DOCUMENT RESUME

ED 112 259 CE 005 285

TITLE Tractor Hydraulics. A Teaching Reference.INSTITUTION American Association for Vocational Instructional

Materials, Athens, Ga.; Farm and Industrial EquipmentInst., Chicago, Ill.

REPORT NO VT-102-063NOTE 48p.; Illustrations have color keying which will not

reproduce

EDRS PRICE MF-$0.76 HC-$1.95 Plus PostageDESCRIPTORS *Agricultural Machinery; Fluid Power Education;

*Hydraulics; Instructional Materials; *Manuals; PowerMechanics; *Tractors; Trade and IndustrialEducation

ABSTRACTThe manual was developed to help provide a better

understanding of how and why hydraulic principles serve the purposesof weight reduction, increase of physical effort, and more precisecontrol to machines of all types. The four components that arenecessary to have a workable hydraulic system - -a reservoir, a pump, avalve, and a motor (cylinder) are described in detail. Specialemphasis is given to farm and industrial equipment applications.(VA)

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102. 063)

A Teaching ReferenceAvailable through the cooperative efforts of the ...

FARM and INDUSTRIALEQUIPMENT INSTITUTE and the

AAVIMAMERICAN ASSOCIATIONFOR VOCATIONALINSTRUCTIONAL MATERIALS

Introduction

CE005285CONTENTS

Definition of Hydraulics

Hydraulic Theory

Basic System

Reservoir

3

3

5

6

9

Pumps 10

Reciprocating 11

External Gear 11Internal Gear 12Gear-Like 12Screw 13

Vane 13

Radial Piston 14Axial Piston 15

Centrifugal 15

Combination 16

ValvesCock 17

Globe 18

Gate 18

Flapper 19

Ball 19

Needle 19

Spool 19

Rotary 22

Directional Poppet 22

Combination 24Flow Control and Flow Divider 24

'Cylinders 24

Seals and Packing 28

0-Rings 29

"U" and "V" Packing 29

Cup and Flange Seals 29

Mechanical Seals 29

Metallic Seals 29

Compression Packing 30

Compression Gaskets 30

4

Hydraulic Lines and Fittings 30

Piping and Tubing .7 30Flexible Hose 30Fittings 31

Accessories 32

Phenomena 33

Ruptured Line or Exploded Cylinder in System at Rest 33Collapsed Suction Hose 34Pressure Drop Caused by Friction Losses 34Effect of Using Wrong Size Pump in System 35

The Effect of "Hammer" in a System 35

Systems 35

Blocked Return Line System 36Basic Open. Center System 36

Tandem Open Center System 36Through Flow System 37

Closed Center System 37

Hydraulic Motors 38

Transmissions 39

Hydrostatic 39

Hydrodynamic 39

Hydraulic Couplings 39

Torque Converters 39

Fluids 41

Functions of Hydraulic Fluid 41Viscosity 41

Viscosity Index 41

Oxidation Resistance 41

Rust and Corrosion Resistance 42

Oiliness 42

Water Separation 42

Foaming Resistance 42

Aniline Point 42

Other Properties 43

System MaintenanceStorage of FluidDrain ScheduleCleaning the System

Trouble Shooting

43

434343

45

INTRODUCTION

THE Wright brothers shifted their weighton a seat to control their early airplane. Latermodels employed wires to the movable sur-faces of wings and tail, which were pulled byhand or foot levers to control the flight. Thesize, weight and complexity of modern mili-tary and commercial machines require a muchmore powerful and flexible means of control.Present day farm and industrial equipment isalso larger, more powerful and performs morejobs than it did only a few years ago.

These are only a few of the many kinds ofmodern mechanical units which would not be

practical had not a better means of controlbeen found. Application of the age old princi-ples of hydraulics has provided designers witha practical means to reduce weight, multiplyphysical effort and furnish more precise con-trol to machines of all types. This has been amajor factor in achieving greater factory andfarm production per manhour. The uses towhich hydraulics are being put increases daily.

This manual has been developed to helpprovide a better understanding of how andwhy hydraulic principles serve these purposes.

DEFINITION OF HYDRAULICS

Hydraulics may be defined, in a strict sense,as the science of fluid forces. In modern usage,hydraulics has come to mean the use of fluidto transfer power, or to change a power sourceinto useful force.

While modern farm and industrial equip-ment hydraulic uses are relatively new,. the

3

use. of hydraulic forces water power, forexample, dates back to early times. In the sev-enteenth century, Pascal discovered the funda-mental law upon which modern machinehydraulics is based. Simply stated, Pascal'slaw is that pressure applied at any point in astatic fluid is the same in all directions andacts with equal force on equal areas (Illust. 1).

AREA = 1 SQ. IN.

AREA =1 SQ. IN.

1. PISTONS OF EQUAL AREA; OUTPUT FORCE EQUALS INPUT FORCE.

AREA 1 SQ. IN.

AREA= 2 SQ.IN.

2. OUTPUT PISTON AREA INCREASED; OUTPUT FORCE IS INCREASED.

AREA = 2 SQ.IN.

AREA 1 SQ. IN.

AREA =4 SQ.IN.

AREA -3 SQ.IN.

3. MULTIPLE PISTONS: OUTPUT FORCES PROPORTIONAL TO PISTON AREAS.

Illust. 1 Application of Pascal's law.

Since fluids are nearly incompressible, theapplication of Pascal's law means that me-chanical forces may be transmitted, multi-plied, or controlled by means of fluids under

4

pressure. One of the earliest applications ofthe principle was the hydraulic press shownin Illust. 2.

CYLINDER A

1 SQ. IN.

Illust. 27Principle of early hydraulic press.

It was not until the late eighteenth centurythat pistons and cylinders could be made withclose enough "fit" to make full use of hydrau-lic principles. The development of present-day hydraulic systems dates from about 1900.

Hydraulics offer a simple method of apply-, ing large forces with a flexibility of controls

that is practically limitless. Hydraulic equip-ment is so versatile that it can be easilyadapted to provide multiples of interdepend-ent and coordinated motions of force whichwould be economically impossible by mechani-cal linkages.

Some of the main features that makehydraulics so adaptable are:

1. Simplicity in design.2. Extreme flexibility as regards location of

components.3. Complete automation of sequence is

possible.4. Simplicity of speed control.

CYLINDER B

10 SQ. IN.

5. Limitless variety of speeds, controls, andforces.

6. Reduction of wear on moving parts by:(a) Controlled acceleration and decel-eration; (b) automatic release of pres-ure at overload; (c) absence of vibra-tion; (d) automatic lubrication.

7. Efficient and economical to operate.8. Large forces can be controlled by much

smaller ones.9. Power and friction losses are compar-

ably small.

No mechanical device is perfect, and hy-draulics have some drawbacks also:

1. Pressures often are very high. Two orthree thousand p.s.i. is not uncommon.High pressures require heavy tubing,tight joints, and intelligent maintenance.

2. Operating efficiency can be severely re-duced, or operation halted, by rust, cor-rosion, high temperatures, dirt, and theproducts of fluid deterioration.Cleanliness is all-important!

HYDRAULIC THEORY

Some knowledge of hydraulic theory is nec-essary in order to fully understand the appli-cation of hydraulics to farm and industrialequipment and the reasons that these applica-tions act as they do.

1. IncompressibilityFluids are very nearly incompressible. For

this reason, fluid in a confined space can be

5

used to transmit force. By Pascal's law, whenpressure is exerted by a piston in a cylinder,the pressure is transmitted undiminishedthroughout the fluid.

Although fluid is said to be incompressible,in a high-pressure hydraulic system, fluid doescompress slightly. For example, at 3,000 p.s.i.,fluid "loses" about 1.2 percent of its volume.

This is a minor item of concern mainly todesign engineers.

2. Multiplication of Forces

Pascal's law explains the ability of confinedfluid to multiply forces. Since the pressure atany point in confined fluid is the same in alldirections, the total thrust on any piston canbe computed by multiplying p.s.i. by the areaof the piston head in square inches.

For example, fluid in a closed system,brought to a pressure of 100 p.s.i. by meansof a small pump, will exert a thrust of 500pounds on a piston with an area of five squareinches. Thus, a small force exerted on the fluid

of a hydraulic system may be multiplied intoa greatly increased work force.

3. Hydrostatic and Hydrodynamic

Hydraulic power may be classified in twogeneral categories: Hydrostatic: where the po-tential (static) energy of fluid under pressureis used to perform work. Examples: the hy-draulic press, hydraulic brakes, hydrauliclifts, hydraulic motors. This includes most ofthe present applications of hydraulics to farmand industrial equipment.

Hydrodynamic: where the kinetic (dy-namic) energy of fluid in motion is used toperform work. Examples: the hydraulic cou-pling, the torque converter.

BASIC SYSTEM

Modern hydraulic systems on many types ofequipment seem complicated, and some are.Actually, however, their fundamental designis quite simple. This can be illustrated bybuilding up a basic system, piece by piece. (II-lust. 2.)

In Illustration 2, it will be noticed that, ifthe piston in cylinder A is moved ten inches, itwill displace 10 cubic inches of fluid (10inches movement times 1 square inch pistonarea). Since fluid is essentially incompressi-

CYLINDER A

ble, the displaced fluid will go to cylinder B.To find the distance this piston would bemoved, simply divide 10 cubic inches by thearea of piston B (10 square inches), and thedistance will be found to be one inch.

To alter this illustration of Pascal's law intoa hydraulic system, it is necessary to changethe design of cylinder B, add a spring to pro-vide a force against which the piston will push,and straighten out the connecting tubing, asin Il lust. 3.

TUBINGCYLINDER B

II lust. 3Simple hydraulic press with spring added.

6

SPRING

In this illustration, cylinder A is actually asimplified hand pump, and any force devel-oped by it will exert a greater force on thepiston in cylinder B, according to the previ-ously explained multiplication of forces. Sev-eral strokes of the hand pump will be requiredto move piston B through its complete stroke.

AIRVENT

RESERVOIR

CHECK VALVES(ARROWS SHOWFREE FLOW)

the spring will force the piston up and thefluid will be delivered back to the reservoir.

Because it is usually desired to operate sucha cylinder at frequent intervals, a motordriven pump is added (Illust. 5). As therotary pump supplies fluid continuously, the

CYLINDER

HAND PUMP

TUBING

!Dust. 4Reservoir and check valves added.

This necessitates an additional quantity of flu-id, which can be obtained from a reservoir,as shown in 'Must. 4. Also, check valves areneeded to permit:0e fluid to be pumped fromthe reservoir (which is at atmospheric pres-sure), to the cylinder at an increased pressure.

