve.

The pump oil flows pass the load check valve and around the valve spool to thehead end of the cylinder.

The oil in the rod end of the cylinder flows past the valve spool to the tank.

The reverse applies when the DCV is placed in the Lower position.


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Check Valve

Figure 96

The purpose of a check valve is to readily permit oil flow in one direction, but prevent(check) oil flow in the opposite direction.

The check valve is sometimes called a ’one way’ check valve.

Most check valves consist of a spring and a tapered seat valve as in the figure above.However, a round ball is sometimes used instead of the tapered seat valve. In somecircuits, the check valve may be free floating (has no spring).

In the valve on the left (Figure 96), when the pump oil pressure overcomes the oilpressure in back of the check valve plus the check valve slight spring force, the checkvalve opens and allows the oil to flow to the implement.

In the valve on the right, when the pressure of the pump oil is less than the oilpressure in the implement, the check valve closes and prevents implement oil flowback through the valve.

Closed centre, directional control valve

Figure 97 - Closed Centre valve with bypass passage plugged

Closed centre valves do not have a bypass passage and block all flow through thevalve when the spool is in the neutral, or centre position. These types of valves areused with variable displacement pumps where system flow in the neutral position doesnot exist as the pump will be in its “cut off” or “standby” position. Figure 97 illustrates atypical closed centre mobile spool valve. Closed centre valves can also be acombination of inlet spool with outlet poppets; or, inlet poppets with outlet poppets.These designs feature independent control of the valve inlet from the valve outlet withresulting flexibility to the control features of the valve.

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Figure 98 - multiple spool valve with centre bypass construction

Figure 98 shows a multiple spool, open centre valve, with a centrallylocated bypass passage.

In Figure 98 valve section 1 is in the neutral position and the bypass feature is portingflow at the bypass pressure setting to the following valve sections. Valve Section 2illustrates the bypass operation with the valve shifted out for flow through Port B; and,in Section 3, flow to Port A Before the flow in either section 2 or section 3 can beuseable, the bypass must first be restricted or dosed down, or else the fluid willcontinue through the bypass and do no work (flow takes the path of least resistance).As illustrated in section 2, when the spool is shifted far enough, the bypass will becompletely shut off and a flow will then pass to the work port (port “B”). Return flow,coming back through the return line, is free to flow to the outlet, or tank, passage tothe reservoir. The reverse of this is illustrated in the third section.

Multiple spool valves are typically either series design or series parallel design.Series design valves are typically the lowest cost and are used in many cost sensitiveapplications where maximum system pressure is not very high, generally 2000 psi(13790kPa) or less. All of the flow is available to each of the valve sections, but at thehighest pressure demanded. As such, they are not advantageous in applicationswhere system heat level or energy consumption is critical.

Series parallel designs (Figure 98) are the most commonly used type of multiplespool valve. While they permit independent pressure operation for each section, oilflow will follow the path of least resistance and the section with the lowest pressurewill tend to have all the flow unless the operator is able to minimize this through theuse of metering.


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PRESSURE CONTROL VALVESAlso known as relief valves. The function of a relief valve is to provide protection to ahydraulic system so that the system components do not malfunction, seize or burstand the fluid lines do not burst or leak at their connections. The relief valve performsthis function by providing a means for system fluid to be diverted to the reservoirwhen the valve pressure setting is reached.

The opening of the relief valve is accomplished when pressure of the fluid in thesystem exceeds a pressure set by a spring force in the relief valve. The spring holdsthe relief valve in its closed position. As the fluid pressure rises to a level that exceedsthe force of this spring, the relief valve opens and creates a flow path to the reservoir.This action results in the “relieving”, or limiting, of fluid pressure in the system to thevalue of the spring force in the relief valve.

Figure 99 - Basic direct acting relief valves

Figure 99 illustrates a basic direct acting relief valve where either a ball or a poppet isheld in the closed position by an adjustable spring. This blocks the flow path to tank.This valve can be adjusted manually. When pump pressure (p) exceeds springtension, the valve is forced off its seat, allowing oil to flow directly to the tank (T) thusreducing or relieving system pressure.

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Simple Pressure Relief Valve, Cracking Pressure

Figure 100

Figure 100 above shows a simple relief valve in the ‘cracking pressure’ position.

The simple relief valve (also called direct acting relief valve) is kept closed by springforce. The spring tension is set to the ’relief pressure’ setting. However, the reliefpressure setting is not the pressure at which the valve first begins to open.

When a condition develops that causes a resistance to the normal oil flow in thecircuit, excessive oil flow causes the oil pressure to increase. The increasing oilpressure is sensed at the relief valve.

When the force of the increasing oil pressure overcomes the force of the relief valvespring, the valve moves against the spring and begins to open. The pressure requiredto begin valve opening is called the ’cracking pressure’. The valve opens just enoughto allow excess oil to flow through the valve.

