外文翻译--液压系统和气压系统.doc

HydraulicsystemandPeumaticSystemHui-xiongwan1,JunFan21.SchoolofmechanicalscienceandEngineening,Huazhonguniv.ofscienceandtechnology,Wuhan,430074,China2.SchoolofLogisticsEngineening,Wuhan,univ.ofscienceandtechnology,Wuhan,430063,ChinaAbstract:Hydraulicsystemiswidelyusedinindustry,suchasstamping,grindingofsteeltypeworkandgeneralprocessingindustries,agriculture,mining,spacetechnology,deepseaexploration,transportation,marinetechnology,offshoregasandoilexplorationindustries,inshort,Fewpeopleintheirdailylivesdonotgetcertainbenefitsfromthehydraulictechnology.Successfulandwidelyusedinthehydraulicsystemssecretliesinitsversatilityandeaseofmaneuverability.Hydraulicpowertransmissionmechanicalsystemsasbeingnotlikethemachinegeometryconstraints,Inaddition,thehydraulicsystemdoesnotliketheelectricalsystem,asconstrainedbythephysicalpropertiesofmaterials,itpassedalmostnoamountofpowerconstraints.Keywords:Hydraulicsystem,Pressuresystem,FluidThehistoryofhydraulicpowerisalongone,datingfrommansprehistoriceffortstoharnesstheenergyintheworldaroundhim.Theonlysourcereadilyavailablewerethewaterandthewindtwofreeandmovingstreams.Thewatermill,thefirsthydraulicmotor,wasanearlyinvention.OneispicturedonamosaticattheGreatPalaceinByzantium,datingfromtheearlyfifthcentury.ThemillhadbeenbuiltbytheRomans.Butthefirstrecordofawatermillgoesbackevenfurther,toaround100BC,andtheoriginsmayindeedhavebeenmuchearlier.Thedomesticationofgrainbegansome5000yearsbeforeandsomeenterprisingfarmerisboundtohavebecometiredofpoundingorgrindingthegrainbyhand.Perhaps,infact,theinventorweresomefarmerswives.Sincetheoftendrewtheheavyjobs.Fluidisasubstancewhichmayflow；thatis,itsconstituentparticlesmaycontinuouslychangetheirpositionsrelativetooneanother.Moreover,itoffersnolastingresistancetothedisplacement,howevergreat,ofonelayeroveranother.Thismeansthat,ifthefluidisatrest,noshearforce(thatisaforcetangentialtothesurfaceonwhichitacts)canexistinit.FluidmaybeclassifiedasNewtonianornon-Newtonian.InNewtonianfluidthereisalinearrelationbetweenthemagnitudeofappliedshearstressesandtheresultingrateofangulardeformation.InnonNewtonianfluidthereisanonlinearrelationbetweenthemagnitudeofappliedshearstressandtherateofangulardeformation.Theflowoffluidsmaybeclassifiedinmanyways,suchassteadyornonsteady,rotationalorirrotational,compressibleorincompressible,andviscousornoviscous.AllhydraulicsystemsdependonPascalslaw,suchassteadyorpipeexertsequalforceonallofthesurfacesofthecontainer.Inactualhydraulicsystems,Pascalslawdefinesthebasisofresultswhichareobtainedfromthesystem.Thus,apumpmovestheliquidinthesystem.Theintakeofthepumpisconnectedtoaliquidsource,usuallycalledthetankorreservoir.Atmosphericpressure,pressingontheliquidinthereservoir,forcestheliquidintothepump.Whenthepumpoperates,itforcesliquidfromthetankintothedischargepipeatasuitablepressure.Theflowofthepressurizedliquiddischargedbythepumpiscontrolledbyvalves.Threecontrolfunctionsareusedinmosthydraulicsystems:(1)controloftheliquidpressure,（2）controloftheliquidflowrate,and(3)controlofthedirectionofflowoftheliquid.Hydraulicdrivesareusedinpreferencetomechanicalsystemswhen(1)powersistobetransmittedbetweenpointtoofarapartforchainsorbelts；(2)hightorqueatlowspeedinrequired；(3)averycompactunitisneeded；(4)asmoothtransmission,freeofvibration,isrequired；(5)easycontrolofspeedanddirectionisnecessary；and(6)outputspeedisvariedsteplessly.Fig.1givesadiagrammaticpresentationofthecomponentsofahydraulicinstallation.Electricallydrivenoilpressurepumpsestablishanoilflowforenergytransmission,whichisfedtohydraulicmotorsorhydrauliccylinders,convertingitintomechanicalenergy.Thecontroloftheoilflowisbymeansofvalves.Thepressurizedoilflowproduceslinearorrotarymechanicalmotion