The system shown in Illust. 4 would beimpractical, since once the piston is extended,the fluid cannot get back to the reservoir. Forthis reason, a directional control valve and areturn line to the reservoir must be installedas in Il lust. 5. A directional control valve is adevice capable of directing the flow of fluid inone of several directions. When the valve is inthe position shown by the solid lines, the fluidmay be pumped into the cylinder and thepiston will be forced down. When the valve isin the position indicated by the dotted lines,

SPRING

check valves have been removed. However, arelief valve has been added.

Il lust. 5 represents a simple hydraulic sys-tem which is operable. If it is advisable to usea cylinder which does not contain a springto return the piston to its "home" position,a four-way directional valve installed as shownin Illust. 6, so that fluid can be supplied toeither end of the piston. Then, regardless ofwhich side of the piston pressure is applied,the fluid on the opposite side of the piston willreturn to the reservoir.

The relief valve, added in Il lust. 5, is afundamental part of nearly all hydraulic sys-tems.

Without a relief valve, the pump wouldhave to be stopped at the exact moment the

10

piston reached the end of its stroke (in eitherdirection), or when the directional valve isplaced in a position that stops the flow of fluid.If the pump were not stopped, the pressure inthe system would quickly build up to a pointwhere rupture of parts or stalling of the driv-ing motor would occur. The relief valve is setat a predetermined pressure, according to thesystem involved, so that excessive pressure

AIRVENT

RETURN LINE

will cause it to open and allow fluid to escapeto the reservoir.

All hydraulic systems can be broken downinto simplified diagrams as shown in Il lusts. 5and 6, adding units as necessary. Multiple pis-tons, multiple pumps with a common reser-voir, rotary power units, or any combinationof components is possible with proper valuingand flow design.

CYLINDER

AIRVENT

PUMP

DIRECTIONALCONTROL VALVE

II lust. 5Directional control valve, rotary pump, relief valve, and return line added.

CYLINDER

PUMP

DIRECTIONAL CONTROL VALVE

II lust. 6Four-way valve and double acting piston added.

8

RESERVOIR

Every hydraulic system must have a reser-voir. In most farm and industrial equipmentapplications, the reservoir is a built-in unit.For the sake of compactness, some comprom-ises with ideal design are made. Ease of clean-ing, large capacity, ideal return line, baffleand filter installation are often impossible toobtain with built-in reservoir systems.

The size of the reservoir depends on theamount of fluid required to operate the system.An ideal size is two or three times the capacityper minute of the pump, except where extralarge cylinders or heat problems require

may be required to accomplish thisseepage 32.)

4. Allow air and foreign matter to separatefrom the fluid.

The design characteristics shown in Illust. 7may be incorporated, in one form or another,in any hydraulic reservoir.

Most of these features are self-explanatory;however, some emphasis should be placed onsome of them. All openings 'into the reservoirshould be sealed to prevent the entry of dirt,water and other contaminants.

The vertical baffle plate (if used) withopenings at the bottom, helps to segregatereturn fluid from that entering the pump.

DipsUck forchecking fluidlevel

I Pump lntskc filter

Illust. 7A drawing of the ideal features of a fluid reservoir.

greater quantities. At any rate, the* reservoirshould be large enough to:

1. Hold all of the fluid that can drain backinto the reservoir by gravity flow.

2. Maintain the fluid level above the open-ing of the suction line at all times.

3. Dissipate excess heat generated duringnormal operation. (An accessory oil cooler

912

This allows slower and more complete fluidcirculation, and helps air and foreign matterseparate from the fluid.

If a pump intake filter is provided, it shouldbe mounted at least one inch away from thesides or bottom of the reservoir.

The dipstick, or fluid level plug, whicheveris used, should show the proper level whenthe pump is not running, with the cylinders

collapsed. Too little fluid will allow air toenter the pump; too much will not providesufficient air space, and may cause damageto the reservoir. The dipstick or plug must fittightly in the reservoir.

The filler hole must be tightly capped, anda strainer-equipped funnel should be used tokeep contaminants out of the reservoir whenadding fluid. The drain plug should be located

in such a manner that all the fluid will bedrained when it is removed.

An air vent, or breather, which may be fittedwith a filter, is very important. As fluid ismoved to and from the hydraulic reservoir,air will be sucked in and pushed out by thefluctuating level of fluid in the reservoir.Filtered air helps prevent moisture, dirt, andother contaminants from reaching the fluid.

HYDRAULIC PUMPS

The pump is the power supply of thehydraulic system, and it is a precision-builtunit, of rugged design and high qualitymaterials.

At this point it is well to mention thatinstallation, trouble-shooting, and operatinginstructions for hydraulic pumps must be fol-lowed to the letter.

Design features, tolerances, and lubricationrequirements that are not "spelled out" havebeen taken into consideration. It is advisablethat service personnel take advantage of the ex-perience and research that have been put intothese manuals.

Pumps are commonly thought of as com-pressors, but this is not true of hydraulicpumps. Since fluids are virtually incompressi-ble, except at extremely high pressures, thepump serves only to transmit force.

In the very simplest type of system, such ashydraulic brakes, force is merely transmittedfrom one point to another. Pushing down onthe brake pedal moves the master cylinderpiston toward the discharge end of the cylin-der, and fluid flows under pressure to thecylinders at the wheels. Through one simplemovement, force is transmitted from one pointto oilier points.

Rods, shafts and levers can, and are, usedto obtain the same effect; however, fluid-filledlines give more desirable results.

10

In the hydraulic braking system, fluid flowstops when all the brake shoes make contactwith the brake drums. In this case, the pri-mary purpose of the pump (master cylinder)is to transmit force hydraulically. In morecomplex hydraulic systems, the pump providesa steady flow of fluid under pressure, thusgiving both transmission of force and motion.

Pumps may be classified into two types,based on performance: (1) Fixed deliverywhen running at a given speed, and (2) Vari-able delivery when running at a given speed.

At the present time, most farm and indus-trial equipment pumps are of the fixed deliv-ery type.

Variable delivery pumps are usually run ata constant speed, and are so designed that thevolume output can be easily varied from zeroto maximum. These pumps are more complex,therefore more expensive, than constant deliv-ery types, so their use is limited to applica-tions where the variable delivery feature is anecessity.

Clearances are extremely critical in hydrau-lic pumps, as in most hydraulic system com-ponents. The formation of contaminants ineven tiny amounts can result in operating dif-ficulties. The efficiency of a hydraulic pumpdepends upon a minimum of wear throughoutits life. Abnormal wear, for example, causesincreased internal slippage of the fluid in thepump. (Internal slippage is internal leakagefrom the high pressure side of the pumpthrough the pump parts.) This results in

power loss, reduced output, and increasedoperating temperature.

Since the hydraulic fluid is nearly alwaysthe lubricant for the moving parts of the sys-tem, it is essential that the recommended fluidbe used in the system.

The principles of operation of most of thecommon types of hydraulic pumps are de-scribed on the following pages. Illustrations,in many instances highly simplified, will helpexplain how these pumps put fluids in motion.

RECIPROCATING PUMPSPumps with reciprocating piston action (this

classification does not include the rotary pistonpumps covered later) are useful in extremelyhigh pressure applications because of lowleakage losses and in applications where largevolumes of fluid are required.

The pulsating nature of the discharge fromreciprocating pumps may cause objectionablevibration in many installations. Pumps withthree or more cylinders on the same shaft aredesigned to reduce pulsation by overlappingthe pumping strokes of each cylinder, in muchthe same manner as internal combustionengines.

Most of these pumps are of the fixed dis-placement type, although one variation is pro-vided with a stroke-transformer which permitsvariable delivery by varying the length of thepiston stroke. Reciprocating pumps are capa-ble of operating under adverse conditions(such as extreme cold and heat), are compar-atively trouble-free and will handle tremen-dous pressure. Il lust. 8 is a self-explanatoryschematic of a simple reciprocating pump.

EXTERNAL GEAR PUMPSExternal gear pumps are of the constant

delivery type, and operate on the simple prin-ciple that as gears revolve, fluid trapped in be-tween the gear teeth and the housing is carriedfrom the suction (inlet) side to the discharge(outlet) side of the pump. (Illust. 9). Onegear is driven, the other "follows." Externaltiming is not necessary.

The efficiency of this pump is largely deter-mined by the close fit of the working parts.

11

CHECKVALVES

CHECKVALVES

Illust. 8A reciprocating pump showing the principal workingparts.

2. Fluid is picked up andcarried in these spaces tothe outlet side of the pump.

3. Mating teeth displacefluid and force it throughoutlet.

1. Parting gears createslight suction here thathelps to draw in fluid.

Illust. 9Operating principle of the external gear pump.

They are used principally where relativelylow pressures, volumes, economy of cost, andrestricted space are factors.

More quiet and somewhat smoother opera-tion plus slightly higher pressures may be ob-tained by using helical or herringbone gears.

14

Pressures up to 3000 p.s.i. are available by"pressure loading" loose fitting bearings sothat a balance is set up between pressurewithin the gear cavity and the thrust of thebearings against the gears. This enables thepump to be driven at very high speeds, withincreased pump efficiency.

Gear pumps have a tendency to churnfluids, which may raise their temperature.This action hastens the mixture of contam-inants with the fluid.

INTERNAL GEAR PUMPSInternal gear pumps are a modification of

the gear pump principle, in that pressure isdeveloped by trapping fluid between meshinggears. Either the internal gear or the externalring may be the driven member. See Il lust. 10.

The crescent shaped block (Illust. 10) actsas a division between suction and discharge.

INNER GEAR(DRIVEN)

IN

OUTER GEAR

Must. 10Internal gear pump.

STATIONARYCRESCENT

DIVIDER

In another design, (Illust. 11) the crescentis omitted and sealing of discharge from suc-tion is accomplished by having the teeth inconstant sliding contact throughout the cycle.Also, there is one less tooth on the internalgear than on the external ring. See Il lust. 11.

l2

FLUID OUT(PORT NOT SHOWN)

FLUID IN(PORT NOT SHOWN)

Illust. I I Internal gear pump.

Internal gear pumps have about the sameeffect on fluid as the external gear pumpsalready described.

GEAR-LIRE PUMPSGear like pumps are similar to both gear

types, in that they have rotating memberswith lobes resembling teeth, which inter-mesh(Illust. 12).

1. Timing gearssuch as these. . .

2. are required to drive these gear-likerotors, because the shape of the rotorteeth makes it impossible for one rotorto drive the other.

3. Fluid drawn In here is carriedthrough the pump and discharged atthe other side in the same way asIn a simple gear pump.

Illust. 12Operating principle of a gear-like pump.

ts

One rotor cannot be used to drive the other,so external timed driving gears are necessary.They are generally used for transferring fluids,not for providing pressure in a hydraulicsystem.