Simple Pressure Relief Valve, Relief Pressure Setting

Figure 101

An increase in the resistance to oil flow (Figure 101) increases the volume of excessoil and increases the circuit pressure. The increase in circuit pressure overcomes thenew spring tension and further opens the relief valve.

The process is repeated until the maximum volume of oil (full pump flow) is flowingthrough the relief valve. This is the relief pressure setting.

The simple relief valve is commonly used where the volume of excess oil flow is lowor where there is a need for a quick response. This makes the simple relief valve idealfor relieving shock pressures or as a safety valve.


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Pilot Operated Relief Valve

Closed Position

Figure 102

The pilot operated relief valve (Figure 102) is often used in systems that require alarge volume of oil and a small differential between the cracking pressure and the fullflow pressure.

In the pilot operated relief valve, a pilot valve (simple relief valve) controls the mainunloading valve. It is much smaller and does not handle a large volume oil flow. It’sspring is much smaller allowing more precise pressure control. The differencebetween the pilot valve cracking and maximum pressure is minimal.

The unloading valve can handle the complete pump flow at the designed maximumrelief pressure. It uses system oil pressure to keep closed. Therefore, the unloadingvalve spring does not need to be strong and heavy and allows a more precise openingpressure.

The system oil flows into the relief valve housing, through the unloading valve orificeand fills the unloading valve spring chamber. The oil in the unloading valve springchamber contacts a small area of the pilot valve. This allows the pilot valve to use asmall spring to control a high pressure. When the oil pressure increases in thesystem, the same pressure is in the unloading valve spring chamber and the oilpressure is the same on both sides of the unloading valve.

The combined force of the oil pressure in the unloading valve spring chamber and thespring force on the top of the unloading valve is greater than the force of the oilpressure against the bottom of the valve. The combined force in the spring chamberkeeps the unloading valve closed.

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Open Position

Figure 103

When the system oil pressure exceeds the pilot valve spring setting (Figure 103) thepilot valve opens. The open pilot valve allows the oil in the unloading valve springchamber to flow to the tank. The pilot valve opening (orifice) is larger than theunloading valve orifice.

Therefore, oil flows pass the pilot valve much faster than through the unloading valveorifice. This allows the pressure to decrease in the unloading valve spring chamber.The force of the higher system oil pressure moves the unloading valve against thespring. The excessive pump oil flows through the throttling holes in the unloadingvalve to the tank. The throttling holes allow the unloading valve to dump the volume ofoil necessary to maintain the desired relief pressure.

FLOW CONTROL VALVESFlow control consists of controlling or regulating the volume of oil flow in or out of acircuit to a rate which is below that of the system pump output flow. Controlling flow ina hydraulic circuit can be accomplished in several ways.

The most common way of controlling flow is by installing an orifice. When an orifice isinstalled, the orifice presents a higher than normal restriction to the pump flow. Thehigher restriction increases the oil pressure. The increase in oil pressure causessome of the oil to take another path.

The path may be through another circuit or it may be over a relief valve.

There are four basic types of flow controls. These are:

Non-compensated flow controls

Flow dividers

Pressure compensated flow controls

Temperature compensated flow controls.


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Non-compensated Flow ControlsThe simplest type of flow control is the non-compensated flow control. Non-compensated flow control valves are used as restrictors to control the rate of flow intoa branch circuit at a given input pressure to the valve. As system pressure increases,flow through the non-compensated flow control will increase with a correspondingincrease in pressure drop. If outlet or load pressure increases, flow may decreaseproportionally.

The simplest form of a non-compensated flow control is an orifice. An orifice canconveniently be inserted into a hydraulic line as a stand alone valve, or more typically,into a hydraulic fitting as a means of restricting the flow into a branch circuit.

Needle or glob type valves are also commonly used as simple flow restrictions thatcontrol the flow into a branch circuit.

An orifice is a small opening in the oil flow path. Flow through an orifice is affected byseveral factors.

Three of the most common are:

1. The temperature of the oil

2. The size of the orifice

3. The pressure differential across the orifice.

TemperatureThe oil viscosity changes with changes in temperature. Viscosity is a measurement ofthe oil’s resistance to flow at a specific temperature.

Hydraulic oil becomes thinner and flows more readily as the temperature increases.

Orifice Size The size of the orifice controls the flow rate through the orifice. A common example isa hole in a garden hose. A small pinhole will leak in the form of a drip or a fine spray.A larger hole will leak in the form of a stream. The hole, whether small or large,meters a flow of water to the outside of the hose. The amount of water metereddepends on the size of the hole (orifice).

The orifice size may be fixed or variable.

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Check Valve with a Fixed Orifice

Figure 104

Figure 104 shows an example of a check valve with a fixed orifice that is commonlyused in construction equipment.

The fixed orifice is a hole through the centre of the check valve.

When oil flow is in the normal direction, the valve opens and allows oil to flow aroundthe valve as well as through the orifice. When oil attempts to flow in the reversedirection, the valve closes. All reverse flowing oil must flow through the orifice thatcontrols the flow rate.

