固体火箭发动机原理两相流动力学跟踪研究正文大学论文.docx
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固体火箭发动机原理两相流动力学跟踪研究正文大学论文.docx
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固体火箭发动机原理两相流动力学跟踪研究正文大学论文
Drumbrake
Adrumbrakewiththedrumremovedasusedontherearwheelofacarortruck.Notethatinthisinstallation,acable-operatedparkingbrakeusestheserviceshoes.
Adrumbrakeisabrakeinwhichthefrictioniscausedbyasetofshoesorpadsthatpressagainstarotatingdrum-shapedpartcalledabrakedrum.
Theterm"drumbrake"usuallymeansabrakeinwhichshoespressontheinnersurfaceofthedrum.Whenshoespressontheoutsideofthedrum,itisusuallycalledaclaspbrake.Wherethedrumispinchedbetweentwoshoes,similartoaconventionaldiskbrake,itissometimescalleda"pinchdrumbrake",althoughsuchbrakesarerelativelyrare.Arelatedtypeofbrakeusesaflexiblebeltor"band"wrappingaroundtheoutsideofadrum,calledabandbrake.
History
Adrumbrakeattherearwheelofamotorbike
KawasakiW800
Themodernautomobiledrumbrakewasinventedin1902byLouisRenault,thoughaless-sophisticateddrumbrakehadbeenusedbyMaybachayearearlier.Inthefirstdrumbrakes,theshoesweremechanicallyoperatedwithleversandrodsorcables.Fromthemid-1930stheshoeswereoperatedwithoilpressureinasmallwheelcylinderandpistons(asinthepicture),thoughsomevehiclescontinuedwithpurely-mechanicalsystemsfordecades.Somedesignshavetwowheelcylinders.
Theshoesindrumbrakesaresubjecttowearandthebrakesneededtobeadjustedregularlyuntiltheintroductionofself-adjustingdrumbrakesinthe1950s.Inthe1960sand1970sbrakedrumsonthefrontwheelsofcarsweregraduallyreplacedwithdiscbrakesandnowpracticallyallcarsusediscbrakesonthefrontwheels,withmanyofferingdiscbrakesonallwheels.However,drumbrakesarestilloftenusedforhandbrakesasithasprovenverydifficulttodesignadiscbrakesuitableforholdingacarwhenitisnotinuse.Moreover,itisveryeasytofitadrumhandbrakeinsideadiscbrakesothatoneunitservesasbothservicebrakeandhandbrake.
Earlytypebrakeshoescontainedasbestos.Whenworkingonbrakesystemsofoldercars,caremustbetakennottoinhaleanydustpresentinthebrakeassembly.TheUnitedStatesFederalGovernmentbegantoregulateasbestosproduction,andbrakemanufacturershadtoswitchtonon-asbestoslinings.Ownersinitiallycomplainedofpoorbrakingwiththereplacements;however,technologyeventuallyadvancedtocompensate.Amajorityofdaily-drivenoldervehicleshavebeenfittedwithasbestos-freelinings.Manyothercountriesalsolimittheuseofasbestosinbrakes.Earlyautomotivebrakesystems,aftertheeraofhandleversofcourse,usedadrumdesignatallfourwheels.Theywerecalleddrumbrakesbecausethecomponentswerehousedinarounddrumthatrotatedalongwiththewheel.Insidewasasetofshoesthat,whenthebrakepedalwaspressed,wouldforcetheshoesagainstthedrumandslowthewheel.Fluidwasusedtotransferthemovementofthebrakepedalintothemovementofthebrakeshoes,whiletheshoesthemselvesweremadeofaheat-resistantfrictionmaterialsimilartothatusedonclutchplates.Thisbasicdesignprovedcapableundermostcircumstances,butithadonemajorflaw.Underhighbrakingconditions,likedescendingasteephillwithaheavyloadorrepeatedhigh-speedslowdowns,drumbrakeswouldoftenfadeandloseeffectiveness.Usuallythisfadingwastheresultoftoomuchheatbuild-upwithinthedrum.Rememberthattheprincipleofbrakinginvolvesturningkineticenergy(wheelmovement)intothermalenergy(heat).Forthisreason,drumbrakescanonlyoperateaslongastheycanabsorbtheheatgeneratedbyslowingavehicle'swheels.Oncethebrakecomponentsthemselvesbecomesaturatedwithheat,theylosetheabilitytohaltavehicle,whichcanbesomewhatdisconcertingtothevehicle'soperator.
Self-applyingcharacteristic
Drumbrakeshaveanatural"self-applying"characteristic.[1]Therotationofthedrumcandrageitherorbothoftheshoesintothefrictionsurface,causingthebrakestobiteharder,whichincreasestheforceholdingthemtogether.Thisincreasesthestoppingpowerwithoutanyadditionaleffortbeingexpendedbythedriver,butitdoesmakeitharderforthedrivertomodulatethebrake'ssensitivity.Italsomakesthebrakemoresensitivetobrakefade,asadecreaseinbrakefrictionalsoreducestheamountofbrakeassist.
Discbrakesexhibitnoself-applyingeffectbecausethehydraulicpressureactingonthepadsisperpendiculartothedirectionofrotationofthedisc.Discbrakesystemsusuallyhaveservoassistance("BrakeBooster")tolessenthedriver'spedaleffort,butsomediscbrakedcars(notablyracecars)andsmallerbrakesformotorcycles,etc.,donotneedtouseservos.
