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An initiative of:

3

BASIC LEVEL

MECHANICS

HYDRAULICS

UNIDO Headquarters Vienna International CentreP.O. Box 300. A-1400 Vienna Austria Tel: +43 (1) 26026-3752Email: [email protected]

www.lkdfacility.orgAn initiative of:

This curriculum has been developed as part of the Learning and Knowledge Development (LKD) Facility, initiated by the Swedish International Development Agency (Sida) and the United Nations Industrial Development Organization (UNIDO). The LKD Facility is a platform to promote industrial skills development among young people in emerging economies. Working with the private sector through Public Private Development Partnerships, the LKD Facility supports the establishment and upgrading of local industrial training academies to help meet the labour market’s increasing demand for skilled employees, ultimately contributing to inclusive and sustainable industrial development.

TRANSMISSIONS AND DRIVETRAIN

HYDRAULICS

BASIC LEVEL MECHANICS 3

Table of Contents1. Basic principles of hydraulics 1

1.1 Additional component needed to run a Hydraulic System 3

2. Control valve (4/3 valve) 4

2.1 A Pressure Relief Valve 4

2.2 The Pros and Cons of Hydraulics 4

2.3 Two major Types of Hydraulic systems are used today 5

2.4 Variations on Open and Closed Center Systems 6

2.5 Control Valves 8

2.6 Open Center System with Series Connection 9

2.7 Open Center System with Series / Parallel Connection 10

3. Hydraulic pumps 11

3.1 Positive Displacement pump 12

3.1.1 Fixed Displacement 12

3.1.2 Variable Displacement 12

3.2 Types of Hydraulic Pumps 13

3.3 Axial Piston Pump (Variable Displacement) 14

3.4 Gear pumps 14

3.4.1 External Gear Pump 15

3.4.2 Internal Gear Pump 16

3.5 Vane pumps 17

3.5.1 Balanced vane pump 17

3.5.2 Unbalanced vane pump 18

3.6 Piston pumps 20

3.6.1 Axial Piston Pumps 20

3.6.1.1 In-line Axial Piston Pumps 21

3.6.1.2 Axial Piston Pump 22

3.6.1.3 Bend-Axis Axial Piston Pump 24

3.6.2 Radial Piston Pumps 25

3.7 Hydraulic Pump Efficiency 26

3.8 Pump Qualities 26

3.9 Pump Delivery, Pressure and Speed 27

4. Safety 28

5. Hydraulic valves 29

5.1 Introduction 29

HYDRAULICS


5.2 Pressure Control Valves 30

5.3 Relief Valves 30

5.3.1 Direct Acting Relief Valves 30

5.3.2 Pilot Operated Relief Valves 32

5.4 Pressure Reducing Valves 33

5.5 Pilot Operated Types 34

5.6 Pressure Sequence Valves 35

5.7 Unloading Valves 36

5.8 Unloading Valves for Accumulator Circuits 36

6. Hydraulic cylinders 37

6.1 Introduction 37

6.2 Double Acting Cylinders 40

6.3 Cushions (Hydraulic Buffer) 41

6.4 Maintanance of Cylinders 42

6.5 Bleeding Air from Remote Cylinders 43

6.6 Caution 44

6.7 Basic Hydraulic System 45

7. Hydraulic accumulators 46

7.1 Introduction 46

7.2 Pneumatic Accumulators 48

7.3 Bladder Type Accumulators 48

7.4 Precautions 50

7.4.1 Checking Precharged Accumulator on the Machine 50

7.4.2 Before Removing Accumulator from Machine 51

7.4.3 Repairing Accumulator 51

7.4.4 Precharging Accumulator 52

7.4.5 Installing Accumulator on Machine 52

7.5 New Developments 52

8. Practical Exercise 53

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1. Basic Principles of HydraulicsThe basic principles of hydraulics are:

• Liquidshavenoshapeoftheirown

• Liquidsarepracticallyincompressible

• Liquidstransmitappliedpressureinalldirections

• Liquidsprovidegreatincreasesinworkforce

A) Liquids have no shape of their ownTheyacquiretheshapeofanycontainer.Becauseofthis,oilinahydraulicsystemwillflowinanydirectionandintoapassageofanysizeorshape.

F

FP

A

A

=

= = 120 N /

5 C 2m

C 2m

600 N=

B) Liquid transmit applied pressure in all directionsTheexperimentintheglassjarwillshattertheglassjarandalsoshowedhowliquidstransmitpressureinalldirectionswhentheyareputundercompression.Thisisveryimportantinahydraulicsystem.

Aforceof600NontopofthecorkwilltransmitapressureofP = 600 : 5 = 120 N/cm²

Thispressureiscreatedthroughoutthesystem,andanequalforceof120N/cm2isappliedtoallthewalls(andcork)ofthejar.

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C) Liquids are practically incompressibleForsafetyreasons,weobviouslywouldn’tperformtheexperimentshown.However,ifweweretopushdownonthecorkofthetightlysealedjar,theliquidinthejarwouldnotcompress.Thejarwouldshatterfirst.

Note:Liquidswillcompressslightlyunderpressure,butforourpurposestheyareincompressible.

D) Liquids provide great increases in work force

Let’stakeabottlejackasanexample.

Thepumpplungerhasanareaof7,07cm2andthelargeliftingpistonhasanareaof314cm2.

Weightofthevehicleonthelargepistonis1800kg1800dividedby314is40,5(kilo)

Requiredforceonthesmallplungeris40,5kilogram.Asmallforceonthesmallplungerisabletoliftaheavyload(1800kg)

Letter: Description:A PumpplungerwithhandleB Pomp-plunger(withsmallplungerarea)C Releasevalve(forloweringplungerD)D Largepumpplunger(liftingpiston)E Fillerplug(oil)F HydraulicoilG Checkvalve

Fortransportinghighoilpressures,specialhosesarerequired,reinforcedwithmetallayers.

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Pressurecanbemeasuredbyusingamanometer.

How a hydraulic system works

Abasichydraulicsystemhastwoparts:

• Apumpwhichtransportstheoil

• Thecylinderorhydraulicmotorwhichusesthemovingoiltodowork.

Ineffect,thepumpconvertsamechanicalforcetohydraulicpower,whilethecylinderconvertsthehydraulicpowerbacktomechanicalforcetodowork.

1.1 Additional component needed to run a hydraulic system:

A) Check valves:toholdtheoilinthecylindersbetweenstrokesandtopreventoilfromreturningtothereservoirduringthepressurestroke.Theball-typevalvesopenwhenoilisflowingbutclosewhentheflowstops.

B) A hydraulic tank (reservoir):tostoretheoil.Ifyoukeeponoperatingthepumptoraisetheweight,asupplyofextraoilisneeded.Thereservoirhasanairventwhichallowsoiltobeforcedintothepumpbygravityandatmosphericpressurewhenthepumppistonisretracted.

C) A control valve: directstheoil.Thisallowstheoperatortocontroltheconstantsupplyofoilfromthepumptoandfromthehydrauliccylinder.Whenthecontrolvalveisintheneutralposition,theflowofoilfromthepumpgoesdirectlythroughthevalvetoalinewhichcarriestheoilbacktothereservoir.Atthesametime,thevalvehastrappedoilonbothsidesofthehydrauliccylinder,thuspreventingitsmovementineitherdirection.

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2. Control valve (4/3 way valve)

2.1 A pressure relief valve

Basically protectsthesystemfromhighpressures.Ifthepressurerequiredtolifttheloadistoohigh,thisvalveopensandrelievesthepressurebydumpingtheoilbacktothereservoir.

Thereliefvalveisalsorequiredwhenthepistonreachestheendofthestroke.Atthistimethereisnootherpathfortheoilanditmustbereturnedtothereservoirthroughthereliefvalve.

2.2 The pros and cons of hydraulics

Asyouhaveseeninthesimplehydraulicsystem,wehavejustdeveloped,thepurposeistotransmitpowerfromasource(engineormotor)tothelocationwherethispowerisrequiredforwork.

Tolookattheadvantagesanddisadvantagesofthehydraulicsystem,let’scompareittotheothercommonmethodsoftransferringthispower.Thesewouldbemechanical(shafts,gears,orcables)orelectrical.

A) Advantages

1. Flexibility: Unlikethemechanicalmethodofpowertransmissionwheretherelativepositionsoftheengineandworksitemustremainrelativelyconstantwiththeflexibilityofhydrauliclines,powercanbemovedtoalmostanylocation.

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2. Multiplication of force: Inthehydraulicsystem,verysmallforcescanbeusedtomoveverylargeloadssimplybychangingcylindersizes.3. Simplicity: Thehydraulicsystemhasfewermovingparts,fewerpointsofwear.Anditlubricatesitself.

4. Compactness: Comparethesizeofasmallhydraulicmotorwithanelectricmotorofequalhorsepower.Thenimaginethesizeofthegearsandshaftswhichwouldberequiredtocreatetheforceswhichcanbeattainedinasmallhydraulicpress.Thehydraulicsystemcanhandlemorehorsepowerforitssizethaneitheroftheother systems.

5. Economy: Thisisthenaturalresultofthesimplicityandcompactnesswhichproviderelativelylowcostforthepowertransmitted.Also,powerandfrictionallossesarecomparativelysmall.

