ELEVATORSAFETY:GIVETHEMINERABRAKEABSTRACTOverafive-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,theaccidenthistoryhasproventhatthisisnotthebestcontrolstrategy2,3.Theseaccidentsoccurredwhentheretardingeffortofthedrivemotorwasdefeatedwhenthemechanicalbrakeswereinoperative.Thisallowedthecounterweighttofalltothebottomoftheshaft,causingthecartooverspeedandstriketheheadframe.Thehigh-speedelevatorcrashesintotheoverheadstructurecausedextensivemechanicaldamageandpotentiallyfatalinjuries.ELEVATORDESIGNAbasicunderstandingofelevatoroperationisrequiredinordertoassessthesafetyhazardspresentanddeterminetheaccidentpreventmethodsavailable.Figure1showsacompleteviewofamineelevator.Fig.1MicoElevatorInatypicalelevator,theearisraisedandloeredbysixtoeightmotordrivenwireropesthatareattachedtothetopofthecaratoneend,travelaroundapairofsheaves,andareagainattachedtoacounterweightattheotherend.Thecounterweightaddsacceleratingforcewhentheelevatorcarisascendingandprovidesaretardingeffortwhenthecarisdescendingsothatlessmotorhorsepowerisrequired.Thecounterweightisacollectionofmetalweightsthatisequaltotheweightofthecarcontainingabout45%ofitsratedload.AsetofchainsareloopedfromthebottomofthecounterweighttotheundersideofthecartohelpmaintainbalancebyoffsettingtheweightofthesuspensionropesGuiderailsthatrunthelengthoftheshaftkeepthecarandcounterweightfromswayingSUSPENSIONriCPCSortwistingduringtheirtravel.Rollersareattachedtothecarandthecounterweighttoprovidesmoothtravelalongtheguiderails.Thetractiontoraiseandlowerthecarcomesfromthefrictionofthewireropesagainstthegroovedsheaves.Themainsheaveisdrivenbyanelectricmotor.Motor-generator(M-G)setstypicallypro-videtodcpowerforthedrivemotor.Newersystemsuseastaticdrivecontrol.Theelevatorcontrolsvarythemotorsspeedbasedonasetoffeedbacksignalsthatindicatethecarspositionintheshaftway.Asthecarapproachesitsdestination,aswitchnearthelandingsignalsthecontrolstostopthecaratfloorlevel.Additionalshaftwaylimitswitchesareinstalledtomonitorovertravelconditions.Theworstfearoflitanypassengersisthattheelevatorwillgooutofcontrolandfallthroughspaceuntilitsmashesintothebottomoftheshaft.Thereareseveralsafetyfeaturesinmodernelevatorstopreventthisfromoccurring.Thefirstisthehigh-strengthwireropesthemselves.Each0.625-in-diameterextra-high-strengthwireropecansupport32,000lb,orabouttwicetheaverageweightofamineelevatorfilledwith20passengers.Forsafetyssakeandtoreducewear,eachcarhassixtoeightofthesecables.Inaddition,elevatorshavebuffersinstalledattheshaftbottomthatcanstopthecarwithoutkillingitspassengersiftheyarestruckatthenormalspeedoftheelevatorAspreviouslydiscussed,modernelevatorshaveseveralspeedcontrolfeatures.Iftheydonotwork,thecontrolswilldisconnectthemotorandapplythemachinebrake.Finally,theelevatoritselfisequippedwithsafetiesmountedunderneaththecar.Ifthecarsurpassestheratedspeedby15to25%,thegovernorwilltrip,andthesafetieswillgriptheguiderailsandstopthecar.Thiswastheinventionthatmadeelevatortransportationacceptableforthegeneralpublic.SAFETYHAZARDSAhistoricalperspectiveofelevatordevelopmentcanaccountfortodaysproblemswithelevatorsafetyrulesandregulations4.Inthebeginningofmodernelevatorhistory,itwasrealizedthatalthoughtherewereseveralfactorsofsafetyinthesuspensionropedesign,thequalityofconstructionandperiodicinspectioncouldnotbeassured.Therefore,theelevatorcarwasequippedwithreliablestandbysafetiesthatwouldstopthecarsafelyifthesuspensionropesfailed.In1853,Elisha.Otis,aNewYorkmechanic,designedanddemonstratedaninstantaneoussafetycapableofsafelystoppingafreefallingcar.Thisaddressedthehazardshowninfigure2.Latero