Variable Orifice

Figure 105

Figure 105 shows a variable orifice in the form of a needle valve. In the needle valve, thepositioning of the valve tip in relation to the valve seat changes the size of the orifice.


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The oil flow through the needle valve must make a 90° turn and pass between thevalve tip and the valve seat. The needle valve is one of the most frequently usedvariable orifices.

When the valve stem is turned counter-clockwise, the orifice becomes larger and theflow increases through the valve.

When the valve stem is turned clockwise, the orifice becomes smaller and the flowdecreases through the valve.

Figure 106

Figure 106 is an example of how a variable orifice can be used to control theoperating speed of a cylinder.

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Flow Dividers

Figure 107

Flow dividers are either gear type or spool type. Their function is to divide the flowinto two different streams. These two streams typically are equal in flow rate; i.e.,each stream is half of the available flow. However, through the use of springs andspool design, or the use of different gear displacements, is possible to obtain differentflow ratios for the two flow streams.

Gear type flow dividers (Figure 107) are similar to a double (or triple, or more) gearmotor. They use a common inlet and stacked gear sections that are maintained at thesame rotational rate by a common shaft. By this means, the incoming flow is dividedinto a stream of flow for each gear section. It is possible to have more than twosections to produce more than two flow streams. The volume of flow from eachstream will be effected by the volumetric efficiency of each gear section.


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Figure 108

Spool type flow dividers (Figure 108) are more commonly used as they are typicallymore convenient to install in the hydraulic circuit, or can be integrated into the outletcover of a pump. Their design is such that they provide either proportional output flowstreams, or a priority output flow stream. When used as a priority valve, the priorityflow stream provides a set rate of flow while the secondary stream provides only theflow that is available after the priority stream has been satisfied.

When the accuracy of the output flow streams is critical to the application, a pressurecompensation feature can be added to the spool type flow divider in order to preventchanges in output flow rate as inlet or outlet pressure changes.

Pressure Compensated Flow ControlsPressure compensated flow control valves are used to provide a constant output flowrate into a branch circuit regardless of system input pressure or load pressure. Theycan be either piston type or spool type. In both cases constant output flow rate isachieved by using springs to maintain a constant pressure drop across the meteringorifice. As input pressure increases, output flow is restricted as either the piston orspool is closed off. This prevents an increase of output flow as the increase of inputStem pressure tries to force more flow through the valve.

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Figure 109

Figure 109 illustrates a bypass type, pressure compensated flow control with integralrelief valve. The difference is the output flow is controlled by an orifice inserted in theregulated output line. This line maintains a constant flow to a work circuit, all excessflow being diverted to the reservoir through the tank line.

As the work load increases, pressure in the output line increases, as does the pressurein the spring chamber at the right side of the "hydrostat" piston. Inlet pressure, to the leftof the hydrostat piston, will also increase to a level sufficient to overcome the pressure atthe right of the hydrostat, plus the value of the spring. At this point, the hydrostat willslide to the right, allowing the excess flow to exit through the tank port. Thus, regardlessof the pressure on the right side of the hydrostat, the pressure on the left will be above itby virtue of the spring value. It is this constant pressure drop across the orifice thatmaintains the pressure compensated flow to the regulated outlet.

If the regulated outlet pressure exceeds the pilot spring setting inside the hydrostat, thepressure in the spring chamber will be limited, and the valve will function as a relief valve.


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Figure 110

Figure 110, an alternative design; output flow is controlled by an orifice in the mainspool. The spring at the right of the spool maintains a constant pressure drop, andtherefore a constant flow across the orifice, and excess flow is diverted to the excessflow port. A separate relief valve protects the regulated flow circuit.

By-pass Pressure Compensated Flow Control Valve

Figure 111

Figure 111 shows an illustration of a by-pass type pressure compensated flow controlvalve. The by-pass type pressure compensated flow control valve automaticallyadjusts to flow and load changes.

The amount of flow through the valve depends on the size of the orifice. Any changein oil flow through the orifice creates a change in pressure on the upstream side of theorifice. The same pressure change acts against the dump valve and spring.

When the pump flow is within the design flow of the orifice, the force of the upstreamoil pressure acting on the dump valve is less than the combined force of thedownstream oil pressure and the spring. The dump valve remains closed and all ofthe pump oil flows through the orifice.

When the pump flow is more than the design flow of the orifice, the force of theupstream oil pressure acting on the dump valve is greater than the combined force ofthe downstream oil pressure and the spring. The dump valve opens and the excess oilflows through the dump valve to the tank.

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Combined Orifice and Dump Valve

Figure 112

The more common type of flow control valve is shown in Figure 112.

This valve combines the action of the orifice and dump valve in one moving part.The pressure compensating operation is the same as the by-pass pressurecompensated flow control valve.

The graphic on the left shows flow through the valve that is either at the flow rating orless than the flow rating of the valve.