.Thekineticenergyoftheoilflowiscomparativelylow,andthereforethetermhydrostaticdriverissometimesused.Thereislittleconstructionaldifferencebetweenhydraulicmotorsandpumps.Anypumpmaybeusedasamotor.Thequantityofoilflowingatanygiventimemaybevariedbymeansofregulatingvalves(asshowninFig.7.1)ortheuseofvariable-deliverypumps.Theapplicationofhydraulicpowertotheoperationofmachinetoolsisbynomeansnew,thoughitsadoptiononsuchawidescaleasexistsatpresentiscomparativelyrecent.Itwasinfactindevelopmentofthemodernself-containedpumpunitthatstimulatedthegrowthofthisformofmachinetooloperation.Hydraulicmachinetooldriveoffersagreatmanyadvantages.Oneofthemisthatitcangiveinfinitely-variablespeedcontroloverwideranges.Inaddition,theycanchangethedirectionofdriveaseasilyastheycanvarythespeed.Asinmanyothertypesofmachine,manycomplexmechanicallinkagescanbesimplifiedorevenwhollyeliminatedbytheuseofhydraulics.Theflexibilityandresilienceofhydraulicpowerisanothergreatvirtueofthisformofdrive.Apartfromthesmoothnessofoperationthusobtained,agreatimprovementisusuallyfoundinthesurfacefinishontheworkandthetoolcanmakeheaviercutswithoutdetrimentandwilllastconsiderablylongerwithoutregrinding.HydraulicandpneumaticsystemThereareonlythreebasicmethodsoftransmittingpower:electrical,mechanical,andfluidpower.Mostapplicationsactuallyuseacombinationofthethreemethodstoobtainthemostefficientoverallsystem.Toproperlydeterminewhichprinciplemethodtouse,itisimportanttoknowthesalientfeaturesofeachtype.Forexample,fluidsystemscantransmitpowermoreeconomicallyovergreaterdistancesthancanmechanicaltypes.However,fluidsystemsarerestrictedtoshorterdistancesthanareelectricalsystems.Hydraulicpowertransmissionsystemareconcernedwiththegeneration,modelation,andcontrolofpressureandflow，andingeneralsuchsystemsinclude:1.Pumpswhichconvertavailablepowerfromtheprimemovertohydraulicpowerattheactuator.2.Valveswhichcontrolthedirectionofpump-flow,thelevelofpowerproduced,andtheamountoffluid-flowtotheactuators.Thepowerlevelisdeterminedbycontrollingboththeflowandpressurelevel.3.Actcatorswhichconverthydtaulicpowertousablemechanicalpoweroutputatthepointrequired.4.Themedium,whichisaliquid,providesrigidtransmissionandcontrolaswellaslubricationofcomponts,sealinginvalves,andcoolingofthesystem.5.Conncetotswhichlinkthevarioussystemcomponents,providepowerconductorsforthefluidunderpressure,andfluidflowreturntotank(reservoir).6.Fluidstorageandconditioningequipmentwhichensuresufficientqualityandquantityaswellascoolingofthefluid.Hydraulicsystemsareusedinindustrialapplicationssuchasstampingpresses,steelmills,andgeneralmanufacturing,agriculturalmachines,miningindustry,aviation,spacetechnology,deep-seaexploration,transportion,marinetechnology,andoffshoregasandpetroleumexploration.Inshort,veryfewpeoplegetthroughadayoftheirliveswithoutsomehowbenefitingfromthetechnologyofhydraulicks.Thesecretofhydraulicsystemssuccessandwidespreaduseisitsversatilityandmanageability.Fluidpowerisnothinderedbythegeometryofthemachineasisthecaseinmechanicalsystems.Also,powercanbetransmittedinalmostlimitlessquantitiesbecausefluidsystemsarenotsolimitedbythephysicallimitationsofmaterialsasaretheelectricalsystems.Forexample,theperformanceofanelectromangnetislimitedbythesaturationlimitofsteel.Ontheotherhand,thepowerlimitoffluidsystemsislimitedonlybythestrengthcapacityofthematerial.Industryisgoingtodependmoreandmoreonautomationinordertoincreaseproductivity.Thisincludesremoteanddirectcontrolofproductionoperations,manufacturingprocesses,andmaterialshandling.Fluidpoweristhemuscleofautomationbecauseofadvantagesinthefollowingfourmajorcategories.1.Easeandaccuracyofcontrol.Bytheuseofsimpleleversandpushbuttons,theoperatorofafluidpowersystemcanreadilystart,stop,speedupo