英文资料.docx
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英文资料.docx
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英文资料
SandCasting
Thefirststageintheproductionofsandcastingsmustbethedesignandmanufactureofasuitablepattern.Castingpatternsaregenerallymadefromhardwordandthepatternhastobemadelargerthanthefinishedcastingsizetoallowfortheshrinkagethattakesplaceduringsolidificationandcooling.Theextentofthisshrinkagevarieswiththetypeofmetaloralloytobecast.Forallbutthesimplestshapesthepatternwillbemadeintwoormorepiecestofacilitatemoulding.Ifahollowcastingistobemadethepatterndesignwillincludeextensionpiecessothatspacestoacceptthesandcorearemouldedintosand.Theseadditionalspacesinthemouldaretermedcoreprints.
Sandmouldsfortheproductionofsmallandmedium-sizedcastingsaremadeinamouldingbox.Themouldismadeintwoormorepartsinorderthatthepatternmayberemoved.
Thedraghalfofthemouldboxisplacedonaflatfirmboardandthedraghalfofhepatternplacedinposition.Facingsandissprinkledoverthepatternandthenthemouldboxisfilledwithmouldingsand.Thesandisrammedfirmlyaroundthepattern.Thisprocessoffillingandrammingmaybedonebyhandbutmouldproductionisautomatedinalargefoundrywiththemouldboxesmovingalongaconveyor,firstlytobefilledwithsandfromhoppersandthentopassundermechanicalhammersforramming.Whenrammingofthesandiscomplete,excesssandisremovedtoleaveasmoothsurfaceflushwiththeedgesofthemouldingbox.
Thecompleteddragisnowturnedoverandtheupper,orcope,portionofthemouldingboxpositionedoverit.Thecopehalfofthepatternisplacedinposition,correctalignmentbeingensuredbymeansofsmalldowelpins.Patternsforthenecessaryfeeder,runnerandrisersarealsoplacedsoastogiveanevendistributionofmetalintothemouldcavity.Therisersshouldcoincidewiththehighestreadilyescapefromthemould.Thesizesofrisersshouldbesuchthatthemetalinthemdoesnotfreezetoorapidly.Animportantfunctionofariseristoactasreservoirofliquidmetaltofeedsolidificationwithinthemould.Athincoatingofdrypartingsandissprinkledintomouldatthisstage.Thisistopreventthecopeanddragstickingtogetherwhenthecopehalfismoulded.Thecopeisnowfilledwithmouldingsandandthisisrammedfirmlyintoshapeinthesamemannerasinthemakingofthedrag.
Aftertherammingofsandinthecopeiscompletedthetwohalvesofthemouldingboxarecarefullyseparated.Atthisstageventingofthemouldingboxarecarefullyseparated.Atthisstageventingofthemouldcanbedone,ifnecessary,toincreasethepermeabilityofthemould.Afterventingthepatternsarecarefullyremovedfrombothcopeanddrag,andagateorgatesarecarefullycuttoconnecttherunnerchannelwiththemaincavity.Gatesshouldbesitedtoallowforentryintomouldwithaminimumofturbulence.Anyloosesandisgentlyblownawayandifacoreistobeuseditthecopeuponthedraganditisthenreadyforuse.Liquidmetalispouredsmoothlyintothemouldviathefeeder.Pouringceaseswhenliquidmetalappearsatthetopoftherisersandthefeederchannelisalsofull.
Whenthemetalthathasbeenpouredintoasandmouldhasfullysolidifiedthemouldisbrokenandcastingisremoved.Thecastingstillhastherunnerandrisersattachedtoitandtherewillbesandadheringtoportionsofthesurface.Runnersandrisersarecutoffandreturnedtothemeltingfurnace.Sandcoresarebrokenandadherentsandiscleanedfromthesurfacebyvibrationorbysandblastingwithdrysand.Anyfinsormetalflashformedatmouldpartinglinesareremovedbygrindingandthecastingsarethenreadyforinspection.
Engine
Anengineisadevicethatusestheenergyinafueltodowork.Theenergyinthechemicalsofthefuelisturnedintoheatenergy.Theheatisthenusedtomovethemetalpartsofamachine.Therearemanykindsoffuel.Mostenginesusegasoline,oil,kerosene,coal,orcoke.Theheatthatcomesfromburningthefuelmakesagasexpand.Thisexpandgasdrivespistonsorturbineblades.Thepistonsorturbinesturnshafts.Theturningshaftsmovegearsandotherwheels.Weusetheserotatingwheelsandshaftstomoveautomobiles,airplanes,andothertransport.Wecanalsousethemforpumping,drilling,digging,andothersuchactivities.
Earlyenginesburnedcoalorwoodtoheatwater.Thesteamwasusedtodrivesteamengine.Untilthemiddleofthiscentury,mostlocomotiveswerepoweredbysteam.Atthebeginningofthecentury,evensomeautomobileswererunonsteam.Westillusesteamengines,butmostofthemarebeingreplacedbymoreefficientengines.Todaywehavepowerfulgasolineanddieselenginestoworkforus.