The graphic on the right shows that flow is beginning to exceed the flow rating of the valve,the pressure differential resulting from the flow across the orifice becomes great enough tobegin compressing the spring and start dumping the excess oil as shown.

If the flow through the valve increases, the action of the orifice will cause the spring tocompress still more, and more flow will be dumped. The controlled (metered) flowremains fairly constant as flow to the valve increases or decreases.

Temperature Compensated Flow ControlsTemperature compensated flow control valves are used to maintain a constant outputflow rate where a change in the fluid temperature would cause a change. Temperaturecompensation is usually accomplished through a variable orifice that is controlled bya temperature sensitive, bi-metallic lever or rod.


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GRAPHIC SYMBOLS - VALVES

Directional Control Valve

Figure 113 - Valve Envelopes & Ports

The basic directional control valve symbol (Figure 113) consists of one or more basicenvelopes as seen in the top row of symbols. The number of envelopes used representsthe number of positions that a valve can be shifted to. Shifting and valve positions will bediscussed in more detail later.

Next are valve ports, or the connection points for inlet, outlet and working lines. Thefirst symbol to the left has two ports and is commonly referred to as a two-way valve,not to be confused with a two-position valve as seen in the middle of the top row.Valves may have as many positions or ports as needed, although most are in therange of 1-3 positions and 5 or fewer ports.

The other two symbols represent the typical port arrangements for three and four-wayvalves. The terms two, three, and four-way valve does not necessarily mean the valve willhave two, three or four ports but rather designates the number of ports flow is occurringbetween. An example of this will be seen later after discussing internal passages next.

Figure 114 - Internal Passageways

Lines and arrows inside the envelope are used to represent flow paths and directionbetween ports (Figure 114). Two basic types of valves are seen here, normallyblocked and normally open. These terms represent the internal flow condition of thevalve in its neutral state. Valve symbols are always drawn in their neutral or

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unactivated position. Valves which change position due to flow or pressure are in theirneutral condition when there is no flow or pressure in the circuit. That is, the externallines are connected to the ports of the envelope with internal passagewaysrepresenting the valve's neutral condition. The shifting of valve spools or envelopesusing graphic symbols must be done with your imagination. In full machineschematics, components are always drawn in their neutral position. Later we will seean example of how symbols should be shifted using your imagination.

The top row of symbols represents one, two and three position valves in which flow isnormally blocked in neutral. While the bottom row of symbols depict one, two and threeposition valves which are normally open to flow in their neutral position. Let's take a closerlook at the top row of symbols to further understand the internal flow conditions.

Figure 115

The first symbol seen to the left in Figure 115 represents a normally closed infinitepositioning valve. This means the valve spool can be shifted either partially, in an infinitenumber of positions to meter the flow through the valve, or fully open. The arrow heredenotes flow is normally in one direction, from the top port to the bottom port.

The middle symbol in Figure 115 represents a two-position normally closed valve.The function of this valve is very similar to the function of the valve just discussed. Butrather than allowing infinite positioning of the valve spool to meter flow, this valve iseither completely closed or fully open. In the neutral position flow is blocked, indicatedby the small 'IT" symbol. In the fully open position, as shown by the right-handenvelope, flow is allowed in either direction. This is denoted by the two headed arrow.Compare this to the previous symbol in which flow was only in one direction, denotedby a single headed arrow.

The symbol seen to the right of Figure 115 represents a normally closed three positionvalve. The far left envelope shows that when the spool is shifted right connecting the flowpaths in the left enclosure with the external port lines, parallel flow is allowed betweenthe valve ports. The far right envelope shows cross flow is achieved between the portswhen the spool is shifted left. Typically, whenever a valve symbol is shown by itself, theports are labelled to denote supply, return and working ports. But, when shown as a partof a complete circuit schematic, the connecting lines must be followed out to determinewhich ports are the supply, return and working ports.

Two, three and other multiple position valves can also be shown to have infinitepositioning or flow metering capability. This is done by the addition of two horizontalbars drawn parallel to the valve envelopes as will be seen in the next slide. Valvesdrawn with only one envelope as previously seen are assumed to have infinitepositioning capability and do not require the additional horizontal bars.


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Figure 116

Shown to the left of Figure 116 are partial cutaway drawings of directional four-waycontrol valves in their neutral positions. Seen to the right are the symbols for only thecentred or neutral condition of the valves. The labelled port abbreviations are:

p - Supply from pump

T - Return to tank

A - Work port

B - Work port.

The valve cutaway to the upper left contains an open-centred type spool. Supply flowfrom the pump is unrestricted to either working ports “A” and “B”, or return to tank. Inthis case supply flow will take the path of least resistance. The symbol to the rightindicates that all ports are tied together (junction dots) which would allow unrestrictedflow between any of the ports.

Figure 117

The cutaway in Figure 117 represents a closed-centre type spool. Pump supplyflow is blocked by the spool lands and valve housing to the right and left ofworking ports “A” and “B.” Ports “A” and “B” are also blocked by the spool fromdraining to tank. The symbol to the right utilizes small “T” crossed lines to denoteflow is blocked internally on all four ports.

