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,harborcraneswereintroducedtoloadandunloadshipsandassistwiththeirconstructionsomewerebuiltintostonetowersforextrastrengthandstability.Theearliestcraneswereconstructedfromwood,butcastironandsteeltookoverwiththecomingoftheIndustrialRevolution.Formanycenturies,powerwassuppliedbythephysicalexertionofmenoranimals,althoughhoistsinwatermillsandwindmillscouldbedrivenbytheharnessednaturalpower.Thefirstmechanicalpowerwasprovidedbysteamengines,theearlieststeamcranebeingintroducedinthe18thor19thcentury,withmanyremaininginusewellintothelate20thcentury.Moderncranesusuallyuseinternalcombustionenginesorelectricmotorsandhydraulicsystemstoprovideamuchgreaterliftingcapabilitythanwaspreviouslypossible,althoughmanualcranesarestillutilizedwheretheprovisionofpowerwouldbeuneconomic.Cranesexistinanenormousvarietyofformseachtailoredtoaspecificuse.Sizesrangefromthesmallestjibcranes,usedinsideworkshops,tothetallesttowercranes,usedforconstructinghighbuildings.Forawhile,mini-cranesarealsousedforconstructinghighbuildings,inordertofacilitateconstructionsbyreachingtightspaces.Finally,wecanfindlargerfloatingcranes,generallyusedtobuildoilrigsandsalvagesunkenships.Thisarticlealsocoversliftingmachinesthatdonotstrictlyfittheabovedefinitionofacrane,butaregenerallyknownascranes,suchasstackercranesandloadercranes.2.HistoryAncientGreeceThecraneforliftingheavyloadswasinventedbytheAncientGreeksinthelate6thcenturyBC.Thearchaeologicalrecordshowsthatnolaterthanc.515BCdistinctivecuttingsforbothliftingtongsandlewisironsbegintoappearonstoneblocksofGreektemples.Sincetheseholespointattheuseofaliftingdevice,andsincetheyaretobefoundeitherabovethecenterofgravityoftheblock,orinpairsequidistantfromapointoverthecenterofgravity,theyareregardedbyarchaeologistsasthepositiveevidencerequiredfortheexistenceofthecrane.Theintroductionofthewinchandpulleyhoistsoonleadtoawidespreadreplacementoframpsasthemainmeansofverticalmotion.Forthenexttwohundredyears,Greekbuildingsiteswitnessedasharpdropintheweightshandled,asthenewliftingtechniquemadetheuseofseveralsmallerstonesmorepracticalthanoffewerlargerones.Incontrasttothearchaicperiodwithitstendencytoever-increasingblocksizes,GreektemplesoftheclassicalageliketheParthenoninvariablyfeaturedstoneblocksweighinglessthan15-20tons.Also,thepracticeoferectinglargemonolithiccolumnswaspracticallyabandonedinfavorofusingseveralcolumndrums.Althoughtheexactcircumstancesoftheshiftfromtheramptothecranetechnologyremainunclear,ithasbeenarguedthatthevolatilesocialandpoliticalconditionsofGreeceweremoresuitabletotheemploymentofsmall,professionalconstructionteamsthanoflargebodiesofunskilledlabor,makingthecranemorepreferabletotheGreekpolisthanthemorelabor-intensiverampwhichhadbeenthenormintheautocraticsocietiesofEgyptorAssyria.ThefirstunequivocalliteraryevidencefortheexistenceofthecompoundpulleysystemappearsintheMechanicalProblems(Mech.18,853a32-853b13)attributedtoAristotle(384-322BC),butperhapscomposedataslightlylaterdate.Aroundthesametime,blocksizesatGreektemplesbegantomatchtheirarchaicpredecessorsagain,indicatingthatthemoresophisticatedcompoundpulleymusthavefounditswaytoGreekconstructionsitesbythen