Other forms may be used in a gear likepump, such as two lobe or three lobe cams.

SCREW PUMPSScrew pumps are simple, rugged and are

capable of working at very high speeds. Theyare especially useful in handling fluid with agreat deal of entrapped air. This type of pumpis not used to provide hydraulic power. It is

3. Fluid is drawnin here. .

used for transferring fluids from one point toanother. The operating principle is outlinedin Illust. 13.

VANE PUMPSVane pumps are a popular means of pro-

ducing hydraulic power. Unless special adap-tations are made (see below) they are con-stant delivery type pumps. They produce a rea-sonably steady flow. Vane pumps are capableof handling large volumes of fluid at relativelyhigh pressure. Wear does not greatly reduceefficiency because the vanes will move fartherout in their slots to maintain contact with the

4. and carried along inthese spaces

t. FA VA

\11Mt Atr"wit

5. to the discharge.

2, are rotated by thisdriving screw. . .

1. As this raw'turns counter-clockwise. . .

1. As these screws. . .

Illust. 13Operating principle of the screw pump.

J5. Conversely, over areascovered by these dischargeports, the volume betweenadjacent vanes decreasesas rotor turns, thus fluidle forced out of the pump.

2. these vanes arethrown out by cen-trifugal force andfluid pressure. . .

3. against this oval -shaped housing.

4. The volume betweenadjacent vanes increasesas the rotor turns overareas covered by theseintake ports. Thus, fluid13 drawn Into the pump.

Illust. 14Operating principle of the vane type pump.

13

housing. Dirt and sludge may cause the vanesto stick in their slots, thus seriously affect-ing operation. For operating principle, seeIl lust. 14.

The pump in Il lust. 14 is called a "hydrau-lically balanced" pump because it has twoeach diametrically opposed inlets and outlets.

In another design, the rotor is mountedeccentrically with respect to a circular hous-ing: See Il lust. 15.

VANESSLIDE INSLOTS FLUID PRESSURE BEHIND THE

VANES AIDS CENTRIFUGALFORCE TO HOLD THEM AGAINSTTHE BODY SURFACE

Illust. 15Vane pump with the rotor mounted eccentrically to acircular housing.

A further refinement will give the variabledelivery feature to this pump. By adding a

spring-loaded pressure plate and a movableinner housing, delivery can be either governoror hand controlled. See Illust. 16.

AADIAL PISTON PUMPSRadial piston pumps are compact and rug-

ged. They are capable of high pressure, highvolume and high speed, and are very efficient.Because of the large number of closely fitted

ATTACHED TOOUTER CASINGOF PUMP

GOVERNOR VARIESECCENTRICITY OFPRESSURE RING

lfPRESSURERING

Insist. 16Governor operated, variable delivery, vane type pump.

3. in thi circular housing. 4. Assuming clockwise rotation,fluid drawn in through theseports. . .

1 I

2. this rotating cylinderblock mounted eccentrically. . .

These partsdo not rotate.

1. Pump pistons reciprocate asthey are carried around by. . .

4

Spindle earns as a barrierbetween low and high prceeurefluid.

Illust. 17Operating principle of the radial piston pump.

5. and dischargedthrough theme ports.

parts, wear is an extremely important consid-eration. Fluid recommended for use in systemswith radial piston type pumps must be cleanand it must contain properties to lubricatethe closely fitted parts. (Illust. 17).

Unless special valving design is used, radialpiston pumps generally have an odd numberof pistons.

Radial piston pumps may be given thevariable delivery feature by making it possibleto adjust the eccentricity of the rotor to thecircular housingeither manually or gover-nor controlled.

AXIAL PISTON PUMPSAxial piston pumps have the same general

characteristics as radial piston pumps. Insteadof the pistons being mounted at right anglesto the driving shaft, they are parallel to it.(Illust. 18)

Pump body design variations provide thevariable delivery feature by making it possibleto alter the angle of the cylinder block to thedriving member during operation. Either man-ual or governor controls may be used.

CENTRIFUGAL PUMPS

Centrifugal pumps (Illust. 19) are of theconstant delivery type and are unique in thatthey deliver a completely non-pulsating flowof fluid. It is impossible to overload them be-cause the fluid in the pump will start to rotatewith the blades if the design pressure is ex-ceeded. They are not often used to develop

5. this discharge opening.

4. into this galley, whichcollects moving fluid andleads it to.

3. and is thrown off bycentrifugal force.. .

2. fills these spaces.. .

1. Fluid drawn in here. . .

Illust. 19 Operofing principle of the centrifugol type pump.

3. both cylinder block andpistons are attached to thisdriving member, which isrotated by a motor.

2. these pistons rotatewith it, because.. .

4. In addition to rotating,pistons reciprocate, becausethe axis of the cylinder blockis at an angle to the axis ofthe driving shaft.

1. As this cylinder blockrotates about its axis. .

5. Thum this piston, now at theintake end of its stroke, will beat the discharge/end of its strokewhen it reaches the position ofthe piston at the bottom

6. Fluid flow is controlled by valvingarrangement at this end of the cylinderblock. Fluid enters and leaves pumpthrough lines (not shown), which areindexed with the intake and dischargeports.

Illust. 18Operoting principle of the oxiol piston type pump.

15

18

hydraulic pressure. They are better adaptedfor transferring large quantities of fluids. Cen-trifugal pumps are most often found in enginecooling systems.

COMBINATION PUMPS

Many hydraulic applications involve two ormore distinct functions of the hydraulic fluid.Multi-stage pumps are used to provide fluid at

HIGH PRESSURERELIEF VALVE

OUTLET

more than one pressure. For example, a ma-chine that requires rapid movement to engagethe work, followed by slow, high pressurework force uses large volume, low pressureflow for rapid travel, and low volume, highpressure flow for the work force. This can beprovided by a two pressure (two-stage) pumpsuch as the one shown in Illust. 20.

Combination pumps may also be usedwhere pressure higher than that available

MEDIUM PRESSURERELIEF VALVE

LOW PRESSURE,FLOW-THROUGHCHECK VALVE

SMALL VOLUME HIGHPRESSURE PUMP INLET LARGE VOLUME, LOW

PRESSURE PUMP

Illust. 20A two pressure gear type pump.

HIGH PRESSURERELIEF VALVE

21A two-stoge series pump.

16 ft9

from a single pump is desired. Two single-stage pumps operated in series will give thiseffect. See Illust. 21.

Greater volume of fluid delivery is also pos-sible in this manner.

--A duplex system (not illustrated) consistsof two pumps, mounted on_ the same shaft,with a common reservoir, but each having

separate suction and delivery lines. This is

the adaptation used for tractor hydraulics (largevolume pump) and power steering (low volumepump).

Another adaptation of the combinationpump idea is found in hydraulic transmis-sions, many of which use a "multi- stage" cen-trifugal pump arrangement. This will be dis-cussed later in the manual.

HYDRAULIC VALVESIt was mentioned earlier that hydraulic

systems are basically simple and they are, ifthe presence of many valves, of several types,is not allowed to confuse understanding.

Nearly all the valves used in hydraulicsystems may be classified in these three cate-gories:

L Directional control2. Volume control3. Pressure controlLeakage of fluid past the moving parts of

valves may occur, and may be objectionableif back pressures build up. In some designs,this fluid may be internally drained to thesuction side of the pump, or to the reservoir.

Hydraulic fluid is generally the lubricantfor valves, as with pumps, and clearances areagain extremely small, so use of clean fluid,of the recommended type, is essential.

Control of valves may be manual, mechani-cal, electrical, pneumatic, or hydraulic. Cir-cuits can be constructed so that the entiresequence is automatic.

Until recently, valves were the. only meansof controlling fluids in a hydraulic system.With the advent of "variable delivery" pumps,certain valves in systems having this type ofpump may be eliminated. However, valvesare still the most important method of con-trolling fluid pressure and flow, and obtainingwide flexibility in hydraulic systems.

Specific types of valves are described andillustrated in the following section. Valves

17

used in hydraulic systems are usually moredetailed, and their applications more complexusually with elaborations and additions tothe basic valvebut the basic operating principles will be the same. If the basic operationof a valve is understood, then its refinementswill be more easily understood.

COCK VALVESCocks are very simple valVes, usually in

small sizes. They are used to bleed air out of asystem, turn gauges on and off, drain a system,

Illust. 22A cock valve in the open position.

etc. Il lust. 22 shows a cock in the open posi-tion. A quarter-turn of the handle will shut itoff. The one illustrated is designed for moder-ate pressures. Modification will enable cocksto be used at much higher pressures.

GLOBE VALVES

Globe valves are also very simple in design,and very reliable (Illust. 23). They should beused either fully open or fully closed, as leaksalong the stem are more likely in an inter-mediate position. Particles of dirt on the seator disc (washer) usually cause leakage when

Disc (Washer) May beflat or conical; shapedto fit seat, usuallyreplaceable.

Seat. In many designsseat is replaceable, asshown here.

Illust. 23A globe valve in the closed position.

the valve is closed, but will not seriouslyhamper operation. Globe valves offer some re-sistance to flow, and may cause turbulence inthe fluid.

GATE VALVES

Gate valves have several advantages overglobe valves. Mainly, at full open position,they offer almost no resistance to flow, andthey are easily controlled with automaticmechanisms. From their design, (Illust. 24),note that serious wear may occur at partiallyopened positions, as fluid flowing past the

FLOW IN THIS DIRECTION ONLY

Stem. In design shown, stemrises as valve is opened. Largevalves often have stationarystems on which disc moves upand down.

Disc. Design variesSome discs are solid,others split.

MAY BBREPLACZABLE,.....

Illust. 24A gate valve in the closed position.

Must. 25Action of a flapper valve.

valve parts will erode them. Particles of dirtcan interfere with the movement of the gate.

The gate of the valve may be a solid wedgeor a two piece "disc" with an eccentric actionon the discs, so that tightening the valve stemspreads the discs apart for a tighter fit on theirseats.

FLAPPER VALVES

Flapper valves are essentially check valvesin that they permit flow, in only one direction.They are obtainable in all sizesvery smallto very large. They offer little resistance toflow when fully open. (Illust. 25). Althoughthey are usually installed so that gravity andpressure close them, they are sometimesequipped with a spring to start the flappertoward its closed position. The back pressurecauses the flapper valve to seal tightly.

BALL VALVES

Like flapper valves, ball valves permit flowin just one direction, so they also are oftenused as check valves. Their design and opera-tion prevents their use in the larger sizes(above 1" ball size is unusual). They areoften spring-loaded, so that a certain prede-signed pressure is required to open them. Thisadaptation may sometimes be adjustable, andis called a pressure relief valve. (Illust. 26).Dirt can easily make a ball valve inefficient.The normal chatter of an operating ball valvecan be minimized or overcome by variousmodifications.

