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外文翻译--起重机的用途与历史.doc

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外文翻译--起重机的用途与历史.doc

TheUseandHistoryofCraneEverytimeweseeacraneinactionweremainswithoutwords,thesemachinesaresometimesreallyhuge,takinguptonsofmaterialhundredsofmetersinheight.Wewatchwithamazementandabitofterror,thinkingaboutwhatwouldhappeniftheloadcomesofforifthemovementofthecranewaswrong.Itisareallyfascinatingsystem,surprisingbothadultsandchildren.Theseareespeciallytowercranes,butinrealitythereareplentyoftypesandtheyareinuseforcenturies.Thecranesareformedbyoneormoremachinesusedtocreateamechanicaladvantageandthusmovelargeloads.Cranesareequippedwithawinder,awireropeorchainandsheavesthatcanbeusedbothtoliftandlowermaterialsandtomovethemhorizontally.Itusesoneormoresimplemachinestocreatemechanicaladvantageandthusmoveloadsbeyondthenormalcapabilityofahuman.Cranesarecommonlyemployedinthetransportindustryfortheloadingandunloadingoffreight,intheconstructionindustryforthemovementofmaterialsandinthemanufacturingindustryfortheassemblingofheavyequipment.1.OverviewThefirstconstructioncraneswereinventedbytheAncientGreeksandwerepoweredbymenorbeastsofburden,suchasdonkeys.Thesecraneswereusedfortheconstructionoftallbuildings.Largercraneswerelaterdeveloped,employingtheuseofhumantreadwheels,permittingtheliftingofheavierweights.IntheHighMiddleAges,harborcraneswereintroducedtoloadandunloadshipsandassistwiththeirconstruction–somewerebuiltintostonetowersforextrastrengthandstability.Theearliestcraneswereconstructedfromwood,butcastironandsteeltookoverwiththecomingoftheIndustrialRevolution.Formanycenturies,powerwassuppliedbythephysicalexertionofmenoranimals,althoughhoistsinwatermillsandwindmillscouldbedrivenbytheharnessednaturalpower.Thefirstmechanicalpowerwasprovidedbysteamengines,theearlieststeamcranebeingintroducedinthe18thor19thcentury,withmanyremaininginusewellintothelate20thcentury.Moderncranesusuallyuseinternalcombustionenginesorelectricmotorsandhydraulicsystemstoprovideamuchgreaterliftingcapabilitythanwaspreviouslypossible,althoughmanualcranesarestillutilizedwheretheprovisionofpowerwouldbeuneconomic.Cranesexistinanenormousvarietyofforms–eachtailoredtoaspecificuse.Sizesrangefromthesmallestjibcranes,usedinsideworkshops,tothetallesttowercranes,usedforconstructinghighbuildings.Forawhile,minicranesarealsousedforconstructinghighbuildings,inordertofacilitateconstructionsbyreachingtightspaces.Finally,wecanfindlargerfloatingcranes,generallyusedtobuildoilrigsandsalvagesunkenships.Thisarticlealsocoversliftingmachinesthatdonotstrictlyfittheabovedefinitionofacrane,butaregenerallyknownascranes,suchasstackercranesandloadercranes.2.HistoryAncientGreeceThecraneforliftingheavyloadswasinventedbytheAncientGreeksinthelate6thcenturyBC.Thearchaeologicalrecordshowsthatnolaterthanc.515BCdistinctivecuttingsforbothliftingtongsandlewisironsbegintoappearonstoneblocksofGreektemples.Sincetheseholespointattheuseofaliftingdevice,andsincetheyaretobefoundeitherabovethecenterofgravityoftheblock,orinpairsequidistantfromapointoverthecenterofgravity,theyareregardedbyarchaeologistsasthepositiveevidencerequiredfortheexistenceofthecrane.Theintroductionofthewinchandpulleyhoistsoonleadtoawidespreadreplacementoframpsasthemainmeansofverticalmotion.Forthenexttwohundredyears,Greekbuildingsiteswitnessedasharpdropintheweightshandled,asthenewliftingtechniquemadetheuseofseveralsmallerstonesmorepracticalthanoffewerlargerones.Incontrasttothearchaicperiodwithitstendencytoeverincreasingblocksizes,GreektemplesoftheclassicalageliketheParthenoninvariablyfeaturedstoneblocksweighinglessthan1520tons.Also,thepracticeoferectinglargemonolithiccolumnswaspracticallyabandonedinfavorofusingseveralcolumndrums.Althoughtheexactcircumstancesoftheshiftfromtheramptothecranetechnologyremainunclear,ithasbeenarguedthatthevolatilesocialandpoliticalconditionsofGreeceweremoresuitabletotheemploymentofsmall,professionalconstructionteamsthanoflargebodiesofunskilledlabor,makingthecranemorepreferabletotheGreekpolisthanthemorelaborintensiverampwhichhadbeenthenormintheautocraticsocietiesofEgyptorAssyria.ThefirstunequivocalliteraryevidencefortheexistenceofthecompoundpulleysystemappearsintheMechanicalProblemsMech.18,853a32853b13attributedtoAristotle384322BC,butperhapscomposedataslightlylaterdate.Aroundthesametime,blocksizesatGreektemplesbegantomatchtheirarchaicpredecessorsagain,indicatingthatthemoresophisticatedcompoundpulleymusthavefounditswaytoGreekconstructionsitesbythen.AncientRomeTheheydayofthecraneinancienttimescameduringtheRomanEmpire,whenconstructionactivitysoaredandbuildingsreachedenormousdimensions.TheRomansadoptedtheGreekcraneanddevelopeditfurther.Wearerelativelywellinformedabouttheirliftingtechniques,thankstoratherlengthyaccountsbytheengineersVitruviusDeArchitectura10.2,110andHeronofAlexandriaMechanica3.25.TherearealsotwosurvivingreliefsofRomantreadwheelcranes,withtheHateriitombstonefromthelatefirstcenturyADbeingparticularlydetailed.ThesimplestRomancrane,theTrispastos,consistedofasinglebeamjib,awinch,arope,andablockcontainingthreepulleys.Havingthusamechanicaladvantageof31,ithasbeencalculatedthatasinglemanworkingthewinchcouldraise150kg3pulleysx50kg150,assumingthat50kgrepresentthemaximumeffortamancanexertoveralongertimeperiod.HeaviercranetypesfeaturedfivepulleysPentaspastosor,incaseofthelargestone,asetofthreebyfivepulleysPolyspastosandcamewithtwo,threeorfourmasts,dependingonthemaximumload.ThePolyspastos,whenworkedbyfourmenatbothsidesofthewinch,couldalreadylift3000kg3ropesx5pulleysx4menx50kg3000kg.Incasethewinchwasreplacedbyatreadwheel,themaximumloadevendoubledto6000kgatonlyhalfthecrew,sincethetreadwheelpossessesamuchbiggermechanicaladvantageduetoitslargerdiameter.Thismeantthat,incomparisontotheconstructionoftheEgyptianPyramids,whereabout50menwereneededtomovea2.5tonstoneblockuptheramp50kgperperson,theliftingcapabilityoftheRomanPolyspastosprovedtobe60timeshigher3000kgperperson.However,numerousextantRomanbuildingswhichfeaturemuchheavierstoneblocksthanthosehandledbythePolyspastosindicatethattheoverallliftingcapabilityoftheRomanswentfarbeyondthatofanysinglecrane.AtthetempleofJupiteratBaalbek,forinstance,thearchitraveblocksweighupto60tonseach,andonecornercorniceblockevenover100tons,allofthemraisedtoaheightofabout19m.InRome,thecapitalblockofTrajansColumnweighs53.3tons,whichhadtobeliftedtoaheightofabout34mseeconstructionofTrajansColumn.ItisassumedthatRomanengineersliftedtheseextraordinaryweightsbytwomeasuresseepicturebelowforcomparableRenaissancetechniqueFirst,assuggestedbyHeron,aliftingtowerwassetup,whosefourmastswerearrangedintheshapeofaquadranglewithparallelsides,notunlikeasiegetower,butwiththecolumninthemiddleofthestructureMechanica3.5.Second,amultitudeofcapstanswereplacedonthegroundaroundthetower,for,althoughhavingalowerleverageratiothantreadwheels,capstanscouldbesetupinhighernumbersandrunbymoremenand,moreover,bydraughtanimals.ThisuseofmultiplecapstansisalsodescribedbyAmmianusMarcellinus17.4.15inconnectionwiththeliftingoftheLateranenseobeliskintheCircusMaximusca.357AD.Themaximumliftingcapabilityofasinglecapstancanbeestablishedbythenumberoflewisironholesboredintothemonolith.IncaseoftheBaalbekarchitraveblocks,whichweighbetween55and60tons,eightextantholessuggestanallowanceof7.5tonperlewisiron,thatispercapstan.Liftingsuchheavyweightsinaconcertedactionrequiredagreatamountofcoordinationbetweentheworkgroupsapplyingtheforcetothecapstans.MiddleAgesDuringtheHighMiddleAges,thetreadwheelcranewasreintroducedonalargescaleafterthetechnologyhadfallenintodisuseinwesternEuropewiththedemiseoftheWesternRomanEmpire.TheearliestreferencetoatreadwheelmagnarotareappearsinarchivalliteratureinFranceabout1225,followedbyanilluminateddepictioninamanuscriptofprobablyalsoFrenchorigindatingto1240.Innavigation,theearliestusesofharborcranesaredocumentedforUtrechtin1244,Antwerpin1263,Bruggein1288andHamburgin1291,whileinEnglandthetreadwheelisnotrecordedbefore1331.Generally,verticaltransportcouldbedonemoresafelyandinexpensivelybycranesthanbycustomarymethods