外文翻译--制动系统.doc

英文原文BrakesystemsWeallknowthatpushingdownonthebrakepedalslowsacartoastop.Buthowdoesthishappen?Howdoesyourcartransmittheforcefromyourlegtoitswheels?Howdoesitmultiplytheforcesothatitisenoughtostopsomethingasbigasacar?Whenyoudepressyourbrakepedal,yourcartransmitstheforcefromyourfoottoitsbrakesthroughafluid.Sincetheactualbrakesrequireamuchgreaterforcethanyoucouldapplywithyourleg,yourcarmustalsomultiplytheforceofyourfoot.Itdoesthisintwoways:Mechanicaladvantage(leverage)HydraulicforcemultiplicationThebrakestransmittheforcetothetiresusingfriction,andthetirestransmitthatforcetotheroadusingfrictionalso.Beforewebeginourdiscussiononthecomponentsofthebrakesystem,wellcoverthesethreeprinciples:LeverageHydraulicsFrictionLeverageandHydraulicsInthefigurebelow,aforceFisbeingappliedtotheleftendofthelever.Theleftendoftheleveristwiceaslong(2X)astherightend(X).Therefore,ontherightendoftheleveraforceof2Fisavailable,butitactsthroughhalfofthedistance(Y)thattheleftendmoves(2Y).Changingtherelativelengthsoftheleftandrightendsoftheleverchangesthemultipliers.Thebasicideabehindanyhydraulicsystemisverysimple:Forceappliedatonepointistransmittedtoanotherpointusinganincompressiblefluid,almostalwaysanoilofsomesort.Mostbrakesystemsalsomultiplytheforceintheprocess.Hereyoucanseethesimplestpossiblehydraulicsystem:YourbrowserdoesnotsupportJavaScriptoritisdisabled.SimplehydraulicsystemInthefigureabove,twopistons(showninred)arefitintotwoglasscylindersfilledwithoil(showninlightblue)andconnectedtooneanotherwithanoil-filledpipe.Ifyouapplyadownwardforcetoonepiston(theleftone,inthisdrawing),thentheforceistransmittedtothesecondpistonthroughtheoilinthepipe.Sinceoilisincompressible,theefficiencyisverygood-almostalloftheappliedforceappearsatthesecondpiston.Thegreatthingabouthydraulicsystemsisthatthepipeconnectingthetwocylinderscanbeanylengthandshape,allowingittosnakethroughallsortsofthingsseparatingthetwopistons.Thepipecanalsofork,sothatonemastercylindercandrivemorethanoneslavecylinderifdesired,asshowninhere:YourbrowserdoesnotsupportJavaScriptoritisdisabled.MastercylinderwithtwoslavesTheotherneatthingaboutahydraulicsystemisthatitmakesforcemultiplication(ordivision)fairlyeasy.IfyouhavereadHowaBlockandTackleWorksorHowGearRatiosWork,thenyouknowthattradingforcefordistanceisverycommoninmechanicalsystems.Inahydraulicsystem,allyouhavetodoischangethesizeofonepistonandcylinderrelativetotheother,asshownhere:YourbrowserdoesnotsupportJavaScriptoritisdisabled.HydraulicmultiplicationTodeterminethemultiplicationfactorinthefigureabove,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).ASimpleBrakeSystemBeforewegetintoallthepartsofanactualcarbrakesystem,letslookatasimplifiedsystem:YourbrowserdoesnotsupportJavaScriptoritisdisabled.AsimplebrakesystemYoucanseethatthedistancefromthepedaltothepivotisfourtimesthedistancefromthecylindertothepivot,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