Two more examples are seen here of valves and their related centre position symbol.The top cutaway and symbol is representative of a closed centre motor spool in whichports "A" and "B" are both open to tank while pump supply is blocked. The symbol tothe right shows that ports "All and "B" are tied together and drain to tank, while pumpsupply is blocked.

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The lower cutaway and symbol are representative of a tandem-type open centrevalve. Pump supply is open to tank by means of a passageway inside of the spool,while ports "A" and "B" are both blocked. The symbol to the right shows the pumpsupply port is tied to tank return and ports "All and "B" are blocked. Although, nothingfrom the symbol shows its done by means of a hollow spool. once again you mustkeep in mind symbols are used to represent the components resulting function notnecessarily the component's construction which achieves this function. This valve isreferred to as a tandem-type valve because rather than having pump flow go directlyto tank through port 'IT," it could be diverted to another valve section from port 'IT."Therefore, the two valve sections are in tandem or series, one behind the other, thesecond valve being fed by the return oil of the first. An example of this type valvearrangement will be seen later.

PRESSURE CONTROL VALVE

Figure 118

The symbol in Figure 118 shows a single valve envelope which is normally closedto flow. It also shows that system pressure is sensed through the pilot line andworks against a spring tension which can be varied. Nothing from the symbolidentifies whether the relief has both a pilot and relief poppet, or that pressure issensed through an orifice.


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Graphic Sysmbols - Flow Control Valves

Figure 119 - Flow Control

HYDRAULIC FLUIDThe selection and care of the hydraulic fluid will have an important effect on thelife of the system. Just like the hardware components of a hydraulic system, thehydraulic fluid must be selected on the basis of its characteristics and propertiesto accomplish the designed task.

The fluid used in a hydraulic system is as important as any other component in thesystem, for the system to operate correctly and have an acceptable service life.

Service records show that incorrect oil or oil containing dirt or other contaminantscause approximately seventy percent of hydraulic system problems.

The fluid in the hydraulic system is the component that is used to transfer the flowenergy generated by the pump to the mechanical components which convert fluidenergy into mechanical energy, to do work. Examples of these components may becylinders and hydraulic motors.

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Functions of Hydraulic Liquids

Figure 120

Liquids are virtually incompressible. Therefore, fluids can transmit powerinstantaneously in a hydraulic system. For example, petroleum oil compresses atapproximately 1-1.5% at a pressure of 20,685kPa (3000psi). Therefore, petroleum oilcan maintain a constant volume under high pressure.

Petroleum oil is the primary fluid used in developing most hydraulic oils. The primaryfunctions of hydraulic fluids are:

power transmission

lubrication

sealing

cooling.

Power Transmission Because hydraulic fluids are virtually incompressible, once the hydraulic system isfilled with fluid it can instantly transmit power from one area to another. However, thisdoes not mean that all hydraulic fluids are equal and will transmit power with thesame efficiency. Choosing the correct hydraulic fluid depends on the application andthe operating conditions.

LubricationHydraulic fluid must lubricate the moving parts of the hydraulic system. Therotating or sliding components must be able to function without touching othersurfaces. The hydraulic fluid must maintain a thin film between the two surfacesto prevent friction, heat and wear.

SealingMany hydraulic components are designed to use hydraulic fluid instead of mechanicalseals within the component. The viscosity of the fluid helps to determine its ability tofunction as a seal.


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Cooling The hydraulic system develops heat as it transfers mechanical energy to hydraulicenergy and hydraulic energy back to mechanical energy.

As the fluid moves throughout the system, heat flows from the warmer components tothe cooler fluid. The fluid gives up the heat to the reservoir or to coolers that aredesigned to maintain fluid temperatures within design limits.

Properties of Hydraulic Fluids

ViscosityViscosity is a measurement of a fluid’s resistance to flow at a specific temperature. Inother words, the thickness of the oil, at a set temperature. A fluid that flows easily hasa low viscosity. Viscosity in hydraulic oil is very important because if an oil becomestoo thin (low viscosity as the temperature rises), it may leak past seals, joints, valvesand internally leak in pumps and motors. Any area of leakage will effect theperformance of the system.

If the hydraulic oil has a high viscosity, (too thick), sluggish operation of the systemwill result requiring extra power to be used just to push the oil around the system.Viscosity of the oil also effects its ability to lubricate the moving parts of the system.

A fluid’s viscosity is affected by temperature. When a fluid becomes warmer, the fluid’sviscosity becomes lower. Likewise, when a fluid cools, the viscosity increases. Vegetableoil is a very good example of how viscosity changes with a change in temperature. Whenvegetable oil is very cold, vegetable oil thickens and is very slow to pour. As vegetable oilis heated, vegetable oil becomes thinner and pours more readily.