.Typicalareasofapplicationwereharbors,mines,and,inparticular,buildingsiteswherethetreadwheelcraneplayedapivotalroleintheconstructionoftheloftyGothiccathedrals.Nevertheless,botharchivalandpictorialsourcesofthetimesuggestthatnewlyintroducedmachinesliketreadwheelsorwheelbarrowsdidnotcompletelyreplacemorelaborintensivemethodslikeladders,hodsandhandbarrows.Rather,oldandnewmachinerycontinuedtocoexistonmedievalconstructionsitesandharbors.Apartfromtreadwheels,medievaldepictionsalsoshowcranestobepoweredmanuallybywindlasseswithradiatingspokes,cranksandbythe15thcenturyalsobywindlassesshapedlikeashipswheel.Tosmoothoutirregularitiesofimpulseandgetoverdeadspotsintheliftingprocessflywheelsareknowntobeinuseasearlyas1123.Theexactprocessbywhichthetreadwheelcranewasreintroducedisnotrecorded,althoughitsreturntoconstructionsiteshasundoubtedlytobeviewedincloseconnectionwiththesimultaneousriseofGothicarchitecture.Thereappearanceofthetreadwheelcranemayhaveresultedfromatechnologicaldevelopmentofthewindlassfromwhichthetreadwheelstructurallyandmechanicallyevolved.Alternatively,themedievaltreadwheelmayrepresentadeliberatereinventionofitsRomancounterpartdrawnfromVitruviusDearchitecturawhichwasavailableinmanymonasticlibraries.Itsreintroductionmayhavebeeninspired,aswell,bytheobservationofthelaborsavingqualitiesofthewaterwheelwithwhichearlytreadwheelssharedmanystructuralsimilarities.StructureandplacementThemedievaltreadwheelwasalargewoodenwheelturningaroundacentralshaftwithatreadwaywideenoughfortwoworkerswalkingsidebyside.Whiletheearliercompassarmwheelhadspokesdirectlydrivenintothecentralshaft,themoreadvancedclasparmtypefeaturedarmsarrangedaschordstothewheelrim,givingthepossibilityofusingathinnershaftandprovidingthusagreatermechanicaladvantage.Contrarytoapopularlyheldbelief,cranesonmedievalbuildingsiteswereneitherplacedontheextremelylightweightscaffoldingusedatthetimenoronthethinwallsoftheGothicchurcheswhichwereincapableofsupportingtheweightofbothhoistingmachineandload.Rather,craneswereplacedintheinitialstagesofconstructionontheground,oftenwithinthebuilding.Whenanewfloorwascompleted,andmassivetiebeamsoftheroofconnectedthewalls,thecranewasdismantledandreassembledontheroofbeamsfromwhereitwasmovedfrombaytobayduringconstructionofthevaults.Thus,thecranegrewandwanderedwiththebuildingwiththeresultthattodayallextantconstructioncranesinEnglandarefoundinchurchtowersabovethevaultingandbelowtheroof,wheretheyremainedafterbuildingconstructionforbringingmaterialforrepairsaloft.Lessfrequently,medievalilluminationsalsoshowcranesmountedontheoutsideofwallswiththestandofthemachinesecuredtoputlogs.MechanicsandoperationIncontrasttomoderncranes,medievalcranesandhoistsmuchliketheircounterpartsinGreeceandRomewereprimarilycapableofaverticallift,andnotusedtomoveloadsforaconsiderabledistancehorizontallyaswell.Accordingly,liftingworkwasorganizedattheworkplaceinadifferentwaythantoday.Inbuildingconstruction,forexample,itisassumedthatthecraneliftedthestoneblockseitherfromthebottomdirectlyintoplace,orfromaplaceoppositethecentreofthewallfromwhereitcoulddelivertheblocksfortwoteamsworkingateachendofthewall.Additionally,thecranemasterwhousuallygaveordersatthetreadwheelworkersfromoutsidethecranewasabletomanipulatethemovementlaterallybyasmallropeattachedtotheload.Slewingcraneswhichallowedarotationoftheloadandwerethusparticularlysuitedfordocksideworkappearedasearlyas1340.Whileashlarblocksweredirectlyliftedbysling,lewisordevilsclampGermanTeufelskralle,otherobjectswereplacedbeforeincontainerslikepallets,baskets,woodenboxesorbarrels.Itisnoteworthythatmedievalcranesrarelyfeaturedratchetsorbrakestoforestalltheloadfromrunningbackward.Thiscuriousabsenceisexplainedbythehighfrictionforceexercisedbymedievaltreadwheelswhichnormallypreventedthewheelfromacceleratingbeyondcontrol.HarborusageAccordingtothepresentstateofknowledgeunknowninantiquity,stationaryharborcranesareconsideredanewdevelopmentoftheMiddleAges.Thetypicalharborcranewasapivotingstructure

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