机械 车辆工程 外文翻译 外文文献 英文文献 制动器.docx
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机械 车辆工程 外文翻译 外文文献 英文文献 制动器.docx
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机械车辆工程外文翻译外文文献英文文献制动器
外文出处:
TheBrakeBible
附件2:
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.
1:
Basicbrakerotor.2:
Groovedrotor-thegroovesgivemorebiteandthusmorefrictionastheypassbetweenthebrakepadsTheyalsoallowgastoventfrombetweenthepadsandtherotor.3:
Grooved,drilledrotor-thedrilledholesagaingivemorebite,butalsoallowaircurrents(eddies)toblowthroughthebrakedisctoassistcoolingandventilatinggas.4:
Dualventilatedrotors-sameasbeforebutnowwithtworotorsinsteadofone,andwithvanesinbetweenthemtogenerateavortexwhichwillcooltherotorsevenfurtherwhilsttryingtoactually'suck'anygasawayfromthepads.
Animportantnoteaboutdrilledrotors:
Drilledrotorsaretypicallyonlyfound(andtobeusedon)racecars.Thedrillingweakenstherotorsandtypicallyresultsinmicrofracturestotherotor.Onracecarsthisisn'taproblem-thebrakesarechangedaftereachraceorweekend.Butonaroadcar,thiscaneventuallyleadtobrakerotorfailure-notwhatyouwant.Ionlymentionthisbecauseofalotofperformancesupplierswillsupplyyouwithdrilledrotorsforstreetcarswithoutmentioningthislittlefact.
Bigrotors.
Howdoesallthisapplytobiggerbrakerotors-acommonsportscarupgrade?
Sportscarsandracebikestypicallyhavemuchbiggerdiscsorrotorsthanyouraveragefamilycar.Abiggerrotorhasmorematerialinitsoitcanabsorbmoreheat.Morematerialalsomeansalargersurfaceareaforthepadstogeneratefrictionwith,andbetterheatdissipation.Largerrotorsalsoputthepointofcontactwiththepadsfurtherawayfromtheaxleofrotation.Thisprovidesalargermechanicaladvantagetoresisttheturningoftherotoritself.Tobestillustratehowthisworks,imagineaspinningsteeldisconanaxleinfrontofyou.Ifyouclampedyourthumbseithersideofthediscclosetothemiddle,yourthumbswouldheatupveryquicklyandyou'dneedtopushprettyhardtogeneratethefrictionrequiredtoslowthediscdown.Nowimaginedoingthesamethingbutclampingyourthumbstogetherclosetotheouterrimofthedisc.Thediscwillstopspinningmuchmorequicklyandyourthumbswon'tgetashot.That,inanutshellexplainsthewholeprinciplebehindwhybiggerrotors=betterstoppingpower.
Thedifferenttypesofbrake.
Allbrakesworkbyfriction.Frictioncausesheatwhichispartofthekineticenergyconversionprocess.Howtheycreatefrictionisdowntothevariousdesigns.
Bicyclewheelbrakes
IthoughtI'dcoverthesebecausethey'reaboutthemostbasictypeoffunctioningbrakethatyoucansee,watchworking,andunderstand.Theconstructionisverysimpleandout-in-the-open.Apairofrubberblocksareattachedtoapairofcaliperswhicharepivotedontheframe.Whenyoupullthebrakecable,thepadsarepressedagainstthesideorinneredgeofthebicyclewheelrim.Therubbercreatesfriction,whichcreatesheat,whichisthetransferofkineticenergythatslowsyoudown.There'sonlyreallytwotypesofbicyclebrake-thoseonwhicheachbrakeshoesharesthesamepivotpoint,andthosewithtwopivotpoints.Ifyoucanlookatabicyclebrakeandnotunderstandwhat'sgoingon,therestofthispageisgoingtocauseyouabitofaheadache.
Drumbrakes-singleleadingedge
Thenext,morecomplicatedtypeofbrakeisadrumbrake.Theconcepthereissimple.Twosemicircularbrakeshoessitinsideaspinningdrumwhichisattachedtothewheel.Whenyouapplythebrakes,theshoesareexpandedoutwardstopressagainsttheinsideofthedrum.Thiscreatesfriction,whichcreatesheat,whichtransferskineticenergy,whichslowsyoudown.Theexamplebelowshowsasimplemodel.Theactuatorinthiscaseistheblueellipticalobject.Asthatistwisted,itforcesagainstthebrakeshoesandinturnforcesthemtoexpandoutwards.Thereturnspringiswhatpullstheshoesbackawayfromthesurfaceofthebrakedrumwhenthebrakesarereleased.Seethelatersectionformoreinformationonactuatortypes.
The"singleleadingedge"referstothenumberofpartsofthebrakeshoewhichactuallycontactthespinningdrum.Becausethebrakeshoepivotsatoneend,simplegeometrymeansthattheentirebrakepadcannotcontactthebrakedrum.Theleadingedgeisthetermgiventothepartofthebrakepadwhichdoescontactthedrum,andinthecaseofasingleleadingedgesystem,it'sthepartofthepadclosesttotheactuator.Thisdiagram(right)showswhathappensasthebrakesareapplied.Theshoesarepressedoutwardsandthepartofthebrakepadwhichfirstcontactsthedrumistheleadingedge.Theactionofthedrumspinningactuallyhelpstodrawthebrakepadoutwardsbecauseoffriction,whichcausesthebrakesto"bite".Thetrailingedgeofthebrakeshoemakesvirtuallynocontactwiththedrumatall.Thissimplegeometryexplainswhyit'sreallydifficulttostopavehiclerollingbackwardsifit'sequipped
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