.AncientRomeTheheydayofthecraneinancienttimescameduringtheRomanEmpire,whenconstructionactivitysoaredandbuildingsreachedenormousdimensions.TheRomansadoptedtheGreekcraneanddevelopeditfurther.Wearerelativelywellinformedabouttheirliftingtechniques,thankstoratherlengthyaccountsbytheengineersVitruvius(DeArchitectura10.2,1-10)andHeronofAlexandria(Mechanica3.2-5).TherearealsotwosurvivingreliefsofRomantreadwheelcranes,withtheHateriitombstonefromthelatefirstcenturyADbeingparticularlydetailed.ThesimplestRomancrane,theTrispastos,consistedofasingle-beamjib,awinch,arope,andablockcontainingthreepulleys.Havingthusamechanicaladvantageof3:1,ithasbeencalculatedthatasinglemanworkingthewinchcouldraise150kg(3pulleysx50kg=150),assumingthat50kgrepresentthemaximumeffortamancanexertoveralongertimeperiod.Heaviercranetypesfeaturedfivepulleys(Pentaspastos)or,incaseofthelargestone,asetofthreebyfivepulleys(Polyspastos)andcamewithtwo,threeorfourmasts,dependingonthemaximumload.ThePolyspastos,whenworkedbyfourmenatbothsidesofthewinch,couldalreadylift3000kg(3ropesx5pulleysx4menx50kg=3000kg).Incasethewinchwasreplacedbyatreadwheel,themaximumloadevendoubledto6000kgatonlyhalfthecrew,sincethetreadwheelpossessesamuchbiggermechanicaladvantageduetoitslargerdiameter.Thismeantthat,incomparisontotheconstructionoftheEgyptianPyramids,whereabout50menwereneededtomovea2.5tonstoneblockuptheramp(50kgperperson),theliftingcapabilityoftheRomanPolyspastosprovedtobe60timeshigher(3000kgperperson).However,numerousextantRomanbuildingswhichfeaturemuchheavierstoneblocksthanthosehandledbythePolyspastosindicatethattheoverallliftingcapabilityoftheRomanswentfarbeyondthatofanysinglecrane.AtthetempleofJupiteratBaalbek,forinstance,thearchitraveblocksweighupto60tonseach,andonecornercorniceblockevenover100tons,allofthemraisedtoaheightofabout19m.InRome,thecapitalblockofTrajansColumnweighs53.3tons,whichhadtobeliftedtoaheightofabout34m(seeconstructionofTrajansColumn).ItisassumedthatRomanengineersliftedtheseextraordinaryweightsbytwomeasures(seepicturebelowforcomparableRenaissancetechnique):First,assuggestedbyHeron,aliftingtowerwassetup,whosefourmastswerearrangedintheshapeofaquadranglewithparallelsides,notunlikeasiegetower,butwiththecolumninthemiddleofthestructure(Mechanica3.5).Second,amultitudeofcapstanswereplacedonthegroundaroundthetower,for,althoughhavingalowerleverageratiothantreadwheels,capstanscouldbesetupinhighernumbersandrunbymoremen(and,moreover,bydraughtanimals).ThisuseofmultiplecapstansisalsodescribedbyAmmianusMarcellinus(17.4.15)inconnectionwiththeliftingoftheLateranenseobeliskintheCircusMaximus(ca.357AD).Themaximumliftingcapabilityofasinglecapstancanbeestablishedbythenumberoflewisironholesboredintothemonolith.IncaseoftheBaalbekarchitraveblocks,whichweighbetween55and60tons,eightextantholessuggestanallowanceof7.5tonperlewisiron,thatispercapstan.Liftingsuchheavyweightsinaconcertedactionrequiredagreatamountofcoordinationbetweentheworkgroupsapplyingtheforcetothecapstans.MiddleAgesDuringtheHighMiddleAges,thetreadwheelcranewasreintroducedonalargescaleafterthetechnologyhadfallenintodisuseinwesternEuropewiththedemiseoftheWesternRomanEmpire.Theearliestreferencetoatreadwheel(magnarota)reappearsinarchivalliteratureinFranceabout1225,followedbyanilluminateddepictioninamanuscriptofprobablyalsoFrenchorigindatingto1240.Innavigation,theearliestusesofharborcranesaredocumentedforUtrechtin1244,Antwerpin1263,Bruggein1288andHamburgin1291,whileinEnglandthetreadwheelisnotrecordedbefore1331.Generally,verticaltransportcouldbedonemoresafelyandinexpensivelybycranesthanbycustomarymethods.Typicalareasofapplicationwereharbors,mines,and,inparticular,buildingsiteswherethetreadwheelcraneplayedapivotalroleintheconstructionoftheloftyGothiccathedrals.Nevertheless,botharchivalandpictorialsourcesofthetimesuggestthatnewlyintroducedmachinesliketreadwheelsorwheelbarrowsdidnotcompletelyreplacemorelabor-intensivemethodslikeladders,hodsandhandbarrows.Rather,oldandnewmachinerycontinuedtocoexistonmedievalconstructionsitesandharbors.Apartfromtreadwheels,medievaldepictionsalsoshowcranestobepoweredmanuallybywindlasseswithradiatingspokes,cranksandbythe15thcenturyalsobywindlassesshapedlikeashipswheel.Tosmoothoutirregularitiesofimpulseandgetoverdead-spotsintheliftingprocessflywheelsareknowntobeinuseasearlyas1123.Theexactprocessbywhichthetreadwheelcranewasreintroducedisnotrecorded,althoughitsreturntoconstructionsiteshasundoubtedlytobeviewedincloseconnectionwiththesimultaneousriseofGothicarchitecture.Thereappearanceofthetreadwheelcranemayhaveresultedfromatechnologicaldevelopmentofthewindlassfromwhichthetreadwheelstructurallyandmechanicallyevolved.Alternatively,themedievaltreadwheelmayrepresentadeliberatereinventionofitsRomancounterpartdrawnfromVitruviusDearchitecturawhichwasavailableinmanymonasticlibraries.Itsreintroductionmayhavebeeninspired,aswell,bytheobservationofthelabor-savingqualitiesofthewaterwheelwithwhichearlytreadwheelssharedmanystructuralsimilarities.StructureandplacementThemedievaltreadwheelwasalargewoodenwheelturningaroundacentralshaftwithatreadwaywideenoughfortwoworkerswalkingsidebyside.Whiletheearliercompass-armwheelhadspokesdirectlydrivenintothecentralshaft,themoreadvancedclasp-armtypefeaturedarmsarrangedaschordstothewheelrim,givingthepossibilityofusingathinnershaftandprovidingthusagreatermechanicaladvantage.Contrarytoapopularlyheldbelief,cranesonmedievalbuildingsiteswereneitherplacedontheextremelylightweightscaffoldingusedatthetimenoronthethinwallsoftheGothicchurcheswhichwereincapableofsupportingtheweightofbothhoistingmachineandload.Rather,craneswereplacedintheinitialstagesofconstructionontheground,oftenwithinthebuilding.Whenanewfloorwascompleted,andmassivetiebeamsoftheroofconnectedthewalls,thecranewasdismantledandreassembledontheroofbeamsfromwhereitwasmovedfrombaytobayduringconstructionofthevaults.Thus,thecranegrewandwanderedwiththebuildingwiththeresultthattodayallextantconstructioncranesinEnglandarefoundinchurchtowersabovethevaultingandbelowtheroof,wheretheyremainedafterbuildingconstructionforbringingmaterialforrepairsaloft.Lessfrequently,medievalilluminationsalsoshowcranesmountedontheoutsideofwallswiththestandofthemachinesecuredtoputlogs.MechanicsandoperationIncontrasttomoderncranes,medievalcranesandhoists-muchliketheircounterpartsinGreeceandRome-wereprimarilycapableofaverticallift,andnotusedtomoveloads