压力控制阀和流量控制阀外文翻译.docx
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压力控制阀和流量控制阀外文翻译.docx
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压力控制阀和流量控制阀外文翻译
Pressure-ControlValvesandFlow-ControlValves
Pressure-controlvalvesareusedinhydrauliccircuitstomaintaindesiredpressurelevelsinvariouspartsofthecircuits.Apressure-controlvalvemaintainsthedesiredpressurelevelby
(1)divertinghigher-pressurefluidtoalower-pressurearea,therebylimitingthepressureinthehigher-pressurearea,or
(2)restrictingflowintoanotherarea.Valvesthatdivertfluidcanbesafety,relief,counterbalance,sequence,andunloadingtypes.Valvesthatrestrictflowintoanotherareacanbeofthereducingtype.
Apressure-controlvalvemayalsobedefinedaseitheranormallyclosedornormallyopentwo-wayvalve.Relief,sequence,unloadingandcounterbalancevalvesarenormallyclosed,two-wayvalvesthatarepartiallyorfullyopenedwhileperformingtheirdesignfunction,Areducingvalveisanormallyopenvalvethatrestrictsandfinallyblocksfluidflowintoasecondaryarea.Witheithertypeofoperation,thevalvecanbesaidtocreateautomaticallyanorificetoprovidethedesiredpressurecontrol.Anorificeisnotalwayscreatedwhenthevalveispilotedfromanexternalsource.Onevalveofthistypeistheunloading——itisnotself-operating;itdependsonasignalfromanexternalsource.Relief,reducingcounterbalance,andsequencevalvescanbefullyautomaticinoperation,withtheoperatingsignaltakenfromwithintheenvelope.Inthischapterweshallstudythedifferenttypesofpressure-controlvalvesandlearnhowtheyareusedinvarioushydrauliccircuits.
TypesofPressure-ControlValves
Eightpopulardevicesforpressure-controlserviceare:
Safetyvalve.Usuallyapoppet-typetwo-wayvalveintendedtoreleaseasecondaryareawhenthefluidpressureapproachesthesetopeningpressureofthevalve.Thistypeofvalveprotectspipingandequipmentfromexcessivepressure.
Reliefvalve.Valvewhichlimitsthemaximumpressurethatcanbeappliedinthatportionofthecircuittowhichitisconnected.
Counterbalancevalve.Valvewhichmaintainsresistanceagainstflowinonedirectionbutpermitsfreeflowintheotherdirection.
Sequencevalve.Valvewhichdirectsflowtomorethanoneportionofafluidcircuit,insequence.
Unloadingvalve.Valvewhichallowspressuretobuilduptoanadjustablesetting,thenbypassestheflowaslongasaremotesourcemaintainsthepresetpressureonthepilotport.
Pressure-reducingvalve.Valvewhichmaintainsareducedpressureatitsoutletregardlessofthehigherinletpressure.
Hydraulicfuse.Deviceequippedwithafrangiblediskwhichestablishesthemaximumpressureinahydrauliccircuitbyrupturingatapresetpressurevalve.
Pressureswitch.Switchoperatedbyfluidpressureandresponsivetoariseorfallinfluidpressure.
CompoundReliefValve
InthestudyofISOhydraulicsymbolsitwasstatedthatsimplifiedsymbolsarewidelyused.Becauseofthis,pressure-reliefvalvesusedincommonhydrauliccircuitsarerarelyshowncompletewithallauxiliarydevicesandconnections.Instead,thesimplifiedsymbolshowsonlythebasicreliefvalve,pressureinput,tankconnection,valvespring,andtheoffsetarrowindicatingthatthevalveisnormallyclosed.Aslasharrowasshownonthebias-springofthepilot-reliefvalveinmaybeaddedtothebias-springsymbolsofthesimplifiedvalveasshowninifthevalveisadjustable,particularlyifthisinformationissignificanttocircuitoperation.Figure3-20ashowsthecompletesymbolsforacompoundreliefvalve.Alladjacentcontrolsareshown,alongwiththemainreliefelement.Theenvelopesurroundingalltheelementsmayhavefiveconnections.Theseare
(1)pressureinput,
(2)tankconnection,(3)remote-control-stationconnections,(4)teststation,and(5)externaldrainforthepilotrelief(notshown)thatisprovidedonlyonspecialorder.
(a)Completesymbolforacompoundreliefvalve
(b)Simplifiedsymbolforcompoundreliefvalve
Theinputpressureandtankconnectionprovidethemajorflowthroughthevalve.Onlyenoughfluidneedflowtothetest-stationandremote-controlconnectionsfortherespectivefunctions.Theteststationisgenerallyusedforagaugeconnectiontocheckfluidpressure.Thisdoesnotrequireaflowoffluid.Theremote-controlconnectionpassesthequantityoffluidcomingthroughthefixedinternalorificeattherateestablishedbythespringinthemainreliefelement.Anexternaldrainfromthepilot-reliefvalve,iffitted,willnotpassmorefluidthanpassesthroughthefixedinternalorifice.
Figure3-21showsacutawayviewofcompoundreliefvalve.Notethatthemainspoolisheldbythespringinapositionthatblocksthepassagefromthepressureinputporttothetankport,justasthesymbolinshows.
