完整版汽车制动系统英文文献及翻译.docx

完整版汽车制动系统英文文献及翻译.docx

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完整版汽车制动系统英文文献及翻译

Brakesystems

Weallknowthatpushingdownonthebrakepedalslowsacartoastop.Buthowdoesthishappen?

Howdoesyourcartransmittheforcefromyourlegtoitswheels?

Howdoesitmultiplytheforcesothatitisenoughtostopsomethingasbigasacar?

BrakeImageGallery

Layoutoftypicalbrakesystem. Seemorebrakeimages.

Whenyoudepressyourbrakepedal,yourcartransmitstheforcefromyourfoottoitsbrakesthroughafluid.Sincetheactualbrakesrequireamuchgreaterforcethanyoucouldapplywithyourleg,yourcarmustalsomultiplytheforceofyourfoot.Itdoesthisintwoways:

∙Mechanicaladvantage（leverage）

∙Hydraulicforcemultiplication

Thebrakestransmittheforcetothetiresusingfriction,andthetirestransmitthatforcetotheroadusingfrictionalso.Beforewebeginourdiscussiononthecomponentsofthebrakesystem,we'llcoverthesethreeprinciples:

∙Leverage

∙Hydraulics

∙Friction

LeverageandHydraulics

Inthefigurebelow,aforceFisbeingappliedtotheleftendofthelever.Theleftendoftheleveristwiceaslong（2X）astherightend（X）.Therefore,ontherightendoftheleveraforceof2Fisavailable,butitactsthroughhalfofthedistance（Y）thattheleftendmoves（2Y）.Changingtherelativelengthsoftheleftandrightendsoftheleverchangesthemultipliers.

Thepedalisdesignedinsuchawaythatitcanmultiplytheforcefromyourlegseveraltimesbeforeanyforceiseventransmittedtothebrakefluid.

Thebasicideabehindanyhydraulicsystemisverysimple:

Forceappliedatonepointistransmittedtoanotherpointusinganincompressiblefluid,almostalwaysanoilofsomesort.Mostbrakesystemsalsomultiplytheforceintheprocess.Hereyoucanseethesimplestpossiblehydraulicsystem:

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Simplehydraulicsystem

Inthefigureabove,twopistons（showninred）arefitintotwoglasscylindersfilledwithoil（showninlightblue）andconnectedtooneanotherwithanoil-filledpipe.Ifyouapplyadownwardforcetoonepiston（theleftone,inthisdrawing）,thentheforceistransmittedtothesecondpistonthroughtheoilinthepipe.Sinceoilisincompressible,theefficiencyisverygood--almostalloftheappliedforceappearsatthesecondpiston.Thegreatthingabouthydraulicsystemsisthatthepipeconnectingthetwocylinderscanbeanylengthandshape,allowingittosnakethroughallsortsofthingsseparatingthetwopistons.Thepipecanalsofork,sothatonemastercylindercandrivemorethanoneslavecylinderifdesired,asshowninhere:

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Mastercylinderwithtwoslaves

Theotherneatthingaboutahydraulicsystemisthatitmakesforcemultiplication（ordivision）fairlyeasy.IfyouhavereadHowaBlockandTackleWorksorHowGearRatiosWork,thenyouknowthattradingforcefordistanceisverycommoninmechanicalsystems.Inahydraulicsystem,allyouhavetodoischangethesizeofonepistonandcylinderrelativetotheother,asshownhere:

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Hydraulicmultiplication

Todeterminethemultiplicationfactorinthefigureabove,startbylookingatthesizeofthepistons.Assumethatthepistonontheleftis2inches（5.08cm）indiameter（1-inch/2.54cmradius）,whilethepistonontherightis6inches（15.24cm）indiameter（3-inch/7.62cmradius）.TheareaofthetwopistonsisPi*r2.Theareaoftheleftpistonistherefore3.14,whiletheareaofthepistonontherightis28.26.Thepistonontherightisninetimeslargerthanthepistonontheleft.Thismeansthatanyforceappliedtotheleft-handpistonwillcomeoutninetimesgreaterontheright-handpiston.So,ifyouapplya100-pounddownwardforcetotheleftpiston,a900-poundupwardforcewillappearontheright.Theonlycatchisthatyouwillhavetodepresstheleftpiston9inches（22.86cm）toraisetherightpiston1inch（2.54cm）.

ASimpleBrakeSystem

Beforewegetintoallthepartsofanactualcarbrakesystem,let'slookatasimplifiedsystem:

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Asimplebrakesystem

Youcanseethatthedistancefromthepedaltothepivotisfourtimesthedistancefromthecylindertothepivot,sotheforceatthepedalwillbeincreasedbyafactoroffourbeforeitistransmittedtothecylinder.

Youcanalsoseethatthediameterofthebrakecylinderisthreetimesthediameterofthepedalcylinder.Thisfurthermultipliestheforcebynine.Alltogether,thissystemincreasestheforceofyourfootbyafactorof36.Ifyouput10poundsofforceonthepedal,360pounds（162kg）willbegeneratedatthewheelsqueezingthebrakepads.

Thereareacoupleofproblemswiththissimplesystem.Whatifwehavealeak?

