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HOISTS10-15Table10.2.10TypicalDataforElectricChainHoists*Table10.2.11TypicalDataforElectricWireRopeHoists*SomeairhoistsalsoincludeaWeston-typeloadbrake.Thehoistmaybesuspendedbyahook,lug,ortrolley；thelattermaybeplain,geared,orair-motor-driven.Horizontalmovementislimitedtoabout25ft(7.6m)becauseoftheairhose,althougharunwaysystemisavailablewithaseriesofnormallyclosedportsthatareopenedbyaspecialtrolleytosupplyairtothehoist.Airhoistsprovideinfinitelyvariablespeed,accordingtothemovementofthecontrolvalve.Veryhighspeedsarepossiblewithlightloads.Whenseverelyoverloaded,theairmotorstallswithoutdamage.Airhoistsaresmallerandlighterthanelectrichoistsofequalcapacityandcanbeoperatedinexplosiveatmospheres.Theyaremoreexpensivethanelectrichoists,requiremufflersforreasonablyquietoperation,andnormallyarefittedwithautomaticlubricatorsintheairsupply.JacksJacksareportable,hand-operateddevicesformovingheavyloadsthroughshortdistances.Therearethreetypesincommonuse:screwjacks,rack-and-leverjacks,andhydraulicjacks.Bell-bottomscrewjacks(Fig.10.2.23)areavailableincapacitiesto24tonsandliftingrangesto14in.ThescrewisrotatedbyabarinsertedinholesinthescrewheadorByaratchetleverfittedtothehead.Gearedbridgejackswillliftupto50shorttons(45tonnes).Aleverratchetmechanismturnsabevelpinion；aninternalthreadinthegearraisesthenonrotatingscrew.Rack-and-leverjacks(Fig.10.2.24)consistofacast-steelormalleable-ironhousinginwhichtheleverpivots.Theracktoothedbarpassesthroughthehollowhousing；theloadmaybeliftedeitheronthetoporonatoeextendingfromthebottomofthebar.Theleverpawlmaybebiasedeithertoraiseortolowerthebar,thehousingpawlholdingtheloadonthereturnleverstroke.Rack-and-leverjacksto20shorttons(18tonnes)aredirectacting.Leveroperatedgearedjacksrangeupto35shorttons(32tonnes).Liftingheightsto18in(0.46m)areprovided.Trackjacksarerack-and-leverjackswhichmaybetrippedtoreleasetheload.Theyareusedforrailroad-trackworkbutnotforindustrialservicewherethetrippingfeaturesmightbehazardous.Hydraulicjacks(Fig.10.2.25)consistofacylinder,apiston,andalever-operatedpump.Capacitiesto100shorttons(91tonnes)andliftingheightsto22in(0.56m)areavailable.Jacks25shorttons(22.7tonnes)andlargermaybeprovidedwithtwopumps,thesecondpumpbeingahigh-speedunitforrapidtravelatpartialload.MINEHOISTSANDSKIPSbyBurtGarofabTherearetwotypesofhoists,thedrumhoistandthefrictionhoist.Onadrumhoist,theropeisattachedtothedrumandiswoundaroundandstoredonthedrum.Adrumhoistmaybesingle-drumordouble-drum.Therearealsoseveraldifferentconfigurationsofboththesingle-drumandthedouble-drumhoist.Adrumhoistmaybefurtherdividedaseitherunbalanced,partiallybalanced,orfullybalanced.Onafrictionhoist,whichisoftenreferredtoasaKoepehoist,theropeisnotwoundaroundthedrum,butratherpassesoverthedrum.Africtionhoistmayutilizeasingleropeormultipleropes.Theoperationofhoistsmaybecontrolledmanually,automatically,orsemiautomatically.Therearealsoarrangementsthatusecombinationsofthedifferentcontroltypes.Therearecurrentlythreetypesofgroovingbeingusedonnewdrumsfordrumhoists.Theyarehelicalgrooving,counterbalance(Lebus)grooving,andantisynchronousgrooving.Helicalgroovingisusedprimarilyforapplicationswhereonlyasinglelayerofropeiswoundaroundthedrum.Counterbalanceandantisynchronousgroovingareusedforapplicationswheremultiplelayersofropearewoundaroundthedrum.Minehoistsaregenerallydividedinto(1)Metal-minehoists(e.g.,iron,copper,zinc,salt,gypsum,silver,gold,ores)and(2)coal-minehoists.Theseclassessubdivideintomainhoists(forhandlingoresorcoal)andhoistsformen,timbers,andsupplies.Theyaredesignedforoperating(1)mineshafts,verticalandinclined,balancedandunbalanced；and(2)slopes,balancedandunbalanced.Whenanemptycageorplatformdescendswhiletheloadedcageorplatformascends,aswhenbothcablesarewoundonasingledrum,themachineisreferredtoasabalancedhoist.Mostmedium-andlarge-sizedhoistsnormallyoperateinbalance,asthetonnageobtainedforagivenloadandropespeedisaboutdoublethatforanunbalancedhoistandthepowerconsumptionpertonhoistedislower.Balancedoperationcanalsobeobtainedbyacounterweight.Thecounterweight(approximatelyequaltoallthedeadloadsplusone-halftheliveload)isusuallyinstalledinguideswithinasingleshaftcompartment.Theaveragedepthoforeminesisabout2,000ft(610m),andthatofcoalminesiscloseto500ft(152m).Mostoreminehoistsareofthedouble-drumtype,normallyhostinginbalance,eachdrumbeingprovidedwithafrictionclutchforchangingtherelativepositionsofthetwoskipswhenoperatingfromvariouslevels.Hoistsforcoalminesareprincipallyofthekeyed-drumtype,foroperatinginbalancefromonelevel.Forhighropespeedsinshallowshafts,itisgenerallyadvantageoustousecombinedcylindricalandconicaldrums.Thecylindroconicaldrumplacesthemaximumropepull(weightofropeandloadedskip)onthesmalldiametersothatduringtheaccelerationperiodofthecycle,theweightoftheopposingskipisofferingthegreatestcounterbalancetorque,reducingmotorpeakloadsandslightlyreducingthepowerconsumption.Thepeakreductionobtainedbecomesgreaterwhen

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