毕业设计电梯外文翻译Word格式文档下载.docx
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Overafive-yearperiod,therewereatleast18documentedcasesofascendingelevatorsstrikingtheoverhead.Insomecases,theaccidentsresultedinseriousinjuriesorfatalities.Theseaccidentsoccurredoncounterweightedelevatorsasaresultofelectrical,mechanical,andstructuralfailures.Elevatorcarsarefittedwithsafetiesthatgriptheguiderailsandstopafallingcar;
however,thesedevicesdonotprovideprotectionintheuowarddirection.
Rulesandregulatioinsapplyingtoelevatorsafetyhavecomeunderreviewinresponsetotheseaccidents.Somegoverningauthoritieshavealreadyrevisedtheirregulationstorequireascendingcaroverspeedprotection.thispaperwilldiscussbasicelevatondesign,hazard,regulation,andemergencybrakingsystemsdesignedtoprovideascendingcaroverspeedprotection.Inaddition,acase-studyreportonapneumaticropebrakesysteminstalledandtestedonamineelevatorwillbediscussed.
INTRODUCTION
Elevatorsincorporateseveralsafetyfeaturestopreventthecarfromcrashingintothebottomoftheshaft.safetiesinstalledonthecarcanpreventthistypeofaccidentformoccurringwhenthemachinebrakefailsorthewireropessuspendingthecarbreak.However,theinherentdesignofthesafetiesrendertheminoperativeintheascendingdirection.
Intheupwarddirection,themachinebrakeisrequiredtostopthecagewhenanemergencyconditionoccurs.Undernormaloperation,themachinebrakeservesonlyasaparkingbrakedtoholdthecageatrest.However,whenanemergencyconditionisdetected,modernelevatorcontrolsystemdesignsrelysolelyonthemachinebraketostopthecar.
IntheUnitedStatesminingindustry,theaccidenthistoryhasproventhatthisisnotthebestcontrolstrategy[2],[3].Theseaccidentsoccurredwhentheretardingeffortofthedrivemotorwasdefeatedwhenthemechanicalbrakewereinoperative.Thisallowedthecounterweighttofalltothebottomoftheshaft,causingthecartooverspeedandstriketheheadframe.Thehigh-speedelevatorcrashesintotheoverheadstructurecausedextensivemechanicaldamageandpotentiallyfatalinjuries.
ELEVATORDESIGN
Abasicunderstandingofelevatoroprationisrequiredinordertoassessthesafetyhazardspresentanddeterminetheaccidentpreventmethodsavailable.Figure1showsacompleteviewofamineelevator.
Inatypicalelevator,thecarisraisedandloweredbysixtoeightmotor-drivenwireropesthatareattachedtothetopofthecaratoneend,travelaroundapairofsheaves,andareagainattachedtoacounterweightattheotherend.
Thecounterweightaddsacceleratingforcewhentheelevatorcarisascendingandprovidesaretardingeffortwhenthecarisdescendingsothatlessmotorhorsepowerisrequired.Thecounterweightisacollectionofmetalweightsthatisequaltotheweigheofthecarcontainingabout45%ofitsratedload.Asetofchainsareloopedfromthebottomofthecounterweighttotheundersideofthecartohelpmaintainbalancebyoffsettingtheweightofthesuspensionropes.
Guiderailsthatrunthelengthoftheshaftkeepthecarandcounterweightfromswayingortwistingduringtheirtravel.Rollersareattachedtothecarandthecounterweighttoprovidesmoothtravelalongtheguiderails
Thetractiontorasieandlowerthecarcomesfromthefrictionofthewireropeagainstthegroovedsheaves.Themainsheaveisdrivenbyanelectricmotor.
Mostelevatorsuseadirectcurrentmotorbecauseitsspeedcanbepreciselycontrolledtoallowsmoothaccelerationanddeceleration.Motor-generator(M-G)setstypicallyprovidetodopowerforthedrivemotor.Newersystemsuseastaticdrivecontrol.Theelevatorcontrolsvarythemotor’sspeedbasedonasetoffeedbacksignalsthatindicatethecar’spositionintheshaftway.Asthecarapproachesitsdestination,aswitchnearthelandingsignalsthecontrolstostopthecaratfloorlevel.Additionalshaftwaylimitswitchesareinstalledtomonitorovertravelconditions.
Theworstfearofmanypassengersisthattheelevatorwillgooutofcontrolandfallthroughspaceuntilitsmashesintothebottomoftheshaft.Thereareseveralsafetyfeatureinmodernelevatorstopreventthisfromoccurring.
Thefirstisthehigh-strengthwireropesthemselves.Each0.625-in-diameterextra-high-strengthwireropecansupport32,0001b,orabouttwicetheaverageweightofamineelevatorfilledwith20passengers.Forsafety’ssakeandtoreducewear,eachcarhassixtoeightofthesecables.Inaddition,elevatorshavebuffersintstalledattheshaftbottomthatcanstopthecarwithoutkillingitspassengersiftheyarestruckatthenormalspeedoftheelevator.
Aspreviouslydiscussed,modernelevatorshaveseveralspeedcontrolfeatures.Iftheydonotwork,thecontrolswilldisconnectthemotorandapplythemachinebrake.Finally,theelevatoritselfisequippedwithsafetiesmountedunderneaththecar.Ifthecarsurpassestheratedspeedby15to16%,thegovernorwilltrip,andthesafetieswillgriptheguiderailsandstopthecar.Thiswastheinventionthatmadeelevatortransportationacceptableforthegeneralpublic.
