跨越70年·中国的故事(湖南篇)--湖南频道--人民网

0SYKl-news 发布时间: 2020-07-06 12:26:20

“pk10冠 亚 军 和 值 群”Nocomputations,drawings,ordemonstrationsofanykindwillbeemployedtorelievethemindofthereaderfromthecareofrememberingandadependenceonhisownexertions.Drawings,constants,formul?,tables,rules,withallthatpertainstocomputationinmechanics,arealreadyfurnishedinmanyexcellentbooks,whichleavenothingtobeadded,andsuchbookscanbestudiedatthesametimewithwhatispresentedhere.Drilling,morethananyotheroperationinmetalcutting,requiresthesenseoffeeling,andisfartherfromsuchconditionsasadmitofpowerfeeding.Thespeedatwhichadrillmaycutwithoutheatingorbreakingisdependentuponthemannerinwhichitisgroundandthenatureofthematerialdrilled,theworkingconditionsmaychangeatanymomentasthedrillingprogresses;sothathandfeedismostsuitable.Drillingmachinesarrangedwithpowerfeedforboringshouldhavesomemeansofpermanentlydisengagingthefeedingmechanismtopreventitsuseinordinarydrilling.

3.Thevalvemustmoveafterthehammerstops.

CHAPTERXXII.PATTERN-MAKINGANDCASTING.

Byexaminingthehistoryofgreatachievementsinthemechanicarts,itwillbeseenthatsuccesshasbeenmainlydependentuponpredictingfuturewants,aswellasuponanabilitytosupplysuchwants,andthatthecommercialvalueofmechanicalimprovementsisoftenmeasuredbyconditionsthattheimprovementsthemselvesanticipate.Theinventionofmachine-madedrills,forexample,wasbutasmallmatter;butthedemandthathasgrownupsince,andbecauseoftheirexistence,hasrenderedthisimprovementoneofgreatvalue.Mouldedbearingsforshaftswerealsoatriflingimprovementwhenfirstmade,butithassinceinfluencedmachineconstructioninAmericainawaythathasgivengreatimportancetotheinvention.

Water-wheels,nexttosteam-engines,arethemostcommonmotiveagents.Forcenturieswater-wheelsremainedwithoutmuchimprovementorchangedowntotheperiodofturbinewheels,whenitwasdiscoveredthatinsteadofbeingaverysimplematter,thescienceofhydraulicsandwater-wheelsinvolvedsomeveryintricateconditions,givingrisetomanyproblemsofscientificinterest,thatintheendhaveproducedtheclassknownasturbinewheels.CHAPTERXXVI.STEAM-HAMMERS.

Planingmachinesofthemostimprovedconstructionaredrivenbytwobeltsinsteadofone,andmanymechanicalexpedientshavebeenadoptedtomovethebeltsdifferentially,sothatbothshouldnotbeonthedrivingpulleyatthesametime,butmoveonebeforetheotherinalternateorder.Thisiseasilyattainedbysimplyarrangingthetwobeltswiththedistancebetweenthemequaltooneandone-halforoneandthree-fourthtimesthewidthofthedrivingpulley.Theeffectisthesameasthataccomplishedbydifferentialshiftinggearing,withtheadvantageofpermittinganadjustmentoftherelativemovementofthebelts.

Itwillbepropertomentionhere,whatwillbemorefullypointedoutintheIntroduction,thatalthoughworkshopprocessesmaybescientificallyexplainedandproved,theymustneverthelessbelearnedlogically.Thisview,itishoped,willnotleadtoanythinginthebookbeingconstruedasadisparagementoftheimportanceoftheoreticalstudies.

Oneofthepeculiaritiesoftrip-hammersasmachinesisthemechanicaldifficultiesinconnectingthemwiththedrivingpower,especiallyincaseswherethereareanumberofhammerstobedrivenfromoneshaft.

