机械设计外文翻译机器和机器零件的设计.docx
- 文档编号:17185511
- 上传时间:2023-07-22
- 格式:DOCX
- 页数:20
- 大小:50.38KB
机械设计外文翻译机器和机器零件的设计.docx
《机械设计外文翻译机器和机器零件的设计.docx》由会员分享,可在线阅读,更多相关《机械设计外文翻译机器和机器零件的设计.docx(20页珍藏版)》请在冰点文库上搜索。
机械设计外文翻译机器和机器零件的设计
Designofmachineandmachineelements
Machinedesign
Machinedesignistheartofplanningordevisingneworimprovedmachinestoaccomplishspecificpurposes.Ingeneral,amachinewillconsistofacombinationofseveraldifferentmechanicalelementsproperlydesignedandarrangedtoworktogether,asawhole.Duringtheinitialplanningofamachine,fundamentaldecisionsmustbemadeconcerningloading,typeofkinematicelementstobeused,andcorrectutilizationofthepropertiesofengineeringmaterials.Economicconsiderationsareusuallyofprimeimportancewhenthedesignofnewmachineryisundertaken.Ingeneral,thelowestover-allcostsaredesigned.Considerationshouldbegivennotonlytothecostofdesign,manufacturethenecessarysafetyfeaturesandbeofpleasingexternalappearance.Theobjectiveistoproduceamachinewhichisnotonlysufficientlyruggedtofunctionproperlyforareasonablelife,butisatthesametimecheapenoughtobeeconomicallyfeasible.
Theengineerinchargeofthedesignofamachineshouldnotonlyhaveadequatetechnicaltraining,butmustbeamanofsoundjudgmentandwideexperience,qualitieswhichareusuallyacquiredonlyafterconsiderabletimehasbeenspentinactualprofessionalwork.
Designofmachineelements
Theprinciplesofdesignare,ofcourse,universal.Thesametheoryorequationsmaybeappliedtoaverysmallpart,asinaninstrument,or,toalargerbutsimilarpartusedinapieceofheavyequipment.Innoease,however,shouldmathematicalcalculationsbelookeduponasabsoluteandfinal.Theyareallsubjecttotheaccuracyofthevariousassumptions,whichmustnecessarilybemadeinengineeringwork.Sometimesonlyaportionofthetotalnumberofpartsinamachinearedesignedonthebasisofanalyticcalculations.Theformandsizeoftheremainingpartsaredesignedonthebasisofanalyticcalculations.Ontheotherhand,ifthemachineisveryexpensive,orifweightisafactor,asinairplanes,designcomputationsmaythenbemadeforalmostalltheparts.
Thepurposeofthedesigncalculationsis,ofcourse,toattempttopredictthestressordeformationinthepartinorderthatitmaysagelycarrytheloads,whichwillbeimposedonit,andthatitmaylastfortheexpectedlifeofthemachine.Allcalculationsare,ofcourse,dependentonthephysicalpropertiesoftheconstructionmaterialsasdeterminedbylaboratorytests.Arationalmethodofdesignattemptstotaketheresultsofrelativelysimpleandfundamentaltestssuchastension,compression,torsion,andfatigueandapplythemtoallthecomplicatedandinvolvedsituationsencounteredinpresent-daymachinery.
Inaddition,ithasbeenamplyprovedthatsuchdetailsassurfacecondition,fillets,notches,manufacturingtolerances,andheattreatmenthaveamarketeffectonthestrengthandusefullifeofamachinepart.Thedesignanddraftingdepartmentsmustspecifycompletelyallsuchparticulars,mustspecifycompletelyallsuchparticulars,andthusexercisethenecessaryclosecontroloverthefinishedproduct.
Asmentionedabove,machinedesignisavastfieldofengineeringtechnology.Assuch,itbeginswiththeconceptionofanideaandfollowsthroughthevariousphasesofdesignanalysis,manufacturing,marketingandconsumerism.Thefollowingisalistofthemajorareasofconsiderationinthegeneralfieldofmachinedesign:
①Initialdesignconception;
②Strengthanalysis;
③Materialsselection;
④Appearance;
⑤Manufacturing;
⑥Safety;
⑦Environmenteffects;
⑨Reliabilityandlife;
Strengthisameasureoftheabilitytoresist,withoutfails,forceswhichcausestressesandstrains.Theforcesmaybe;
①Graduallyapplied;
②Suddenlyapplied;
③Appliedunderimpact;
④Appliedwithcontinuousdirectionreversals;
⑤Appliedatloworelevatedtemperatures.
Ifacriticalpartofamachinefails,thewholemachinemustbeshutdownuntilarepairismade.Thus,whendesigninganewmachine,itisextremelyimportantthatcriticalpartsbemadestrongenoughtopreventfailure.Thedesignershoulddetermineaspreciselyaspossiblethenature,magnitude,directionandpointofapplicationofallforces.Machinedesignismot,however,anexactscienceanditis,therefore,rarelypossibletodetermineexactlyalltheappliedforces.Inaddition,differentsamplesofaspecifiedmaterialwillexhibitsomewhatdifferentabilitiestoresistloads,temperaturesandotherenvironmentconditions.Inspiteofthis,designcalculationsbasedonappropriateassumptionsareinvaluableintheproperdesignofmachine.