SAFETYHAZARDS
Ahistoricalperspectiveofelevatordevelopmentcanaccountfortoday’sproblemswithelevatorsafetyrulesandregulations[4].Inthebeginningofmodernelevatorhistory,itwasrealizedthatalthoughtherewereseveralfactorsofsafetyinthesuspensionropedesign,thequalityofconstructionandperiodicinspectioncouldnotbeassured.Therefore,theelevatorcaiwasequippedwithreliablestandby”safeties”thatwouldstopthecarsafelyifthesuspensionropefailed.In1853,ElishaOtis,aNewYorkmechanic,desingnedanddemonstratedaninstantaneoussafetycapableofsafelystoppingafree-fallingcar.Thisaddressedthehazardshowninfigure2.
Lateron,itwasrealizedthatpassengersmaybeinjuredwhenthecaroverspeedsinthedowndirectionwithsuspensionropesintact,asshowninfigure3.Topreventthishazard,anoverspeedgovernorwithgraduallyappliedsafetieswasdeveloped.Itdetectedtheoverspeedingconditionandactivatedthesafeties.
Furthermore,itwasnoticedthatfrequentapplicationofsafetiescausedmechanicalstressontheelevatorstructureandsafetysystem.Therefore,agovernoroverspeedswitchwasinstalledthatwouldtrytostopthecarbymachinebrakebeforethesafetiesactivated.Theswitchwasausefulideabecauseitcouldalsoinitiatestoppinginthecaseofoverspeedingintheupdirectionaswell.
Theproblemstartedinthe1920’swhentheAmericanElevatorSafetyCodewasdeveloped.ThewritersmostlikelylooledatthetechnologythatwasavailableatthattimeandsubsequentlyrequireditonallelevatorscoveredbytheCode.
Thewritersweresoconcentratedondescribingthedesignoftherequireddevicesthattheyforgottoacknowledgethehazards’thatthedevicesareguardingagainstandtheelevatorcomponentsthatmayfailandcausethehazards.Theydidnotconsiderthefactthatfor90%oftheelevatortrips,theelevatorispartiallyloaded(i.e.lessthan45%ofratedload)[5].Therefore,ifabrakefailureoccurs,theelevatorwilloverspeedandcrashintheupdirectionasshowninfigure4.
Untilrecently,elevatorsafetysystemshavenotdifferedsignificantlyfromtheearly1990’sdesigns.Theproblemarisesbecauserulemakingcommitteesandregulatoryauthoritiesarereluctanttorequirenewsafeguardswhenthetechnologyhasnotbeenfullydeveloped.Conversely,theelevatormanufacturingindustrycannotjustifytheproductdevelopmentexpenseforanewsafetydevicewithlittlemarketability.Thisproblemwillbeaddressedinthefollowingsections.
RULESANDREGULATIONS
Severalrulemakingcommitteesandgovernmentsafetyauthoritieshaveaddressedthedeficienciesintheexistingelevatorregulationsandhaveproposedrevisionstotheelevatorsafetycodes.
Thereport-formtheAmericanSocietyofMechanicalEngineers-A17MechanicalDesignCommitteeon“Carsascendingintothebuildingoverhead”-datedSeptember1987,containedthetypeoffailuresthatcouldresultinelevatorsacceleratingintooverheadstructureandananalysisofthepossiblesolutions.Inaddition,aproposaltotheA17.1CommitteeforanewcodeRule205.6wasintroducedasfollows:
R205.6(“Preventionofoverspeedingcarfromstrikingtheoverheadstructure”):
Alltractionelevarorsshallbeprovidedwithameanstopreventanascendingcarfromstrikingtheoverheadstructure.Thismeansshallconformtothefollowingrequirements:
1.Priortothetimewhenthecounterweighestrikesitsbuffer,itshallreducethespeedofthecartothespeedforwhichthecounterweightbufferisdesigned.
2.Itshallnotdevelopanaverageretardationofthecarinexcessof32.2ft/s2(9.81m/s2)duringthestoppingphase.
3.Itshallbeamechanicalmeansindependentofthedrivingmachinebrake.
4.Itshallpreventoverspeedingoftheelevatorsystemthroughthecontrolofoneormoreofthefollowing:
a.counterweight
b.car
c.suspensionorcompensatingropesystem.
Thisproposedruleiscurrentlyundercommitteereview,andcondiderationhasbeengiventorequiringprotectiontopreventthecarfromleavingthelandingwiththedoorsopenedorunlocked.
PennsyIvaniaBureauofDeepMineSafety
AnascendingelevatorcaraccidentoccurredatawesternPennsylvaniacoalmineonFebruary4,1987andcausedextensivestructuraldamageanddisabletheelevatorfortwomonths.Followingthisaccident,thePennsylvaniaBureauofDeepMineSafetyestablishedanadvisorycommitteetodeterminethesedevicesthatareavailableoprovideascendingcaroverspeedprotectionfornewandexistingmineelevatorinstallations.
Thefollowingfourpreotectivemethodsweredeterminedtobefeasiblebase