6. Safety: Therearefewermovingpartssuchasgears,chains,beltandelectricalcontactsthaninothersystems.Overloadscanbemoreeasilycontrolledbyusing reliefvalvesthanispossiblewiththeoverloaddevicesontheothersystems.

B) Disadvantages

1. Need for cleanliness: Hydraulicsystemscanbedamagedbyrust,corrosion,dirt,heatandbreakdownoffluids.Cleanlinessandpropermaintenancearemorecritical inthehydraulicsystemthanintheothermethodsoftransmission.

2.3 Two major types of hydraulic systems are used today:

•Open-Center Systems

•Closed-Center Systems

A) Open Center System

Thesimplehydraulicsystemshownonpage4iswhatwecallanOpencenter system.Thissystemrequiresthecontrolvalvespooltobeopeninthecentertoallowpumpflowtopassthroughthevalveandreturntothereservoir.Thepumpwehaveusedsuppliesaconstantflowofoilandtheoilmusthaveapathforreturnwhenitisnotrequiredtooperateafunction.

IntheClosedcentersystem,thepumpiscapableof“takingabreak”whenoilisnotrequiredtooperateafunction.Therefore,thecontrolvalveisclosedinthecenter,whichstops(deadends)theflowofoilfromthepump-the“closedcenter”feature.

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Fixed displacement pumps:arecommonlyusedincombinationwithanOpencentersystem

Variable displacement pumps: arecommonlyusedincombinationwithaClosedcentersystem

B) Closed Center System

Valve Stops Oil, But OilStays at Full System Pressure

Trapped OilHolds Cylinder Piston in place

This Pump Can StopsPumping During Neutral

2.4 Variations on Open- and Closed center systems

Tooperateseveralfunctionsatonce,hydraulicsystemshavethefollowingconnections:

A) Open center systems

• Open-CenterwithSeriesConnection

• Open-CenterwithSeriesParallelConnection

• Open-CenterwithFlowDivider

B) Closed center systems

• Closed-CenterwithFixedDisplacementPumpandAccumulator

• Closed-CenterwithVariableDisplacementPump

A familyofgraphicsymbolshasbeendevelopedtorepresentfluidpowercomponentsandsystemsonschematicdrawings.IntheUnitedStates,theAmericanNationalStandardsInstitute(ANSI)isresponsibleforsymbolinformation.TheInstitutecontrolsthemake-upofsymbolsandmakeschangesoradditionsasrequired.

http://hydraulicspneumatics.com/other-technologies/fluid-power-tips-issue-11-its-really-just-bunch-lines-and-shapes

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HYDRAULICS


TheInternationalStandardsOrganization(ISO)hasthesamecontroloversymbolsusedinternationally.Bothsystemshavealmostthesameformat(especiallysinceANSIchangeditssymbolsin1966toeliminateallwritteninformation).Standardsymbolsallowfluidpowerschematicdiagramstobereadandunderstoodbypersonsinmanydifferentcountries,evenwhentheydon’tspeakthesamelanguage.Eithersymbolset(ANSIorISO)maybe--andis--usedintheUnitedStates.However,manycompaniestodayusetheISOsymbolsastheirstandardforworkwithforeignsuppliersandcustomers.Inordertosimplifydrawingsofhydraulicsystems,wewillcontinuethisbookusingISOsymbols.

Insteadofdrawingcomplicatedcircuitdiagrams,usingpicturesoforiginalmachinecomponents,hydraulicsymbolsareeasytodrawandInternationalyrecognisableasuniformlanguageonhydraulicsystems.

Example:

Number: Description:1 Hydraulicoiltank(reservoir)2 Suctionfilter(strainer)3 Hydraulicoilpump4 Hydrauliccontrolvalve5 Hydrauliccylinder(doubleacting

Theadvantageofusingsymbolsinsteadofpicturesoforiginalcomponents,symbolsaredesignedinsuchawaythatthekindofvalvecanberecognisedeasily.Number4 inthepictureabove,isa4/3wayvalvewithopencentre.Thesymbolhasalltheinformationyouneedtoknowwhatkindofvalveitis.

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2.5 Control valves

Controlvalvescanbeoperatedbyhand,byoil(servo)andelectric(solenoid).

Thespoolvalveitselfconsistsofanumberoflandshavingslots“A”tometertheoil.Thegrooveslabeled“B”areusedasakindofhydraulicbearing/seal.Spoolslotsareavailableinallkindofshapes,dependingonthesensitivityrequiredforeachparticularsystem.

SolenoidoperatedvalvesareavailableinOn/OffshiftingvalvesandProportionaloperatedvalves.

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2.6 Open-Center System with Series Connection

Oilfromthepumpisroutedtothetwocontrolvalvesinseries.Whenthe1stvalveisoperated,nooilwillreachthe2ndvalve.

Inneutral,theoilpassesthroughthevalvesinseriesandreturnstothereservoirasshownbythearrows.Whenacontrolvalveisoperated,incomingoilisdivertedtothecylinderwhichthatvalveserves.

Thissystemissatisfactoryaslongasonlyonevalveisoperatedatatime.Inthiscasethefulloutputofthepumpatfullsystempressureisavailabletothatfunction.However,ifmorethanonevalveisoperated,thetotalofthepressuresrequiredforeachindividualfunctioncannotexceedthesystemreliefsetting.

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2.7 Open-Center System with Series / Parallel Connection

Thissystem,shownbelow,isavariationontheseriesconnectedtype.Oilfromthepumpisroutedthroughthecontrolvalvesinseries-butalsoinparallel.Thevalvesaresometimes“stacked”toallowfortheextrapassages.

Inneutral,theoilpassesthroughthevalvesinseriesasshownbythearrows.Valve2and3areacombinationofseriesandparallelconnection.Whenvalve2isoperated,valve3isalsoreceivingoilandcanbeoperatedatthesametime.

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HYDRAULICS


3. Hydraulic pumpThepumpistheheartofthehydraulicsystem.Itcreatestheflowoffluidwhichsuppliesthewholecircuit.

Thehumanheartisapump.Sowastheoldwaterpumponcefoundonthefarm.

Oncetheterm“hydraulics”meantthestudyoffluidsinmotion.Therefore,anypumpwhichmovedfluidswasconsideredahydraulicpump.

Buttoday,“hydraulics”meansthestudyoffluidpressureandflow-fluidsinmotionplus theabilitytodowork.

When is a pump hydraulicAllpumpscreateflow.Theyoperateonaprinciplecalleddisplacement. Thefluidistakeninanddisplacedtoanotherpoint.

Displacement can be done in two ways:

• Non-PositiveDisplacement

• PositiveDisplacement

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3.1 Positive displacement pump

Butthepositivedisplacementpump,usedinhydraulicstoday,notonlycreatesflow,italsobacksitup. Noticethesealedcasearoundthegear.Thistrapsthefluidandholdsitwhileitmoves.Asthefluidflowsouttheotherside,itissealedagainstback-up.Thissealingisthe“positive”partofdisplacement.Withoutit,thefluidcouldneverovercometheresistanceoftheotherpartsinthesystem.Whenhighpressureisneededinacircuit,apositivedisplacementpumpisamust.Thisistrueforallmodernhydraulicsystemswhichprovidefluidpower.

Inlow-pressuresystems,suchaswatercoolingorcropsprayingtypes,theoldnon-positivedisplacementpumpstillworks.

Inthischapter,wewilldiscussonlythepositive displacement pumpwhichistheheartofmodernoilhydraulicsystems.Thispumpisatruehydraulicpump.

Displacement of hydraulic pump“Displacement”isthevolumeofoilmovedordisplacedduringeachcycleofapump.

In this sense, hydraulic pumps fall into two broad types:

• FixedDisplacementPumps

• VariableDisplacementPumps

3.1.1 Fixed displacement

Pumpsmovethesamevolumeofoilwitheverycycle.Thisvolumeisonlychangedwhenthespeedofthepumpischanged.

Volumecanbeaffectedbythepressureinthesystem,butthisisduetoanincreaseinleakagebacktothepumpinlet.Usuallythisoccurswhenpressurerises.Thisleakagemeansthatfixeddisplacementpumpsareusuallyfoundinlowerpressuresystemsorasaidstoanotherpumpinahigherpressuresystem.

3.1.2 Variable displacement

Variabledisplacementpumpscanvarythevolumeofoiltheymovewitheachcycle-evenatthesamespeed.Thesepumpshaveaninternalmechanismwhichvariestheoutputofoil,usuallytomaintainaconstantpressureinthesystem.

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• When system pressure drops, volume increases• As pressure rises, volume decreases.

Remember:

A hydraulic pump does not create pressure: it creates flow.

Pressure is caused by resistance to flow !

3.2 Types of hydraulic pumps

Nowthatwehaveseenwhathydraulicpumpsareandwhattheycando,let’stakean“inside”look.a)

b)

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HYDRAULICS


3.3 Axial PistonPump (Variable Displacement)

Wewillshowhoweachtypeofpumpoperatesandhowitisused.Ahydraulicsystemmayuseoneofthesepumps,oritmayusetwoormoreincombination.

