Counterbalance valves.docx

Counterbalance valves.docx

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Counterbalancevalves

Counterbalancevalves

Someactuatorswithrunning-away（oroverrunning）loadswilllettheloadfreefallwhenthedirectionalvalvethatcontrolstheactuatorshiftstolowertheload.Cylinderswithlargeplatensandtoolingorhydraulicmotorsonwinchdrivesaretwoexamplesofsuchactuators.Whenthedirectionalvalveshifts,anoverrunningloadforcestheactuatortomovefasterthanpumpflowcanfillit.Oilathighvelocityleavesoneendwhiletheoppositesidestarvesforfluid.Avacuumvoidformsintheinletsideoftheactuatorthatmustbefilledbeforeapplyingforce.Anyrunning-awayoroverrunningloadneedssomemethodtoretarditsmovement.

Ameter-outflowcontrolisonewaytocontrolarunning-awayloadataconstantspeed.Unlesspumpflowneverchanges,settingflowpreciselyonthistypecontroliscritical.Settingtheflowcontrolforminimumpumpflowwillwasteenergywhenpumpflowishigh.Settingtheflowcontrolformaximumpumpflowletsthecylinderrunaheadwhenpumpflowislow.Incorporatingapressurecontrolvalvecalledacounterbalanceisabetterwaytocontrolrunning-awayloads.Acounterbalancekeepsanactuatorfromrunningawayevenwithvariableflowrates.

Fig.5-1:

Internallypilotedcounterbalancevalve

Figure5-1showsthesymbolforaninternallypilotedcounterbalancevalve.Useaninternallypilotedcounterbalancetoholdaloadbackwhentheactuatordoesnotneedfullpowerattheendofstroke.Thistypeofcounterbalancevalveretardsflowcontinuously,soitresistsflowevenafterworkcontactstopstheactuator.Notethatitisnecessarytoadjustaninternallypilotedcounterbalancevalveeverytimetheloadchanges.Thefollowingcircuitsshowthesecharacteristicsandhowtodesignaroundthem.

Fig.5-2:

Externallypilotedcounterbalancevalve

Figure5-2showsthesymbolforanexternallypilotedcounterbalancevalve.Thisvalve’spilotsupplyisfromasourceotherthanthecontrolledload.Anexternallypilotedcounterbalancedoesnotwasteenergyattheendofstrokeanddoesnotrequireadjustmentforchangingloads.However,anexternallypilotedcounterbalancevalvedoeswastealittlemoreenergywhenmovingtheloadtothework.

Fig.5-3:

Internally/externallypilotedcounterbalancevalve

Figure5-3showsthesymbolforaninternally/externallypilotedcounterbalancevalve.Thisvalvehasthebestofbothsystems.Astheloadextends,internalpilotsupplygivessmoothcontrolwithlittleenergyloss.Afterworkcontact,assystempressurebuilds,theexternalpilotfullyopensthecounterbalancetorelieveallbackpressureinthecylinder.

Counterbalancevalvesaremanufacturedinbothspoolandpoppetdesigns.Spooldesignsleakatarateof3to5in.3/min,thusallowingactuatorcreep.Poppetdesignsleakatonly3to5drops/min,minimizingcylindercreep.（Becausehydraulicmotorsbypassinternally,acounterbalanceonlyworkswithamovingload.Thedesignershouldapplyabrakingmethodtoholdahydraulicmotoratrest.）

Internallypilotedcounterbalancevalve

Figure5-4picturesacircuitwitharunning-awayload.Thiscircuitdemonstratestheoperationofaninternallypilotedcounterbalancevalve.ThecylinderinFigure5-4hasastaticpressureof566psiintherodendduetothe15,000-lbloadonthe26.51-in.2area.（15,000/26.51=566psi）.Anopen-centerdirectionalvalveunloadsthepumpandkeepsbackpressureoffthecounterbalancevalveoutletandpilotport.Thecylinderholdsinanypositionifthecounterbalancevalveissetcorrectlyanddoesnotleak.Setthecounterbalanceapproximately100to150psihigherthantheload-inducedpressure.

Fig.5-4:

Internallypilotedcounterbalancevalveatrestwithpumprunning

Normalprocedureforsettingacounterbalancevalveistoturntheadjustingscrewtoitshighestpressurebeforeraisingthecylinder.Afterstartingthepump,energizethedirectionalvalveandcarefullyraisetheloadashortdistance.Withtheloadsuspended,deenergizethedirectionalvalve.AworkingcounterbalancewillholdtheloadsuspendedandgaugePG3willshowtheload-inducedpressure.Nowstartloweringthecounterbalancepressuresettingslowly.Whenthecylinderbeginstocreepdownward,increasethepressureuntilcreepingstops.Thencontinueturningtheadjustingcontrolinthesamedirectionanothertoturn.Aftersettingthecounterbalancethisway,powerthecylinderdownandnoticethepressurereadingongaugePG3.Pressureshouldbeapproximately700to750psi.Anytimethecylinderloadingchanges,repeattheaboveprocess.Resettingthecounterbalancevalvekeepsthecylinderfromrunningawayandreducesenergylosswithalighterload.

