教学材料《汽车构造英语》-Unit9

UnitNineBrakeSystemBasically，allcarbrakesarefrictionbrakes.Whenthedriverappliesthebrake，thecontroldeviceforcesbrakeshoes，orpads，againsttherotatingbrakedrumsordisksatwheels.Frictionbetweentheshoesorpadsandthedrumsordisksthenslowsorstopsthewheelssothatthecarisbraked.Brakingactionstartsatthebrakepedal.Whenthepedalispusheddown，brakefluidissentfromthemastercylindertothewheels.Atthewheels，thefluidpushesbrakeshoes，orpads，againstrevolvingdrumsordisks.Thefrictionbetweenthestationaryshoes，orpads，andtherevolvingdrumsordisksslowsandstopsthem.上一页返回下一页UnitNineBrakeSystemThisslowsorstopstherevolvingwheels，which，inturn，slowsorstopsthecar.Fig.9-1showsthebrakelines，ortubes，throughwhichthefluidflows.Therearetwochambersandtwopistonsinthemastercylinder.Onechamberisconnectedtothefront-wheelbrakes.Theotherchamberisconnectedtotherear-wheelbrakes.Thisiscalledadual-brakingsystem.Thereisareasonforsplittingthesystemintotwosections.Ifonesectionshouldfail，theothercanstillwork.Itwillprovidebrakinguntilthefailedsectioncanbefixed.上一页返回下一页UnitNineBrakeSystemDrumBrakesThedrumbrake(showninFig.9-2)hasaniron，steel，oraluminumdrumtowhichthewheelisbolted.Thedrumandwheelrotatetogether.Insidethedrumattachedtothesteeringknuckleoraxlehousingisthebrakeassembly.Atthefrontwheels，thebrakeisattachedtothesteeringknuckle.Attherearwheels，thebrakeisattachedtotheaxlehousing.上一页返回下一页UnitNineBrakeSystemTherearetwobrakeshoesateachwheel.Thebottomsoftheshoesareheldapartbyanadjuster.Thetopsoftheshoesareheldapartbyawheelcylinder.1Theshoesaremadeofmetal.Glued(calledbonded)orrivetedtoeachshoeisafacingoffrictionmaterial.Thefacingiscalledthebrakelining.Thesebrakeliningsaremadeofatoughmaterialsuchasasbestos.Itcanholdupundertherubbingpressureandheatproducedduringbraking.上一页返回下一页UnitNineBrakeSystemThebrakebackingplateisboltedtotheaxlehousingattherearwheels，ortothesteeringknuckleatthefrontwheels.Eachshoeisattachedatonepointtothebrakebackingplate.Theattachmentisloosesothattheshoecanmoveslightly.DiskBrakes

Thediskbrake(showninFig.9-3)hasametaldisk(alsocalledarotor)insteadofadrum.Aflatshoe，ordisk-brakepad，islocatedoneachsideofthedisk.Inoperationthesetwoflatshoesareforcedtightlyagainsttherotatingdisk.上一页返回下一页UnitNineBrakeSystemTheshoessqueezetherotatingdisktostopthecar.Fluidfromthemastercylinderforcesthepistonstomovein，towardthedisk.Thisactionpushesthefrictionpadstightlyagainstthedisk.Thefrictionbetweentheshoesandthediskslowsandstopsit.Thisprovidesthebrakingaction.Pistonsaremadeofeitherplasticormetal.上一页返回下一页UnitNineBrakeSystemTherearethreegeneraltypesofdiskbrakes.Theyarethefloating-calipertype，thefixed-calipertype，andthesliding-calipertype.Floating-caliperandsliding-caliperdiskbrakesuseasinglepiston.Fixed-caliperdiskbrakeshaveeithertwoorfourpistons.上一页返回下一页UnitNineBrakeSystemMasterCylinderThemastercylinderdisplaceshydraulicbrakefluidunderpressuretotherestofbrakesystem.Whenthebrakepedalisdepressed，thepushrodmovestheprimarypistonforwardinthecylinder.Thehydraulicpressurecreatedandtheforceoftheprimarypistonspringmovesthesecondarypistonforward.Whentheforwardmovementofthepistonscausestheirprimarycupstocoverthebypassholes，hydraulicpressurebuildsupandistransmittedtothewheelcylinders.上一页返回下一页UnitNineBrakeSystemWhenthepedalretracts，thepistonsallowthefluidfromthereservoirtofillthechamberifneeded.Specialsensorswithinthemastercylinderareusedtomonitorthelevelofthefluidinthereservoir，andalertthedriverifapressureimbalancedevelops.上一页返回下一页UnitNineBrakeSystemBrakeLines

