机械工程专业英语 第一课文及阅读材料.docx
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机械工程专业英语第一课文及阅读材料
Lesson1themechanicaldesignprocess
1,themechanicaldesignprocess
Theultimateobjectiveofmechanicaldesignistoproduceausefulproductthatsatisfiestheneedsofacustomerandthatissafe,efficient,reliable,economical,andpracticaltomanufacture.thinkbroadlywhenansweringthequestion,”whoisthecustomerfortheproductorsystemIamabouttodesign?
”
itisessentialthatyouknowthedesiresandexpectationsofallcustomersbeforeproductdesign.marketingprofessionalsareoftenemployedtomanagethedefinitionofcustomerexpectations,butdesignerswilllikelyworkwiththemasapartofaproductdevelopmentteam.
manymethodsareusedtodeterminewhatthecustomerwants.Onepopularmethod,calledqualityfunctiondeploymentorQFD,seeks
(1)toidentifyallofthefeaturesandperformancefactorsthatcustomersdesireand
(2)toassesstherelativeimportanceofthesefactors.theresultoftheQFDprocessisadetailedsetoffunctionsanddesignrequirementsfortheproduct.
Itisalsoimportanttoconsiderhowthedesignprocessfitswithallfunctionsthatmusthappentodeliverasatisfactoryproducttothecustomerandtoservicetheproductthroughoutitsliftcycle.infact,itisimportanttoconsiderhowtheproductwillbedisposedofafterithasserveditsusefullife.thetotalofallsuchfunctionthataffecttheproductissometimescalledtheproductrealizationprocessorPRP.someofthefactorsincludedinPRPareasfollows:
1marketingfunctionstoassesscustomerrequirements
2researchtodeterminetheavailabletechnologythatcanreasonablybeusedintheproduct
3availabilityofmaterialsandcomponentsthatcanbeincorporatedintotheproduct
4productdesignanddevelopment
5performancetesting
6documentationofthedesign
7vendorrelationshipsandpurchasingfunctions
8work-forceskills
9physicalplantandfacilitiesavailable
10capabilityofmanufacturingsystems
11productionplanningandcontrolofproductionsystems
12productionsupportsystemsandpersonnel
13qualitysystemsrequirements
14salesoperationsandtimeschedules
15costtargetsandothercompetitiveissues
16customerservicerequirements
17environmentalconcernsduringmanufacture,operationanddisposaloftheproduct
18legalrequirements
19availabilityoffinancialcapital
Canyouaddtothislist?
Youshouldbeabletoseethatthedesignofaproductisbutonepartofacomprehensiveprocess.inthistext,wewillfocusmorecarefullyonthedesignprocessitself,buttheproducibilityofdesignsmustalwaysbeconsidered.thissimultaneousconsiderationofproductdesignandmanufacturingprocessdesignisoftencalledconcurrentengineering.
2skillsneededinmechanicaldesign
Productengineersandmechanicaldesignersuseawiderangeofskillsandknowledgeintheirdailywork.theseskillsandknowledgeareincludedinthefollowing:
1sketching,technicaldrawing,andcomputer-aideddesign
2propertiesofmaterials,materialsprocessing,andmanufacturingprocesses
3applicationsofchemistry,suchascorrosionprotection,plating,andpainting
4statics,dynamics,strengthofmaterials,kinematics,andmechanisms
5fluidmechanics,thermodynamics,andheattransfer
6fluidpower,thefundamentalsofelectricalphenomena,andindustrialcontrols
7experimentaldesignandperformancetestingofmaterialsandmechanicalsystems
8stressanalysis
9specializedknowledgeofthebehaviorofmachineelements,suchasgears,beltdrives,chaindrives,shafts,bearings,keys,splines,couplings,seals,spring,connections(bolted,riveted,welded,adhesive),electricmotors,linearmotiondevices,clutches,andbrakes
10creativity,problemsolving,andprojectmanagement
11oralcommunication,listening,technicalwriting,andteamworkskills
3functions,designrequirementsandevaluationcriteria
Section1emphasizedtheimportanceofcarefullyidentifyingtheneedsandexpectationsofthecustomerpriortobeginningthedesignofamechanicaldevice.youcanformulatethesebyproducingclear,completestatementsoffunctions,designrequirements,andevaluationcriteria:
1Functionstelluswhatthedevicemustdo,usinggeneral,nonquantitativestatementsthatemployactionphrasessuchas“tosupportaload”,“toliftacrate”,“totransmitpower”,or“toholdtwostructuralmemberstogether”,etc
2Designrequirementsaredetailed,usuallyquantitativestatementsofexpectedperformancelevels,environmentalconditionsinwhichthedevicemustoperate,limitationsonspaceorweight,oravailablematerialsandcomponentsthatmaybeused
3Evaluationcriteriaarestatementsofdesirablequalitativecharacteristicsofadesignthatassistthedesignerindecidingwhichalternativedesignisoptimum-thatis,thedesignthatmaximizesbenefitswhileminimizingdisadvantages
Togethertheseelementscanbecalledthespecificationsforthedesign
Mostdesignsprogressthroughacycleofactivitiesareoutlinedinfigure1.1.youshouldtypicallyproposemorethanonepossiblealternativedesignconcept.Thisiswherecreativityisexercisedtoproducetrulynoveldesigns.Eachdesignconceptmustsatisfythefunctionsanddesignrequirements.acriticalevaluationofthedesirablefeatures,advantages,anddisadvantagesofeachdesignconceptshouldbecompleted.thenarationaldecisionanalysistechniqueshouldusetheevaluationcriteriatodecidewhichdesignconceptistheoptimumand,therefore,shouldbeproduced.