Inputpressureisdirectedtothebottomofthespoolbelowthespringcavitywithoutrestriction.Thesupplylinetothespringcavityisrestrictedbyanorificeintheline.Theareaofeachendofthemainspoolisthesame.Incertainpoppetdesigns;theareasmaynotbeexactlyequal.Oneendmayhavealargerareatoensurecertainfunctionalactions.Inoperation,iffluidcannotescapethroughadjustmentB,abalanceisprovidedbytheequalareasateachendofthespool.Thespringthenmaintainsthespoolinthepositionwhereitblocksthepaththroughthevalvefrompressureinputtotank.WhenthepressureinthespringchamberabovethemainspoolrisestoapointwhereitcanunseattheconeatadjustmentB,aportionofthefluidwillbepassedtothetankport.Whenthevolumeofthisflowexceedsthatpassingthroughtheorificeintothemainspringchamber,thepressurethatiseffectiveontheupperspoolareamaybelessthanthatonthelowerareasothatbalanceisnolongermaintained.AsthepressurecontinuestoriseatthelowerendofthevalvespoolandtheflowcontinuestoincreasethroughadjustmentBwhilethedegreeofunbalanceofthemainspoolbecomesmorepronounced,thepressurewillforcethemainspoolagainstthespring.Thiscreatesapathfromthepressureinputtothetankmuchlikethatcreatedinadirectedinadirectspring-operatedreliefvalve.
Sliding-spool-typecompoundreliefvalve
inputsupplylineorificeconnection(vent)chamberABtankspool
AdjustmentAwouldprovideaspecificmaximumrelievingpressureifadjustmentBwerecompletelyrelaxed.WhenadjustmentBisinuse,providinganadditivepressuretothemain-spoolspring,theminimumrelievingpressurewillbefixedbyadjustmentA.TherelievingpressurecanneverbelessthanthatestablishedbyadjustmentAinthevalve.Pressureinthecircuitcouldbelessiftherewererelaxationthroughsomeotherpath.ResistancetopilotfluidflowcreatedbyadjustmentBmaybeconsideredasahydraulicadditivetothevalveofadjustmentspringA.Inmanyvalves,themain-spoolspringisnotadjustable.
Thepocketcontainingthemain-spoolspringiscalledthecontrolchamber.Itwillbetorememberthisterm,asitiswidelyusedinindustrialhydraulics.Notetheauxiliaryventconnectionintheupperleftsideofthevalvein.Thisport(whichisconnectedtothecontrolchamber)permitstheescapeoffluiddirectlytothetankwithoutrestriction.Thus,therecanbenohydraulicadditivepressuretothemainspoolspring.
Ifasmallreliefvalveisplacedinthecircuitwithaconnectiontotheventport,themaximumrelievingpressurewillbeestablishedbythisadditiveataremotepoint.
VolumeControls
Volumeorflowcontrolvalvesareusedtoregulatespeed.Aswasdevelopedinearlierchapters,thespeedofanactuatordependsonhowmuchoilispumpedintoitperunitoftime.Itispossibletoregulateflowwithavariabledisplacementpump,butinmanycircuitsitismorepracticaltouseafixeddisplacementpumpandregulateflowwithavolumecontrolvalve.
FlowControlMethods
Therearethreebasicmethodsofapplyingvolumecontrolactuatorspeeds.Theyaremeter-in,meter-outandbleed-off.
Meter-InCircuitInmeter-inoperation,theflowcontrolvalveisplacedbetweenthepumpandactuator.Inthisway,itcontrolstheamountoffluidgoingintotheactuator.Pumpdeliveryinexcessofthemeteredamountisdivertedtotankoverthereliefvalve.
Withtheflowcontrolvalveinstalledinthecylinderlineasshown,flowiscontrolledinonedirection.Acheckvalvemustbeincludedintheflowcontrolorplacedinparallelwithitforreturnflow.Ifitisdesiredtocontrolspeedinbothdirections,theflowcontrolcanbeinstalledinthepumpoutletlinepriortothedirectionalvalve.
Themeter-inmethodishighlyaccurate.Itisusedinapplicationswheretheloadcontinuallyresistsmovementoftheactuator,suchasraisingaverticalcylinderloadorpushingaloadatacontrolledspeed.
Meter-OutCircuit.Meter-outcontrolisusedwheretheloadmighttendto“runaway.”Theflowcontrolislocatedwhereitwillrestrictexhaustflowfromtheactuator.
Toregularspeedinbothdirections,thevalveisinstalledinthetanklinefromthedirectionalvalve.Moreoftencontrolisneededinonlyonedirectionanditisplacedinthelinebetweentheactuatoranddirectionalvalve.Heretooabypasscheckvalvewouldberequiredforarapidreturnstroke.
Bleed-OffCircuit.Inableed-offarrangement,theflowcontrolisteedoffthesupplylinefromthepumpanddeterminestheactuatorspeedbymeteringaportionofthepumpdeliverytotank.Theadvantageisthatthepumpoperatesatthepressurerequiredbythework,sinceexcessfluidreturnstotankthroughtheflowcontrolinsteadofthroughthereliefvalve.
Itsdisadvantageissomelossofaccuracybecausethemeasuredflowistotankratherthanintothecylinder,makingthelattersubjecttovariationsinthepumpdelive
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