The Saybolt Viscosimeter

Figure 121

The most common tool of measuring viscosity is the Saybolt Viscosimeter, inventedby George Saybolt.

The Saybolt Viscosimeter unit of measurement is the Saybolt Universal Second(SUS). In the original viscosimeter a container of fluid was heated to a specifictemperature. When the temperature was reached, a stopcock (orifice) was openedand the fluid flowed out of the container and into a 60 ml. flask. A stopwatch was usedto measure the time it took to fill the flask.

The viscosity was recorded as the number of seconds the flask took to fill at agiven temperature.

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If a fluid, when heated to a temperature of 24° C, took 115 seconds to fill the flask, itsviscosity was 115 SUS @ 24° C. If the same fluid was heated to 38° C and took90 seconds to fill the flask, its viscosity would be 90 SUS @ 38° C.

Viscosity Index

Viscosity index (VI) is a measure of a fluid’s change in thickness with respect tochanges in temperature. If a fluid’s consistency remains relatively the same overvarying temperatures, the fluid has a high VI. If a fluid becomes thick at lowtemperatures and very thin at high temperatures, the fluid has a low VI. In mosthydraulic systems, fluids with a high VI are desirable over fluids with a low VI.

Viscosity Improver

These types of additives help maintain the oil’s viscosity over a wide range intemperatures. When the oil is cold the system will operate correctly and when the oilis hot the system will also operate correctly.

Anti-wear Additives Hydraulic oil contains a selected number of additives to increase and insure its anti-wear capabilities. It must provide good lubrication to lower and hold to a minimum, thefriction between components in the system.

Anti-foamingFoaming in hydraulic oil refers to the oil being mixed with air bubbles.

A liquid can not be compressed but air can, these anti foaming additives help the oilabsorb the air so that it does not affect the operation of the system. If the oil is mixedwith an amount of air that is greater than it can absorb, the oil will foam. When the oilbecomes foamed the system will operate with a slower response to direction changesand load changes, in other words an unsatisfactory operation. The air in the systemwill also affect the oils ability to lubricate the system components, cause systemoverheating and erratic operation.

Water Resistant SeparationWater vapour enters the system through the reservoir, and any small leaks in the system.

As the oil is moving around the system it is subjected to violent mixing, churning andcontinual recirculation (flowing from the mechanical work area back to the reservoir).This action mixes the oil and water together and it becomes what is known as anemulsion. This emulsion promotes rust, acids and forms sludge in the system. It willalso greatly reduce the ability of the oil to lubricate system components. Additives tothe oil help separate the water from the oil.


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Types of Hydraulic Fluids

Petroleum Oil All petroleum oil becomes thin as the temperature goes up and thickens as thetemperature goes down. If the viscosity is too low, there may be excessive leakagepast seals and from joints. If the viscosity is too high, sluggish operation may be theresults and extra power is needed to push the oil through the system.

Viscosity of petroleum oil is expressed by the Society of Automotive Engineers (SAE)numbers: 5W, 10W, 20W, 30W, 40W, etc. The lower the number, the better the oil willflow at low temperatures.

The higher the number, the more viscous the oil and the more suited to hightemperatures. Viscosity in the ISO standards is expressed in mm2/S, calledcentiStokes (cSt).

Synthetic Oils Synthetic oils are formed by processes that chemically react to materials of a specificcomposition to produce a compound with planned and predictable properties. Syntheticoils are specifically blended for extreme service at both high and low temperatures.

Fire Resistant Fluids There are three basic types of fire resistant fluids: water-glycols, water-oil emulsionsand synthetics. They are used in situations of high fire risk, such as in undergroundmining, steel making and oil wells.

Water-glycol fluids contains 35% to 50% water (water inhibits burning), glycol(synthetic chemical similar to some anti-freeze) and a water thickener. Additivesare added to improve lubrication and to prevent rust, corrosion and foaming.Water-glycol fluids are heavier than oil and may cause pump cavitation(Formation and collapse of vapour bubbles in hydraulic oil, resulting in erosionand pitting of metal surfaces) at high speeds. These fluids may react with certainmetals and seals and cannot be used with some types of paints.

Water-oil emulsion is the least expensive of the fire resistant fluids.

A similar amount (40%) of water is used as in water-glycol fluids to inhibit burning.Water-oil can be used in typical hydraulic oil systems. Additive may be added toprevent rust and foaming.

Oil LifeThe hydraulic oil never wears out, although a breakdown of the chemical additives willeventually cause the base oil to become ineffective as hydraulic oil. The use of filtersto remove solid particles and some chemicals add to the useful life of the oil.

However, eventually the oil will become so contaminated that it will have to bereplaced. In construction machines, the oil is replaced at regular time intervals.

The contaminants in the oil may also be used as indicators of high wear and prospectiveproblem areas. The hydraulic oil should be analysed at scheduled intervals.

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FILTERS & STRAINERS

Filler Screen

Figure 122

The filler screen is usually located in the filler tub. It keeps large contaminants fromentering the tank when the fill cap is removed.

