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附录AAutomobileBrakeSystemThebrakingsystemisthemostimportantsystemincars.Ifthebrakesfail,theresultcanbedisastrous.Brakesareactuallyenergyconversiondevices,whichconvertthekineticenergymomentumofthevehicleintothermalenergyheat.Whensteppingonthebrakes,thedrivercommandsastoppingforcetentimesaspowerfulastheforcethatputsthecarinmotion.Thebrakingsystemcanexertthousandsofpoundsofpressureoneachofthefourbrakes.Twocompleteindependentbrakingsystemsareusedonthecar.Theyaretheservicebrakeandtheparkingbrake.Theservicebrakeactstoslow,stop,orholdthevehicleduringnormaldriving.Theyarefootoperatedbythedriverdepressingandreleasingthebrakepedal.Theprimarypurposeofthebrakeistoholdthevehiclestationarywhileitisunattended.Theparkingbrakeismechanicallyoperatedbywhenaseparateparkingbrakefootpedalorhandleverisset.Thebrakesystemiscomposedofthefollowingbasiccomponentsthemastercylinderwhichislocatedunderthehood,andisdirectlyconnectedtothebrakepedal,convertsdriverfootsmechanicalpressureintohydraulicpressure.Steelbrakelinesandflexiblebrakehosesconnectthemastercylindertotheslavecylinderslocatedateachwheel.Brakefluid,speciallydesignedtoworkinextremeconditions,fillsthesystem.Shoesandpadsarepushedbytheslavecylinderstocontactthedrumsandrotorsthuscausingdrag,whichhopefullyslowsthecar.ThetypicalbrakesystemconsistsofdiskbrakesinfrontandeitherdiskordrumbrakesintherearconnectedbyasystemoftubesandhosesthatlinkthebrakeateachwheeltothemastercylinderFigure.Basically,allcarbrakesarefrictionbrakes.Whenthedriverappliesthebrake,thecontroldeviceforcesbrakeshoes,orpads,againsttherotatingbrakedrumordisksatwheel.Frictionbetweentheshoesorpadsandthedrumsordisksthenslowsorstopsthewheelsothatthecarisbraked.InmostmodernbrakesystemsseeFigure15.1,thereisafluidfilledcylinder,calledmastercylinder,whichcontainstwoseparatesections,thereisapistonineachsectionandbothpistonsareconnectedtoabrakepedalinthedriverscompartment.Whenthebrakeispusheddown,brakefluidissentfromthemastercylindertothewheels.Atthewheels,thefluidpushesshoes,orpads,againstrevolvingdrumsordisks.Thefrictionbetweenthestationaryshoes,orpads,andtherevolvingdrumsordisksslowsandstopsthem.Thisslowsorstopstherevolvingwheels,which,inturn,sloworstopthecar.Thebrakefluidreservoirisontopofthemastercylinder.Mostcarstodayhaveatransparentrreservoirsothatyoucanseethelevelwithoutopeningthecover.Thebrakefluidlevelwilldropslightlyasthebrakepadswear.Thisisanormalconditionandnocauseforconcern.Iftheleveldropsnoticeablyoverashortperiodoftimeorgoesdowntoabouttwothirdsfull,haveyourbrakescheckedassoonaspossible.Keepthereservoircoveredexceptfortheamountoftimeyouneedtofillitandneverleaveacamofbrakefluiduncovered.Brakefluidmustmaintainaveryhighboilingpoint.Exposuretoairwillcausethefluidtoabsorbmoisturewhichwilllowerthatboilingpoint.Thebrakefluidtravelsfromthemastercylindertothewheelsthroughaseriesofsteeltubesandreinforcedrubberhoses.Rubberhosesareonlyusedinplacesthatrequireflexibility,suchasatthefrontwheels,whichmoveupanddownaswellassteer.Therestofthesystemusesnoncorrosiveseamlesssteeltubingwithspecialfittingsatallattachmentpoints.Ifasteellinerequiresarepair,thebestprocedureistoreplacethecompeteline.Ifthisisnotpractical,alinecanberepairedusingspecialsplicefittingsthataremadeforbrakesystemrepair.Youmustneverusecoppertubingtorepairabrakesystem.Theyaredangerousandillegal.Drumbrakes,itconsistsofthebrakedrum,anexpander,pullbacksprings,astationarybackplate,twoshoeswithfrictionlinings,andanchorpins.Thestationarybackplateissecuredtotheflangeoftheaxlehousingortothesteeringknuckle.Thebrakedrumismountedonthewheelhub.Thereisaclearancebetweentheinnersurfaceofthedrumandtheshoelining.Toapplybrakes,thedriverpushespedal,theexpanderexpandstheshoesandpressesthemtothedrum.Frictionbetweenthebrakedrumandthefrictionliningsbrakesthewheelsandthevehiclestops.Toreleasebrakes,thedriverreleasethepedal,thepullbackspringretractstheshoesthuspermittingfreerotationofthewheels.Diskbrakes,ithasametaldiskinsteadofadrum.Aflatshoe,ordiskbrakepad,islocatedoneachsideofthedisk.Theshoessqueezetherotatingdisktostopthecar.Fluidfromthemastercylinderforcesthepistonstomovein,towardthedisk.Thisactionpushesthefrictionpadstightlyagainstthedisk.Thefrictionbetweentheshoesanddiskslowsandstopsit.Thisprovidesthebrakingaction.Pistonsaremadeofeitherplasticormetal.Therearethreegeneraltypesofdiskbrakes.Theyarethefloatingcalipertype,thefixedcalipertype,andtheslidingcalipertype.Floatingcaliperandslidingcaliperdiskbrakesuseasinglepiston.Fixedcaliperdiskbrakeshaveeithertwoorfourpistons.Thebrakesystemassembliesareactuatedbymechanical,hydraulicorpneumaticdevices.Themechanicalleverageisusedintheparkingbrakesfittedinallautomobile.Whenthebrakepedalisdepressed,therodpushesthepistonofbrakemastercylinderwhichpressesthefluid.Thefluidflowsthroughthepipelinestothepowerbrakeunitandthentothewheelcylinder.Thefluidpressureexpandsthecylinderpistonsthuspressingtheshoestothedrumordisk.Ifthepedalisreleased,thepistonreturnstotheinitialposition,thepullbackspringsretracttheshoes,thefluidisforcedbacktothemastercylinderandbrakingceases.Theprimarypurposeoftheparkingbrakeistoholdthevehiclestationarywhileitisunattended.Theparkingbrakeismechanicallyoperatedbythedriverwhenaseparateparkingbrakinghandleverisset.Thehandbrakeisnormallyusedwhenthecarhasalreadystopped.Aleverispulledandtherearbrakesareapproachedandlockedintheonposition.Thecarmaynowbeleftwithoutfearofitsrollingaway.Whenthedriverwantstomovethecaragain,hemustpressabuttonbeforethelevercanbereleased.Thehandbrakemustalsobeabletostopthecarintheeventofthefootbrakefailing.Forthisreason,itisseparatefromthefootbrakeusescableorrodsinsteadofthehydraulicsystem.Antilockbrakesystemsmakebrakingsaferandmoreconvenient,Antilockbrakesystemsmodulatebrakesystemhydraulicpressuretopreventthebrakesfromlockingandthetiresfromskiddingonslipperypavementorduringapanicstop.Antilockbrakesystemshavebeenusedonaircraftforyears,andsomedomesticcarwereofferedwithanearlyformofantilockbrakinginlate1990s.Recently,severalautomakershaveintroducedmoresophisticatedantilocksystem.InvestigationsinEurope,whereantilockbrakingsystemshavebeenavailableforadecade,haveledonemanufacturetostatethatthenumberoftrafficaccidentscouldbereducedbysevenandahalfpercentifallcarshadantilockbrakes.Sosomesourcespredictthatallcarswillofferantilockbrakestoimprovethesafetyofthecar.Antilocksystemsmodulatebrakeapplicationforceseveraltimespersecondtoholdthetiresatacontrolledamountofslipallsystemsaccomplishthisinbasicallythesameway.Oneormorespeedsensorsgeneratealternatingcurrentsignalwhosefrequencyincreaseswiththewheelrotationalspeed.Anelectroniccontrolunitcontinuouslymonitorsthesesignalsandifthefrequencyofasignaldropstoorapidlyindicatingthatawheelisabouttolock,thecontrolunitinstructsamodulatingdevicetoreducehydraulicpressuretothebrakeattheaffectedwheel.Whensensorsignalsindicatethewheelisagainrotatingnormally,thecontrolunitallowsincreasedhydraulicpressuretothebrake.Thisreleaseapplycycleoccursseveraltimepersecondtopumpthebrakeslikeadrivermightbutatamuchfasterrate.Inadditiontotheirbasicoperation,antilocksystemshavetwootherthingsincommon.First,theydonotoperateuntilthebrakesareappliedwithenoughforcetolockornearlylockawheel.Atallothertimes,thesystemstandsreadytofunctionbutdoesnotinterferewithnormalbraking.Second,iftheantilocksystemfailinanyway,thebrakescontinuetooperatewithoutantilockcapability.Awarninglightontheinstrumentpanelalertsthedriverwhenaproblemexistsintheantilocksystem.ThecurrentBoschcomponentAntilockBrakingSystemABSⅡ,isasecondgenerationdesignwildlyusedbyEuropeanautomakerssuchasBWM,MercedesBenzandPorsche.ABSⅡsystemconsistsoffourwheelspeedsensor,electroniccontrolunitandmodulatorassembly.Aspeedsensorisfittedateachwheelsendssignalsaboutwheelrotationtocontrolunit.Eachspeedsensorconsistsofasensorunitandagearwheel.Thefrontsensormountstothesteeringknuckleanditsgearwheelispressedontothestubaxlethatrotateswiththewheel.Therearsensormountstherearsuspensionmemberanditsgearwheelispressedontotheaxle.Thesensoritselfisawindingwithamagneticcore.Thecorecreatesamagneticfieldaroundthewinding,andastheteethofthegearwheelmovethroughthisfield,analternatingcurrentisinducedinthewinding.Thecontrolunitmonitorstherateochangeinthisfrequencytodetermineimpendingbrakelockup.Thecontrolunitsfunctioncanbedividedintothreepartssignalprocessing,logicandsafetycircuitry.Thesignalprocessingsectionistheconverterthatreceivesthealternatingcurrentsignalsformthespeedsensorsandconvertsthemintodigitalformforthelogicsection.Thelogicsectionthenanalyzesthedigitizedsignalstocalculateanybrakepressurechangesneeded.Ifimpendinglockupissensed,thelogicsectionsendscommandstothemodulatorassembly.ModulatorassemblyThehydraulicmodulatorassemblyregulatespressuretothewheelbrakeswhenitreceivescommandsfromthecontrolutuit.Themodulatorassemblycanmaintainorreducepressureoverthelevelitreceivesfromthemastercylinder,italsocanneverapplythebrakesbyitself.Themodulatorassemblyconsistsofthreehighspeedelectricsolenoidvalves,twofluidreservoirsandaturndeliverypumpequippedwithinletandoutletcheckvalves.Themodulatorelectricalconnectorandcontrollingrelaysareconcealedunderaplasticcoveroftheassembly.Eachfrontwheelisservedbyelectricsolenoidvalvemodulatedindependentlybythecontrolunit.Therearbrakesareservedbyasinglesolenoidvalveandmodulatedtogetherusingtheselectlowprinciple.Duringantibrakingsystemoperation,thecontrolunitcyclesthesolenoidvalvestoeitherholdorreleasepressurethebrakelines.Whenpressureisreleasedfromthebrakelinesduringantibrakingoperation,itisroutedtoafluidreservoir.Thereisonereservoirforthefrontbrakecircuit.Thereservoirsarelowpressureaccumulatorsthatstorefluidunderslightspringpressureuntilthereturndeliverypumpcanreturnthefluidthroughthebrakelinestothemastercylinder.Whilediscussingthedifferencesamongengineers,scientists,andmathematiciansinChapter1,wesawthatthewordengineeringisrelatedtobothingeniousanddevise.Creativedesignliesatthecenterofthemechanicalengineeringprofession,andanengineersultimategoalistoproducenewhardwarethatsolvesoneofsocietystechnicalproblems.Beginningeitherfromablanksheetofpaperorfromexistinghardwarethatisbeingmodified,theproductdevelopmentprocessoftenformsthefocusofanengineersactivities.Inkeepingwiththeirprofessionstitle,manyengineerstrulyareingenious,andtheypossessthevisionandskilltomakesuchlastingcontributionsasthosedescribedinthetoptenlistofSection1.3Formaleducationinengineeringisnotaprerequisitetohavingagoodforaneworimprovedproduct.Yourinterestinstudyingmechanicalengineering,infact,mayhavebeensparkedbyyourownideasforbuildinghardware.Theelementsofmechanicalengineeringthatwehaveexamineduptothispointmachinecomponentsandtools,forcesinstructuresandfluids,materialsandstresses,thermalandenergysystems,andthemotionofmachineryareintendedtohavesetafoundationthatwillenableyoutoapproachmechanicaldesigninamoreeffectiveandsystematicmanner.INthatrespect,approachthetakeninthistextbookisacondensedanalogofthetraditionalengineeringcurriculumApproximation,mathematics,andscienceareappliedtodesignproblemsinordertoincreaseperformanceandreducetrialanderror.ByapplyingtheresourcesofChapter27,youcanselectcertainmachinecomponentsandperformbackoftheenvelopecalculationtoguidedesigndecisions.Suchanalysesarenotmadefortheirsakealonerather,theyenableyoutodesignbetterandfast.
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