English Elevator.docx
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English Elevator.docx
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EnglishElevator
ELEVATORSAFETY:
GIVETHEMINERABRAKEABSTRACT
Overafive-yearperiod,therewereatleast18documentedcasesofascendingelevatorsstrikingtheoverhead.Insomecases,theaccidentsresultedinseriousinjuriesorfatalities.Theseaccidentsoccurredoncounterweightedelevatorsasaresultofelectrical,mechanical,andstructuralfailures.Elevatorcarsarefittedwithsafetiesthatgriptheguiderailsandstopafallingcar;however,thesedevicesdonotprovideprotectionintheupwarddirection.Rulesandregulationsapplyingtoelevatorsafetyhavecomeunderreviewinresponsetotheseaccidents.Somegoverningauthoritieshavealreadyrevisedtheirregulationstorequireascendingcaroverspeedprotection.Thispaperwilldiscussbasicelevatordesign,hazards,regulations,andemergencybrakingsystemsdesignedtoprovideascendingcaroverspeedprotection.Inaddition,acase-studyreportonapneumaticropebrakesysteminstalledandtestedonamineelevatorwillbediscussed.
INTRODUCTIONElevatorsincorporateseveralsafetyfeaturestopreventthecarfromcrashingintothebottomoftheshaft.Safetiesinstalledonthecarcanpreventthistypeofaccidentfromoccurringwhenthemachinebrakefailsorthewireropessuspendingthecarbreak.However,theinherentdesignofthesafetiesrendertheminoperativeintheascendingdirection.Intheupwarddirection,themachinebrakeisrequiredtostopthecageIrvhenanemergencyconditionoccurs.Undernormaloperation,themachinebrakeservesonlyasaparkingbrakedtoholdthecageatrest.However,whenanemergencyconditionisdetected,modernelevatorcontrolsystemdesignsrelysolelyonthemachinebraketostopthecar.IntheUnitedStatesminingindustry,theaccidenthistoryhasproventhatthisisnotthebestcontrolstrategy[2],[3].Theseaccidentsoccurredwhentheretardingeffortofthedrivemotorwasdefeatedwhenthemechanicalbrakeswereinoperative.Thisallowedthecounterweighttofalltothebottomoftheshaft,causingthecartooverspeedandstriketheheadframe.Thehigh-speedelevatorcrashesintotheoverheadstructurecausedextensivemechanicaldamageandpotentiallyfatalinjuries.ELEVATORDESIGNAbasicunderstandingofelevatoroperationisrequiredinordertoassessthesafetyhazardspresentanddeterminetheaccidentpreventmethodsavailable.Figure1showsacompleteviewofamineelevator.
Fig.1MicoElevatorInatypicalelevator,theearisraisedandloeredbysixtoeightmotor–drivenwireropesthatareattachedtothetopofthecaratoneend,travelaroundapairofsheaves,andareagainattachedtoacounterweightattheotherend.Thecounterweightaddsacceleratingforcewhentheelevatorcarisascendingandprovidesaretardingeffortwhenthecarisdescendingsothatlessmotorhorsepowerisrequired.Thecounterweightisacollectionofmetalweightsthatisequaltotheweightofthecarcontainingabout45%ofitsratedload.AsetofchainsareloopedfromthebottomofthecounterweighttotheundersideofthecartohelpmaintainbalancebyoffsettingtheweightofthesuspensionropesGuiderailsthatrunthelengthoftheshaftkeepthecarandcounterweightfromswayingortwistingduringtheirtravel.
Rollersareattachedtothecarandthecounterweighttoprovidesmoothtravelalongtheguiderails.Thetractiontoraiseandlowerthecarcomesfromthefrictionofthewireropesagainstthegroovedsheaves.Themainsheaveisdrivenbyanelectricmotor.Motor-generator(M-G)setstypicallypro-videtodcpowerforthedrivemotor.Newersystemsuseastaticdrivecontrol.Theelevatorcontrolsvarythemotor'sspeedbasedonasetoffeedbacksignalsthatindicatethecar'spositionintheshaftway.Asthecarapproachesitsdestination,aswitchnearthelandingsignalsthecontrolstostopthecaratfloorlevel.
Additionalshaftwaylimitswitchesareinstalledtomonitorovertravelconditions.Theworstfearof'litanypassengersisthattheelevatorwillgooutofcontrolandfallthroughspaceuntilitsmashesintothebottomoftheshaft.Thereareseveralsafetyfeaturesinmodernelevatorstopreventthisfromoccurring.Thefirstisthehigh-strengthwireropesthemselves.Each0.625-in-diameterextra-high-strengthwireropecansupport32,000lb,orabouttwicetheaverageweightofamineelevatorfilledwith20passengers.Forsafety'ssakeandtoreducewear,eachcarhassixtoeightofthesecables.Inaddition,elevatorshavebuffersinstalledattheshaftbottomthatcanstopthecarwithoutkillingitspassengersiftheyarestruckatthenormalspeedoftheelevator
Aspreviouslydiscussed,modernelevatorshaveseveralspeedcontrolfeatures.Iftheydonotwork,thecontrolswilldisconnectthemotorandapplythemachinebrake.Finally,theelevatoritselfisequippedwithsafetiesmountedunderneaththecar.Ifthecarsurpassestheratedspeedby15to25%,thegovernorwilltrip,andthesafetieswillgriptheguiderailsandstopthecar.Thiswastheinventionthatmadeelevatortransportationacceptableforthegeneralpublic.