StrainersInlet strainers are usually mounted inside the reservoir submerged in the oil. Normalflow is through the filtering element.

Should the filter become blocked the pressure on the inside will decrease (pump’sucking’) and oil can flow past the bypass valve. A strainer not fitted with a bypasswill damage a pump very quickly should it become blocked.

Filters

Figure 123


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Oil filters may be located in a number of positions in the hydraulic circuit. Inlet filtersare mounted in the reservoir or in the line going to the pump. Strainers are preferredbecause they are not as fine as high-pressure filters.

High-pressure filters protect sensitive valves in the system.They are usually located after the pump and can be recognised by their heavypressure proof housing.

Return line filters are mounted in the line returning oil back to the reservoir. Thissystem has a major disadvantage in that it is filtering oil after it leaves the circuit.This type of filter is housed in a low-pressure housing or may be a spin-on type.

Figure 124

A full flow system filters the entire supply of oil each time it circulates in the hydraulicsystem. For this reason they are the most commonly used system.

Normal flow is from the outside of the filtering element to the centre of the filter.Should the filter become blocked, pressure will build up around the outside of the filterand open the bypass valve.

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Filter Construction

Surface Filter

Figure 125

As the name implies they trap contaminants on the surface of the element. Theelement is usually constructed of treated porous paper and is pleated to increase thesurface area. This is the most common type of filter element.

A partial flow or bypass filter system only filters part of the oil that flows through thesystem. They rely on the oil flowing through the system several times to clean the oilproperly. They are used in special applications only and have the advantage that theycan remove very fine contaminants.

Bypass Valves

Figure 126

They are fitted to prevent pressure building up. All filters containing a bypass aremarked to show oil flow, i.e. IN and OUT.

Maintenance of filters is probably the most important factor in obtaining long hydraulicsystem life. Neglect the filters and the system will rapidly breakdown or fail.


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Visual inspection gives no indication that the oil is bypassing the filter, filters must beregularly serviced. Follow manufacturer’s specification on the recommended intervalsand use a filter with the same micron rating.

Some filters are fitted with an indicator that signals the operator to clean or replacethe element. They are simply a pressure gauge that tells the operator the flow isrestricted and pressure is building up.

COOLERSAny fluid used in mobile machinery absorbs and carries heat away from heatgenerating components such as cylinders and pumps. The fluid then must be allowedto circulate as much as possible against the heat dissipating sides of a reservoirbefore it is allowed to reenter the pump.

Some system designs may not allow sufficient transfer of fluid to the reservoir,particularly with long lines from the rod end of cylinders. This can cause a buildup ofheat and oxidized fluid in an isolated segment of a circuit and result in destruction ofthe fluid and components. Provision should be made in machine design to circulatethe oil through oil coolers.

Inefficiency in the form of heat can be expected in all hydraulic systems. Even welldesigned hydraulic systems can be expected to turn some portion of its inputhorsepower into heat. Hydraulic reservoirs are sometimes incapable of dissipating allthis heat. In these cases a cooler is used.

Figure 127

Coolers are divided into air coolers and water coolers.

Even a well designed system may convert 20% of its power into heat.

Air coolerIn an air cooler (Figure 127, right), fluid is pumped through tubes to which fins areattached. To dissipate heat, air is blown over the tubes and fins by a fan. Theoperation is exactly like an automobile radiator.

Air coolers are generally used where water is not readily available or too expensive.

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Water coolerA water cooler basically consists of a bundle of tubes encased in a metal shell(Figure 127, left). In this cooler, a system's hydraulic fluid is usually pumped throughthe shell and over the tubes which are circulated with cooling water.

This cooler is also known as a shell-and-tube type heater exchanger. It is a true heatexchanger since hydraulic fluid can also be heated with this device by simply runninghot water through the tubes.

Coolers in a circuitCoolers are usually rated at a relatively low operating pressure (150 PSI). Thisrequires that they be positioned in a low pressure part of a system. If this is notpossible, the cooler may be installed in its own separate circulating system.

To insure that a pressure surge in a line does not damage a shell-and-tube typecooler, they are generally piped into a system in parallel with a 65 PSI check valve.

Coolers can be located in a system's return line, after a relief valve, or in a case drainline of a variable volume, pressure compensated pump.

GRAPHIC SYMBOLS - FLUID CONDITIONER

Figure 128 - Fluid Conditioners

Fluid ConditionersThe general symbol used for a fluid conditioner (Figure 128) is an empty diamond asseen at the top. Several different types of fluid conditioner symbols can be made bymaking slight changes or additions to this basic symbol.

First is a filter or strainer which is shown with a dashed vertical line inside the basicsymbol. This represents the filtering media which the fluid or gas must flow through.The separator with a manual drain. The line drawn horizontally across the bottom


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portion of the symbol represents the separated matter which has settled to the aswater from fuel The short line extending from the bottom of the symbol represents thedrain. Nothing on the symbol will designate it has a manual drain. It must be assumedmanual. Automatic drains will be represented by the small "V" placed below thehorizontal separation line as seen in the bottom symbol. To the right are the symbolsfor combination filter separators with manual and automatic drains. Further right is thesymbol for an in circuit oil cooler which can be air cooled in water cooled type.