Machineswithdirectaction,suchaspunches,shears,orrolls,requirefirstatrainofmechanismofsomekindtoreducethemotionfromthedrivingpowersoastoattainforce;andsecondly,[105]thisforcemustbebalancedorresistedbystrongframing,shafts,andbearings.Apunching-machine,forexample,musthaveframingstrongenoughtoresistathrustequaltotheforceappliedtothework;hencetheframesofsuchmachinesarealwaysahugemass,disposedinthemostadvantageouswaytomeetandresistthisreactiveforce,whilethemaindetailsofadrop-machinecapableofexertinganequalforceconsistonlyofablockandapairofguidestodirectitscourse.

Agreatshareofthehandlingaboutanengineeringestablishmentisavoided,ifmaterialcanbestoredandreceivedonahigherlevelthantheworkingfloors;if,forinstance,coal,iron,andsandisreceivedfromrailwaycarsatanelevationsufficienttoallowittobedepositedwhereitisstoredbygravity,itisequivalenttosavingthepowerandexpenserequiredtoraisethematerialtosuchaheight,ormoveitandpileitup,whichamountstothesamethingintheend.Itisnotproposedtofollowthedetails[74]ofshoparrangement,fartherthantofurnishacluetosomeofthegeneralprinciplesthatshouldberegardedindevisingplansofarrangement.Suchprinciplesaremuchmoretoberelieduponthanevenexperienceinsuggestingthearrangementofshops,becauseallexperiencemustbegainedinconnectionwithspeciallocalconditions,whichoftenwarpandprejudicethejudgment,andleadtoerrorinformingplansundercircumstancesdifferentfromthosewheretheexperiencewasgained.

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网友评论

  • 雨花郡主8M4N5

    Movementsandstrainsmaybecalledtwooftheleadingconditionsuponwhichdesignsformachinesarebased:movementsdeterminegeneraldimensions,andstrainsdeterminetheproportionsandsizesofparticularparts.Movementandstraintogetherdeterminethenatureandareaofbearingsorbearingsurfaces.

     

  • 雪灵芝6dIU3

  • 如萱紫儿5nIu8

  • 但法国cN7MX

     6.Handlingmaterialinmachineconstructionisoneoftheprincipalexpensestobedealtwith;eachtimeapieceismoveditscostisenhanced,andusuallyinamuchgreaterdegreethanissupposed.

  • 大笨蛋UP4Pe

    v

  • 扒皮成Q1a5a

    Opposedtothemaintenanceofstandarddimensionsarethevariationsinsizeduetotemperature.Thisdifficultyappliesaliketogaugingimplementsandtopartsthataretobetested;yetinthis,asinnearlyeveryphenomenonconnectedwithmatter,wehavesucceededinturningittosomeusefulpurpose.Bandsofiron,suchasthetiresofwheelswhenheated,canbe'shrunk'on,andacompressiveforceandsecurityattained,whichwouldbeimpossiblebyforcingthepartstogetherbothatthesametemperature.Shrinkinghas,however,beenalmostentirelyabandonedforsuchjointsascanbeaccuratelyfitted.

     

  • 草莓果hVTFt

    Machinesarecomposedofpartsconnectedtogetherbyrigidandmovablejoints;rigidjointsarenecessarybecauseoftheexpense,andinmostcasestheimpossibility,ofconstructingframingandotherfixeddetailinonepiece.

     

  • 绿卡纸cqmA7

     (1.)Whycannottheregularworkingdrawingsofamachinebeemployedtoconstructpatternsby?—(2.)Whatshoulddeterminethequalityordurabilityofpatterns?—(3.)Howcanthearrangementofpatternsaffectcertainpartsofacasting?—(4.)Whatmeanscanbeemployedtoavoidinherentstrainincastings?—(5.)Whyisthetopofacastinglesssoundthanthebottomordragside?—(6.)Whatarecoresemployedfor?—(7.)Whatismeantbyventingamould?—(8.)Explainthedifferencebetweengreenanddrysandmouldings.—(9.)Whyissandemployedformoulds?—(10.)Whatgenerallycausesthedisarrangementofcoresincasting?—(11.)Whyarecastingsoftensprungorcrooked?—(12.)Whatshoulddeterminetheamountofdraughtgiventopatterns?—(13.)Whatarethemeansgenerallyadoptedtoavoidcoolingstrainsincastings?

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