外文翻译制动系统设计.docx
《外文翻译制动系统设计.docx》由会员分享,可在线阅读,更多相关《外文翻译制动系统设计.docx(19页珍藏版)》请在冰豆网上搜索。
外文翻译制动系统设计
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:
YourbrowserdoesnotsupportJavaScriptoritisdisabled.
Simplehydraulicsystem
Inthefigureabove,twopistons(showninred)arefitintotwoglasscylindersfilledwithoil(showninlightblue)andconnectedtooneanotherwithanoil-filledpipe.Ifyouapplyadownwardforcetoonepiston(theleftone,inthisdrawing),thentheforceistransmittedtothesecondpistonthroughtheoilinthepipe.Sinceoilisincompressible,theefficiencyisverygood--almostalloftheappliedforceappearsatthesecondpiston.Thegreatthingabouthydraulicsystemsisthatthepipeconnectingthetwocylinderscanbeanylengthandshape,allowingittosnakethroughallsortsofthingsseparatingthetwopistons.Thepipecanalsofork,sothatonemastercylindercandrivemorethanoneslavecylinderifdesired,asshowninhere:
YourbrowserdoesnotsupportJavaScriptoritisdisabled.
Mastercylinderwithtwoslaves
Theotherneatthingaboutahydraulicsystemisthatitmakesforcemultiplication(ordivision)fairlyeasy.IfyouhavereadHowaBlockandTackleWorksorHowGearRatiosWork,thenyouknowthattradingforcefordistanceisverycommoninmechanicalsystems.Inahydraulicsystem,allyouhavetodoischangethesizeofonepistonandcylinderrelativetotheother,asshownhere:
YourbrowserdoesnotsupportJavaScriptoritisdisabled.
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:
YourbrowserdoesnotsupportJavaScriptoritisdisabled.
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.
YourbrowserdoesnotsupportJavaScriptoritisdisabled.
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.
YourbrowserdoesnotsupportJavaScriptoritisdisabled.
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.
Figure10.