Note:
Inmostdesigns,the"selfapplying"effectonlyoccursononeshoe.Whilethisshoeisfurtherforcedintothedrumsurfacebyamomentduetofriction,theoppositeeffectishappeningontheothershoe.Thefrictionforceistryingtorotateitawayfromthedrum.Theforcesaredifferentoneachbrakeshoeresultinginoneshoewearingfaster.Itispossibletodesignatwo-shoedrumbrakewherebothshoesareself-applying(havingseparateactuatorsandpivotedatoppositeends),buttheseareveryuncommoninpractice.
Drumbrakedesigns
Renderingofadrumbrake
Drumbrakesaretypicallydescribedaseitherleading/trailingortwinleading.[1]
Reardrumbrakesaretypicallyofaleading/trailingdesign(ForNonServoSystems),or[Primary/Secondary](ForDuoServoSystems)theshoesbeingmovedbyasingledouble-actinghydrauliccylinderandhingedatthesamepoint.[1]Inthisdesign,oneofthebrakeshoeswillalwaysexperiencetheself-applyingeffect,irrespectiveofwhetherthevehicleismovingforwardsorbackwards.[1]Thisisparticularlyusefulontherearbrakes,wherethefootbrake(handbrakeorparkingbrake)mustexertenoughforcetostopthevehiclefromtravellingbackwardsandholditonaslope.Providedthecontactareaofthebrakeshoesislargeenough,whichisn'talwaysthecase,theself-applyingeffectcansecurelyholdavehiclewhentheweightistransferredtotherearbrakesduetotheinclineofaslopeorthereversedirectionofmotion.Afurtheradvantageofusingasinglehydrauliccylinderontherearisthattheoppositepivotmaybemadeintheformofadoublelobedcamthatisrotatedbytheactionoftheparkingbrakesystem.
Frontdrumbrakesmaybeofeitherdesigninpractice,butthetwinleadingdesignismoreeffective.[1]Thisdesignusestwoactuatingcylindersarrangedsothatbothshoeswillutilizetheself-applyingcharacteristicwhenthevehicleismovingforwards.[1]Thebrakeshoespivotatoppositepointstoeachother.[1]Thisgivesthemaximumpossiblebrakingwhenmovingforwards,butisnotsoeffectivewhenthevehicleistravelinginreverse.[1]
Theoptimumarrangementoftwinleadingfrontbrakeswithleading/trailingbrakesontherearallowsformorebrakingforcetobedeployedatthefrontofthevehiclewhenitismovingforwards,withlessattherear.Thishelpstopreventtherearwheelslockingup,butstillprovidesadequatebrakingattherearwhenitisneeded.[1]
Thebrakedrumitselfisfrequentlymadeofcastiron,althoughsomevehicleshaveusedaluminumdrums,particularlyforfront-wheelapplications.Aluminumconductsheatbetterthancastiron,whichimprovesheatdissipationandreducesfade.Aluminumdrumsarealsolighterthanirondrums,whichreducesunsprungweight.Becausealuminumwearsmoreeasilythaniron,aluminumdrumswillfrequentlyhaveanironorsteellinerontheinnersurfaceofthedrum,bondedorrivetedtothealuminumoutershell.
Advantages
Drumbrakesareusedinmostheavydutytrucks,somemediumandlightdutytrucks,andfewcars,dirtbikes,andATV's.Drumbrakesareoftenappliedtotherearwheelssincemostofthestoppingforceisgeneratedbythefrontbrakesofthevehicleandthereforetheheatgeneratedintherearissignificantlyless.Drumbrakesallowsimpleincorporationofaparkingbrake.Drumbrakesarealsooccasionallyfittedastheparking(andemergency)brakeevenwhentherearwheelsusediskbrakesasthemainbrakes.Inthissituation,asmalldrumisusuallyfittedwithinoraspartofthebrakediskalsoknownasabanksiabrake.
Inhybridvehicleapplications,wearonbrakingsystemsisgreatlyreducedbyenergyrecoveringmotor-generators(seeregenerativebraking),sosomehybridvehiclessuchastheGMCYukonhybridandToyotaPrius(exceptthethirdgeneration)usedrumbrakes.
Discbrakesrelyonpliabilityofcalipersealsandslightrunouttoreleasepads,leadingtodrag,fuelmileageloss,anddiscscoring.Drumbrakereturnspringsgivemorepositiveactionand,adjustedcorrectly,oftenhavelessdragwhenreleased.
Certainheavierdutydrumbrakesystemscompensateforloadwhendeterminingwheelcylinderpressure;afeatureunavailablewhendisksareemployed.OnesuchvehicleistheJeepComanche.TheComanchecanautomaticallysendmorepressuretothereardrumsdependingonthesizeoftheload,whereasthiswouldnotbepossiblewithdisks.
Duetothefactthatadrumbrakesfrictioncontactareaisatthecircumferenceofthebrake,adrumbrakecanprovidemorebrakingforcethananequaldiameterdiscbrake.Theincreasedfrictioncontactareaofdrumbrakeshoesonthedrumallowsdrumbrakeshoestolastlongerthandiscbrakepadsusedinabrakesystemofsimilardimensionsandbrakingforce.Drumbrakesretainheatandaremorecomplexthandiscbrakesbutareoftentimesthemoreeconomicalandpowerfulbraketypetouseinrearbrakeapplicationsduetothelowheatgenerationofrearbrakes,adrumbrakesselfapplyingnature,largefrictio
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