Fig.5-5:

Internallypilotedcounterbalancevalvewithcylinderextending

WhenthedirectionalvalveshiftstoextendthecylinderinFigure5-5,oilfromthepumpflowsintothecapendofthecylinderandpressurestartstobuild.Whenpressureinthecapendofthecylinderreachesabout75psi,thecylindershouldstarttostroke.（Thisisbecauseitbuildsanextra140psiintherodend,addingtotheload’s566psi.）Atthispointthecylinderstartstoextendandcontinuestomoveaslongasthepumpsuppliesoilat75psiorhighertothecylindercapend.Ifpumpflowchanges,cylinderspeedchangesalso.

Fig.5-6:

Internallypilotedcounterbalancevalvewithcylinderretracting

Whilethecylinderisretracting,Figure5-6,pumpflowbypassesthecounterbalancevalvethroughtheintegralbypasscheck.Thecounterbalancevalveoffsetsthepotentialenergyoftheweightontherodendofacylinder.The15,000-lbforceinthisfigurecannotdousefulworkwhenusinganinternallypilotedcounterbalancevalve.（SeeFigures5-13through5-15forenergylossandheatgenerationfordifferenttypesofcounterbalancecircuits.）

Externallypilotedcounterbalancevalve

Figure5-7showsanexternallypilotedcounterbalancevalvecircuit.（Thisisthesamecylinderarrangementshownpreviously.）Noticethecounterbalancesettingof150psiwiththiscircuit.Becausethisisanexternallypilotedcounterbalance,itoperatesatamuchlowersetting.Iftheloadchanges,thecounterbalancesettingdoesnotchange.Load-inducedpressuredoesnotaffectsetpressure.Theoretically,setpressurecouldbe1psiandtheloadwouldnotmove.Thecylinderwouldonlyextendwhenthecounterbalancevalve’sexternalpilotportsensed1psiatthecylindercapend.

Fig.5-7:

Externallypilotedcounterbalancevalveatrestwithpumprunning

Onmostexternallypilotedcounterbalancevalves,theminimumpressuresettingis100to200psi.Thiskeepsthecounterbalancevalvefromhunting.Huntingstartswhenthevalveseesenoughpressuretoopen,butthenopenstoowide.Thecylinderrunsawaywhenthevalveopenstoomuchandpilotpressuredrops.Whenpilotpressuredrops,thecounterbalancevalveclosesandthecylinderstops.Afterthecylinderstops,pilotpressurebuildsagain.Theprocessrepeatsandcontinuestotheendofstroke.Thehighertheload-inducedpressure,thegreaterthehuntingproblem.（Onsomesystemsitispossibletoaddanorificeinthepilotlinetoslowthepilotsupplyresponseandreducehunting.Thisorificefixisdifficulttogetrightandmaycauseothercircuitproblems.）

Fig.5-8:

Externallypilotedcounterbalancevalvewithcylinderextending

EnergizingthedirectionalvalvetoextendthecylinderinFigure5-8sendspilotpressuretothecounterbalancevalvefromthecapendcylinderline.Oncepressureinthecapendcylinderlinereaches150psi,thecounterbalanceopensandthecylinderextends.Aslongasthereisenoughpilotpressuretokeepthecounterbalanceopen,thecylindermovesforward.Increasing,decreasing,orstoppingpumpflowcausesthecylindertorespondaccordingly,butnevertorunaway.

Fig.5-9:

Externallypilotedcounterbalancevalvewithcylinderretracting

Whenthecylindermeetstheload,pressureinthepilotportofthecounterbalancecontinuestoincrease.Whenpilotpressuregoesabovethecounterbalancesetting,thevalveopensfullyanddropsallbackpressureonthecylinderrodend.Withnobackpressureontherodend,theweightenergygeneratesextradownwardforce.Theexternallypilotedsystemsavesenergybyeliminatingallbackpressurewhenperformingwork.Figure5-9showstheflowpathsafterthedirectionalcontrolvalveshiftstoretractthecylinder.

Internally/externallypilotedcounterbalancevalve

Somemanufacturersmakecounterbalancevalveswithinternalandexternalpilots.Theseinternally/externallypilotedvalvesprovidethebestofbothsystems.Theyusetheinternalpilottolowertheloadsmoothlyandtheexternalpilottodropallbackpressurewhenperformingwork,thusavoidinglossofdownforce.Inaddition,internally/externallypilotedcounterbalancevalvesdon’thunt.

Fig.5-10:

Internally/externallypilotedcounterbalancevalveatrestwithpumprunning

Figure5-10showsaschematicdrawingwithaninternally/externallypilotedcounterbalancevalve.Intheatrestcondition,theexternalpilotdrainstotankthroughthedirectionalvalve.Theinternalpilothasstaticpressurefromtheload-inducedpressureontherodendarea.Settingthecounterbalancepressureapproximately25%higherthanstaticpressure（1.25X566=707psi）meansthatwhenpressureatthecylindercapendrisestoapproximately75psi,thecylinderstartstostroke.

Fig.5-11:

Internally/externallypilotedcounterbalancevalvewithcylinderextending

Whenthedirectionalvalveshifts,Figure5-11,thecylinderbeginstoextend.Internalpilotpressureopensthecounterbalancevalveenoughforthecylindertomove.Pilotpressurebuiltbythepumpp