汽车制动系统-英文文献及翻译

BrakesystemsWeallknowthatpushingdownonthebrakepedalslowsacartoastop.Buthowdoesthishappen?Howdoesyourcartransmittheforcefromyourlegtoitswheels?Howdoesitmultiplytheforcesothatitisenoughtostopsomethingasbigasacar?BrakeImageGallery

Layoutoftypicalbrakesystem.

Seemorebrakeimages.Whenyoudepressyourbrakepedal,yourcartransmitstheforcefromyourfoottoitsbrakesthroughafluid.Sincetheactualbrakesrequireamuchgreaterforcethanyoucouldapplywithyourleg,yourcarmustalsomultiplytheforceofyourfoot.Itdoesthisintwoways:Mechanicaladvantage(leverage)HydraulicforcemultiplicationThebrakestransmittheforcetothetiresusingfriction,andthetirestransmitthatforcetotheroadusingfrictionalso.Beforewebeginourdiscussiononthecomponentsofthebrakesystem,we'llcoverthesethreeprinciples:LeverageHydraulicsFrictionLeverageandHydraulicsInthefigurebelow,aforceFisbeingappliedtotheleftendofthelever.Theleftendoftheleveristwiceaslong(2X)astherightend(X).Therefore,ontherightendoftheleveraforceof2Fisavailable,butitactsthroughhalfofthedistance(Y)thattheleftendmoves(2Y).Changingtherelativelengthsoftheleftandrightendsoftheleverchangesthemultipliers.

Thepedalisdesignedinsuchawaythatitcanmultiplytheforcefromyourlegseveraltimesbeforeanyforceiseventransmittedtothebrakefluid.Thebasicideabehindanyhydraulicsystemisverysimple:Forceappliedatonepointistransmittedtoanotherpointusinganincompressiblefluid,almostalwaysanoilofsomesort.Mostbrakesystemsalsomultiplytheforceintheprocess.Hereyoucanseethesimplestpossiblehydraulicsystem:YourbrowserdoesnotsupportJavaScriptoritisdisabled.

SimplehydraulicsystemInthefigureabove,twopistons(showninred)arefitintotwoglasscylindersfilledwithoil(showninlightblue)andconnectedtooneanotherwithanoil-filledpipe.Ifyouapplyadownwardforcetoonepiston(theleftone,inthisdrawing),thentheforceistransmittedtothesecondpistonthroughtheoilinthepipe.Sinceoilisincompressible,theefficiencyisverygood--almostalloftheappliedforceappearsatthesecondpiston.Thegreatthingabouthydraulicsystemsisthatthepipeconnectingthetwocylinderscanbeanylengthandshape,allowingittosnakethroughallsortsofthingsseparatingthetwo

Figure1.Locationofdrumbrakes.

Seemoredrumbrakepictures.Manycarshavedrumbrakesontherearwheelsanddiscbrakesonthefront.Drumbrakeshavemorepartsthandiscbrakesandarehardertoservice,buttheyarelessexpensivetomanufacture,andtheyeasilyincorporateanemergencybrakemechanism.InthiseditionofHowStuffWorks,wewilllearnexactlyhowadrumbrakesystemworks,examinetheemergencybrakesetupandfindoutwhatkindofservicingdrumbrakesneed.

Figure2.Drumbrakewithdruminplace

Figure3.DrumbrakewithoutdruminplaceLet'sstartwiththebasics.TheDrumBrakeThedrumbrakemaylookcomplicated,anditcanbeprettyintimidatingwhenyouopenoneup.Let'sbreakitdownandexplainwhateachpiecedoes.Figure4.PartsofadrumbrakeLikethediscbrake,thedrumbrakehastwobrakeshoesandapiston.Butthedrumbrakealsohasanadjustermechanism,anemergencybrakemechanismandlotsofsprings.First,thebasics:Figure5showsonlythepartsthatprovidestoppingpower.YourbrowserdoesnotsupportJavaScriptoritisdisabled.

