CIRCUITDESINGSummaryTheselectionofhydrauliccomponentsforuseinagivenapplicationisdeterminedbytheirabilitytomeettherequiredspecificationwithinthedesiredcostframework.Avarietyofcomponentscanbearrangedtofulfilagivenfunctionbyusingdifferentcircuitconfigurationsasthefluidpowersystemdesignerhasthefreedom,withintheconstraintssetbythepreferencesofthemachinebuilderand/ortheuser,toselectcomponentsofhischoice.Thisfreedommakesitdifficulttosummarisecircuitondesignhowever,thedesignerneedtobeablejustifythecircuitonthebasisoftechnicalconsiderations.Thischapterthereforedescribesand,whereapplicable,evaluatesvarietyofcircuitoptionsthatcanbeusedfortherangeoffunctionsgenerallyencounteredintheapplicationoffluidpowersystems.1.IntroductionToaverylargedegreethemainfunctionofhydrauliccircuitsistocontroltheflowtooneorseveralactuatorsasrequiredbytheapplication.Thereare,however,avarietyofmethodsforcontrollingflow,someofwhichactindirectlybyusingpressureasthecontrollingparameter.Thecircuitsdiscussedinthischapterinclude:Directionalcontrolandvalveconfigurations.Velocitycontrolswithconstantsupplypressure.Velocitycontrolswithloadsensing.Variabledisplacementpumpcontrols.Hydrostatictransmissions.Loadcontrol.Contaminationcontrol.2.PressureandFlowHydraulicsystemsprovideflowfromthepumpthatisdirectedtooneormoreactuators(motors)atapressurelevelthatsatisfiesthehighestdemand.Whereasingleoutputisbeingdriventhepumppressurewillfloattotheleveldemandedbytheload.However,evenforsuchsimplesystemsthemethodthatisemployedtoprovidevariableflowneedstobeevaluatedinordertoensurethatbestefficiencyisobtained.Incircuitswithmultipleoutputsthisaspectcanbemoredifficulttoevaluate.Foroperationatpressuresandflowsthatarelowerthantherequiredmaximumvaluestheefficiencyofthesystemwilldependonthetypeofpumpbeingused(i.e.fixedorvariabledisplacement).ThiscanberepresenteddiagrammaticallyasinFigure1.ForfixeddisplacementpumpsystemitisclearfromFigure1thatexcesspumpflowwillhavetobereturnedtothereservoirsothatthepowerrequiredbythepumpisgreaterthanthatbeingsuppliedtotheload.Thelevelofinefficiencyincurredisdependentontheratiobetweenthepressurerequiredbytheloadandthatatthepumpoutletwhichcanbecontrolledatthemaximumlevelbythereliefvalveoratlowerpressuresbyvarioustypesofbypassvalves.Figure1FlowandpressurevarlotionForvariabledisplacementpumpsthegenerationofexcessflowcanbeavoided.However,theleverofpumppressurewilldependonthemethodthatisusedforcontrollingthedisplacementbutclearlythereisscopeforachievingmuchhigherefficienciesthanwithfixeddisplacementpumps.Eachofthesecontrolmethodswillrequireaparticularcircuitdesignemployingcomponentsthathavebeendescribedinthepreviouschapters.3.DirectionalcontrolValvesusedforcontrollingthedirectionoftheflowcanbeputintofixedpositionsforthispurposebutmanytypesarefrequentlyusedinacontinuouslyvariablemodewheretheyintroducearestrictionintotheflowpath.3.1TwopositionvalvesAfour-wayvalvewithtwopositionsforchangingdirectionoftheflowtoandfromanactuatorisshowninFigure2.Forsupplyflow,Q,theactuatorvelocitieswillbe:ExtendUE=Q/Ap；RetractUR=Q/AAHere,theactuatorareasareApforthepistonandAAfortheannulusorrodendtheactuator.Hence,URUEasApARAnyexternalforces(F)thatareactingontheactuatorrodmustbeinoppositiontothedirectionmotion.Forreversingforceapplicationsitwillbenecessarytoapplyrestrictorcontrolwhichwillbediscussedlaterinthechapter.Theseforceswillcreateasupplypressurethatis=F/AporF/AAFigure2Twopositionfour-wayvalveThree-wayvalvesareusedinapplicationswhereonlyonesideoftheactuatorneedsaconnectionfromthesupply.Atypicalexampleforthisistheoperationoftheliftmechanismonaforklifttruck,asshowninFigure3wheretheactuatorisloweredundertheactionoftheweight.Figure3Twopositionthree-way3.2ThreepositionvalvesThreepositionvalveshaveathird,centralpositionthatcanbeconnectedindifferentconfigurations.Thesevariantsaredescribed.ClosedCentreValves(Figure4)Closedcentrevalvesblockallofthefourports.Thispreventstheactuatorfrommovingundertheactionofanyforcesontheactuator.Thesupplyflowportisalsoblockedwhichmayrequiresomemeansoflimitingthesupplypressuresupplypressurecanbemadebyappropriatepumpcontrolsorbyareliefvalve.Figure4TandemCentrevalves(Figgure5)Tandemcentrevalvesblocktheactuatorportsbutthesupplyisreturnedtothetankatlowpressure.Ifothervalvesarebeingsuppliedfromthesamesourcethistypeofvalvemaynotbeused-unlessconnectedinseries.Figure5OpenCentreValves(Figure6)Opencentrevalvesconnectallofthefourportstothetanksothatthesupplyandtheactuatorpressureareatlowpressure.Thisallowstheactuatortoactuatortobefreetobemoveundertheactionofanyexternalforces.Figure6WhereitisnecessarytoblockthesupplyflowtheconfigurationshowninFigure7canbeused.Figure74.LoadholdingvalvesTheradialclearancebetweenthevalveanditshousingofspoolvalvesiscarefullycontrolledinthemanufacturingprocesstolevelsofaround2micron.Theleakagethroughthisspace,evenathighpressures,issmallbutforapplicationswhereitisessentialthattheactuatorremainsintheselectedpo