Brakelinesaresteeltubingwithcopperandleadcoatingstopreventrustandcorrosion.2Asthebrakepedalisdepressed，itmovesthepistonwithinthemastercylinderandforcesthehydraulicbrakefluidthroughoutthebrakesystemandintothewheels(orbrake)cylinders.Thepressureplaceduponthefluidcausesthecylinderpistonstomove，forcingthebrakeshoesorfrictionpadsandbrakedrumsorrotorstoslowthevehicle.上一页返回下一页UnitNineBrakeSystemBrakeFluidBrakefluidisaspecialliquidforuseinhydraulicbrakesystems，whichmustmeetexactingconditions;i.e.，itmustbeimpervioustoheat，freezing，thickening，bubbling.3Differenttypesofbrakefluidareusedindifferentsystems，andshouldnotbemixed.上一页返回下一页UnitNineBrakeSystemOneoftheworstthingsthatcanhappentoyourbrakesiswhenthefluidbecomescontaminated，becausethatmeansthatallofthepistonsealsandhosesmustbereplaced，sobecarefulofwhatyouputinthemasterreservoir!Itshouldbenotedthatbrakefluidishighlycorrosivetopaint，andcareshouldbeusednottogetitonyourcar’sfinish.4上一页返回下一页UnitNineBrakeSystemNotes1.Therearetwobrakeshoesateachwheel.Thebottomsofshoesareheldapartbyanadjuster.Thetopsofshoesareheldapartbyawheelcylinder.每个车轮上都有两个制动蹄片。每个蹄片的底部由调节器将其分离固定，顶部由车轮制动缸将其分离固定。2.Brakelinesaresteeltubingwithcopperandleadcoatingstopreventrustandcorrosion.制动管路是镀铜包铅的钢制管路，以防生锈、腐蚀。上一页返回下一页UnitNineBrakeSystem3.Brakefluidisaspecialliquidforuseinhydraulicbrakesystems，whichmustmeetexactingconditions;i.e.,itmustbeimpervioustoheat，freezing，thickening，bubbling.制动液是用在液压制动系统上的特殊液体，其必须符合严格的条件，也就是必须耐热、耐冻、不增稠和不起泡。4.Itshouldbenotedthatbrakefluidishighlycorrosivetopaint，andcareshouldbeusednottogetitonyourcar’sfinish.应该注意制动液对油漆具有高腐蚀性，要小心，勿将其接触到汽车的光洁表面。上一页返回下一页UnitNineBrakeSystemExtraReadingAntilockBrakeSystemsAnantilockbrakesystem(ABS)isacomputer-controlledbrakesystemthathelpspreventwheellockupduringbraking.Whenawheellocks，itstopsrotatingandthetireskidsorslidesovertheroadsurface.Whenatireisskidding，thefrictionbetweenthetireandtheroadsurfaceiskineticfriction.Whenthewheelisnotskidding，thefractionisstaticfriction.Kineticfrictionisnotasgreatasstaticfriction;therefore，whenskiddingoccurs，acarrequiresagreaterdistancetostopthanifthetirewerenotskidding.1

上一页返回下一页UnitNineBrakeSystemSkiddingoccursmorereadilyonawetorslipperyroadsurfacethanonadrysurface.Thisisbecausethereislessstaticfrictionbetweenthetiresandtheroadsurfacewhentheroadiswetorslipperythanwhentheroadisdry.Staticfrictiontendstokeepthetiresrotatingontheroadsurface；therefore，withlessstaticfriction，thewheelslockeasilywhenthebrakesareapplied.