Illust.26Two designs of the ball valve principle.

19

NEEDLE VALVES

A needle valve is mainly used for meteringfluid flow very carefully and exactly. It canbe used in any position from fully closed tofully open, and is usually manually operated.(Illust. 27). Its design makes it almost im-possible to cause an abrupt change in the rateof flow, and therefore it is often used in sys-tems containing any extremely sensitive com-ponents. Dirt will easily interfere with itsaction.

Illust. 27A needle valve in the closed position.

SPOOL VALVES

Spool valves are used to control directionof flow. They are very popular in hydraulicequipment because of quick, positive action.By increasing the number of "lands" andadding appropriate ports, one spool valve canbe made to handle flows in many directions.Also, these valves can be "stacked" so thatsupply ports of the added valves are in linewith the discharge ports of the preceding ones.Grooves are often machined around the landsso that rings of fluid retained in the grooveswill help keep the spool centered and willlubricate it. Mating surfaces of spool valvesmust be accurately machined and fitted. Wornparts will cause leakage and erratic or sluggishaction. Dirt will cause sticking and erratic ac-tion. If the valve is actuated hydraulically,

22

OPEN CENTER SPOOL VALVE

From cylinder(Cylinder flow shut off)

From pump

To reservoir

CLOSED CENTER SPOOL VALVE

From pump

From cylinderPump flowshut off

WA,-91tailmaislisftwel

Valved fluid linesconnected hereprovide hydrauliccontrol of valves

'Pump flowstraight through

To reservoir

Flow to one endof hydraulic cylinder

From pump

From cylinder From cylinder

Pressurefrom

pilot valve

To reservoir

rReturn line to pilot valve I

ITo reservoir I

From pump From cylinder

From cylinder

Flow to otherend of hydrauliccylinder

Returnto pilot

valve

ITo reservoir Pressurefrom

pilot valve

Illust. 28Open and closed center spool valves.

20

z3

To reservoir

dirt and wear may upset delicate hydraulicbalances and thus cause erratic action, or per-haps completely halt operation.

There are two basic types of spool valvesthe open center and the closed center. (Illust.28). The illustration shows that the differencein operation between them is simply thatpump flow passes through the open centertype in the neutral position, while the cylin-

FROM PUMP

FROM CYL 1 TO CYL

TO RESERVOIR

der ports are blocked. In the closed centervalve illustrated, pump flow is "blocked" inthe neutral position and the cylinder ports areopen (float position). In some designs, allthree ports are blocked.

The operating principle of these valves isthe same, whether the effect is obtained withbored passages, (Illust. 29) or with slottedlands, (Illust. 30), or with plain lands(Illust. 28).

FROM PUMP

CENTERED POSITION

FROM PUMP

TO RESERVOIR

TO RESERVOIR

Illust. 29Open center type spool valve with bored passages.

FROM PUMP

FROM CYL TO CYL

TO RESERVOIR

FROM PUMP

TO RESERVOIR

TO RESERVOIR

Illust. 30Open center type spool valve with slotted lands.

24

Slotting or notching of the lands in a spoolvalve is often used as a method of "feather-ing" the closing or opening of a valve to pre-vent "fluid hammer" (see page 35) and shockto the system.

Spool valves may be spring or hydraulicallybalanced. Neutral position can be at eitherend or on center, as desired. In some installa-tions, a "detent mechanism" is used to holdthe spool in work position. This mechanismwill hold the spool until its force is overcome,sometimes by hydraulic pressure. Usually, theoperator will overcome this force by manipu-lating the controls.

ROTARY VALVES

Rotary valves are also used to control direc-tion of flow. Sometimes they are used by them-selves, but usually they are used as pilot valvesto control the movement of spool valves. Byadding extra ports and passages it is possibleto control flow through several lines with onevalve. Rotary valves handle a wide range ofpressures and will operate efficiently in thepresence of relatively high amounts of dirt

TO RESERVOIRTO VALVE OR

ORK

TO RESERVOIRTO VALVE OR

WORK

TO RESERVOIRFROM VALVE

ORWORK

\ X

ORFROM VALVE FROM TO VALVE OR/

WORK WORKOR `' WORK, -,., ..

FROM PUMP FROM PUMP FROM PUMPROTARY VALVE IN THREE POSITIONS

Must. 31Operating principle of the rotary valve.

22

and sludge. Their operating principle is verysimple. See Illust. 31.

Rotary valves are usually operated manu-ally, but it is also possible to operate themhydraulically and electrically.

DIRECTIONAL POPPET VALVES

One kind of poppet valve has already beendiscussed the simple spring-loaded ballvalve. A poppet valve is usually spring loaded.It is any valve which is opened either hydrau-lically or mechanically, and in which flow offluid is past the sealing member, through theseat itself. The sealing member of a poppetvalve may be ball-shaped, bluntly spear-shaped, mushroom shaped, or any of a num-ber of other shapes.

This discussion is limited to directionalpoppet valves, which are gaining popularityin farm and industrial equipment applica-tions. They have the advantage over spooltype directional valves of being positivelyseated, thus allowing no leakage past theirinternal members, and consequent cylindersettling.

One disadvantage of directional poppetvalves is that it is difficult to obtain the finemetering action possible with spool valves.Another is that it is very hard to obtain theshock lessening feathering action possible withspool valves.

The operating principles of the cone typepoppet valve (Illust. 32) are very simple. Thecams, which may be operated by hand ormechanical means, are turned to open onehigh pressure valve and one return valve.When cylinder .movement in the other direc-tion is desired, thi" camshaft is turned so thatthe other two valves in the block are opened.

The operating principle of the mushroomtype poppet valve (Illust. 33) is also verysimple. As the cam shaft is turned, it over-comes the spring pressure which holds thevalves seated, and allows fluid to flow, asdirected by the two opened valves. To reversethe flow, the cam shaft is turned until theother two valves are opened.

PISTON LINE

THIS CAM OPERATESTHE HIGH PRESSURE SIDE

ROD LINE

THIS CAM OPERATESTHE LOW PRESSURE SIDE

ROTATE 180°TO CHANGEDIRECTION

TO AND FROMPISTON ENDOF CYLINDER

PISTON LINE

Illust. 32Cone type cam operated poppet valve.

SPRINGS

;,111LW

TO VALVE ORRESERVOIR

TO AND FROM ROD ENDOF CYLINDER

SINGLE INNERECCENTRIC

FROM PUMPOR VALVE

Illust. 33Mushroom type cam operated poppet valve.

23

26

DOUBLEOUTERECCENTRIC

CROSS SECTIONOF CAM

COMBINATION

It was mentioned earlier that spool valvesmay be "stacked" so that more than one opera-tion is possible with one pump and reservoir.This type of valve block is especially welladapted to farm and industrial equipment.

Other combinations are also commonlyused, such as, a block with both pressure regu-lating and check valves, or check valves anddirectional valves together. Valves are oftenincorporated into the body of the pump.

These combinations will, unless they arethoroughly understood, confuse the service-man. It is very important, therefore, to under-stand the purposes of a valve block and theworkings of basic valves, before attempting totear down a block which has not been seenbefore.

FLOW CONTROL ANDFLOW DIVIDER VALVES

It is relatively easy to understand the opera-tion of a simple relief valve, and what ismeant by the term "relief valve." Most valvesin any system are relatively easy to under-stand. However, the terms flow control andflow divider are highly confusing, and theiroperational differences must be thoroughlyunderstood.

To begin with, flow control valves are alarge grouping of valves, which include flowdividers. Flow control valves control volumeflow, usually through a measured, "designedin" orifice.

Flow dividers perform this function, andalso divide the flow into two or more circuits

which operate at different pressure and vol-ume specifications, usually by controllingvolume through measured orifices.

Flow divider valves fall into three cate-gories.

The first is the priority type, wherein all thefluid delivered by the pump goes into one partof the system (for example, where the tractorhydraulic pump also supplies fluid to thepower steering) until pump delivery buildsup to a point where there is extra deliverybeyond that required for the priority delivery.The "extra" deliver.), is then made availableto other parts of the system.

Next is the adjustable priority type, whichsimply means that the delivery to the prioritypart of the system is adjustable, either exter-nally, with levers; electric solenoids, or hy-

"positioners," or internally, by chang-- ing spring tension in the valve, or changingshims, etc.

The third is the ratio type, wherein thereis delivery to all parts of the system (at differ-ing volume and pressure specifications to eachpart) at all times, but delivery may not beenough to operate the system until the fluid"warms up," or until the power source is oper-ating at full speed.

If pump delivery is lowered for any reason,the ratio type flow delivery valve will causedelivery to lower in all parts of the system atthe same rate. Division of flow in the ratiotype valve is also accomplished through me-tered ports or past spring loaded valves withinthe block.

CYLINDERS

The hydraulic cylinder is by far the mostpopular method of turning fluid under pres-sure into a work force. Other methods are inuse, of course, and will be mentioned later.

24

Cylinders may be classified into two generalcategoriessingle acting, (Illust. 34) whereinthe weight of the load returns the piston to itshome positionand double acting, wherein

27

...:.., ....."....... ....,...

....,N....,... '...."

.c. '........

..... ....

....,.... .....

*......

N.%.N. N.N. N.N. s.N.

.4.:... N.N. N.N. NN NN. N..... 'N.N. N.N. N.N N.N. '...N. ,41.1...

N.N ..........e.'N.

N..... .........

Illust. 34A single acting hydraulic cylinder.

Illust. 36A piston modified to provide equal force in bothdirections.

the piston is under pressure in both directions.See Illust. 35.

The double acting cylinder in Illust. 35 iscalled a differential type Lecause the pistonarea at the bottom is larger, providing aslower, more powerful force upward, and afaster, less powerful downward stroke becauseof the difference in piston area, if the samepsi is applied to move the piston in one direc-

Must. 35A double acting hydraulic cylinder.

.21.7=EZat

r711/1. 1/;1 0

A

, t/// /////// ////////

Illust. 37A "stepped piston" type cylinder.

tion as is applied to move it in the other dirtion.

If equal force in both directions is desired,the piston rod is designed to extend throughboth ends of the cylinder. See Illust. 36.

The "stepped piston" provides a means fora rapid approach stroke at low pressure and aslower, more powerful work stroke. A valveadmits fluid first against the smaller part ofthe piston, which moves rapidly until the workis contacted; the entire piston surface takesover for the power stroke. See Illust. 37.