Allthreedesignsworkontherotaryprinciple:arotatingunitinsidethepumpmovesthefluid.Arotarypumpcanbebuiltverycompact,yetdisplacethenecessaryvolumeoffluid.Thisisthenumberoneneedinamobilesystemwherespaceislimited.

3.4 Gear pumps

Gearpumpsareofthefixeddisplacementtype.TwotypesbeingExternalgeartypeandInternalgeartype.Theyarewidelyusedbecausetheyaresimpleandeconomical.Whilenotcapableofvariabledisplacement,theycanproducethevolumeneededbymostsystemsusingfixeddisplacement.Oftentheyareusedaschargingpumpsforlargersystempumpsofothertypes.Servosystemsalsomakeuseofgearpumps.

Basic types of gear pumps are used:

• ExternalGearPumps

• InternalGearPumps

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3.4.1 External gear pump

Externalgearpumpsusuallyhavetwogearsinmesh,closelyfittedinsideahousingThedriveshaftdrivesonegear,whichinturndrivestheothergear.Shaftbushingsandmachinedsurfacesorwearplatesareusedtosealintheworkinggears.

OperationOperationisquitesimple.Asthegearsrotateandcomeoutofmesh,theytrapinletoilbetweenthegearteethandthehousing.Thetrappedoiliscarriedaroundtotheoutletchamber.Asthegearsmeshagaintheyformasealwhichpreventsoilfrombackinguptotheinlet.Theoilisforcedoutattheoutletportandsentthroughthesystem.

Thisoilispushedoutbythecontinuousflowoftrappedoilcomingintotheoutletchamberwitheachrotationofthegears.Attheinletside,gravityfeedsinmoreoilfromthereservoirtoreplacethatdrawnoutbytheturninggears.

Somegearpumpsuseapressurizedplateworkingagainstthegearstoincreasepumpefficiency.Asmallamountofpressureoilisfedunderthebackingplate,pressingitagainstthegearsandformingatightersealagainstleakage.

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3.4.2 Internal gear pumpTheinternalgearpumpalsousestwogears,butnowaspurgearismountedinsidealargergear.Thespurgearisinmeshwithonesideofthelargergear,andbothgearsaredividedontheothersidebyacrescent-shapedseparator.Thedriveshaftturnsthespurgear,whichdrivesthelargergear.

Operation

Operationisbasicallythesameasfortheexternalgearpump.Themajordifferenceisthatbothgearsturninthesamedirection.

Asthegearscomeoutofmesh,oilistrappedbetweentheirteethandtheseparatorandiscarriedaroundtotheoutletchamber.Asthegearsmeshagain,asealisformed,preventingback-upoftheoil.Acontinuousflowofoiltotheoutletpushesthefluidoutintothecircuit.Gravitykeepsfeedingoilintothepumpinlettofillthepartialvacuumcreatedasoilisdrawninbythegears.

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3.5 Vane pumps

Vanepumpsarefairlyversatilepumpsandcanbedesignedassingle,double,oreventripleunits.

Allvanepumpsmoveoilusingarotatingslottedrotorwithvanesfittedintotheslots.

Two types of vane pumps are most often used:

• BalancedVanePumps• UnbalancedVanePumps

Thebalancedvanepumpisstrictlyafixeddisplacementtype.Theunbalancedvanecanhaveafixedoravariabledisplacement.

3.5.1 Balanced vane pumps

Inthebalancedvanepump,therotorisdrivenbythedriveshaftandturnsinsideanovalrotorring.

Thevanesarefittedintotherotorslotsandarefreetomoveinorout.

The“balanced”partofthispumpisshownbythepositionoftheoilports.Thepumphastwoinletports,locatedoppositeeachother.Andithastwooutletports,alsoonoppositesidesofthepump.Bothsetsareconnectedtoacentralinletandoutlet.

Slot

Inlet Port

Rotor Ring

Rotor

Outlet Port

Outlet Port

Inlet

Vane

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OperationAstherotorturns,thevanesarethrownoutagainsttheinsidesurfaceoftheringbycentrifugalforce.Asthevanesfollowthecontouroftheoval-shapedring,theydividethecrescent-shapedareasbetweentherotorandtheringintotwoseparatechambers.Thesechambersarecontinuallyexpandingandshrinkinginsize-twiceduringeachrevolution.Theinletportsarelocatedwhereeachchamberbeginstoexpand:theoutletportsarelocatedwhereeachchamberbeginstoshrink.

Asthechamberbeginstoexpand,inletoilrushesintofillthepartialvacuum.Thisoiliscarriedaroundbythevanes.Astheoilchamberbeginstoreduce,theconfinedoilisforcedoutattheoutletport.

Inthesecondhalfoftherevolution,thisactionisrepeatedatthesecondsetofinletandoutletports.

3.5.2 Unbalanced vane pumps Theunbalancedvanepumpusesthesamebasicprincipleofaturningrotorwithvanesworkinginsideafixedrotorring.

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However,theoperatingcycleonlyhappensonceeachrevolution.Sothispumphasonlyoneinletandoneoutletport.Also,theslottedrotorisnowsetoffsideinacircularring.

Inoperation,theoilchamberstartstoexpandattheinletport,andfinishesitscontractingattheoutletport.Oilisdrawninbythepartialvacuum,andforcedoutbytheshrinkingofthechamber,thesameasinthebalancedvanepump.

However,thedesignoftheunbalancedvanepumpisdifferentfromthebalancedtype,aswe’llexplainnow.Balancedvanepumpscannotoperateasvariabledisplacementpumps.Unbalancedpumpscanbeusedasavariabledisplacementpump.

Thebalancedvanepumpisreallyarefinementoftheunbalancedmodel.Whywasthisrefinementneeded?

Theunbalancedvanepumpseemedtohavefrequentbearingfailures.Thecausewasfoundtobeforceontheshaftandbearingsofthebackpressurefromoilbeingexpelledattheoutletsideofthepump.Noequalforcewasexertedontheoppositeside,sincetheinletoilwasunderlittleornopressure.

Thebalancedvanepumpwasasolutiontothisproblem.Tobalanceofftheoutletpressuresontheshaft,two(2)outletportswereused,directlyoppositeeachother.hisequalizedtheforces,increasedbearinglife,andmadethepumpworklonger.

Whilethebalancedvanepumpsolvedoneproblem,itposedanotherone:itcouldonlybeusedforfixeddisplacement.Theoutletportpositionscannotbechangedorthebalancewouldbeupset.

Theunbalancedmodelcanbeusedeitherforfixedorvariabledisplacement.Byspecialdesign,thepositionofitsrotorringandoilportscanbechangedinrelationtotheoffsetoftherotor.Thischangesthesizeofthechamberswhichthevanescreate,thustheamountofoileachcarries.Theresult:avariabledisplacementpump.

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3.6 Piston pumpsPistonpumpsareoftenfavoredonmodernhydraulicsystemswhichusehighspeedsandhighpressures.However,pistonpumpsaremorecomplexandmoreexpensivethantheothertwotypes.Pistonpumpscanbedesignedforeitherfixedorvariabledisplacement.

• Axial Piston Pumps

• Radial Piston Pumps

3.6.1 Axial Piston pumps

AXIALpistonmeansthatthepistonsaremountedinlinesparallelwiththepump’s“axis”(alinedownthecenter).

RADIALpistonmeansthatthepistonsaresetperpendiculartothepump’scenterlikethesun’srays.

Bothstylesofpistonpumpsoperateusingpistonswhichpumpoilbymovingbackandforthincylinderbores.(Anothertermforthismovementis“reciprocate.”)

Axialandradialpistonpumpsusereciprocatingpistonsbutdrivethembytherotaryprinciple.Inthiswaytheefficiencyofthereciprocatingmethodiscombinedwiththecompactnessoftherotaryoperatingpump.

Theresultisapumpwhichisefficient,yetcanfitintoamachinehydraulicsystem.

Axialpistonpumpsusuallyfallintotwobroadtypes:inlineandbent-axis.

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3.6.1.1 In-line Axial Piston Pumps

Inthispump,thecylinderblockismountedonadriveshaftandrotateswiththeshaft.Thepistonsworkinboresinthecylinderblockwhichareparalleltotheaxisoftheblock.Theheadsofthepistonsareincontactwithatiltedplatecalledaswashplate.

Theswashplatedoesnotturnbutitcanbetiltedbackandforth.Itmountsonapivotandiscontrolledeithermanuallyorbyanautomatic“servo”device.

Sincetheswashplatecontrolstheoutputofthepistons,thispumphasavariabledisplacement.

Rememberthattheangleoftheswashplatecontrolsthedistancethatthepistonscanmovebackandforthintheirbores.Thegreatertheangle,thefartherthepistonstravelandthemoreoilthatisdisplacedbythepump.

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Whentheswashplateistiltedasshown.Asthecylinderblockrotates,pistonboresalignwiththisportandoilisforcedintotheboresbythesmallchargingpump.Thisoilpushesthepistonsagainsttheswashplate.Thenastheyrevolve,thesepistonsfollowthetiltoftheswashplateandforcetheoiloutoftheirboresintotheoutletport.Iftheangleoftheswashplatewasfixed,thepumpwouldoperateasafixeddisplacementtype,puttingoutthesameamountofoilwitheachrevolution.

