机械 车辆工程 外文翻译 外文文献 英文文献 制动器Word格式.docx
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外文原文
TheBrakeBible
Brakes-whatdotheydo?
Thesimpleanswer:
theyslowyoudown.
Thecomplexanswer:
brakesaredesignedtoslowdownyourvehiclebutprobablynotbythemeansthatyouthink.Thecommonmisconceptionisthatbrakessqueezeagainstadrumordisc,andthepressureofthesqueezingactioniswhatslowsyoudown.Thisinfactisonlypartoftheequation.Brakesareessentiallyamechanismtochangeenergytypes.Whenyou'
retravellingatspeed,yourvehiclehaskineticenergy.Whenyouapplythebrakes,thepadsorshoesthatpressagainstthebrakedrumorrotorconvertthatenergyintothermalenergyviafriction.Thecoolingofthebrakesdissipatestheheatandthevehicleslowsdown.It'
stheFirstLawofThermodynamics,sometimesknownasthelawofconservationofenergy.Thisstatesthatenergycannotbecreatednordestroyed,itcanonlybeconvertedfromoneformtoanother.Inthecaseofbrakes,itisconvertedfromkineticenergytothermalenergy.
Angularforce.Becauseoftheconfigurationofthebrakepadsandrotorinadiscbrake,thelocationofthepointofcontactwherethefrictionisgeneratedalsoprovidesamechanicalmomenttoresisttheturningmotionoftherotor.
Thermodynamics,brakefadeanddrilledrotors.
Ifyourideamotorbikeordrivearacecar,you'
reprobablyfamiliarwiththetermbrakefade,usedtodescribewhathappenstobrakeswhentheygettoohot.Agoodexampleiscomingdownamountainpassusingyourbrakesratherthanyourenginetoslowyoudown.Asyoustarttocomedownthepass,thebrakesonyourvehicleheatup,slowingyoudown.Butifyoukeepusingthem,therotorsordrumsstayhotandgetnochancetocooloff.Atsomepointtheycan'
tabsorbanymoreheatsothebrakepadsheatupinstead.Ineverybrakepadthereisthefrictionmaterialthatisheldtogetherwithsomesortofresinandoncethisstartstogettoohot,theresinstartstovapourise,formingagas.Becausethegascan'
tstaybetweenthepadandtherotor,itformsathinlayerbetweenthetwowhilsttryingtoescape.Thepadslosecontactwiththerotor,reducingtheamountoffrictionandvoila.Completebrakefade.
Thetypicalremedyforthiswouldbetogetthevehicletoastopandwaitforafewminutes.Asthebrakecomponentscooldown,theirabilitytoabsorbheatreturnsandthenexttimeyouusethebrakes,theyseemtoworkjustfine.Thistypeofbrakefadewasmorecommoninoldervehicles.Newervehiclestendtohavelessoutgassingfromthebrakepadcompoundsbuttheystillsufferbrakefade.Sowhy?
It'
sstilltodowiththepadsgettingtoohot.Withnewerbrakepadcompounds,thepadstransferheatintothecalipersoncetherotorsaretoohot,andthebrakefluidstartstoboilformingbubblesinit.Becauseairiscompressible(brakefluidisn'
t)whenyousteponthebrakes,theairbubblescompressinsteadofthefluidtransferringthemotiontothebrakecalipers.Voila.Modernbrakefade.
Sohowdotheengineersdesignbrakestoreduceoreliminatebrakefade?
Foroldervehicles,yougivethatvapourisedgassomewheretogo.Fornewervehicles,youfindsomewaytocooltherotorsoffmoreeffectively.Eitherwayyouendupwithcross-drilledorgroovedbrakerotors.Whilegroovingthesurfacemayreducethespecificheatcapacityoftherotor,itseffectisnegligibleinthegrandschemeofthings.However,underheavybrakingonceeverythingishotandtheresinisvapourising,thegroovesgivethegassomewheretogo,sothepadcancontinuetocontacttherotor,allowingyoutostop.
Thewholeunderstandingoftheconversionofenergyiscriticalinunderstandinghowandwhybrakesdowhattheydo,andwhytheyaredesignedthewaytheyare.Ifyou'
veeverwatchedFormula1racing,you'
llseethefrontwheelshavehugescoopsinsidethewheelpointingtothefront(seethepictureabove).ThisistoductairtothebrakecomponentstohelpthemcooloffbecauseinF1racing,thebrakesareusedviciouslyeveryfewsecondsandspendalotoftheirtimetryingtostayhot.Withoutsomeformofcoolingassistance,thebrakeswouldbefineforthefirstfewcornersbutthenwouldfadeandbecomenearuselessbyhalfwayaroundthetrack.
Rotortechnology.
Ifabrakerotorwasasinglecastchunkofsteel,itwouldhaveterribleheatdissipationpropertiesandleavenowhereforthevapourisedgastogo.Becauseofthis,brakerotorsaretypicallymodifiedwithallmannerofextradesignfeaturestohelpthemcooldownasquicklyaspossibleaswellasdissapateanygasfrombetweenthepadsandrotors.Thediagramhereshowssomeexamplesofrotortypeswiththevariousmodificationthatcanbedonetothemtohelpthemcreatemorefriction,dispersemoreheatmorequickly,andventilategas.Fromlefttoright