25

ZS

/ mimmaiimmarmuoimmiammaimmmm

Illust. 38A hydraulic cylinder with provision for a cushion at theend of the stroke.

Another important refinement of the hy-draulic cylinder provides a "cushion" for theend of the stroke of the piston. Since pistonsare often under heavy load, and moving rap-idly, this is very important in many applica-tions. (Illust. 38). The boss on the pistonenters a bore in the end of the cylinder, shut-ting off the normal discharge port. The re-maining fluid is then forced through a smallerport, often adjustable by means of a ball orneedle valve.

Some other refinements often found inhydraulic cylinders are as follows:

1. A thermal safety valve, set far higherthan system pressure, to relieve any pressurescaused by thermal expansion while the systemis at rest.

2. Using a double-walled cylinder, to pro-vide fluid passages, instead of external hoseor tubing. This is found in many power steer-ing applications, including International Har-ester self-propelled combines.

3. Stroke control valves which are adjust-able to stop the cylinder at any point in itstravel, by shutting off the flow of fluid to the

cylinder, or otherwise closing a valve. SeeIllust. 39.

4. In some cases, the cylinder moves, andthe piston is fastened to the base.

5. Another refinement is the telescopingpiston. See Illust. 40.

The piston acts as though it were one pieceuntil the outer section reaches its stop, where-upon the inner section will continue to theend of its stroke. They may be designed so thatthe inner section will move first, followed bythe outer section. The speeds of the varioussections are dependent upon the square inchesof pushing surface in each section, also adesigned-in feature.

Illust. 40Cross section of a telescoping piston type cylinder.

Another feature which should be mentionedis that of multiplication of forces, as usedespecially in fork lifts. See Illust. 41.

Many other design features are possiblewith hydraulic cylinderstwo pistons in onecylinder, internal stroke adjustment, speedvariability, and many others, which are ofrelatively small importance to farm and indus-trial equipment.

It has been a rather common practice forsome dealers and servicemen to attempt toinstall hydraulic cylinders in applications for

Must. 39A hydraulic cylinder with a stroke control valve.

26

Illust. 4IPrinciple of using cables or chains and pulleys to multiply length of cylinder stroke.

P (pivot)

WEIGHT

1500 LB.

(a)

2'

Illust. 42Basic lever principle.

which no provision has been made for attach-ing them. Of course, it is possible for an imple-ment, wagon, or truck to be equipped with ahydraulic lifting mechanism, with properstrengthening and mounting. However, theattaching points for adding a hydraulic cylin-der are very critical, and no attempt shouldbe made to install one in an application whichhas no attaching points, without seeking ex-pert advice.

L

1500 LB.

W

4'

Must. 42 shows the basic lever principle.The formula for figuring the force (F), re-quired to lift the weight (W), is F x 1Wx (L 1) (force multiplied by distance frompivot to point of force (1) equals weight multi-plied by distance from pivot to weight (L 1).In (a), then, the force required would beF x 2 = 1500 x 4, or F/2 6000, or F = 3000pounds upward force.

In (b), having lengthened the distance (L),F 4500 pounds, according to the formula.

27

30

If an attempt is made to lift 1500 lbs. with atwo ton cylinder as in (b), then, obviously,it will not work.

This explains why, after a wagon bed hasbeen rebuilt, the cylinder, which was properlyinstalled for its previous dimensions, some-times will not handle the rebuilt bed.

With this principle in mind, it is also under-standable why a loader will lift a heavy weightpart way, and then stop. As the loader armsgo up, they also follow an arc which graduallymoves the weight farther away from tlie liftingpoint, thus increasing the force necessary to

a

raise the load. Also, this is the same thing thathappens when the drawbars of a tractor willnot lift a load when improperly attached tothe tractor, even though the weight may beless than the lifting ability of the drawbar. Insimple terms, what has happened in thesecases is that distance (L) becomes too greatfor the lifting force of a given cylinder.

When further complications are added tothe problem of attaching a cylinder such as ascissors type lift, or drawbar lift, it becomeseven more clear that it is very wise for thedealer or serviceman to get expert advice oninstalling cylinders.

SEALS AND PACKING

b d

(A) CUP PACKING(B) FLANGE PACKING(C) U-PACKING(D) V-PACKING(E) EXPANDING METALLIC SEAL

(c) a

(F) NON-EXPANDING METALLIC SEAL(G) MECHANICAL SEAL(H) COMPRESSION PACKING(I) SPRING-LOADED LIP SEAL(J) 0-RING

Illust. 43Various types of seals and packing.

None of the components of a hydraulicsystem so far discussed would operate withoutproper seals and packing to hold the fluidunder pressure in the system. They may beclassified as to their use:

28

1. StaticUsed as a gasket to seal non-moving parts.

2. DynamicUsed to seal moving parts.They may also be classified according to

their shape: U-packing, chevron or V-packing,

0-rings, cup and flange seals, metallic seals,compression packing, mechanical seals, andcompression gaskets. (Illust. 43). The type tobe used is decided by the manufacturer, andmanufacturers recommendations must befollowed.

0-RINGSPerhap-s' the most popular seal in use in

farm and industrial hydraulics is the 0-ring.It is used as both a static and a dynamic seal,and is usually made of synthetic rubber. It isused in sealing pistons in cylinders, sealingthe rod end of the cylinder, sealing movingvalve parts (dynamic applications), and insealing boss or flange type fittings, sealing thevarious parts of a cylinder together, and toseal pump and. valve body sections together(static applications).

They are designed for installation in agroove, so that when they are in use, the meet-ing members compress the ring about ten per-cent. In dynamic applications, their life is de-pendent upon the smoothness of the movingparts, and the closeness of fit. 0-rings are notused where they must cross openings or passcorners under pressure. In static applicationsunder high pressure, they are often strength-ened by use of a fiber or leather back-up ring,so that they will not squeeze out of theirgroove. When a back-up washer is used, itis almost always installed on the low Pres-sure side of the 0-ring. If two are used, thereis one on each side.

"U" AND "V "- PACKING

"U" and "V"-packings (chevron packings)are dynamic seals used to seal pistons and rodends of cylinders, and to seal pump shafts.They are made of leather, synthetic and nat-ural rubber, plastics, and other material.

They are installed with the open side, orlip, toward the pressure so that pressure willpush the lip against the stationary member, toform a tight seal. Chevron packings made upof several single "U" or "V"-packings are usedin packing glands, and are very popular forsealing rotating shafts, pistons, and rod endsof cylinders. Packing glands are cases or com-pressing units that hold this packing in onepiece.

2932

CUP AND FLANGE SEALS

Cup and flange seals are dynamic seals, andare made of leather, synthetic rubber, plastics,and other material. Surfaces are sealed by theexpansion of the "lip" or beveled edges. Theyare used in pistons and on piston rods.

MECHANICAL SEALS

These seals are designed to eliminate someof the problems attached to using chevronpacking for rotating shafts. They are dynamicseals, made of metal and rubber, usually.Sometimes the rotating portion of the seal ismade of carbon, "backed up" with steel.

The seal (Illust. 43) consists of a fixedmember, (stationary seal) attached to thehousing against which a rotating member at-tached to the shaft revolves. A spring holdsthe two parts of the seal tightly together. Arubber ring (flange-shaped) or diaphragm isusually included to permit lateral flexibility,and to keep the rotating part of the seal inmotion.

METALLIC SEALS

Metallic seals used on pistons and pistonrods are very similar to the piston rings usedin automotive engines. They may be eitherexpanding or non-expanding. They are dy-namic seals, and are usually made of steel.

Unless extremely close tolerances are ob-served, non- expanding seals are subject to con-siderable leakage. Expanding seals (for use onpistons) and contracting seals (for use onpiston rods) are subject to moderate frictionand leakage losses.

Metallic seals are therefore used whereeconomy is a very important factor in thedesign. Precision metallic ,seals, however, arenot so badly subject to leakage, and are espe-cially well adapted for use in extremely hightemperatures.

Since metallic seals of all kinds are moresubject to leakage than others, fluid wiper sealsin connection with external drains to the reser-voir, are often used.

COMPRESSION PACKING

Compression packing (jam packing) is usedin dynamic applications, and is made of plas-tics, asbestos cloth, rubber laminated cotton,fibrous or plastic cores with metallic foiljackets, and vulcanized fibrous binders whichare impregnated with grease, graphite, asbes-tos or metal particles.

It is used for the same purposes, and inmuch the same manner as "U" and "V"-pack-ings. It comes as coils, and as endless ringssized to fit, from which properly sized piecesmay be cut.

Generally, compression packing is suitablefor the lower pressures. Lubrication of this

type packing is extremely important, as scor-ing of moving parts will result if it is allowedto run dry.

COMPRESSION GASKETSGaskets are suitable, of course, only for

static applications, such as assembly of cou-plings in hydraulic lines, parts of the body ofpumps, valves, cylinders, and for sealingcovers, etc., on hydraulic reservoirs. A gas-keted joint is sealed by the molding of thegasket material into the imperfections of themating surfaces of the joint.

Gaskets are made of many materials, bothmetallic and non-metallic, and of course, comein hundreds of shapes.

HYDRAULIC LINES AND FITTINGS

No hydraulic system can be expected tooperate without proper connections for mov-ing the fluid between the various units of thesystem. Hydraulic lines must be designed andinstalled with the same care applied to theother parts of the system. They should beleakproof and strong enough to stand themaximum pressure, temperature, and vibra-tion of the system involved. They should notbe constructed of materials, or designed in away, that will cause restriction of flow andturbulence.

They should be large enough to carry themaximum pump output without excessive fric-tion losses or turbulence. They should be asshort as possible and have as few bends aspossible. Hydraulic lines may be of piping,tubing or flexible hose.

PIPING AND TUBINGPiping is usually measured according to its

inside diameter, and tubing, by its outsidediameter, although there are exceptions to thisrule. Choice between piping or tubing andassociated fittings is a design decision. Galvan-

ized pipe should never be used, as it is likelyto "flake,"' and these metal chips will causeserious damage.

When installing piping or tubing, make surethe inside is bright and clean. Try to have eachsegment continuous, and where bends are nec-essary, they must not be too sharp. The bend-ing radius is ideally not less than three timesthe I.D. of the tube.

Piping and tubing must be adequately sup-ported to minimize vibration. Vibration, if notcompensated for, can set up stresses in themetal which will cause it to rupture.

Copper has been extensively used in thepast for hydraulic lines, but its use is dimin-ishingsteel and standard iron pipe is morepractical in today's systems.

FLEXIBLE HOSEFlexible hose is used where the parts con-

nected to each other by the hose must havefreedom to move. It is made of syntheticrubber, steel, and sometimes cotton cords inlayers. See Illust. 44.