SwashPlate BendAxis

Thebendaxisshownaboveisofthefixeddisplacementtype.Itisoftenusedashydraulicmotordrivingthetracksofhydraulicexcavators.

3.6.1.2 Axial piston pump

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Number: Description:1 Portplate(inletandoutletports)2 Heavycoilspring3 Cylinderblock(holdingthepistons/plungers)4 Plungers(orpistons)5 Pistonslippersretractionring6 Swashplate7 Servocontrolledoperationofswashplate8 Bearingsshellsofswashplate(oftenneedlebearing)9 Driveshaftofcylinderblock

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Totilttheswashplate,thecontrolleverisactuated,movingthedisplacementcontrolvalvetotheleft.Thisdirectsoilfromthechargingpumpintotheupperservocylinder,movingthepistonwhichtiltstheswashplate.

Meanwhilethepistoninthelowerservocylinderispushedinbythelowerpartoftheswashplate,forcingitsoilbackthroughthevalvetothepumpcase.

Whentheswashplatereachestheanglesetbythecontrollever,thecontrolvalvereturnstoneutralandtrapstheoilintheservocylinders.Thisholdstheswashplateuntilthecontrolleverismovedagain.

Thepumpkeepsonpumpingasexplainedbefore,drawinginoilatthetopandpushingoutoilatthebottomofeachrevolution.

Iftheswashplateweretiltedtheoppositeway,theinlet-outletcycleofthepumpwouldbereversed.Oilwouldbedrawninatthebottomandpushedoutatthetop.Sotheservodevicenotonlycontrolsthepumpdisplacementbutalsothedirectionofthisoil.

3.6.1.3 Bend-Axis Axial Piston Pump

Asexplainedearlier,thebendaxisisafixeddisplacementtypeofpump.

Hydrostaticdrivenwheelloadersusedtobeequippedwithbentaxishydraulicmotors.Sincemoredifferenceinspeedandtorqueisneeded,thelatesttypeofwheelloadersareequippedwithaaxialpistonpump,variabledisplacement,twodirectionsofflow.ThehydraulicmotorsarenolongeroffixeddisplacementtypebutarevariableandsomeWheelloaderhavetwoofthemfixedontothetransferbox(highandlowspeed).

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Radial piston pumpsRadialpistonpumpsareamongthemostsophisticatedofallpumps.Theyarecapableofhighpressures,highvolumes,highspeeds,andvariabledisplacement.

Thebasicoperationissimple,butbyusingextravalvesandotherdevices,thispumpcanbeadaptedtomanysystemsandneeds.

Thispumpiscloselyfitted,sowearcanbeaproblemunlesscleanoilisused.Andtheoilmustcontainpropertieswhichlubricatethecloselyfittedparts.

Radialpistonpumpsaredesignedtooperateintwoways.

Inthe“rotatingcam”pump,thepistonsarelocatedinafixedpumpbody.Thecentershafthasacamwhichdrivesthepistonsasitrotates.

Inthe“rotatingpiston”pump,thepistonsarelocatedinarotatingcylinder.Asthecylinderrotates,thepistonsarethrownoutagainsttheouterhousing.Sincetherotatingcylinderissetoffsideinthehousing,thepistonsaremovedbackandforthastheyfollowthehousing.

ExcavatorsusedtobeequippedwithRadialpistonpumps.Todaymostheavymachineryandequipmentuseaxialpistonpumps.

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In summary:

1. A hydraulic pump converts mechanical force into hydraulic or fluid power-in other words, induces fluid to work.2. Of the two main types of pumps, positive displacement and non-positive displacement,

the positive displacement type is best suited for power hydraulics due to its ability to produce a steady flow against the high pressures in the system.

3. A hydraulic pump can be designed to produce either a specific volume of fluid at a specific speed, or to produce a variable volume of fluid at a constant speed . . . fixed displacement or variable displacement.

4. The three types of pumps most often used in machine hydraulic Systems are gear, vane, and piston.

5. These three pumps operate on a rotary principle. This allows them to be constructed as small units, yet still have the ability to produce the required volume of fluid.

6. The preceding text covers only basic hydraulic pumps and there are a great number of variations on all of the pumps selected.

3.7 Hydraulic pump efficiency

Thusfar,wehaveonlydescribedthethreemostpopulartypesofpumps.This,ofcourse,isnotthewholestoryonhydraulicpumps.Theirapplicationandefficiencyisjustasimportantastheiroperationandthismayhelplaterindiagnosinghydraulicproblems.

3.8 Pump Qualities

Becauseofthewidevarietyofpumpsandhydraulicsystems,wecouldnotpossiblyprescribeaparticularmodelofpumpforaparticularsystemwithouthavingfullinformationaboutthesystem.However,wecandescribethedesirableandundesirablequalitiesofthethreepumpsandletyoujudgeforyourselfthereasonswhyaparticularpumpisusedinahydraulicsystem.

Oneofthefirstfactorstoconsiderwhenchoosingapumpforamachinesystemisthepump’sphysicalsize.Mostofthesesystemshavelittleornoroomtospareandmayallowjustasmallareaforthepump.

Fortunately,withthewidevarietyofpumpsandpumpsizesavailable,thisisnotabigproblemunlessthesystemrequiresafunctionthatapumpcannotprovideexceptasalargeunit.Inthiscase,spaceforthepump,regardlessofsize,willbemadeavailable,becausetheothermoreimportantrequirementsofthesystemcannotbesacrificed.

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3.9 Pump Delivery, Pressure and Speed

Oneotherrequirementisthevolumeoffluidthatthepumpproduces.

Mostpumpsareratedbyvolume,whichisusuallyexpressedinLitersperminute(orgallonsperminute(gpm).Thisratingiscalledseveralnames-deliveryrate,discharge,capacity,orsize.Regardlessoftherating,itcannotstandalone.Itmustbeaccompaniedbyafigurestatingtheamountofbackpressurethatthepumpcanwithstandandstillproducethegpmrating:foraspressureincreases,internalpumpleakageincreasesandusablevolumedecreases.

Pumpspeedmustalsobeincludedwiththevolumeratingfortworeasons.

First,inafixeddisplacementpump,flowisdirectlyrelatedtothespeedofthepump-thefasterthespeed,themorefluidpumped.

Second,howfastthepumpmustgotoproduceacertainflowindicatesatwhatspeedthedrivingmechanismforthepumpmusttravel(inrevolutionsperminuteorrpm).Addthistothedeliveryrateofapump,andhereisanexampleofhowaratingcouldread:“11.5gpmwith2000psiat2100rpm.”

Occasionally,apumpwillhaveanalternatedeliveryrate,referredtoasanintermittentdeliveryrate.Thisratingindicatesthehighestlevelapumpcanoperate,intermsofdelivery,pumpspeedandpressure,foraperiodoftimeandstillmaintainsatisfactoryservicelife.

Pump efficiencyTheefficiencyofapump(howwellitdoesitsjob)isalsoimportantinselectingapump.

Wemayhaveapumpthatmeetsthedeliveryrequirementsofasystemundertheexistingpressureinthesystematthespeedthatisavailabletodrivethepump-wemayhaveallthisandmorebutwhatifwefindthatthepumprequiresagreatamountofmechanicalpowertoattainthisdeliveryrate?Orwhatifwefindthatthematerialsinthepumpmustbespeciallyandexpensivelyconstructedtowithstandthepressureorfrictioninthesystem?Thisiswhytheknowledgeofpumpefficiencyisimportantbeforeselectingaparticularpump.Wearenotonlylookingfordeliveryrate,butdeliveryrateprovidedbyefficientandeconomicaloperatingmeans.

Pump quality is judged by three ratings:• VolumetricEfficiency• MechanicalEfficiency• Over-AllEfficiency

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Volumetric efficiencyistheratiooftheactualoutputofthepumptotheoreticaloutput(theamountitshouldputoutunderidealconditions).Thedifferenceisusuallyduetointernalleakageinthepump.

Mechanical efficiencyistheratiooftheoverallefficiencyofthepumptovolumetricefficiency.Thisdifferenceisusuallyduetowearandfrictiononthepump’sworkingparts.

Over-all efficiencyistheratioofthehydraulicpoweroutputtothemechanicalpowerinputofthepump.Thisistheproductofbothmechanicalefficiencyandvolumetricefficiency.

4. Safety Bealwaysalertofthehighpressuresinhydraulicsystems.

Pressuresover300bar(kg/cm2)areverynormalintoday’shydraulicsystems.Peekpressurecanevendoublethisamount.

Anaveragepistonpumpcandeliverplus/minus200litersperminute.Incaseofahoseburstorotherwayofleakageinthesystem,alotofoilwillbelost.Alwaystop-upthereservoir,afterlosingoil.

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5. Hydraulic Valves

5.1 Introduction

Valvesarethecontrolsofthehydraulicsystem.Theyregulatethepressure,direction,andvolumeofoilflowinthehydrauliccircuit.

Valves can be divided into three major types:

• PressureControlValves

• DirectionalControlValves

• VolumeControlValves

Pressure control valvesareusedtolimitorreducesystempressure,unloadapump,orsetthepressureatwhichoilentersacircuit.Pressurecontrolvalvesincludereliefvalves,pressurereducingvalves,pressuresequencevalves,andunloadingvalves.