Thesteamengineisanexternalcombustionengine.Thismeansthatfueliscombustion,orburned,outsidethecylinderthatproducespower.SincetheinventionofthesteamenginebyJamesWattin1769s,steamengineshadagreateffectontheindustrialrevolutionintheeighteenthcentury.Untilthemiddleofthetwentiethcentury,mostlocomotiveswerepoweredbysteam.Attheendofthecentury,evensomemobileswererunonsteam.Todaymostofthefunctionsofsteamenginehavebeentakenoverbyinternalcombustionenginesfueledbygasolineanddieseloil.However,therearemanysteamenginesinuseatsea.Steamisalsousedtogenerateelectricity.Thekindofsteamenginethatisusedtodaydoesnothavecylinders.Itisgenerallyasteamturbine.ThesteamturbinewasinventedbySirCharlesParsons.Steamturbinescanhandlesteamathigherpressuresthanpistonenginescan,andtheyaremorecompactthanpistonenginesofsimilarpower.
ThegasolineengineTheengineofmostautomobilesandsmallvehicleusegasolineasfuel.Thegasolineengineisacombustionengine.Thefuelisburnedincombustion(burning)chambersinsidetheengine.Thecombustionchambersareplacedatoneendofthecylinders.Pistonsmoveupanddowninthecylinders.Theyarepushedbythehotgasesfromtheburningfuel.Whenthefuelismixedwithairitburnssoquicklythatitexplodes.Thecombustionchambersandcylinderapartlikeabomb,theexplosionsimplykickshandagainsttheheadofthepiston.Itpushesitasfarasitcan.
Eachmovementofapistonupordowninitscylinderiscalledastroke.Mostgasolineenginesworkonafour-strokecycle.Thismeansthateachpistongoesupanddowntwiceforeachexplosion.Thatmakesfourmovementorstrokes.Thiscycleofeventsisrepeatedoverandoveragain.Onthefirstdownstroke,thepistonmovestothelowestpartofthecylinder.Amixtureofgasolinedropletsandairisdrawnintothecylinderaboveit.Nowthepistonmovesupagain.Thisisitssecondstroke.Itsqueezesthemixtureintoasmallspace.Anelectriclightsthemixture,anditexplodes.Thepistonisforcedownagainforitsthirdstroke.Thisiscalledthepowerstroke.Forthefourthstroke,thepistonmovestothetopagain.Thistimeitpushestheburntgasesoutofthecylinder.Thegasesleavetheengineasexhaustfumes.
Thefirstenginethatusedthefour-strokecyclewasmadeinabout1876.ItwasdesignedbyaGermanengineer,AugustOtto.Heusedcoalgas,notgasoline.ThefirstenginestoburngasolineweredevelopedbyKarlBenzandGottlieb.Thesetwomenwerefamousasautomobilepioneers.
Apistonsimplygoingupanddowncannotpushanautomobilealong.Itsmovementmustbechangedtoaturningmovement.Todothis,acrankshaftisused.Eachpistonoftheengineislinkedtopartofthecrankshaft.Eachpushitgivesmakestheshaftturn.Thespinningshaftpassesthepowerontotheautomobile'stransmissionsystem.Itusuallydoesthisthroughaheavyflywheel.Thetransmissionsystemtransmitspowertotheclutchandtothepropellershaft,throughagearbox.Thepropellershaftdrivestheroadwheelsbymeansofaxles.
Tokeepanautomobileenginegoing,thereneedtobeseveralsystems.Theremustbeafuelsystem.Thishastosupplygasolinetotheenginecylindersintherightamounts.Ithasalsotomixitwiththerightamountofair,sothatitwillexplodeproperly.Theremustbeanignitionsystem.Thishastoprovidesparkstoignitetheexplosivemixturefatexactlytherighttime.Therehastobeacoolingsystem,otherwisetheenginewouldoverheat.Thelubricationsystemmustkeepallthemovingpartsoiledandmovingfreely.Toomuchfrictioncauseswearofthemetalandmakestheengineoverheat.
TheengineunitGasolineenginehastwobasicparts.Theyarecalledthecylinderheadandthecylinderblock.Thecylinderblockismachinedfromsolidmetal.Themetalsusuallycastsidethecylinderblocksarethecylinders.Thewallsofthecylindershavetobeveryaccuratelymade,andarehighlypolished.Thepistonsthatmoveupanddowninthecylindersmustbeaccuratelymade,too.Theyhavespringybandsofmetalaroundthemtopresstightagainstthecylinderwallsandstopgasesleaking.Thebandsarecalledpistonrings.Theyareoftenmadeofaluminumalloyforstrengthandlightness.
Anenginemayhaveanynumberofcylinders.Theymaybearrangedinaline,orinoppositepairs.Theyareoftenarrangedina"V"shape.Inmanyairplaneswithpitonengines,thecylindersarearrangedinaringaroundthecrankshaft.
Thelowerpartofthecylinderblockiscalledthecrankcase.Thisiswherethecrankshaftlies.Thecrankshaftislinkedtoeachpitonbyaconnectingrod.Thecrankshaftismadeinasinglepiece.Itmustbetoughandaccuratelymachined.Itmayspinasmanyas6000timesaminute.Itchangesup-and-downmotion.Itdoesthisbymeansofcranks,oneforeachpiston.Thecranksaresetatdifferentang
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