COTTON ITZIL

Illust. 44Cutaway hydraulic hose.

In use, hose should be installed so that notwist of any kind or sharp bends are present.Bending. radius should not be less than sixtimes the inside diameter. Also, enough slackshould be provided for changes in distancefrom one part to another, and for changes inrelative motion.

FITTINGS

Good high pressure fittings are fabricatedor forged, and are either threaded or flangedfor use with piping, or of the compressiontype for use with tubing. Compression fittingsmay be-of flared or flareless (bite-type) design.(Illust. 45). Due to possibility of leakage,threaded connections are avoided as much as

SLEEVE COUPLING FOR FLEXIBLE HOSE

FLEXIBLE HOSE COUPLING

possible. When they are used, they are oftenof the boss type, with an 0-ring seal. (Illust.46). Threads must be cut clean and smoothand a protective compound should be used tohelp seal the threads and protect theirilromcorrosion. Brass fittings do not always requireuse of a protective compound.

NUT"O"

RING

FITTING

BACK-UPWASHER

"0" RING GROOVE

BOSS

Illust. 46--Boss type hydraulic fitting with an 0-ring.

Flanged fittings may be either threaded tothe pipe ends or welded. Gaskets of softermaterial are often used to make a better seal.Fittings with restricted or stepped-up passagesshould not be used, as they create restrictionand turbulence, thereby causing frictionlosses. Manufacturer's recommendations re-garding tubing, hose, fittings, and the use ofprotective compound during assembly mustbe followed.

FLARELESS (BITE-TYPE) COUPLING

0-RING SEALED FLARELESS TYPE

THREADED COUPLING FOR FLARED TUBING

Illust. 45Various hydraulic line fittings.

31

34

ACCESSORIES

Several accessories are found in manyhydraulic systems. Of these, only two are ofgreat importance in farm and industrialequipment hydraulics.

The heat exchanger, or fluid cooler, is usedto remove heat caused by rapidly movingpressurized fluid. This is often necessary tokeep the viscosity of the fluid at an operablelevel, and to keep the temperature of the fluidfrom reaching a point injurious to the com-ponents, especially seals. Also, excessive heatcontributes to fluid deterioration.

Coolers are made in separate units whichconsist of tubes or places in a metal housingthrough which cooling water circulates.Baffles break up the flow of fluid to providemore cooling surface (Illust. 47). A by-passline should be provided to prevent excessivebuild-up of pressure if the cooler becomesclogged.

Coolers may be located in either the returnor suction line. It may be a separate system,drawing hot fluid from the reservoir, andreturning cool fluid to the reservoir.

The other accessory of importance in thisdiscussion is the filter. Filters or strainers arenecessary in modern, precision-fit hydraulicsystems. The presence of relatively fine abra-sive particles can be very harmful to thesystem.

Many contaminants harmful to a systemwill "settle out" in the reservoir, and will thenbe removed during subsequent cleaning of thereservoir. A wire mesh strainer on the pumpintake will often take care of any remainingcontaminants.

Where contaminants are still present afterthese means are applied, one of the followingthree methods may be employed:

1. Continuous filtration of all the fluid byflow through a filter.

2. Continuous flow of part of the fluidthrough a filter.

3. Removing the fluid from the system,

32rt.

Illust. 47Cutaway fluid coolers.

purifying it, and returning it to the system.

The last method is not practical in farmand industrial equipment hydraulics, becauseof the relatively small amount of fluid usedin these systems, since the equipment forpurifying fluid is rather expensive.

Filters are of three types: mechanical,absorbent, and adsorbent. Mechanical filterscontain fine wire mesh, closely stacked metaldiscs or cloth in the form of a bag. They willremove the relatively large contaminatingparticlek

Absorbent filters contain porous materialssuch as cloth, paper, cotton waste, etc. Theywill remove all contaminating particles, butnot products of fluid deterioration.

Adsorbent filters contain chemically treatedpaper or waste, charcoal, fuller's earth, acti-vated clay, etc., which remove impurities bychemical action. They remove all contaminat-ing particles, and some of the soluble con-taminants. Because they also will remove someof the desirable additives in hydraulic fluid,they are not often recommended for hydraulicsystems.

Filters may be installed so that they willfilter all of the fluid, or only part of it. Thelatter method is usually preferred, as complete

clogging of the filter will not cause stoppingof fluid flow. Filters may be mounted so, thata "bleed-off" from either the pressure line orreturn line is filtered or so that fluid passingthrough the relief valve is filtered. Placementis a design consideration, so detailed infor-mation on placement is not within the scopeof this manual.

Proper servicing of filters is very important.Mechanical filters must be cleaned at recom-mended intervals. Absorbent filter "car-tridges" must be replaced when necessary.With tolerances and clearances in hydraulicsystems getting more and more critical, propermaintenance of the filters and strainers inany hydraulic system is becoming vital. Manu-facturer's instructions must be followed.

PHENOMENAThere are certain easily misunderstood

scientific facts which are responsible for someof the breakdowns in hydraulic systems thathave been difficult to discover, explain orovercome.

RUPTURED LINE OR EXPLODEDCYLINDER IN SYSTEM AT REST

There are two basic reasons for the ruptureof hose or cylinders in a system not in use.One is simply thermal expansion, which isfluid expanding in the system due to an in-crease in outside temperatures. Depending onmany variable conditions, one degree rise inoutside temperature may cause an increase inpressure of approximately 50 to 60 psi in atightly blocked system.

Unless there is some external leakage inthe system, if there is no air in the system,and the fluid is held in one part of the systemby check valves, a given increase in tempera-ture may cause some part of the "blocked off"section of the system to rupture. There is onlyso much room in the cylinder and lines forfluid, and even though fluid is nearly incom-pressible, if it expands even minutely due to

heat, an escape hatch must be provided, ofbreakage will occur.

A thermal safety valve is one method usedto combat this problem. Another is for theoperator to be sure that the system is not leftin a "blocked off" position any time that itis at rest. Cylinders should be drained beforebeing put into storage.

The second reason for rupturing in a systemat rest is internal leaking of pressure fromthe piston head end of the cylinder to thepiston' rod end of the cylinder. -(IlThst. 48)In this example, 1000 psi is required to holdthe implement up. If there were no internalleakage, the pressure at "A" would be at 1000lb. and the pressure at "B" would be at returnline pressure, or perhaps no pressure. Since,in this case, there is internal leakage and thesystem is positively blocked, pressure willtransfer past the piston head until a hydraulic"balance" between "A" and "B" is reached.There will be very little, if any, movement ofthe piston rod in the cylinder. Because of theleakage past the piston head, the systembehaves as though the piston head did notexist. The area of the piston rod cross-section,

33-4-16

POSITIVELY BLOCKED AT SOME POINT

A WEIGHT OFIMPLEMENT

\ :,\&\\A'S,,N

4 SQUARE INCHES I SQUARE INCH

LEAKAGE PAST PISTON

Must. 48Effect of faulty piston seal.

therefore, is all that will hold the imple-ment up.

Since the four square inch piston headrequires 1000 psi to hold the implement up,the one square inch rod will require 4000psi to hold it up. This is a simple applicationof Pascal's law.

Since a system of this sort is ordinarilydesigned for 2500 to 3000 psi maximum pres-sure, it is obvious that the fluid will find anescape somewhere in the system, usually byrupturing a hose, although the cylinder couldexplode, or external seals could break.

A thermal safety valve will often preventthis type of breakdown. The best way, how-ever, of preventing this occurrence is to alwaysleave the implement "down" when parking it,whether it's attached to the tractor or not.This should be emphasized to customers andoperators.

Internal leaks that do not affect operationcan, and often do, cause this type of break-down.

COLLAPSED SUCTION HOSEAnother type of breakdown, seemingly

simple, is the collapsed suction hose. It is

34

entirely possible, even probable, that the innerlayer of rubber of the suction hose will, whenit starts to deteriorate, collapse inward, com-pletely sealing off flow, without showing anysymptoms of collapse externally.

A noisy pump, lack of pressure, "spongy"action, or no action at all are indications of acollapsed suction tube, among other things.

PRESSURE DROP CAUSEDBY FRICTION LOSSES

Another scientific factor, often misunder-stood, which sometimes leads to improperoperation, is that of pressure and volumedrop caused by friction losses. Every com-ponent of any hydraulic system, except thepump itself, causes friction loss in the movingfluid. These friction losses show up in thesystem mostly in the form of heat.

It is not the purpose of this manual to gointo design features, and the subject of fric-tion loss is mostly a designer's responsibility.However, sometimes it is necessary to addhydraulic components to a system for anadded implement, or to obtain certain specificresponses. In this case, it is advisable for theserviceman and the dealer to know that fric-

tion losses may impair operation. It may benecessary to add a larger pump, larger lines,or different valving in order to overcomefriction losses, when other components areadded to an existing system, in order to obtainproper operation.

Recommended line diameter must be knownand proper lines used in any application.

EFFECT OF USING WRONGSIZE PUMP IN SYSTEM

Since pumps are usually spoken of in termsof G.P.M. or gallons per minute, the termG.P.M. will be used in this discussion. Anyserviceman can see that the use of a pumptoo small in G.P.M. will lead to improperoperation.

However, it is also possible to have inoper-able conditions with too large a pump. Sub-stituting a larger pump in a given system will,first of all, create greatly increased frictionlosses. These losses do not increase in directproportion, so the multiple increase in per-centage loss at each point may result in suchlow pressure and volume at the point of usageas to make the system inoperable. All this ad-ditional friction loss will appear in the form ofincreased heat, in itself a danger to properoperation, as well as to the components,especially the seals, in a system.

In some systems that use hydraulicallyactuated detent mechanisms, the momentaryexcessive pressure rise may cause premature

unlatching. Other types of malfunction canresult from using a pump with too high aG.P.M. rating, such as primary power loss,increased "back" pressure, and increasedturbulence in the pump.

Taking these factors into consideration, itis obvious that the recommended pump mustbe used in a given system, unless an increasein components or number of uses forces theuse of a larger pump. In case a larger pumpmust be added to a system, it may be necessaryto increase the size of lines and valves.

THE EFFECT OF"HAMMER" IN A SYSTEM

Everyone has noticed the banging noise thatresults when a valve is closed suddenly. Thiseffect is called "fluid hammer" and will takeplace in any system involving moving fluidwherein the fluid flow is suddenly stopped.This effect' is not always audible, but it is

present.

Hammer, if allowed to continue, can causerupture of parts in a system, especially bysetting up "metal fatigue," so, when it isnoticed, an investigation should be' made im-mediately to determine its cause. It is usuallyovercome in hydraulic systems by slottingvalves, using pilot valves to open larger ones,and by "cushioning" the end of a piston strokeor valve closing. Operating the system athigher than recommended pressure will in-crease the likelihood of hammering. Over-coming hammer is mostly a matter of design.