Directional control valvescontrolthedirectionofoilflowwithinahydraulicsystem.Theyincludecheckvalves,spoolvalves,rotaryvalves,pilotcontrolledpoppetvalves,andelectro-hydraulicvalves.

Volume control valvesregulatethevolumeofoilflow,usuallybythrottlingordivertingit.Theyincludecompensatedandnon-compensatedflowcontrolvalvesandflowdividervalves

Somevalvesarevariationsonthethreemaintypes.Forexample,manyvolumecontrolvalvesuseabuilt-inpressurecontrolvalve.

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Valvescanbecontrolledinseveralways:manually,hydraulically,electrically,orpneumatically.Insomemodernsystems,theentiresequenceofoperationforacomplexmachinecanbemadeautomaticLet’sdiscusseachtypeofvalveindetail,startingwithpressurecontrolvalves.

5.2 Pressure control valves

Pressure control valves are used to:

•Limitsystempressure•Reducepressures•Setpressureatwhichoilentersacircuit•Unloadapump

5.3 Relief valves

Eachhydraulicsystemisdesignedtooperateinacertainpressurerange.Higherpressurescandamagethecomponentsordeveloptoogreataforcefortheworktobedone.

Reliefvalvesremedythisdanger.Theyaresafetyvalveswhichreleasetheexcessoilwhenpressuresgettoohigh.

Types of relief valves are used:• Direct actingreliefvalvesaresimpleopen-closedvalves.•Pilot operatedreliefvalveshavea“trigger”whichcontrolsthemainreliefvalve.

5.3.1 Direct Acting Relief Valves

Whenclosed,thespringtensionisstrongerthaninletoilpressure,holdingtheballclosedonitsseat

Thevalveopenswhenpressurerisesattheoilinletandovercomesthespringforce.Oilthenflowsouttothereservoir,preventinganyfurtherriseinpressure. Thevalveclosesagainwhenenoughoilisreleasedtodroppressurebelowthetensionofthespring.Somereliefvalvesareadjustable.Oftenascrewisinstalledbehindthespring.Byturningthescrewinorout,thereliefvalvecanbeadjustedtoopenatacertainpressure.

.

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Poppet”isatermfortheworkingpartofthevalve.Ballpoppetsaremostcommonlyused(thoughtheymay“chatter”duringfrequentoperation).Otherpoppetsusedareshapedlikebuttonsorlikesmallconesordisks.

Cracking pressure and pressure override“Crackingpressure”isthepressureatwhichthereliefvalvefirstbeginstoopen.“Full-flowpressure”isthepressureatwhichthevalvepassesitsfullquantityofoil.

Full-flowpressureisquiteabithigherthancrackingpressure.Thisisbecausethespringtensionbuildsupasthevalveopensfarther.Thisconditioniscalled“pressureoverride”anditisonedisadvantageofthesimplereliefvalve.

Direct acting relief valvesThesevalvesareusedmainlywherevolumeislow,andforlessfrequentoperations.Theyhaveafastresponse,makingthemidealforrelievingshockpressures.Theyareoftenusedassafetyvalvestopreventdamagetocomponents.

Directactingreliefvalvesalsoserveaspilotvalvesforthepilotoperatedreliefvalves.

Directactingreliefvalvesareverysimple.Iftheyfail,noharmisusuallydone.Theresultingpressurelossinthesystemisapparenttotheoperatorandhecanreplacethebrokenspringorwornvalveorseat.

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5.3.2 Pilot operated Relief Valves

Whenareliefvalveisneededforlargevolumeswithlittlepressuredifferential,apilotoperatedreliefvalveisoftenused.

Thepilotvalveisa“trigger”whichcontrolsthemainreliefvalve.Itisusuallyasmall,spring-loadedreliefvalvebuiltintothemainreliefvalve. Themainreliefvalveisclosedwheninletoilpressureisbelowthevalvesetting.Passage(1)inthemainvalve(6)keepsitinhydraulicbalance,whilespring(5)holdsitclosed.Thepilotvalve(3)isalsoclosedatthistime.Inletpressurethroughsensingpassage(2)islessthanthepilotvalvesetting.

Asinletoilpressurerises,pressureinpassage(2)alsorises.Whenitreachesthepilotvalvesetting,thevalve(3)isopened.Thisreleasesoilbehindthemainvalvethroughpassage(2)andoutthedrainport.Theresultingpressuredropbehindthemainreliefvalve(6)causesittoopen.Nowthemainreliefoperationbeginsasexcessoilisdumpedatthedischargeport,preventingafurtherriseininletpressure.Thevalvescloseagainwheninletoilpressuredropsbelowthevalvesettings.

Pilotoperatedreliefvalveshavelesspressureoverridethanthesimpledirectactingtypes.Whilethedirectactingvalvestartstoopenatabouthalfitsfull-flowpressure,thepilotoperatedvalveopensatabout90percentofitsfull-flowpressure.

Becausethesevalvesdon’tstarttoopenuntilalmostfull-flowpressure,theefficiencyofthesystemisprotected,lessoilisreleased.

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5.4 Pressure reducing valves

Apressure-reducingvalveisusedtokeepthepressureinonebranchofacircuitbelowthatinthemaincircuit.Whennotoperating,apressurereducingvalveisopen.Whenitoperates,ittendstocloseasshown.

Operation:Whenpressurestartstoriseinthesecondarycircuit,forceisexertedonthebottomofthevalvespool,partlyclosingit.Springtensionholdsthevalveagainsttheoilpressuresothatonlyenoughoilgetspastthevalvetoservethesecondarycircuitatthedesiredpressure(Thespringtensioncanbeadjustedusingthescrewshownatthetop)

Thepressuresensingforthevalvecomesfromtheoutletside,Orthesecondarycircuit.Thisvalveoperatesthereverseofareliefvalve,whichsensespressurefromtheinletandisclosedwhennotoperating.

Apressurereducingvalvewilllimitmaximumpressuresinthesecondarycircuit,regardlessofpressurechangesinthemaincircuit,aslongasthesystemworkloaddoesnotcreatebackpressureintothereducingvalveport.Backpressurewouldclosethevalvecompletely

Types of Pressure Reducing Valves

Pressure reducing valves can operate in two ways:

• ConstantReducedPressure

• FixedAmountReduction

Constantreducedpressurevalvessupplyafixedpressureregardlessofmaincircuitpressure(solongasitishigher).

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Fixedamountreducingvalvessupplyafixedamountofpressurereduction,whichmeansthatitvarieswiththemaincircuitpressure.

Forexample,thevalvemightbesettogiveareductionof500psi.Ifsystempressurewas2000psi,thevalvewouldreducepressureto1500psi.Ifsystempressuredroppedto1500psi,thevalvewouldreducepressureto1000psi.

FromMainCircuit

To SecondaryCircuit

Restriction

Constant Reduced Pressure Fixed Amount Reduction

Restriction

FromMainCircuit

Theconstantreducedpressurevalveoperatesbybalancingthesecondarypressureagainstanadjustablespringwhichistryingtoopenthevalve.Whensecondarypressuredrops,thespringopensthevalveenoughtoincreasepressureandtokeepaconstantreducedpressureinthesecondarycircuit.

Thefixedamountreductionvalveoperatesbybalancingthemaincircuitpressurecominginagainstboththesecondarypressureattheoutletandthespringpressure.Sincetheexposedareasoftheinletandoutletsidesareequal,thefixedreductionwillbethatofthespringsetting.

5.5 Pilot Operated Types

Aswithreliefvalves,asmallpilotvalvecanbeaddedtocontrolapressure-reducingvalve.Operationisthesameasdescribedaboveexceptthatthepilotvalveactsfirstto“trigger”thereducingvalve.Usingapilotvalvegivesawiderrangeofadjustmentandamoreconsistentreliefoperation.

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5.6 Pressure Sequence Valves

Pressuresequencevalvesareusedtocontrolthesequenceofflowtovariousbranchesofacircuit.Usuallythevalvesallowflowtoasecondfunctiononlyafterafirsthasbeenfullysatisfied

Whenclosed,thevalvedirectsoilfreelytotheprimarycircuit.

Whenopened,thevalvedivertsoiltoasecondarycircuit.

Thevalveopenswhenpressureoiltotheprimaryreachesapresetpoint(adjustableatthevalvespring).Thevalveisthenliftedoffitsseatasshownandoilcanflowthroughthelowerporttothesecondary.

Oneuseofthesequencevalveistoregulatetheoperatingsequenceoftwoseparatecylinders.Thesecondcylinderbeginsitsstrokewhenthefirstcompletesitsstroke.Herethesequencevalvekeepspressureonthefirstcylinderduringtheoperationofthesecond.Sequencevalvessometimeshavecheckvalveswhichallowareversefreeflowfromthesecondarytotheprimary,butsequencingactionisprovidedonlywhentheflowisfromprimarytosecondary.

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5.7 Unloading valves

Theunloadingvalvedirectspumpoutputoilbacktoreservoiratlowpressureaftersystempressurehasbeenreached.Theymaybeinstalledinthepumpoutletlinewithateeconnection.

Insomehydraulicsystemspumpflowmaynotbeneededduringpartofthecycle.Ifpumpoutputhastoflowthroughareliefvalveatsystempressure,muchhydraulicenergyiswastedasheat.Thisiswhereanunloadingvalveworksbest.

Whenclosed,springpressureholdsthevalveonitsseat.Sensingpressureattheotherendofthevalveislessthanspringpressure.Thereservoiroutletisclosedandnounloadingoccurs.Thevalveopenswhenthesensingpressurerisesandovercomesthespringthrust.

Thevalvemovesback,openingtheoutlettothereservoir.Pumpoutputoilisnowdivertedtothereservoiratlowpressure.

5.8 Unloading Valves for Accumulator Circuits

Anunloadingvalveisoftenusedinanaccumulatorcircuittounloadthepumpaftertheaccumulatorischarged.

Thevalveisclosedwhilethepumpchargestheaccumulatorwithoil.Asthepressurerisesitforcesthesmallsensingpistonagainstthelargevalveandcompressesthespring. Whenaccumulatorpressurereachesthatdeterminedbythespringsetting,thevalveopensbypassingoilandrelievingthepump.Atthistimethelowneutralpressureoilisdirectedtothelargeendofthelargepiston.

Whentheaccumulatordischargesandthesystempressuredrops,thespringmovedthe

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valveagainstthereducedsystempressureinthesmallpistonandtheneutralpressureagainstthelargeendofthevalve.

Thismeansthevalvewillcloseataslightlylowerpressurethanitopens.Thisgivesthevalveanoperatingrangeandpreventschattering.

6. Hydraulic cylinders

6.1 Introduction

Thecylinderdoestheworkofthehydraulicsystem.Itconvertsthefluidpowerfromthepumpbacktomechanicalpower.Cylindersarethe“arms”ofthehydrauliccircuit.

ModuleBasicHydraulicsexplainstheusesofhydraulicsandshowshowcylinderscanbeusedtoactuatebothmountedequipmentanddrawnimplements(remoteuses).Ineithercase,thebasicdesignofthecylinderisthesame:onlytheextrafeaturesaredifferent.

Types of cylinders

Two major types of cylinders are covered in this chapter:

• Singleactingcylinders• Doubleactingcylinders

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Singleactingcylinders-giveforceonlyoneway.Pressureoilisadmittedtoonlyoneendofthecylinder,raisingtheload.Anoutsideforcesuchasgravityoraspringmustreturnthecylindertoitsstartingpoint.

Oilinthecylinderisneededforliftingtheforksofaforklift.Loweringtheforksisdonebyitsownweight(gravityforce).Somesinglesactingcylindersareextendedbyusingoil,andretractingisdonebytheuseofacoilspring.

Doubleactingcylinders-giveforceinbothdirections.Pressureoilisadmittedfirstatoneendofthecylinder,thenattheother,givingtwo-waypower.

Inbothtypesofcylinders,amovablepiston(orrod)slidesinacylinderhousingorbarrelinresponsetopressureoiladmittedtothecylinder.Thepistonmayusevariouspackingsorsealstopreventleakage.

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Asealonthepistonpreventsleakageofoilintothedrysideofthecylinder.Awipersealintherodendofthecylindercleanstherodasitmovesinandoutofthehousing.Insomesingle-actingcylinders,thepistonrodhasnopistonontheinnerend.Instead,theendoftherodservesasthepiston.Thisiscalledaram-typecylinder.Therodisslightlysmallerthantheinsideofthecylinder.(Asmallshoulderorringontheendoftherodkeepstherodfrombeingpushedoutofthecylinder.)

The ram-type construction has several advantages over the piston-type:

1)Therodisbiggerandresistsbendingduetosideloads.

2)Thepackingisontheoutsideandiseasiertoreach.

3)Scoringinsidethecylinderborewillnotdamagepackings.

4)Noairventisneededsinceoilfillsthewholeinnerchamberofthecylinderhousing.

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6.2 Double acting cylinders

Double-actingCylindersprovideforceinbothdirections.Pressureoilentersatoneendofthecylindertoextendit,attheothertoretractit.Oilfromtheoppositeendofthecylinderreturnstoreservoireachtime.

Withthedouble-actingcylinder,boththepistonheadandthepistonrodmustbesealedtopreventoilleakage.

Two types of double-acting cylinders:IntheUNBALANCEDordifferentialtype,totalforceontherodsideofthepistonislessthanthatontheblankside.Thisisbecausetherodfillsinanareanotexposedtopressure.Thiscylinderisusuallydesignedforaslower,morepowerfulstrokewhenitextends,andforafaster,lesspowerfulstrokewhenitretracts.

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IntheBALANCEDcylinder,thepistonrodextendsthroughthepistonheadonbothsides.Thisgivesequalworkingareaonbothsidesofthepistonandbalancestheworkingforceofthecylinderwhetheritisextendingorretracting.

Forkliftsandmobileexcavatorsareequippedwithabalancedtypeofdoubleactingcylinder.

6.3 Cushions (hydraulic buffer)

Acushionisbuiltintosomecylinderstoslowthemdownattheendoftheirstrokes.Thiscushionisusedasa“hydraulicbrake”toprotectagainstimpactdamage.Thecylinderworksnormallyduringitsmainstroke(top),butslowsdownasthepistonsealsofftheoiloutlet(bottom).Nowtheoutletoilmustgothroughthesmallorifice,slowingthepiston.

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TESTING AND DIAGNOSING CYLINDER PROBLEMSCylinderscanbetestedonthemachineforleaksandotherfailures.Thefollowingsectionon“MaintenanceofCylinders”coversafewofthecommonproblems.

6.4 Maintanance of Cylinders

HydraulicCylindersarecompactandrelativelysimple.Thekeypointstowatcharethesealsandthepivots.Hereareafewservicetips:

1.EXTERNALLEAKAGE-Ifthecylinderendcapsareleaking,tightenthecaps.Ifthisfailstostoptheleak,replacethegasket.Ifthecylinderleaksaroundthepistonrod,replacethepacking.Besuretheseallipfacestowardthepressureoil.Ifthesealcontinuestoleak,checkitems5through9below.

2. INTERNALLEAKAGE-Leakagepastthepistonsealsinsidethecylindercancausesluggishmovementorsettlingunderload.Pistonleakagecanbecausedbywornpistonsealsorrings,orscoredcylinderwalls.Thelattermaybecausedbydirtandgritintheoil.

IMPORTANT:Whenrepairingacylinder,besuretoreplaceallsealsandpackings beforere-assembly.

3. CREEPINGOFCYLINDER-Ifthecylindercreepswhenstoppedinmidstroke,checkforinternalleakage(item2).Anothercausecouldbeaworncontrol.

4. SLUGGISHOPERATION-Airinthecylinderisthemostcommoncauseofsluggishaction.(Tobleedair,seeendofthischapter).Internalleakageinthecylinderisanothercause(item2).Ifactionissluggishwhenstartingupthesystem,butspeedsupwhenthesystemiswarm,checkforoiloftoo-highviscosity(seemachineoperator’smanual).Ifthecylinderisstillsluggishafterthesechecks,thewholecircuitshouldbetestedforworncomponents.

5. LOOSEMOUNTING-Pivotpointsandmountsmaybeloose.Theboltsorpinsmayneedtobetightenedortheymaybewornout.

Toomuch“slop”or“float”inthecylindermountingsdamagesthepistonrodseals. Checkallcylindersforloosemountingsperiodically.

6. MISALIGNMENT-Pistonrodsmustworkinlineatalltimes.Iftheyare“sideloaded,”thepistonrodswillbegalledandthepackingswillbedamaged,causingleaks.Eventuallythepistonrodsmaybebentortheweldsbroken.

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7. LACKOFLUBRICATION-Lackofpistonrodlubricationmaycausetherodpackingtoseize,resultinginanerraticstroke,especiallyonsingle-actingcylinders.

8. ABRASIVESONPISTONROD-Whenpistonrodsextend,theycanpickupdirtandothermaterial.Thenwhentherodretracts,itcarriesthegritintothecylinder,damagingtherodseal.Thisiswhyrodwipersareoftenusedattherodendofthecylindertocleantherodasitretracts.Rubberbootsarealsousedovertheendofthecylinderinsomecases.Anotherproblemisrustingofpistonrods.Whenstoringcylinders,alwaysretractthepistonrodstoprotectthem.

9. BURRSONPISTONROD-Exposedpistonrodscanbedamagedbyimpactwithhardobjects.Ifthesmoothsurfaceoftherodismarred,therodsealmaybedamaged.Burrsontherodshouldbecleanedupimmediatelyusingcrocuscloth.

10. CHECKINGAIRVENTS-Single-actingcylinders(exceptram-types)musthaveanairventinthedrysideofthecylinder.Topreventdirtentry,variousfilterdevicesareused.Mostareself-cleaning,buttheyshouldbeinspectedperiodicallytoinsureproperoperation.

6.5 Bleeding Air from Remote Cylinders

Anytimearemotecylinderispluggedinthehydrauliccircuit,alltrappedairmustbebled.Thiswillpreventsluggishactionofthecylinder.