Readingmaterial
Mechanicaldesignistheprocessofdesigningand/orselectingmechanicalcomponentsandputtingthemtogethertoaccomplishadesiredfunction.Ofcoursemachineelementsmustbecompatible,mustfitwelltogether,andmustperformsafelyandefficiently.Thedesignermustconsidernotonlytheperformanceoftheelementbeingdesignedatagiventime,butalsotheelementswithwhichitmustinterface.
Toillustratehowthedesignofmachineelementsmustbeintegratedwithalargermechanicaldesign,letusconsiderthedesignofaspeedreducerforthesmalltractor.supposethat,toaccomplishthespeedreduction,youdecidetodesignadouble-reduction,spurgearspeedreducer.Youspecifyfourgears,threeshafts,sixbearings,andahousingtoholdtheindividualelementsinproperrelationtoeachother.Theprimaryelementsofthespeedreducerare:
1theinputshaftistobeconnectedtothepowersource,agasolineenginewhoseoutputshaftrotatesat2000rpm.aflexiblecouplingistobeemployedtominimizedifficultieswithalignment.
2thefirstpairofgears,aandb,causesareductioninthespeedoftheintermediateshaftproportionaltotheratioofthenumbersofteethinthegears.Gearbandcarebothmountedtointermediateshaftandrotateatthesamespeed.
3akeyisusedattheinterfacebetweenthehubofeachgearandtheshaftonwhichitismountedtotransmittorquebetweenthegearandtheshaft.
4thesecondpairofgears,candd,furtherreducesthespeedofgeardandtheoutputshafttotherangeof290rpmto295rpm.
5theoutputshaftistocarryachainsprocket.thechaindriveultimatelyistobeconnectedtothedrivewheelofthetractor.
6eachofthethreeshaftsissupportedbytwoballbearings.makingthemstaticallydeterminateandallowingtheanalysisofforcesandstressesusingstandardprinciplesofmechanics.
7thebearingsareheldinahousingthatistobeattachedtotheframeofthetractor.notethemannerofholdingeachbearingsothattheinnerracerotateswiththeshaftwhiletheouterraceisheldstationary.
8sealsareontheinputandoutputshaftstoprohibitcontaminantsfromenteringthehousing.
9detailsofhowtheactiveelementsaretobeinstalled,lubricated,andalignedareonlysuggestedatthisstageofthedesignprocesstodemonstratefeasibility.onepossibleassemblyprocesscouldbeasfollows.
Startbyplacingthegearingthegears,keys,spacers,andbearingsontheirrespectiveshafts
Theninsertinputshaftintoitsbearingseatontheleftsideofthehousing.
Inserttheleftendofintermediateshaftintoitsbearingseatwhileengagingtheteethofgearsaandb.
Installthecenterbearingsupporttoprovidesupportforthebearingattherightsideofinputshaft.
Installoutputshaftbyplacingitsleftbearingintotheseatonthecenterbearingsupportwhileengaginggearscandd.
Installtherightsidecoverforthehousingwhileplacingthefinaltwobearingsintheirseats.
Ensurecarefulalignmentoftheshafts.
Placegearlubricantinthelowerpartofhousing.
Thearrangementofthegears,theplacementofthebearingssothattheystraddlethegears,andthegeneralconfigurationofthehousingarealsodesigndecisions.thedesignprocesscannotrationallyproceeduntilthesekindsofdecisionsaremade.whentheoveralldesignisconceptualized,thedesignoftheindividualmachineelementsinthespeedreducercanproceed.youshouldrecognizethatyouhavealreadymademanydesigndecisionsbyrenderingsuchasketch.first,youchoosespurgearsratherthanhelicalgears,awormandwormgear,orbevelgears.infact,othertypesofspeedreductiondevices–beltdrives,chaindrives,ormanyothers—couldbeappreciate.
1gears
forthegearpairs,youmustspecifythenumberofteethineachgear,thepitch(size)oftheteeth,thepitchdiameters,thefacewidth,andthematerialanditsheattreatment.thesespecificationsdependonconsiderationsofstrengthandwearofthegearteethandthemotionrequirements(kinematics).Youmustalsorecognizethatthegearsmustbemountedonshaftsinamannerthatensuresproperlocationofthegears,adequate