Figure5.DrumbrakeinoperationWhenyouhitthebrakepedal,thepistonpushesthebrakeshoesagainstthedrum.That'sprettystraightforward,butwhydoweneedallofthosesprings?Thisiswhereitgetsalittlemorecomplicated.Manydrumbrakesareself-actuating.Figure5showsthatasthebrakeshoescontactthedrum,thereisakindofwedgingaction,whichhastheeffectofpressingtheshoesintothedrumwithmoreforce.Theextrabrakingforceprovidedbythewedgingactionallowsdrumbrakestouseasmallerpistonthandiscbrakes.But,becauseofthewedgingaction,theshoesmustbepulledawayfromthedrumwhenthebrakesarereleased.Thisisthereasonforsomeofthesprings.Otherspringshelpholdthebrakeshoesinplaceandreturntheadjusterarmafteritactuates.BrakeAdjusterForthedrumbrakestofunctioncorrectly,thebrakeshoesmustremainclosetothedrumwithouttouchingit.Iftheygettoofarawayfromthedrum(astheshoesweardown,forinstance),thepistonwillrequiremorefluidtotravelthatdistance,andyourbrakepedalwillsinkclosertothefloorwhenyouapplythebrakes.Thisiswhymostdrumbrakeshaveanautomaticadjuster.

Figure6.AdjustermechanismNowlet'saddinthepartsoftheadjustermechanism.Theadjusterusestheself-actuationprinciplewediscussedabove.YourbrowserdoesnotsupportJavaScriptoritisdisabled.

Figure7.DrumbrakeadjusterinoperationInFigure7,youcanseethatasthepadwearsdown,morespacewillformbetweentheshoeandthedrum.Eachtimethecarstopswhileinreverse,theshoeispulledtightagainstthedrum.Whenthegapgetsbigenough,theadjustingleverrocksenoughtoadvancetheadjustergearbyonetooth.Theadjusterhasthreadsonit,likeabolt,sothatitunscrewsalittlebitwhenitturns,lengtheningtofillinthegap.Whenthebrakeshoeswearalittlemore,theadjustercanadvanceagain,soitalwayskeepstheshoesclosetothedrum.Somecarshaveanadjusterthatisactuatedwhentheemergencybrakeisapplied.Thistypeofadjustercancomeoutofadjustmentiftheemergencybrakeisnotusedforlongperiodsoftime.Soifyouhavethistypeofadjuster,youshouldapplyyouremergencybrakeatleastonceaweek.ServicingThemostcommonservicerequiredfordrumbrakesischangingthebrakeshoes.Somedrumbrakesprovideaninspectionholeonthebackside,whereyoucanseehowmuchmaterialisleftontheshoe.Brakeshoesshouldbereplacedwhenthefrictionmaterialhasworndowntowithin1/32inch(0.8mm)oftherivets.Ifthefrictionmaterialisbondedtothebackingplate(norivets),thentheshoesshouldbereplacedwhentheyhaveonly1/16inch(1.6mm)ofmaterialleft.