Firstattachthecylindertothecircuit.Whilestandingtothesideofthemachine,placethecylinderontheground(oronahanger)withthepistonrodenddown.(Oronmountedcylinders,placetheheadendofthecylinderinitsworkingmount,allowingtherodendfreedomtomoveinandout.)Havesomeoneelsestartthemachineandmovethehydrauliccontrolleverbackandforthsevenoreighttimestoextendandretractthecylinder.Thiswillbleedtheair.(Ondouble-actingcylinders,youmayhavetoturnthecylinderend-for-endandrepeatthecyclingofthecontrollever.)

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6.6 Caution

Longreachdiggerarmcylinderscangeteasilydamaged.

Withanexternalforceactingonthediggerarm,theforceactingonthediggerarmcancausethediggerarm,hydrauliccylindertocracknearthejointwerethecylinderheadisweldedontothecylinder.

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6.7 Basic Hydraulic System

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7. Hydraulic accumulators

7.1 Introduction

Aspringisthesimplestaccumulator.Whencompressed,aspringbecomesasourceofpotentialenergy.Itcanalsobeusedtoabsorbshocksortocontroltheforceonaload.

Hydraulicaccumulatorsworkinmuchthesameway.Basicallytheyarecontainerswhichstorefluidunderpressure.

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Accumulators have four major functions

• StoreEnergy

• AbsorbShocks

• BuildPressureGradually

• MaintainConstantPressure

Whilemostaccumulatorscandoanyofthesethings,theiruseinasystemisusuallylimitedtoonlyone.

Accumulatorswhichstoreenergyareoftenusedas“boosters”forsystemswithfixeddisplacementpumps.Theaccumulatorstorespressureoilduringslackperiodsandfeedsitbackintothesystemduringpeakperiodsofoilusage.Thepumprechargestheaccumulatoraftereachpeak.Sometimestheaccumulatorisusedasaprotectionagainstfailureoftheoilsupply.

Example:powerbrakesonlargermachines.Ifthesystemoilsupplyfails,theaccumulatorfeedsinseveral“charges”ofoilforuseinemergencybraking.

Accumulatorswhichabsorbshockstakeinexcessoilduringpeakpressuresandletitoutagainafterthe“surge”ispast.Thisreducesvibrationsandnoiseinthesystem.Theaccumulatormayalsosmoothoutoperationduringpressuredelays,aswhenavariabledisplacementpumpgoesintostroke.Bydischargingatthismoment,theaccumulator“takesuptheslack.”

Accumulatorswhichbuildpressuregraduallyareusedto“soften”theworkingstrokeofapistonagainstafixedload,asinahydraulicpress.Byabsorbingsomeoftherisingoilpressuretheaccumulatorslowsdownthestroke.

Accumulatorswhichmaintainconstantpressurearealwaysweight-loadedtypeswhichplaceafixedforceontheoilinaclosedcircuit.Whetherthevolumeofoilchangesfromleakageorfromheatexpansionorcontraction,thisaccumulatorkeepsthesamegravitypressureonthesystem.

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The major types of accumulators are:

• Pneumatic(Gas-Loaded)Weight-Loaded

• Spring-Loaded

7.2 Pneumatic accumulators

Welearnedthatfluidswillnotcompress(little),butgaseswill.Forthisreason,manyaccumulatorsuseinertgasasawayof“charging”aloadofoilorofprovidinga“cushion”againstshocks.

“Pneumatic”meansoperatedbycompressedgas.Intheseaccumulators,gasandoiloccupythesamecontainer.Whentheoilpressurerises,incomingoilcompressesthegas.Whenoilpressuredrops,thegasexpands,forcingoutoil.

Inmostcases,thegasisseparatedfromtheoilbyapiston,abladder,oradiaphragm.Thispreventsmixingofthegasandoilandkeepsgasoutofthehydraulicsystem.

7.3 Bladder type accumulators

Aflexiblebagorbladdermadeofsyntheticrubbercontainsthegasandseparatesitfromthehydraulicoil.Thebladderismoldedtothegaschargingstemlocatedatthetopoftheaccumulator.

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Topreventdamagetothebladder,aprotectivebuttonisusedatthebottom.Thisbuttonpreventsthebladderfrombeingdrawnintotheoilportwhenthebladderexpands.Otherwise,thebladdermightbecutortorn.

Bladder-typeaccumulatorscanalsobepre-chargedbeforeuse.

Allhydraulicearthmovingmachinesareusingaccumulators.Tocushionthemainboomwhentraveling(mobileexcavator)orasasuspensionsystemondumptrucksandoff-high-waytrucks.

ButalsotheServosystemisusingaccumulatortoabsorbshockloadswhenoperatingthemaincontrolvalve.Italsoallowsthemainboomtolowersafelytotheground,incaseofenginefailure(notrunning).

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7.4 Precautions

Observethefollowingprecautionswhenworkingonpneumaticaccumulators.Thecorrectproceduresforservicearegivenindetaillaterunder“ServicingandPre-chargingPneumaticAccumulators.”

1. CAUTION: NEVER FILL AN ACCUMULATOR WITH OXYGEN! Anexplosioncouldresultifoilandoxygenmixunderpressure.

2. Neverfillanaccumulatorwithair.Whenairiscompressed,watervaporintheaircondensesandcancauserust.Thisinturnmaydamagesealsandruintheaccumulator.Also,onceairleaksintotheoil,theoilbecomesoxidizedandbreaksdown.

3. Alwaysfillanaccumulatorwithaninert gas suchasdry nitrogen. Thisgasisfreeofbothwatervaporandoxygen:thismakesitharmlesstopartsandsafetouse.

4. Neverchargeanaccumulatortoapressuremorethanthatrecommendedbythemanufacturer.Readthelabelandobservethe“workingpressure.”

5. Beforeremovinganaccumulatorfromahydraulicsystem,release all hydraulic pressure.

6. Beforeyoudisassembleanaccumulator,releasebothgasandhydraulicpressures.

7. Whenyoudisassembleanaccumulator,makesurethatdirtandabrasivematerialdoesnotenteranyoftheopenings.

7.4.1 Checking Precharged Accumulator on the Machine

1. Ifyoususpectexternalgasleaks,applysoapywatertothegasvalveandseamsonthetankatthe“gas”end.Ifbubblesform,thereisaleak.

2. Ifyoususpectinternalleaks,checkforfoamingoilinthesystemreservoirand/ornoactionoftheaccumulator.Thesesignsusuallymeanafaultybladderorpistonsealsinsidetheaccumulator.

3. Iftheaccumulatorappearstobeingoodconditionbutisstillsloworinactive,pre-chargeitasnecessary.

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7.4.2 Before Removing Accumulator from Machine

First be sure all hydraulic pressure is released

Todothis,shutdownthepumpandcyclesomemechanismintheaccumulatorhydrauliccircuittorelieveoilpressure(oropenableedscrew).

REMOVING ACCUMULATOR FROM MACHINEAfterallhydraulicpressurehasbeenreleased,removetheaccumulatorfromthemachineforservice.

7.4.3 Repairing Accumulator

1. Beforedismantlingaccumulator,releaseallgaspressure.Normallyunscrewthegasvalveleververyslowly.Installthechargingvalvefirstifnecessary.Neverreleasethegasbydepressingthevalvecore,asthecoremightberuptured.

2. Disassembletheaccumulatoronacleanbencharea.3. Checkallpartsforleaksorotherdamage.4. Plugtheopeningswithplasticplugsor5. cleantowelsassoonaspartsareremoved.6. Checkbladderorpistonsealsfordamage andreplaceifnecessary.7. Ifgasvalvecoresarereplaced,

besuretousetherecommendedtypes.8. Carefullyassembletheaccumulator.

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7.4.4 Precharging Accumulator

thehosefromaDryNitrogentanktothechargingvalveoftheaccumulatorandopentheaccumulatorchargingvalve.

Openthevalveontheregulatorveryslowlyuntilpressureongaugeissameasthatrecommendedbythemanufacturer.Closethechargingvalveontheaccumulator,thenclosethevalveontheregulator.Removehosefromchargingvalve.

NOTE:Whencheckingpre-chargeonanaccumulatorinstalledonamachine,firstreleasehydraulicpressurefromtheaccumulator.Otherwiseyouwillnotgetatruepressurereading.

7.4.5 Installing Accumulator on Machine

Attachaccumulatortomachineandconnectalllines.Startmachineandcycleahydraulicfunctiontobleedanyairfromthesystem.Thenchecktheaccumulatorforproperaction.

7.5 New Developments

Manufacturesofheavyequipmentarebusymakingtheirmachinesmoreenvironmentalfriendly.ThisnewsystemiscalledHydraulichybridoperated.Accumulatorswillstoreenergyformthedrivelinewhendrivenbytheengine,andfeedtheaccumulatorswhenbrakingthevehicle.Onexcavatorsliftingboomsandswingarealsoavailableintoahydrideversion.

JustvisittheInternetformoreinformationregardingthissubject.

