起重机的用途与历史.docx

起重机的用途与历史.docx

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起重机的用途与历史

TheUseandHistoryofCrane

Everytimeweseeacraneinactionweremainswithoutwords,thesemachinesaresometimesreallyhuge,takinguptonsofmaterialhundredsofmetersinheight.Wewatchwithamazementandabitofterror,thinkingaboutwhatwouldhappeniftheloadcomesofforifthemovementofthecranewaswrong.Itisareallyfascinatingsystem,surprisingbothadultsandchildren.Theseareespeciallytowercranes,butinrealitythereareplentyoftypesandtheyareinuseforcenturies.Thecranesareformedbyoneormoremachinesusedtocreateamechanicaladvantageandthusmovelargeloads.Cranesareequippedwithawinder,awireropeorchainandsheavesthatcanbeusedbothtoliftandlowermaterialsandtomovethemhorizontally.Itusesoneormoresimplemachinestocreatemechanicaladvantageandthusmoveloadsbeyondthenormalcapabilityofahuman.Cranesarecommonlyemployedinthetransportindustryfortheloadingandunloadingoffreight,intheconstructionindustryforthemovementofmaterialsandinthemanufacturingindustryfortheassemblingofheavyequipment.

1.Overview

ThefirstconstructioncraneswereinventedbytheAncientGreeksandwerepoweredbymenorbeastsofburden,suchasdonkeys.Thesecraneswereusedfortheconstructionoftallbuildings.Largercraneswerelaterdeveloped,employingtheuseofhumantreadwheels,permittingtheliftingofheavierweights.IntheHighMiddleAges,harborcraneswereintroducedtoloadandunloadshipsandassistwiththeirconstruction–somewerebuiltintostonetowersforextrastrengthandstability.Theearliestcraneswereconstructedfromwood,butcastironandsteeltookoverwiththecomingoftheIndustrialRevolution.

Formanycenturies,powerwassuppliedbythephysicalexertionofmenoranimals,althoughhoistsinwatermillsandwindmillscouldbedrivenbytheharnessednaturalpower.Thefirst'mechanical'powerwasprovidedbysteamengines,theearlieststeamcranebeingintroducedinthe18thor19thcentury,withmanyremaininginusewellintothelate20thcentury.Moderncranesusuallyuseinternalcombustionenginesorelectricmotorsandhydraulicsystemstoprovideamuchgreaterliftingcapabilitythanwaspreviouslypossible,althoughmanualcranesarestillutilizedwheretheprovisionofpowerwouldbeuneconomic.

Cranesexistinanenormousvarietyofforms–eachtailoredtoaspecificuse.Sizesrangefromthesmallestjibcranes,usedinsideworkshops,tothetallesttowercranes,usedforconstructinghighbuildings.Forawhile,mini-cranesarealsousedforconstructinghighbuildings,inordertofacilitateconstructionsbyreachingtightspaces.Finally,wecanfindlargerfloatingcranes,generallyusedtobuildoilrigsandsalvagesunkenships.Thisarticlealsocoversliftingmachinesthatdonotstrictlyfittheabovedefinitionofacrane,butaregenerallyknownascranes,suchasstackercranesandloadercranes.

2.History

AncientGreece

ThecraneforliftingheavyloadswasinventedbytheAncientGreeksinthelate6thcenturyBC.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.

AncientRome

TheheydayofthecraneinancienttimescameduringtheRomanEmpire,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,thecapitalblockofTrajan'sColumnweighs53.3tons,whichhadtobeliftedtoaheightofabout34m（seeconstructionofTrajan'sColumn）.

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.

MiddleAges

DuringtheHighMiddleAges,thetreadwheelcranewasreintroducedonalargescaleafterthetechnologyhadfallenintodisuseinwesternEuropewiththedemiseoftheWesternRomanEmpire.Theearliestreferencetoatreadwheel（magnarota）reappearsinarchivalliteratureinFranceabout1225,followedbyanilluminateddepictioninamanuscriptofprobablyalsoFrenchorigindatingto1240.Innavigation,theearliestusesofharborcranesaredocumentedforUtrechtin1244,Antwerpin1263,Bruggein1288andHamburgin1291,whileinEnglandthetreadwheelisnotrecordedbefore1331.

Generally,verticaltransportcouldbedonemoresafelyandinexpensivelybycranesthanbycustomarymethods.Typicalareasofapplicationwereharbors,mines,and,inparticular,buildingsiteswherethetreadwheelcraneplayedapivotalroleintheconstructionoftheloftyGothiccathedrals.Nevertheless,botharchivalandpictorialsourcesofthetimesuggestthatnewlyintroducedmachinesliketreadwheelsorwheelbarrowsdidnotcompletelyreplacemorelabor-intensivemethodslikeladders,hodsandhandbarrows.Rather,oldandnewmachinerycontinuedtocoexistonmedievalcons