CONTAMINATION CONTROL

Why Contamination Control?

Figure 129

Customers are demanding:

more power

greater breakout forces

faster cycle times.

The industry now uses:

more electro-hydraulics

higher system pressures

tighter clearances.

Result: today’s fluid systems are more sensitive to contamination in:

hydraulics

transmissions and final drives

fuel systems

engines.

Contaminated fluid systems result in:

shorter component and fluid life

reduced productivity

can lead to catastrophic failure and costly down-time and repairs

75-85% of hydraulic system failures can be traced to contamination.

If we don’t address fluid contamination, we can expect:

warranty costs to increase

rework to increase

repeat failures and associated failures to increase

increased customer dissatisfaction, resulting in lost sales.

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What Is Contamination? dirt

weld spatter

paint

rag fibres

metal wear particles

cigarette ashes

grease

heat

water

air

products from oil oxidation or bacteria growth.

How Much is Too Much? 1/2 Teaspoon of dust in 55 gallons (250 litres) of hydraulic oil exceeds contaminationallowed in new Caterpillar machines.

Pay attention to particles too small to see.

Contamination Sources

1. During the manufacture of the machine

2. During the manufacture of the oil

3. Operational factors, i.e. dusty conditions

4. Careless maintenance

5. Lack of maintenance.

Filtration

Figure 130

Strainer Rating: the degree of filtration of a strainer is measured by its mesh size orsieve number.

Filter Rating: the degree of filtration of a filter is measured by its micron ratio.

1 Micron = .001 mm or .000001 metre (1 millionth of a metre).


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Sizes of Particles & Comparison of Dimensional Units

Table 1

Absolute Rating: this type stops all contaminants larger than the filter rating, i.e. 10micron absolute filter - traps all contaminants larger than 10 microns.

Nominal Rating: this stops most (50 - 95%) of the contaminants.

It is presumed that repeated flows through the filter will trap all particles. This is themore common type of rating.

Magnetic plugs are often found in the reservoir on the drain plug. They removeferrous or magnetic metal contaminants only.

Strainers provide course filtration and are usually constructed of fine wire mesh.

Filters provide fine filtration, they are usually constructed of porous treated paper.The difference between a filter and a strainer is their filtering ability.

Sizes of Familiar Objects Screen Size

Substance Micron Inch Mesh Size

297 .0117 50

238 .009 60

210 .0083 70

149 .0059 100

105 .0041 140

Grain of Table Salt 100 .0039

74 .0029 200

Human Hair 70 .0027

53 .0021 270

44 .0017 325

Lower Limit of 40 .00158

Visibility 25 .001

White Blood Cells 10 .00039 Paper

Talcum Powder 8 .003

Red Blood Cells 5 .00019 Paper

2 .000078

Bacteria (average) 1 .000039

APLTCL025



Roles and ResponsibilitiesEverybody needs to think:

Equipment ManufacturerDesign machines for life-cycle cleanliness

Build & ship clean products & components

New standards developed

Formed Contamination Control Teams

Provide tooling and information support

Educate employees, suppliers, dealers and customers.

Customers Educate operators and service personnel

Implement contamination control measures.

Servicing DealerAssign contamination control administrator

Educate employees and customers

Acquire contamination control tooling

Develop and implement contamination control procedures.

Opportunities to Control ContaminationHousekeeping

Oil storage and transfer

Parts handling and storage

Hose assembly and storage

Component repair and assembly

Field service

Particle Count.


APLTCL025


Housekeeping

Figure 131

Keep work areas clean and organised

Sweep floors daily

Clean up spills immediately

Keep work benches uncluttered and free of debris

Limit use of floor storage.

Oil Transfer and Storage

Figure 132

Filter new oil from bulk tanks and 44-gallon (200 litre) drums

Store oil drums inside

Use drum covers.

APLTCL025



Parts Handling and Storage

Figure 133

Keep components packaged until ready to install

Return parts to storage in packaging

Clean in-process components.

Hose Assembly and Storage

Figure 134

Clean assembled hoses with Cat Hose Cleaner

Protect assembled hoses with caps & plugs

Protect bulk hose with caps and plugs.

Component Repair and Assembly Use absorbent pads - not Oil Dry - to clean up spills

Establish ‘clean’ processes

Use a rotary brush to clean hydraulic cylinders after honing

Separate welding and cleaning operations.

Field Service Keep parts and components packaged until ready to install

Stock vehicles with High Efficiency Filters

Ensure lube trucks supply clean fluids.

APLTCL025 SGD L-01 - Azerfrema · PDF fileMODULE INTRODUCTION APLTCL025 © Caterpillar of...

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