固体火箭发动机原理两相流动力学跟踪研究正文文档格式.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文档格式.docx
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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.Whilethisshoeisfurtherforcedintothedrumsurfacebyamom
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