PhotocourtesyofalocalAutoZonestore

Figure9.BrakeshoeJustasindiscbrakes,deepscoressometimesgetwornintobrakedrums.Ifaworn-outbrakeshoeisusedfortoolong,therivetsthatholdthefrictionmaterialtothebackingcanweargroovesintothedrum.Abadlyscoreddrumcansometimesberepairedbyrefinishing.Wherediscbrakeshaveaminimumallowablethickness,drumbrakeshaveamaximumallowablediameter.Sincethecontactsurfaceistheinsideofthedrum,asyouremovematerialfromthedrumbrakethediametergetsbigger.Figure10.Brakedrum制动系统众所周知，踩下制动踏板可以使汽车减速至停止。但这是如何产生的呢？汽车是如何将力从你的腿传递到车轮的呢？汽车是如何将力放大到足够大以致可以将像汽车一样大的东西制动的呢？制动系统组件当你踩下制动踏板的时候，汽车通过液体把力从脚传递到制动器。因为制动器需要的真正力量比你的腿能提供的要大的多，所以汽车必须放大脚产生的力有两种方式：机械杠杆作用液力放大制动器通过摩擦把力传递给轮胎，并且轮胎也是通过摩擦把力传递给路面的。在我们讨论制动系统的组成之前，先来介绍以下三条原则：杠杆液力摩擦力杠杆和液力在下面的图中，一个力F加在杠杆的左端。左端的杠杆长度（2X）是右端（X）的两倍。因此杠杆右端可施加的力为2F，但是右端移动的距离(Y)是左端距离(2Y)的一半。改变杠杆的左端和右端的长度可以改变放大系数。任何液压系统背后的基本原理都是非常简单的：作用在某一点力通过通常是油一类的不可压缩的液体传递到另一点。大多数的制动系统也在这个过程中放大力。下面的是最简单的液压系统：简单液压系统在上图中，两个活塞放在两个充满油的玻璃液压缸中并且由充满油的管道相连。如果在一个活塞上施加一个向下的力，那么力将通过管道中的油传递到第二个活塞。因为油液是不可压缩的，所以传递效率很好，大部分的作用力都传递到了另一个活塞。液压系统的好处连接两液压缸的管道可以是任何长度和形状，这样就可以使管道弯曲的通过两活塞之间的各种部件。管道也可以是分叉的，如果有需要的话，这样一个主缸可以驱动数个副缸。如下图所示：带有两个副缸的主缸液压系统的另一个好处是产生放大（或者缩小）力相当地容易。如果你一读过滑车设备工作原理或者齿轮齿数比原理，那么你就会知道在机械系统中把力转化为距离处理是很常见的。在液压系统中，我们所要做的就是相对地改变一组活塞和液压缸的尺寸。如下图所示：液压增力原理为了确定上图中的放大因子，先由观察活塞的尺寸开始。假设左边活塞的直径为2英尺（5.08cm而右边的直径为6英尺（15.24cm）。两个活塞的面积是Pi*r2。因此左面活塞的面积是3.14，而右面的面积是28.26。右面活塞的面积是左边的九倍大。这就意味着无论在左面的活塞上施加多大的力，在右面的活塞上就会输出九倍于左面的力。所以，如果在左边活塞上施加100磅向下的力，那么在右面活塞上将产生900磅向上的力。唯一的补偿是左面的活塞要移动9英尺（22.86cm）来使右面提升1英尺（2.54cm）一个简单的制动系统在我们深入了解一个真实的制动系统的各部分之前，让我们先来看一个简化的系统：我们可以看到踏板到枢轴的距离是液压缸到枢轴距离的4倍，所以施加在踏板上的力在传递到液压缸之前将被增加4倍。我们还可以看到制动缸的直径是踏板缸直径的3倍。这就将力进一步放大了九倍。最终这个系统将腿上的力增加了36倍。所以，如果在踏板上施加10磅的力，将在挤压制动带的轮上产生369磅（162kg）的力。下面是这种简单系统所存在的问题。要是系统有泄漏该怎么办呢？如果是轻微泄漏，最终将会没有足够的油使制动缸充满，并且制动器将停止工作。如果是严重泄漏，那么在你制动的第一时间，所有的油液将从泄露处喷射而出，并且制动系统将彻底地不起作用。鼓式制动器的工作原理和盘式制动器是一样的：制动面接触一个磨砂的表面。在这个系统中，那个表面称作制动鼓图1.制动鼓的位置许多汽车的后轮安装鼓式制动器，而盘式制动器安装在前面。鼓式制动器比盘式制动器有更多的零件并且更难检修。但是制造成本相对便宜，还有鼓式制动器容易组装一个紧急使用的制动装置。在本版本的HowStuffWorks中，我们将详尽了解鼓式制动系统是如何工作的。考察紧急制动系统的组成，并且找到鼓式制动器需要何种检修工作。图2.有鼓的鼓式制动器图3.未安装鼓的鼓式制动器让我们基础开始：鼓式制动器鼓式制动器可能看起来比较复杂，它可以是很复杂的，当你打开一个的时候。让我们拆开它，并解释每一块的作用。图4.鼓式制动器的组成如盘式制动器，鼓式制动器有两个制动蹄和一个活塞。但是鼓式制动器也有一个调节机制，紧急刹车机制和大量的弹簧。首先，基础知识：图5显示只有部分提供的制动力。图5.工作状态下的鼓式制动器当你踩下刹车踏板时，活塞推动紧靠着鼓的制动蹄。这是很简单的，但为什