SYSTEMS

In order to be able to fully understand theapplication of hydraulics to equipment, it

is necessary to understand what is meant bythe descriptive terms applied to various kindsof hydraulic systems.

35

Four components are all that are necessaryto have a workable hydraulic system a

reservoir, a pump, a valve, and a motor(cylinder). Review the Basic System Sectionin the "Introduction" for more detailed infor-mation on a basic circuit. The system in the

Introductory section uses a rotary type direc-tional valve, while in most farm and industrialapplications, the spool type directional valveis more popular.

Descriptive titles for the various systemsdiscussed in the following text are based onthe type of valving used, or its operation, sinceit would hardly be possible to differentiatethe type of system by basing. its name uponthe kind of pump or cylinder used.

BLOCKED RETURN LINE SYSTEM

The first to be discussed is the blockedreturn line system. (Illust. 49) It uses a leveroperated check valve, and will operate a singleacting cylinder only. In this system, as thecontrol handle is moved to the right (lift)position, the return to the reservoir (A) isblocked, forcing fluid through the check valve(B) and on to the cylinder. To lower, the

CYLINDER

.00

.0000DROP 0dr----46. LIFT

2N -;-.\ '.X . \ \ N",...,j,...0',..,..0

. I......

/0 4%.%i,........,

4 yr'i at- ,z4..._:,.....-- i A

k4,-..1po

... N

RELIE . VALVE

RESERVOIR

Illust. 49 Blocked return line system in the neutral position.

36

control handle is moved to the left (drop)position, causing the lever (C) to upset thecheck valve, and allowing the fluid to returnto the reservoir as the cylinder piston returnsto its home position by gravity.

BASIC OPEN CENTER SYSTEM

The basic open center system (Illust. 50)uses a three or four-way, open center spoolvalve. The fact that the illustration has drilledpassages should not be allowed to confuse theissueit could have slotted lands or solidlandsit's still a four-way open center spoolvalve.

LIFT

LOWER

CONTROL VALVE

RELIEFVALVE

Must. 50 Basic open center system in the neutral position.

In this system, in the neutral position, thecylinder ports are closed off, and pump flowis directed to the reservoir. Also, in thissystem, double acting cylinder operation ispossible. Action of this valve is describedunder "Spool Valverrin the valve section of44iiss manual.

TANDEM OPEN CENTER SYSTEM

In this system (Illust. 51) two or more opencenter spool valves are "stacked" so that thereturn port of the first valve discharges intothe supply port of the second, and so on downthe line.

:39

CYLINDER 2

O

111

CYLINDER 1

Ar.as

VALVE 1VALVE 2

%.

\gli\Must. 51 Tandem open center system in the neutral posifiop.

RE LIE F °VALVE

Illust. 52Tandem open center system with valve 2 activated.

By activating one of the valves in thissystem, flow will be more easily understood.(Illust. 52) Note that if the first valve had

been activated, there will still be flow fromthe pump to the second one.

This makes it possible to activate two valvesat once, but the cylinder with the lightest loadwill move firstto the end of its strokebefore the more heavily loaded one moves.

THROUGH FLOW SYSTEMThe through-flow system (Illust. 53) may

be considered an adaptation of the open centersystem. In this type of system, the high pres-sure line is taken through, or past, the firstvalve block to another open center valve block.

The first valve block is called a "through-flow" or "power beyond" valve. This type ofinstallation is often used when adding an-other open center valve (or set of valves) toan existing system.

TO RESERVOIR

FROM PUMP

POWER BEYONDOR

TRROUCII FLOWDIRECTIONAL VALVE

Illust. 53Through flow system.

CLOSED CENTER SYSTEM

A closed center system employs closed cen-

tel. spool valves, and uses a pressure regulat-

ing device to relieve pump pressure when the

valve (or valves) is in the neutral position.A familiar variation of the closed center

system is the pilot regulated system.

In this system, the directional control valve

is a closed center type with all ports blocked in

the neutral position. See Illust. 28.

37

e.' 40

With all the ports blocked, there is of

course, no place for pump delivery to goexcept through a second unit which is broughtinto play by the position of the control valve.This second unit, or pressure regulating valve,maintains the system on either high pressureor by-pass pressure. In the neutral position,the small regulator port (A) is closed, so backpressure develops at (B), forcing this valveopen, and allowing fluid to flow to the reser-voir at low, or by-pass, pressure.

When this port is open, the fluid from theregulator line flows to the return line. Thepressure then drops at (B), allowing the valveto close, thus building high pressure in the

system.

Other systems are employed, of course, butthese are the main types presently used in

farm and industrial equipment.

It is sometimes desirable (or necessary) touse combinations of the above systems.

Illust. 54Pilot regulated system in the neutral position.

HYDRAULIC MOTORS

Any device which turns hydraulic pressureinto usable mechanical force may be consid-ered a motor. Hydraulic cylinders, alreadycovered, are a form of motor. Theoreticallyany hydraulic pump (except centrifugal)may be used as a motor. Various refinementsare necessary, however, to enable any of thesepumps to be used as a motor.

In actual use, rotary hydraulic motors areof four typesgear, vane, radial piston andaxial piston.

Gear motors can be either the spur geartype or the internal gear or gerotor type. Gearmotors must be of hydraulically balanceddesign.

Vane motors, like the gear type, must be ofhydraulically balanced design (see Vane

Pumps for explanation) in order to operateproperly. The vanes are usually spring-loadedto hold them against their housing.

Radial and axial piston motors are usuallyused in connection with radial or axial pistonpumps. With a variable delivery pump, thiscombination allows an infinite variety ofspeeds. By tilting the slide block of the pumppast "zero" delivery, it is even possible toobtain reverse rotation of the motor while thepump continues to turn at a constant speed,in the same direction.

Pressures developed in any pump-motorcombination are dependent upon the load onthe motor. If the design characteristics of thepump and/or motor are exceeded, the systemwill "lock."

HYDRAULIC TRANSMISSIONS

Hydraulic transmissions are simply a meansfor transferring power from a power sourceto a driven element. The actual delivery ofpower is by means of fluid under pressure(hydrostatic transmission) or fluid in mo-tion (hydrodynamic transmission) instead ofthrough gears, belts, or other mechanicalmethods. Advantages of hydraulic transmis-sions are:

1. Simplicity of design and lower space andweight requirements in many applications.

2. Fast, simple and infinite speed adjust-ment with power source operating at a con-stant (most efficient) speed.

3. Fast, smooth acceleration and decelera-tion.

4. Smooth reversal of motion.5. Reduction of shock loads to either power

source or driven element.

Hydraulic transmissions are of two generaltypes: Hydrostatic and Hydrodynamic.

HYDROSTATIC TRANSMISSIONS

Hydrostatic transmissions combine positivedisplacement pumps with positive displace-ment motorsusually both pumps and motorsare of the external or internal gear, vane, axialpiston or radial piston types. Some of the pos-sible combinations are:

1. Constant delivery pump with constantdisplacement motorconstant horsepower atconstant torque, unless a variable flow controlvalve is added to the system in order to varypump delivery, which would give, in effect,variable horsepower.

2. Variable delivery pump with constantdisplacement motor---variable horsepower atconstant torque.

3. Constant delivery pumps with variabledisplacement motorconstant horsepower atvariable torque.

4. Variable delivery pump with variabledisplacement motor variable horsepowerand variable torque.

Usually, both pump and motor will be en-closed in a common housing, with a built-inreservoir, and with appropriate porting andvalving.

HYDRODYNAMIC TRANSMISSIONS

Hydrodynamic transmissions use the energyof fluid in motion to transfer power to a drivenelement. A centrifugal type pump (impeller)sets the fluid in motion; the moving fluiddrives the blades of the driven member (run-ner) in the same direction. Hydrodynamictransmissions are of two types: hydrauliccouplings and torque converters.

HYDRAULIC COUPLINGS

Hydraulic couplings, sometimes called fluidclutches, hydraulic clutches, or fluid drives,can be either constant or variable speed de-livery. Variable speed is accomplished byvarying the amount of fluid allowed in thecircuit. Illustration 55 shows the operatingprinciple of hydraulic couplings.

In this type of hydrodynamic transmission,torque output equals torque input, but speedoutput is lower, I ecause of friction, and"slip," which is simply loss of motion of fluidinternally. The greater the "slip" vlie greaterthe power loss in the coupling.

In automotive applications, a fluid flywheel(hydraulic coupling) together with an auto-matic device for shifting gears, makes up theautomatic transmission, allowing smoothacceleration without manual clutching or gearshifting.

TORQUE CONVERTERS

The torque converter (Illust. 56) resemblesthe hydraulic coupling except that it containsa stationary section, called the stator, whichmultiplies the torque delivered by the powersource. In torque converters, torque output isgreater than torque input, but speed output,again, is lower. It is used in automotive and

39

412

other applications to permit the power source speed, while selecting automatically the properto operate continuously at its most efficient output torque for any load.

I3. this set of rotor blades, which use some ofthe energy in the flowing fluid. Then. .

4. this driven member. Here the fluid'direction of flow is again changed and itflows back to the driving member.

3. but this curvature causesfluid to make a turn and flowtoward.. .

5. In effect, fluid swirl thus formedis an "endless corkscrew of fluid"that moves simultaneously in severaldirection.: it spins as shown by thearrow, and also rotates about the aidsof the coupling.

2. centrifugal force caumefluid to flow toward thecircumference at rightangles to the axis ofrotation.. .

1. M this driving memberrotates.. .

II lust. 55Operating principle of the hydraulic coupling.

4. this set of stator blades changes the direction offluid flow and feeds fluid to the second set of rotorblades. After second reversal and use of most ofremaining energy in third Bet of rotor blades. . .

2. this centrifugal pump. M pump Irotates, fluid flow, toward. .

5. fluid, with most of its energyspent, flows out here and returnsto intake side of pump, and thus. . .

6. this shaft turns at a lower Ispeed with a higher torque.

Return Line

1. This shaft, driven at relatively high speedand low torque, drives. . .

Turbine. Designed to run atlower speed than pump.

Must. 56Operating principle of the torque converter.

40 4.3

HYDRAULIC FLUIDS

The purpose of this section of the manualis not to describe how to select a hydraulicfluid. This section will merely give the im-portant qualities of a hydraulic fluid, and willserve to emphasize the importance of usingthe fluids recommended by the manufacturerof the equipment being serviced.