上一页返回下一页UnitNineBrakeSystemInadditiontoagreaterstoppingdistance，thedriverlosesdirectionalcontrolofthevehiclewhenitskids.Thereforetostopacarintheshortestpossibledistanceandinasstraightalineaspossible，thewheelsmustbepreventedfromlocking.2Todothis，thedrivermustpumpthebrakepedalwhenthecarisabouttoskid.Pumpingthebrakepedalmeansalternatelydepressingandreleasingthepedal.Thishelpspreventskidding，becauseifawheelbeginstolock，releasingthepedalallowsthewheeltorollagain.上一页返回下一页UnitNineBrakeSystemInacomputer-controlledbrakesystem，thedriverappliesandholdsthebrakepedal.Then，ifawheelbeginstolock，thecomputerpumpsthebrakes.Itdoesthismuchmorerapidlythanthedrivercould，anditappliesandreleasesthebrakesatpreciselytherighttime.Insomeantilocksystems，thereisasensorlockedateachwheel.Thesensorssensethespeedofrotationofthewheels，andthisspeedinformationissenttothecomputer.上一页返回下一页UnitNineBrakeSystemIfawheelbeginstolockduringbraking，thespeedofrotationofthatwheelbeginstodroprapidly.Thecomputerreactstothisrapiddropinwheelspeedbyremovingthebrakingforcefromthatwheel.Whenthewheelbeginstorollagainanditsspeedbeginstoincrease，thecomputerrestoresthebrakingforcetothewheel.Ifthewheelagainbeginstolock，thecomputerwillagainremovethebrakingforce.Insomeantilocksystemsthecomputercanapplyandreleasethebrakesateachwheelasrapidlyas15timespersecond.上一页返回下一页UnitNineBrakeSystemSomeantilocksystemsuseonlythreewheelsensors.Onesensorisusedateachfrontwheel，andonesensorislocatednearthedriveshafttosenselockupatbothrearwheels.Somelighttruckshaveanantilocksystemthatpreventslockupattherearwheelsonly.Thesearecalledrearantilockbrakesystems(RABS).Asinglespeedsensorinthedifferentialgearassemblysensesrear-wheellockup.上一页返回下一页UnitNineBrakeSystemEachwheel-speedsensorispositionednearatoothedring.Fig9-4showsthesensorandringlocationsforafrontwheelonarear-wheel-drivecar.Theringispartofthehub-and-bearingassembly.Fig9-5showsthesensorandringforafrontwheelonafront-wheel-drivecar，theringispressedontothedriveaxle.Fig9-6showsthesensorandringlocatedforarearwheelonafront-wheel-drivecar.Theringispressedontothehub-and-bearingassembly.上一页返回下一页UnitNineBrakeSystemAsthewheelrotates，theringmovespastthesensor.Magneticlinesofforcepassfromonepoleofthemagnet，throughthetoothedring，throughtheair，andbacktotheotherpoleofthemagnet.Becausethesensorcoilsurroundsthemagnet，thelinesofforcealsopassthroughthesensorcoil.Theringismadeofamagneticmaterial.Amagneticmaterialisamaterial，suchasiron，thatpassesmagneticlinesofforcemuchmorereadilythanairdoes.3上一页返回下一页UnitNineBrakeSystemInthepositionoftheringshowninFig9-4，theairgapbetweentheringandthesensorissmall.Withasmallairgap，thelinesofforceeasilypasstothering，becauseofthis，therearemanylinesofforceandthemagneticfieldisstrong.Inthisposition，theairgapbetweentheringandthesensorislarge.Thelinesofforcedonotreadilypassthroughthelargergap，andthusthemagneticfieldisweak.上一页返回下一页UnitNineBrakeSystemWhenthewheelrotates，theringspinspastthesensor.Aseachtoothoftheringpassesthetoothonthesensor，themagneticfieldisalternatelystrengthenedandweakened.Wheneverachangingmagneticfieldpassesacoilofwire，avoltageisgeneratedinthecoil.Asmallvoltageisgeneratedinthecoileachtimeatoothoftheringpassesthecoil.Thevoltagelastsonlyaveryshorttimeandiscalledavoltage-pulse.OnesystemusedbyFordhas104teeth