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8.Practical Exercise 1) Refer to the appropriate manual and complete Questions A, B and C

a)Drawthesymbolforthefollowing

VentedReservoir

PressurizedReservoir

Returnlineabovetheoillevel

Suction/Returnlinebelowtheoillevel

Reservoirwithsuctionlineat-tachedatthebottom

Lines,TubesandHoses

Pump

Oilflowonewayonly

Oilcanfloweitherway

Variabledisplacementpump

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NonReversiblemotor

Reversiblemotor

Singleactingcylinder

Doubleactingcylinder

Doublerodendcylinder

Normallyclosedreliefvalve

Normallyopenreliefvalve

Pressurereducingvalve

OneWayvalve

ByPassvalve

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b)Writeshortnotesonthefollowingsymbols

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c)DiscussthegivensimpleschematicsandwritedownthefunctionofvalveA,B,CandD

D c

c

D

B

B

A

A

Detent

Float

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ValveA:

ValveB:

ValveC:

ValveD;

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2) On the test bench, build the shown hydraulic circuit and observe its operation

a)

Notes:

b)

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Notes:

3) Refer to the appropriate manual and conduct Inspection and Adjustment of the following hydraulic pressure Inspection and Adjustment in actual machinery by following all the precautions and sequence prescribed

a)Inspecthydraulicpumppressure

Pressureatidlerpm

Pressureat1000rpm

Pressureat1500rpm

b)Inspectandadjustsystemreliefpressureondirectionalcontrolvalve

Pressurebeforeadjustment

Pressureafteradjustment

c)Howwillyouinspectandadjustpressurereducingvalve(PRV)?(selectanyPRV) Notes:

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d)Forthegivenpracticalmachinery,useappropriatespecialservicetoolsandconductvarioussystempressuretestandadjustment. Notes:

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Hydraulic cylinder

Inspection and replacement of seals

Topreventpossibleinjury,donotusecompressedairforremovingthepistonandpistonrodformthecylinderbarrel.

Highoilpressuresaredangerous.

Hotcomponentscancauseburns.

Given a hydraulic cylinder (double acting), you are to carry out the following:

Inspection of piston rod, piston and cylinder barrel

Inspection and replacement of seals:

1) Selectthecorrecttoolsandaccessories

2) Usethecorrectworkshopmanual

3) Useatorquewrench,ifnecessary

4) Measurethepistonrodforstraightness

5) Installnewoilseals,usingtherightprocedure

CAUTION Followallsafetyrecommendationsandsafeshoppracticesoutlinedintherepairmanualortheselectedcylinder.

Alwaysusetoolsandequipmentthatareingoodworkingorder.Useliftingandhoistingequipmentcapableofsafelyhandlingtheload.Remember,thatultimatelysafetyisyourownresponsibility.

Excavator cylinderThecylinderthatisusedtooperatetheexcavatorboomorbucketisequippedwitharodstopper,whichactsasacushiononlywhenthecylinderrodisfullyretracted(andthebucketispulledclosetothearm).Thistypeofcylinderisshowninthelowerdrawing.

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CAUTION

Ventairfromthehydraulicsystembeforedisconnectingcylinderpipingconnections.Usetheleveronthereservoir,whiletheengineisrunning.

Dischargethehydraulicaccumulatorandventresidualtankpressureaftertheengineisshutoff.

Pourcleanreplacementfluidbackintothesystemifexcessivefluidislost.

Bucket Cylinder

Main FrameLift Cylinder

Steering Cylinder

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Digger arm cylinder

Armcylindershaveacushionorstopperforoperationinbothdirections.

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Reference Number: Description:1 Tubeassembly(cylinderbarrel)2 Bushbearing3 Rodassembly4 Bushbearing5 Rodcover

Digger arm cylinder (continued)

Reference

Number:

Description: Reference

Number:

Description:

6 DD-bush 19 Dustring7 Retainingring 20 O-ring8 Bufferseal 21 Back-upring9 U-packing 22 Pistonnut10 Dustwiper 23 Setscrew11 Retainingring 24 Cushionplunger12 O-ring 25 Stopring13 Back-upring 26 Checkvalve14 O-ring 27 Spring15 Cushionring 28 Supportspring16 Piston 29 Hexsocketplug17 Glydring 30 Greasenipple18 Wearring

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DISASSEMBLY

1. Followingremovalofcylinderfromexcavatorattachment,supportcylinderonsometypeofsturdyworkplatformanddrainalloil.Rotatecylindersothatpipingportsareontop,toallowtrappedairtovent.

2. Positionpistonrodsothatitisextendedapproximatelyonehalfmeter(20”).3. Removealltheboltsontheendofthecylinder.

Note:Wrapaclothorotherprotectivematerialaroundthepistonrod,toavoidscratchingorscoringoftherodsurface

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4. Taptwoboltsintocoverofcylinderhead,180°apart.Tightentheminastaggered,evensequence,tobackoffpistonrodendcoverfromedgeofcylinderwall.

Lookforadequateclearancebetweencoverandendofcylinderwallbeforeusingasoft-facedhammerforfinaldisassembly.

5. Beginwithdrawingpistonrodassembly,awayfromcylinder.Attachaliftingsupportwhenfinal1/3ofrodisstillinsidebarrelofcylinder.Preparesupportblocksforpistonrodbeforeithasbeencompletelywithdrawn.

6. Lowerpistonrodtosupportblocksanddetachwearring(outersurface)fromendofrod.

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7. Immobilizepistonrodbyinsertingawoodenorothernon-scoring,nonmetallicsupportthroughendofrod.

8. Removesetscrewbyusingasocketwrench.

9. Fabricateorpurchaseapistonnutremovalwrench.

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10. Useaplastichammertoevenlypulloffrodcoverfromendofpistonrod.Becarefulnottodamagerodbushinganddustwiper,U-packingandotherseals.

11. Useadull,rounded-tiptooltopryoffO-ringandbackupring.

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12. Findascrewdriverwithanappropriatewidthtiptofacilitateremovalofslipperseal,wearringandslideringfromthepiston.

13. PulloffO-ringandbackupringfromcylinderhead.

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14. Duringdisassemblyofcylinderhead,becarefulnottodamagethebuffersealandU-packing.

15. Disassembletheretainingringanddustwiper.Separatetheretainingringandrodbushing.

16. Forceoutpinbushingfrombodyofcylinder.

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ASSEMBLY

NOTE:Assemblesubassembliesofcylinderinthefollowingorder:

1.Cylinderbody

2.Pistonrod

3.Pistonassembly

4.Cylinderheadassembly

1.Assemblethepinbushingtopistonrodandbodyofthecylinder.

2.Followingtheassemblyofrodcovercomponents,installthedustwiperandrodbushingtotherodcover.Inserttheretainingrings.

IMPORTANT Replaceanypartthatshowsevidenceofdamageorexcessivewear.ReplacementofallO-ringsandflexiblesealsisstronglyrecommended.Beforestartingthecylinderassemblyprocedure,allpartsshouldbethoroughlycleanedanddried,and/orpre-lubricatedwithcleanhydraulicfluid.Preparetheworkareabeforehandtomaintaincleanlinessduringtheassemblyprocedure.

3. Pre-lubricatetheO-ringsandsealsbeforeassembly.

4. Beforestartingtorebuildpistonassembly,heatslippersealfor5minutesinanoilbathwarmedto150°-180°C(302°-356°F).Usespecialslippersealjigofspecializedtoolsatthebeginningofthisprocedure)toattachseal.Coolsealbypushingaretractingjigagainstsealforseveralminutes.Applyastripofclean,see-throughsealingtapearoundslippersealtokeepitfreeofdust.

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5. Immobilizepistonrodonsolidsupportblocks.AssembletheO-ringandbackupringPreparetoattachtherodcoverassemblytopistonrod.Pushrodcoverbytighteningpistonthenut.

6.Assemblethecushionringandattachthepistonassemblytothepistonrod.

7. Usespeciallyfabricateorfactorysourcedtooltotightenthepistonnut.

8.Assemblethewearring,slideringandsetscrewtopistonassembly.

9.Immobilizebodyofcylinderbeforeassembly.

10. Applyfastenerlockingcompound(Loctite),oranalternatemanufacturer’sequivalentproduct,toallendcoverretainingbolts.Wrapaprotectivecushionaroundendofrodwhiletighteningfasteners,topreventpossibledamagetopolishedsurfaceofrod,shouldawrenchslipduringretightening.

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General disassembly and assembly instructions,hydraulic cylinders

Removing the seal

Insertasealpullerbehindtherodseal.

Attention:becarefulnottodamagethebaseofthegroove

Usingthesealpuller,pulltherodsealoutofthegroove

Therodsealcannowberemovedwithusingthefinger.

Followthisprocedureforallothersealsduringdisassembly.

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Fitting the piston seals

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Fitting the guide seals

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Practical exercise, hydraulic cylinder overhaulReport your findings in the space below.

An initiative of:

3

BASIC LEVEL

MECHANICS

HYDRAULICS

UNIDO Headquarters Vienna International CentreP.O. Box 300. A-1400 Vienna Austria Tel: +43 (1) 26026-3752Email: [email protected]

www.lkdfacility.orgAn initiative of:

This curriculum has been developed as part of the Learning and Knowledge Development (LKD) Facility, initiated by the Swedish International Development Agency (Sida) and the United Nations Industrial Development Organization (UNIDO). The LKD Facility is a platform to promote industrial skills development among young people in emerging economies. Working with the private sector through Public Private Development Partnerships, the LKD Facility supports the establishment and upgrading of local industrial training academies to help meet the labour market’s increasing demand for skilled employees, ultimately contributing to inclusive and sustainable industrial development.

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