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外文翻译--液压系统和气压系统.doc

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外文翻译--液压系统和气压系统.doc

HydraulicsystemandPeumaticSystemHuixiongwan1,JunFan21.SchoolofmechanicalscienceandEngineening,Huazhonguniv.ofscienceandtechnology,Wuhan,430074,China2.SchoolofLogisticsEngineening,Wuhan,univ.ofscienceandtechnology,Wuhan,430063,ChinaAbstractHydraulicsystemiswidelyusedinindustry,suchasstamping,grindingofsteeltypeworkandgeneralprocessingindustries,agriculture,mining,spacetechnology,deepseaexploration,transportation,marinetechnology,offshoregasandoilexplorationindustries,inshort,Fewpeopleintheirdailylivesdonotgetcertainbenefitsfromthehydraulictechnology.Successfulandwidelyusedinthehydraulicsystemssecretliesinitsversatilityandeaseofmaneuverability.Hydraulicpowertransmissionmechanicalsystemsasbeingnotlikethemachinegeometryconstraints,Inaddition,thehydraulicsystemdoesnotliketheelectricalsystem,asconstrainedbythephysicalpropertiesofmaterials,itpassedalmostnoamountofpowerconstraints.KeywordsHydraulicsystem,Pressuresystem,FluidThehistoryofhydraulicpowerisalongone,datingfrommansprehistoriceffortstoharnesstheenergyintheworldaroundhim.Theonlysourcereadilyavailablewerethewaterandthewindtwofreeandmovingstreams.Thewatermill,thefirsthydraulicmotor,wasanearlyinvention.OneispicturedonamosaticattheGreatPalaceinByzantium,datingfromtheearlyfifthcentury.ThemillhadbeenbuiltbytheRomans.Butthefirstrecordofawatermillgoesbackevenfurther,toaround100BC,andtheoriginsmayindeedhavebeenmuchearlier.Thedomesticationofgrainbegansome5000yearsbeforeandsomeenterprisingfarmerisboundtohavebecometiredofpoundingorgrindingthegrainbyhand.Perhaps,infact,theinventorweresomefarmerswives.Sincetheoftendrewtheheavyjobs.Fluidisasubstancewhichmayflowthatis,itsconstituentparticlesmaycontinuouslychangetheirpositionsrelativetooneanother.Moreover,itoffersnolastingresistancetothedisplacement,howevergreat,ofonelayeroveranother.Thismeansthat,ifthefluidisatrest,noshearforcethatisaforcetangentialtothesurfaceonwhichitactscanexistinit.FluidmaybeclassifiedasNewtonianornonNewtonian.InNewtonianfluidthereisalinearrelationbetweenthemagnitudeofappliedshearstressesandtheresultingrateofangulardeformation.InnonNewtonianfluidthereisanonlinearrelationbetweenthemagnitudeofappliedshearstressandtherateofangulardeformation.Theflowoffluidsmaybeclassifiedinmanyways,suchassteadyornonsteady,rotationalorirrotational,compressibleorincompressible,andviscousornoviscous.AllhydraulicsystemsdependonPascalslaw,suchassteadyorpipeexertsequalforceonallofthesurfacesofthecontainer.Inactualhydraulicsystems,Pascalslawdefinesthebasisofresultswhichareobtainedfromthesystem.Thus,apumpmovestheliquidinthesystem.Theintakeofthepumpisconnectedtoaliquidsource,usuallycalledthetankorreservoir.Atmosphericpressure,pressingontheliquidinthereservoir,forcestheliquidintothepump.Whenthepumpoperates,itforcesliquidfromthetankintothedischargepipeatasuitablepressure.Theflowofthepressurizedliquiddischargedbythepumpiscontrolledbyvalves.Threecontrolfunctionsareusedinmosthydraulicsystems1controloftheliquidpressure,(2)controloftheliquidflowrate,and3controlofthedirectionofflowoftheliquid.Hydraulicdrivesareusedinpreferencetomechanicalsystemswhen1powersistobetransmittedbetweenpointtoofarapartforchainsorbelts2hightorqueatlowspeedinrequired3averycompactunitisneeded4asmoothtransmission,freeofvibration,isrequired5easycontrolofspeedanddirectionisnecessaryand6outputspeedisvariedsteplessly.Fig.1givesadiagrammaticpresentationofthecomponentsofahydraulicinstallation.Electricallydrivenoilpressurepumpsestablishanoilflowforenergytransmission,whichisfedtohydraulicmotorsorhydrauliccylinders,convertingitintomechanicalenergy.Thecontroloftheoilflowisbymeansofvalves.Thepressurizedoilflowproduceslinearorrotarymechanicalmotion.Thekineticenergyoftheoilflowiscomparativelylow,andthereforethetermhydrostaticdriverissometimesused.Thereislittleconstructionaldifferencebetweenhydraulicmotorsandpumps.Anypumpmaybeusedasamotor.ThequantityofoilflowingatanygiventimemaybevariedbymeansofregulatingvalvesasshowninFig.7.1ortheuseofvariabledeliverypumps.Theapplicationofhydraulicpowertotheoperationofmachinetoolsisbynomeansnew,thoughitsadoptiononsuchawidescaleasexistsatpresentiscomparativelyrecent.Itwasinfactindevelopmentofthemodernselfcontainedpumpunitthatstimulatedthegrowthofthisformofmachinetooloperation.Hydraulicmachinetooldriveoffersagreatmanyadvantages.Oneofthemisthatitcangiveinfinitelyvariablespeedcontroloverwideranges.Inaddition,theycanchangethedirectionofdriveaseasilyastheycanvarythespeed.Asinmanyothertypesofmachine,manycomplexmechanicallinkagescanbesimplifiedorevenwhollyeliminatedbytheuseofhydraulics.Theflexibilityandresilienceofhydraulicpowerisanothergreatvirtueofthisformofdrive.Apartfromthesmoothnessofoperationthusobtained,agreatimprovementisusuallyfoundinthesurfacefinishontheworkandthetoolcanmakeheaviercutswithoutdetrimentandwilllastconsiderablylongerwithoutregrinding.HydraulicandpneumaticsystemThereareonlythreebasicmethodsoftransmittingpowerelectrical,mechanical,andfluidpower.Mostapplicationsactuallyuseacombinationofthethreemethodstoobtainthemostefficientoverallsystem.Toproperlydeterminewhichprinciplemethodtouse,itisimportanttoknowthesalientfeaturesofeachtype.Forexample,fluidsystemscantransmitpowermoreeconomicallyovergreaterdistancesthancanmechanicaltypes.However,fluidsystemsarerestrictedtoshorterdistancesthanareelectricalsystems.Hydraulicpowertransmissionsystemareconcernedwiththegeneration,modelation,andcontrolofpressureandflow,andingeneralsuchsystemsinclude1.Pumpswhichconvertavailablepowerfromtheprimemovertohydraulicpowerattheactuator.2.Valveswhichcontrolthedirectionofpumpflow,thelevelofpowerproduced,andtheamountoffluidflowtotheactuators.Thepowerlevelisdeterminedbycontrollingboththeflowandpressurelevel.3.Actcatorswhichconverthydtaulicpowertousablemechanicalpoweroutputatthepointrequired.4.Themedium,whichisaliquid,providesrigidtransmissionandcontrolaswellaslubricationofcomponts,sealinginvalves,andcoolingofthesystem.5.Conncetotswhichlinkthevarioussystemcomponents,providepowerconductorsforthefluidunderpressure,andfluidflowreturntotankreservoir.6.Fluidstorageandconditioningequipmentwhichensuresufficientqualityandquantityaswellascoolingofthefluid.Hydraulicsystemsareusedinindustrialapplicationssuchasstampingpresses,steelmills,andgeneralmanufacturing,agriculturalmachines,miningindustry,aviation,spacetechnology,deepseaexploration,transportion,marinetechnology,andoffshoregasandpetroleumexploration.Inshort,veryfewpeoplegetthroughadayoftheirliveswithoutsomehowbenefitingfromthetechnologyofhydraulicks.Thesecretofhydraulicsystemssuccessandwidespreaduseisitsversatilityandmanageability.Fluidpowerisnothinderedbythegeometryofthemachineasisthecaseinmechanicalsystems.Also,powercanbetransmittedinalmostlimitlessquantitiesbecausefluidsystemsarenotsolimitedbythephysicallimitationsofmaterialsasaretheelectricalsystems.Forexample,theperformanceofanelectromangnetislimitedbythesaturationlimitofsteel.Ontheotherhand,thepowerlimitoffluidsystemsislimitedonlybythestrengthcapacityofthematerial.Industryisgoingtodependmoreandmoreonautomationinordertoincreaseproductivity.Thisincludesremoteanddirectcontrolofproductionoperations,manufacturingprocesses,andmaterialshandling.Fluidpoweristhemuscleofautomationbecauseofadvantagesinthefollowingfourmajorcategories.1.Easeandaccuracyofcontrol.Bytheuseofsimpleleversandpushbuttons,theoperatorofafluidpowersystemcanreadilystart,stop,speeduporslowdown,andpositionforceswhichprovideanydesiredhorsepowerwithtolerancesaspreciseasonetenthousandthofaninch.2.Multiplicationofforce.Afluidpowersystemwithoutusingcumbersomegears,pulleys,andleverscanmultiplyforcessimplyandefficientlyfromafractionofanouncetoseveralhundredtonsofoutput.3.Constantforceortorque.Onlyfluidpowersystemsarecapableofprovidingcontantforceortorqueregardlessofspeedchanges.Thisisaccomplishedwhethertheworkoutputmovesafewinchesperhour,severalhundredinchesperminute,afewrevolutionsperhour,orthousandsofrevolutionsperminute.4.Simplicity,safely,economy.Ingeneral,fluidpowersystemsusefewermovingpartsthancomparablemechanicalorelectricalsystems.Thus,theyaresimplertomaintainandoperate.This,inturn,maximizessafety,companctness,andreliability.Forexample,anewpowersteeringcontroldesignedhasmadeallother

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