Moreover,itisabsolutelyessentialthatadesignengineerknowshowandwhypartsfailsothatreliablemachineswhichrequireminimummaintenancecanbedesigned.Sometimes,afailurecanbeserious,suchaswhenatireblowsoutonanautomobiletravelingathighspeeds.Ontheotherhand,afailuremaybenomorethananuisance.Anexampleisthelooseningoftheradiatorhoseintheautomobilecoolingsystem.Theconsequenceofthislatterfailureisusuallythelossofsomeradiatorcoolant,aconditionwhichisreadilydetectedandcorrected.
Thetypeofloadapartabsorbsisjustassignificantasthemagnitude.Generallyspeaking,dynamicloadswithdirectionreversalscausegreaterdifficultiesthanstaticloadsand,therefore,fatiguestrengthmustbeconsidered.Anotherconcerniswhetherthematerialisductileorbrittle.Forexample,brittlematerialsareconsideredtobeunacceptablewherefatigueisinvolved.
Ingeneral,thedesignengineermustconsiderallpossiblemodesoffailure,whichincludethefollowing:
①Stress;
②Deformation;
③Wear;
④Corrosion;
⑤Vibration;
⑥Environmentaldamage;
⑦Looseningoffasteningdevices.
Thepartsizesandshapesselectedmustalsotakeintoaccountmanydimensionalfactorswhichproduceexternalloadeffectssuchasgeometricdiscontinuities,residualstressesduetoformingofdesiredcontours,andtheapplicationofinterferencefitjoint.
Selectedfrom”designofmachineelements”,6thedition,m.f.sports,prentice-hall,inc.,1985and“machinedesign”,AnthonyEsposito,charlese.,Merrillpublishingcompany,1975.
Mechanicalpropertiesofmaterials
Thematerialpropertiescanbeclassifiedintothreemajorheadings:
(1)physical,
(2)chemical,(3)mechanical
Physicalproperties
Densityorspecificgravity,moisturecontent,etc.,canbeclassifiedunderthiscategory.
Chemicalproperties
Manychemicalpropertiescomeunderthiscategory.Theseincludeacidityoralkalinity,react6ivityandcorrosion.Themostimportantoftheseiscorrosionwhichcanbeexplainedinlayman’stermsastheresistanceofthematerialtodecaywhileincontinuoususeinaparticularatmosphere.
Mechanicalproperties
Mechanicalpropertiesincludeinthestrengthpropertiesliketensile,compression,shear,torsion,impact,fatigueandcreep.Thetensilestrengthofamaterialisobtainedbydividingthemaximumload,whichthespecimenbearsbytheareaofcross-sectionofthespecimen.
ThisisacurveplottedbetweenthestressalongtheThisisacurveplottedbetweenthestressalongtheY-axis(ordinate)andthestrainalongtheX-axis(abscissa)inatensiletest.Amaterialtendstochangeorchangesitsdimensionswhenitisloaded,dependinguponthemagnitudeoftheload.Whentheloadisremoveditcanbeseenthatthedeformationdisappears.FormanymaterialsthisoccursoptoacertainvalueofthestresscalledtheelasticlimitAp.Thisisdepictedbythestraightlinerelationshipandasmalldeviationthereafter,inthestress-straincurve(fig.3.1)
.Withintheelasticrange,thelimitingvalueofthestressuptowhichthestressandstrainareproportional,iscalledthelimitofproportionalityAp.Inthisregion,themetalobeyshookes’slaw,whichstatesthatthestressisproportionaltostrainintheelasticrangeofloading,(thematerialcompletelyregainsitsoriginaldimensionsaftertheloadisremoved).Intheactualplottingofthecurve,theproportionalitylimitisobtainedataslightlylowervalueoftheloadthanthe
elasticlimit.Thismaybeattributedtothetime-lagintheregainingoftheoriginaldimensionsofthematerial.Thiseffectisveryfrequentlynoticedinsomenon-ferrousmetals.
Whichironandnickelexhibitclearrangesofelasticity,copper,zinc,tin,arefoundtobeimperfectlyelasticevenatrelativelylowvalueslowvaluesofstresses.Actuallytheelasticlimitisdistinguishablefromtheproportionalitylimitmoreclearlydependinguponthesensitivityofthemeasuringinstrument.
Whentheloadisincreasedbeyondtheelasticlimit,plasticdeformationstarts.Simultaneouslythespecimengetswork-hardened.Apointisreachedwhenthedeformationstartstooccurmorerapidlythantheincreasingload.ThispointiscalledtheyyieldpointQ.themetalwhichwasresistingtheloadtillthen,startstodeformsomewhatrapidly,i.e.,yield.TheyieldstressiscalledyieldlimitAy.
TheelongationofthespecimencontinuesfromQtoSandthentoT.Thestress-strainrelationinthisplasticflowperiodisindicatedbytheportionQRSTofthecurve.Atthespecimenbreaks,andthisloadiscalledthebreakingload.ThevalueofthemaximumloadSdividedbytheoriginalcross-sectionalareaofthespecimenisreferredtoastheultimatetensilestrengthofthemetalorsimplythetensilestrengthAu.
Logicallyspeaking,oncetheelasticlimitisexceeded,themetalshouldstarttoyield,andfinallybreak,withoutanyincreaseinthevalueofstress.Butthecurverecordsanincreasedstressevenaftertheelasticlimitisexceeded.Tworeasonscanbegivenforthisbehavior:
①Thestrainhardeningofthematerial;
②Thediminishingc
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 机械设计 外文 翻译 机器 零件 设计