SAFETYHAZARDS
Rollersareattachedtothecarandthecounterweighttoprovidesmoothtravelalongtheguiderails.Thetractiontoraiseandlowerthecarcomesfromthefrictionofthewireropesagainstthegroovedsheaves.Themainsheaveisdrivenbyanelectricmotor.Motor-generator(M-G)setstypicallypro-videtodcpowerforthedrivemotor.Newersystemsuseastaticdrivecontrol.Theelevatorcontrolsvarythemotor'sspeedbasedonasetoffeedbacksignalsthatindicatethecar'spositionintheshaftway.Asthecarapproachesitsdestination,aswitchnearthelandingsignalsthecontrolstostopthecaratfloorlevel.Additionalshaftwaylimitswitchesareinstalledtomonitorovertravelconditions.Theworstfearof'litanypassengersisthattheelevatorwillgooutofcontrolandfallthroughspaceuntilitsmashesintothebottomoftheshaft.Thereareseveralsafetyfeaturesinmodernelevatorstopreventthisfromoccurring.Thefirstisthehigh-strengthwireropesthemselves.Each0.625-in-diameterextra-high-strengthwireropecansupport32,000lb,orabouttwicetheaverageweightofamineelevatorfilledwith20passengers.Forsafety'ssakeandtoreducewear,eachcarhassixtoeightofthesecables.Inaddition,elevatorshavebuffersinstalledattheshaftbottomthatcanstopthecarwithoutkillingitspassengersiftheyarestruckatthenormalspeedoftheelevator
Aspreviouslydiscussed,modernelevatorshaveseveralspeedcontrolfeatures.Iftheydonotwork,thecontrolswilldisconnectthemotorandapplythemachinebrake.Finally,theelevatoritselfisequippedwithsafetiesmountedunderneaththecar.Ifthecarsurpassestheratedspeedby15to25%,thegovernorwilltrip,andthesafetieswillgriptheguiderailsandstopthecar.Thiswastheinventionthatmadeelevatortransportationacceptableforthegeneralpublic.
SAFETYHAZARDSAhistoricalperspectiveofelevatordevelopmentcanaccountfortoday'sproblemswithelevatorsafetyrulesandregulations[4].Inthebeginningofmodernelevatorhistory,itwasrealizedthatalthoughtherewereseveralfactorsofsafetyinthesuspensionropedesign,thequalityofconstructionandperiodicinspectioncouldnotbeassured.Therefore,theelevatorcarwasequippedwithreliablestandby'safeties"thatwouldstopthecarsafelyifthesuspensionropesfailed.In1853,Elisha.Otis,aNewYorkmechanic,designedanddemonstratedaninstantaneoussafetycapableofsafelystoppingafree–fallingcar.Thisaddressedthehazardshowninfigure2.
Lateron,itwasrealizedthatpassengersmaybeinjuredwhenthecaroverspeedsinthedowndirectionwithsuspensionropesintact,asshowninfigure3.Topreventthishazard,anoverspeedgovernorwithgraduallyappliedsafetieswasdeveloped.Itdetectedtheoverpeelingconditionandactivatedthesafeties.
Furthermore,itwasnoticedthatfrequentapplicationofsafetiescausedmechanicalstressontheelevatorstructureandsafetysystem.Therefore,agovernoroverspeedswitchwasinstalledthatwouldtrytostopthecarbymachinebrakebeforethesafetiesactivated.Theswitchwasausefulideabecauseitcouldalsoinitiatestoppinginthecaseofoverspeedingintheupdirectionaswell.Theproblemstartedinthe1920'swhentheAmericanElevatorSafetyCodewasdeveloped.ThewritersmostlikelylookedatthetechnologythatwasavailableatthattimeandsubsequentlyrequireditonallelevatorscoveredbytheCode.Thewritersweresoconcentratedondescribingthedesignoftherequireddevicesthattheyforgottoacknowledgethehazardsthatthedevicesareguardingagainstandtheelevatorcomponentsthatmayfailandcausethehazards.Theydidnotconsiderthefactthatfor90%oftheelevatortrips,theelevatorispartiallyloaded(i.e.lessthan45%ofratedload)[5].Therefore,ifabrakefailureoccurs,theelevatorwilloverspeedandcrashintheupdirectionasshowninfigure4
1.Priortothetimewhenthecounterweightstrikesitsbuffer,itshallreducethespeedofthecartothespeedforwhichthecounterweightbufferisdesigned.2.Itshallnotdevelopanaverageretardationofthecarinexcessof32.2ft/s2(9.81m/s2)duringthestoppingphase.3.1tshallbeamechanicalmeansindependentofthedrivingmachinebrake.4.1tshallpreventoverspeedingoftheelevatorsystemthroughthecontrolofoneormoreofthefollowinga.counterweightb.carc.suspensionorcompensatingropesystem.Thisproposedruleiscurrentlyundercommitteereview,andconsiderationhasbeengiventorequiringprotectiontopreventthecarfromleavingthelandingwithth
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