In connection with this discussion, it shouldbe explained that the word fluid as used inthis section means oil with additives. Oil is amisleading term, as it gives no indication thatanything has been added to improve it forhydraulic use, so "fluid" is the word used toindicate that plain oil is not good enough forhydraulic use, generally.

FUNCTIONS OF HYDRAULIC FLUID

First of all, of course, it must be capableof transmitting the power applied to it. Itmust lubricate the internal moving parts. Inmany cases, the fluid acts as a sealing medium.

Furthermore, it should be capable of hold-ing its viscosity through a wide temperaturerange. It should resist oxidation, must protectmachine parts against rust and corrosion andshould be capable of separating air and waterfrom itself quickly.

VISCOSITY

Viscosity is the measurement of the re-sistance to flow of a fluid, and is the mostimportant property of any hydraulic fluid.Fluid used must have the correct viscosity atoperating temperature, and must have theability to readily flow to the various parts ofthe system at the desired rate. Viscosity alsohas a definite influence on correct lubricationof the moving parts.

Proper viscosity is a balance between (1)a fluid with a high enough viscosity to holdwear to a minimum and reduce internal leak-age and, (2) low enough viscosity to permitthe fluid to flow readily through the system.

41

' VISCOSITY INDEX

Viscosity of fluids change with temperature.Viscosity changes more in some fluids than inothers. Viscosity index is simply a measure-ment of the rate of change of viscosity overa certain range of temperature. If a fluid be-comes thick at low temperatures, and verythin at high temperatures, it has a low viscos-ity index (V.I.) . However, if the viscosity re-mains relatively the same at varying tempera-tures, it has a high V.I.

From the earlier discussion of the impor-tance of viscosity, it is obvious that, if a givensystem operates at varying temperatures, afluid with a high V.I. is desirable. Of course,if a system has nearly the same operating tem-perature at all times, then the fluid need nothave a high V.I. In very few systems can theconsideration of V.I. be 'eliminated, and it isalways considered in deciding upon fluidrecommendations.

OXIDATION RESISTANCE

Hydraulic fluids, especially in hard service,are likely to combine with oxygen, which leadsto decomposition. This chemical combinationforms organic acids, which are harmful to themetal parts and many types of seals and pack-ing in the system. This alone may shortenmachine life.

Oxygen also combines with fluid to formsludge. This action is multiplied by the pres-ence of water, high temperatures, and somemetals used in hydraulic systems. Conditionsfound in many systems are ideal for hasteningoxidation and sludge formation, as a result ofcontact with air and/or water. Other con-taminants, such as dust, dirt, and metallic par-ticles, will also speed up oxygen reaction,which is one reason filtering is so important.

Careful refinement and chemical additivesare used to combat oxidation in hydraulicfluids.

4.4

RUST AND CORROSION RESISTANCERusting and corrosion differ in that rusting

adds to the metal (oxygen fuses with the basemetal, making the part larger). Corrosion,however, is an eating away of the base metal.

Corrosion, of course, effects the closenessof fit of parts, permitting excessive leakageand causing erratic action. Rusting will effectthe smoothness of operation, and the loosechips of rust will cause wear.

To prevent rusting and corrosion, technolo-gists have developed additives which keepthese harmful effects to a minimum.

OILINESS

Because of the precision fit of most hydrau-lic components, only a very thin film of fluidis available to provide lubrication of theseparts. To provide this lubrication, the fluidmust have good oiliness characteristics. Oili-ness is simply a term indicating the ability ofa fluid to hold friction between parts to aminimum, and may be further defined as itsability to "stick" to closely fitted, warm parts.

Lubrfcating ability becomes doubly impor-tant when the hydraulic fluid is also expectedto lubricate other moving parts in a machine.An example of this is when the rear frame ofa tractor is a common sump for the.hydraulicsystem, transmission and final drive. Here,the same fluid that operates the hydraulicsystem serves to lubricate the transmission andfinal drive.

WATER SEPARATIONIt is practically impossible to keep all water

out of a hydraulic system, so it is necessaryfor the fluid to be able to rid itself of thewater quickly.

Popular opinion notwithstanding, oil andwater will mix, forming an emulsion, whichleads to the formation of corrosive com-pounds. Also, emulsions often have a slimy,sticky, or pasty consistency which interfereswith normal operation of valves and other

42

components. Different fluids have varyingability to separate water, and recommendedfluids are chosen with this consideration inmind.

FOAMING RESISTANCEA frequent difficulty often encountered in

the operation of hydraulics is the formationof foam in the fluid. This foam is formed bythe constant pumping and churning actionfound in the system, or when the fluid levelgets too low. In many systems, turbulence,which leads to the mixing of air with the fluid,is hard to avoid, and foaming becomes a seri-ous problem.

Since the proper operation of any hydrau-lics system is based on the fact that fluids arebasically incompressible, and foam is compres-sible, the formation of foam is a serious detri-ment to proper action.

Air that enters the system is soluble in fluidto a certain extent. As pressure and tempera-ture increases, the capability of fluid to absorbair increases, but air in solution does not af-fect compressibility. When pressure is re-lieved, however, the air will appear as bubblesor foam. Also, if more air is trapped in thefluid than the fluid will absorb, foam willappear.

Foaming is particularly detrimental in fluidcouplings and torque converters, as the foamwill carry less impact energy than fluid withno air bubbles in it. Recommended fluids con-tain foam inhibitors to overcome this condi-tion.

ANILINE POINT

Aniline point is the numerical index of atest made with aniline dyes. The aniline dyesused in the test will change color at certaintemperatures in a given fluid. The resultingtemperature index is called the aniline pointof the particular fluid involved. This anilinepoint index indicates the effect of the fluidbeing tested upon the synthetic rubber usedin seals. A high index indicates that the rubberwill swell; low, that it will shrink. Aniline

point is, therefore, a very important considera-tion in the recommendation of hydraulicfluids.

OTHER PROPERTIES

Most of the following factors have no im-portancethey are mentioned only to over-come some common misconceptions regardinghydraulic fluids.

Pour point reflects the ability of fluid toflow at low temperatures. Its only importancelies in the fact that hydraulic fluid also lubri-cates the components of a system. It is neces-sary, therefore, that fluids used have a pourpoint corresponding to the lowest startingtemperature of the system involved.

A.P.I. gravity is merely a way of stating thespecific gravity of a fluid, and has very littleimportance in recommendations.

Color is used by refiners to maintain uni-

formity of product, and bears no relationshipto fluid performance.

Flash point is a measurement of the pointat which heated fluid vaporizes, and is of noimportance except where hydraulic equipmentis used in the vicinity of extremely high tem-peratures. It is used mainly by refiners as aproduct control.

Carbon residue is the amount of carbonremaining when fluid is subjected to destruc-tive heating, and has no relation to eitherquality or performance of hydraulic fluids.

Acidity of fluid is a measurement of theamount of acid in it. It has no value in ratingnew fluids, but is often used in laboratoriesto help determine the degree of deteriorationof fluids which have been in use.

Again, it is emphasized that all the impor-tant factors described above are consideredin recommendations of fluid to be used, andthese recommendations must be followed.

SYSTEM MAINTENANCE

Continuing, long term high efficiency ofhydraulic systems is very dependent uponproper maintenance. One very important fac-tor is the addition, when necessary, of onlyclean, fresh fluid of the recommended type.

STORAGE OF FLUIDAn important factor in the maintenance of

fluid is the manner in which it is stored. Must.57 shows both improper storage and the man-ner in which fluid must be stored in order tomaintain its. quality.

DRAIN SCHEDULE

Periodic draining of the entire, system is ofvital importance. This is the only way toremove contaminants, oxidized fluid, and in-jurious particles completely from the system.

A drain schedule, as recommended by the

43

46

manufacturer of the equipment involved,must be maintained. The manufacturer willset forth the method to be used, and the fre-quency, depending on conditions.

CLEANING THE SYSTEM

The cleaning procedure, when a regulardraining schedule is followed, and no gum orlacquer formation is suspected, is very simple.

After the system is drained, solid sedimentin the bottom of the reservoir should be re-moved, and filters must be cleaned or re-placed. It may be advisable to flush out theresidue of old fluid remaining in the system,especially if the fluid is badly contaminated.For this purpose, use some of the fluid recom-mended for the system involved. Finally, re-fill the system with clean fluid of the recom-mended type.

If gums and lacquers have been allowed toform, because of neglect of draining schedule,or other reasons, and the components are notfunctioning properly, (sticky action) it is ad-visable to remove the parts affected and cleanthem thoroughly. When cleaning the disas-sembled parts, extreme care must be taken toprevent harm to closely fitted, finely finishedparts. Use of gum solvent or other non-corro-sive chemical cleaner is permissible on metalparts only. Do not allow these materials to

/1/// / AIR/ /ESCAPING

SPACEAIR SPACEREDUCED

PREVENTIONI. KEEP BUNGS DRAWN TIGHT

come in contact with seals and packing.Cleansed parts should be thoroughly rinsed,and protected immediately with a coating ofoil.

Most hydraulic systems will have with themon delivery an Operator's Manual or Preven-tive Maintenance Manual, or both. The recom-mendations given in these manuals for opera-tion and care must be followed if efficientoperation is expected.

ik *ATE k

Fluid and air in thebarrel expand whenwarm. Some, of theair above the fluidescapes.

2. STORE BARRELS INSIDEWHENEVER POSSIBLE (WARM STORAGEIN WINTER PREFERABLE) OR

3. AT LEAST UNDER COVER4. IF STORED OUTSIDE,LAY BARRELS ON THEIR SIDES5. IF BARRELS CANNOT BE LAID ON THEIR SIDES,TILT

THEM SLIGHTLY AS SHOWN BELOWWATER AROUND BUNG NO WATER AROUND DUNG

MAY BE DRAWN INTO TO DE DRAWN INTODARREL BARREL

WRONG

POP/MAULSUCTIONCREATED

WATER

Eft14111111411

// //,'/ /

Water is drawn in.Fluid and air con-tract when cooled.

RIGHT

Illust. 57Proper fluid storage principles.

44 411

TROUBLE

The following factors are responsible formost complaints of inefficient hydraulic oper-ation:

1. Fluid drain periods not frequent enough.

2. Contamination by cutting or grindingabrasives, air, or water.

3. Air leaks.

4. Failure to use the recommended hydrau-lic fluid.

5. Packing, gaskets and seals faulty.

SHOOTING

45

8

6. Inadequate understanding of the components of the hydraulic system.

Locating trouble in any hydraulic systemis a job for a well-trained service man. Heshould be familiar with the equipment used,its construction, and its operation. He shouldknow enough about hydraulic circuits andcomponents to localize the trouble. He musthave the proper tools, and he must use them.