Ifitisaslowleak,eventuallytherewillnotbeenoughfluidlefttofillthebrakecylinder,andthebrakeswillnotfunction.Ifitisamajorleak,thenthefirsttimeyouapplythebrakesallofthefluidwillsquirtouttheleakandyouwillhavecompletebrakefailure.

Drumbrakesworkonthesameprincipleasdiscbrakes:

Shoespressagainstaspinningsurface.Inthissystem,thatsurfaceiscalledadrum.

Figure1.Locationofdrumbrakes. Seemoredrumbrakepictures.

Manycarshavedrumbrakesontherearwheelsanddiscbrakesonthefront.Drumbrakeshavemorepartsthandiscbrakesandarehardertoservice,buttheyarelessexpensivetomanufacture,andtheyeasilyincorporateanemergencybrakemechanism.

InthiseditionofHowStuffWorks,wewilllearnexactlyhowadrumbrakesystemworks,examinetheemergencybrakesetupandfindoutwhatkindofservicingdrumbrakesneed.

Figure2.Drumbrakewithdruminplace

Figure3.Drumbrakewithoutdruminplace

Let'sstartwiththebasics.

TheDrumBrake

Thedrumbrakemaylookcomplicated,anditcanbeprettyintimidatingwhenyouopenoneup.Let'sbreakitdownandexplainwhateachpiecedoes.

Figure4.Partsofadrumbrake

Likethediscbrake,thedrumbrakehastwobrakeshoesandapiston.Butthedrumbrakealsohasanadjustermechanism,anemergencybrakemechanismandlotsofsprings.

First,thebasics:

Figure5showsonlythepartsthatprovidestoppingpower.

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Figure5.Drumbrakeinoperation

Whenyouhitthebrakepedal,thepistonpushesthebrakeshoesagainstthedrum.That'sprettystraightforward,butwhydoweneedallofthosesprings?

Thisiswhereitgetsalittlemorecomplicated.Manydrumbrakesareself-actuating.Figure5showsthatasthebrakeshoescontactthedrum,thereisakindofwedgingaction,whichhastheeffectofpressingtheshoesintothedrumwithmoreforce.

Theextrabrakingforceprovidedbythewedgingactionallowsdrumbrakestouseasmallerpistonthandiscbrakes.But,becauseofthewedgingaction,theshoesmustbepulledawayfromthedrumwhenthebrakesarereleased.Thisisthereasonforsomeofthesprings.Otherspringshelpholdthebrakeshoesinplaceandreturntheadjusterarmafteritactuates.

BrakeAdjuster

Forthedrumbrakestofunctioncorrectly,thebrakeshoesmustremainclosetothedrumwithouttouchingit.Iftheygettoofarawayfromthedrum（astheshoesweardown,forinstance）,thepistonwillrequiremorefluidtotravelthatdistance,andyourbrakepedalwillsinkclosertothefloorwhenyouapplythebrakes.Thisiswhymostdrumbrakeshaveanautomaticadjuster.

Figure6.Adjustermechanism

Nowlet'saddinthepartsoftheadjustermechanism.Theadjusterusestheself-actuationprinciplewediscussedabove.

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Figure7.Drumbrakeadjusterinoperation

InFigure7,youcanseethatasthepadwearsdown,morespacewillformbetweentheshoeandthedrum.Eachtimethecarstopswhileinreverse,theshoeispulledtightagainstthedrum.Whenthegapgetsbigenough,theadjustingleverrocksenoughtoadvancetheadjustergearbyonetooth.Theadjusterhasthreadsonit,likeabolt,sothatitunscrewsalittlebitwhenitturns,lengtheningtofillinthegap.Whenthebrakeshoeswearalittlemore,theadjustercanadvanceagain,soitalwayskeepstheshoesclosetothedrum.

Somecarshaveanadjusterthatisactuatedwhentheemergencybrakeisapplied.Thistypeofadjustercancomeoutofadjustmentiftheemergencybrakeisnotusedforlongperiodsoftime.Soifyouhavethistypeofadjuster,youshouldapplyyouremergencybrakeatleastonceaweek.

Servicing

Themostcommonservicerequiredfordrumbrakesischangingthebrakeshoes.Somedrumbrakesprovideaninspectionholeonthebackside,whereyoucanseehowmuchmaterialisleftontheshoe.Brakeshoesshouldbereplacedwhenthefrictionmaterialhasworndowntowithin1/32inch（0.8mm）oftherivets.Ifthefrictionmaterialisbondedtothebackingplate（norivets）,thentheshoesshouldbereplacedwhentheyhaveonly1/16inch（1.6mm）ofmaterialleft.

PhotocourtesyofalocalAutoZonestore

Figure9.Brakeshoe

Justasindiscbrakes,deepscoressometimesgetwornintobrakedrums.Ifaworn-outbrakeshoeisusedfortoolong,therivetsthatholdthefrictionmaterialtothebackingcanweargroovesintothedrum.Abadlyscoreddrumcansometimesberepairedbyrefinishing.Wherediscbrakeshaveaminimumallowablethickness,drumbrakeshaveamaximumallowablediameter.Sincethecontactsurfaceistheinsideofthedrum,asyouremovematerialfromthedrumbrakethediametergetsbigger.

Figure