机械工程导论外文.docx

机械工程导论外文.docx

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机械工程导论外文

8.3CASESTUDYINCONCEPTUALDESIGN：

MOUSETRAP-POWEREDVEHICLES

Asappliedtoengineering,theadage"wellbegunishalfdone"reinforcestheimportanceofcreativebrainstormingintheconceptualphaseofthehigh-leveldesignprocedure.Bywayofacasestudyinthissection,wetracetheprogressofahypotheticalteamofengineeringstudentsastheygenerateconceptsfordesigningamousetrap-poweredvehicle.Smalltoyvehiclesthatarepoweredbyhouseholdmousetrapsaresometimesthesubjectofdesignandconstructionprojectsinengineeringandsciencecourses.Readilyvisualizedandbuilt,thesevehiclesareausefulmeansforexperiencingtheconceptualdesignprocessandforgaininganappreciationforthetrade-offsthatmustbemadeforadesigntosatisfyallofitsconstraints.

Inourillustrativecasestudy,teamsofengineeringstudentsarechallengedtobuildsmallvehiclesthattravel10metersasquicklyaspossiblebutthatarepoweredbyonlythepotentialenergystoredinamousetrapspring.Eachvehiclewillbedesigned,built,refined,andoperatedbyateamofthreestudents.Attheconclusionof-theproject,theteamswillcompeteagainstoneanotherinhead-to-headracesduringatournament,sothefinalproductsmustbebothdurableandreusable.Inadditiontotheoverallobjectiveofproducingafastvehicle,severalotherspecificationsmustbemet:

1.Themassofthevehiclecannotexceed500g.

2.Thevehiclemustfitcompletelywithina0.1-m3boxatthestartofeachrace.

3.Eachvehiclewillraceinalanethatis10mlongbutonly1mwide.Ifanypartofthevehiclepassesoutsidethelaneduringa～，theteamwillbedisqualifiedfromthetournament.

4.Thevehiclemustremainincontactwiththesurfaceofthelaneduringtheentirerace.

5.Thevehiclemustbepoweredonlybyastandardhouseholdmousetrap.Energythatisincidentallystoredbyotherelasticelementsorobtainedfromachangeinelevationofthevehicle'scenterofmassmustbenegligible.

6.Tapecannotbeusedasafastenerinthevehicle'sconstruction.

Eachofthesespecificationsconstrains,indifferentways,thehardwarethattheteamswillultimatelyproduce.Ifanysinglerequirementisnotmet,theentiredesignwillbeinadequate,regardlessofhowwellthevehiclemightperformrelativetotheotherrequirements.Forinstance,becausetheracinglaneis10timeslongerthanitiswide,thevehiclemustnotonlybefastbutmustalsotravelinareasonablystraightline.Ifaparticularvehiclesometimesveersoutsidethelane,thenitcouldbedefeatedbyaslowervehicle（evenamuchslowerone）inahead-to-headrace.Thedesignteamsrecognizethatthevehiclesshouldnotbeoptimizedwithrespecttoonlyonespecification,butrathershouldbebalancedtomeetallrequirements.

Wenextfollowthethoughtprocessofahypotheticalteamasitbeginstobrainstormandidentifymultipledesignconcepts.Thestudentsdocumenttheirideasinaboundnotebook,andtheyusewrittencommentsandhanddrawingstodescribeeachconcept.Subsequently,theteamwillrecordprogressasprototypesareconstructedandtheoutcomeoftheirtestinganditerationefforts.Inshort,the"notebookservesasalogtochronicletheteam'sentiredesignexperience.Inindustrialresearch'anddevelopmentsettings,suchnotebooksareoftendated,signed,andevenwitnessedinordertoformallydocumentaproduct'sdevelopment.Withaneyetowardyourownprofessionalcareer,youshouldalsobeginthepracticeofsystematicallyrecording,revising,anddevelopingyouroriginalideas.

FirstConcept:

StringandLeverArm

Anideathatemergesfromtheteam'sfirstbrainstormingsessionisbasedonusingthemousetrap'ssnaparmtopullandunwrapstringfromadriveaxle.Together,theteammemberssketchtheconceptshowninFigure8.4.Asthetrapsnapsshut,stringisunwoundfromaspoolthatisattachedtotherearaxleandthevehicleispropelledforward.Theconceptvehicleincorporatesalever～thatlengthensthesnaparm,pullsmorestringfromtheaxle,andchangesthevelocityratiobetweenthemousetrapandthedrivewheels.

Althoughthisconcepthasthepositiveattributeofbeingsimpleandstraightforwardtoconstruct,theteamrecognizesthatanumberofquestionsareraised,andtheyalsolisttheseintheirnotebook.

·Whatshouldbethelengthoftheleverarmextensionandtheradiusofthespoolattachedtothedriveaxle?

Withalong-enoughstring,:

thevehiclewouldbegraduallypoweredbythemousetrapovertheentire10-mdistance.Ontheotherhand,ifthestringisshorter,themousetrapwillclosesoonerandthevehiclewouldcoastafterbeingpoweredonlyalongthefirstportionoftheracecourse.Theteam'sdiscussionofthisissuepromptstheideaforataperedspool,assketchedinFigure8.4（c）,whichwouldenablethevelocityratiobetweenthemousetrapandthedriveaxletochangeasthemousetrapcloses.

·Shouldthemousetrapbepositionedbehind,above,orinfrontofthedriveaxle?

Intheirconceptsketch,thestudentsdrewthemousetrapdirectlybetweenthefrontandrearwheels.Atthisstage,however,thatplacementisarbitrary,andtheteamhasnoreasontoexpectthatchoicetobebetterthananyother.

Whatshouldbetheradiusofthewheels?

Likethelengthoftheleverarmextensionandtheradiusofthespool,theradiusofthedrivewheelsinfluencesthevelocityofthevehicle.Theteamnotedonitssketchthatcomputercompactdiscscouldbeusedasthewheels,butthevehiclemightpostabetterracetimewithdifferent-diameterwheels.

Theteamrecordsthesequestionsanddiscussiontopicsintheirnotebook,buttheyleavethemforfutureconsideration.Atthisearlypointinthebrainstormingprocess,nodecisionswillbemadeondimensionsormaterials.However,ifthisconcepteventuallyemergesasapromisingcandidate,theteamwillneedtoresolvetheseissuesbeforeaviableprototypecouldbeconstructed.

SecondConcept:

CompoundGeartrain

Astheirdiscussionscontinue,theteamnextdevisestheoptionshowninFigure8.5iswhichacompoundgeartraintransferspowerfromthemousetraptothedriveaxle.Thisvehiclehasonlythreewheels,and“portionofthebodyhasbeenremovedtofurther

reduceweight.Theconceptincorporatesatwo-stagegeartrain,anditsvelocityratioissetbythenumbersofteethonthefourgearsshowninFigure8.5（b）.Theteam'sillustrationofatwo-stagegeartrainisarbitrary；asystemwithonlyonestage,ormorethantwostages,mightbepreferable.However,thestudentsacceptsuchambiguity,andtheyrealizethatadecisionforthegeartrain'svelocityratioisnotnecessaryatthisearlyjuncture.

Duringthegive-and-takeoftheirmeetings,theteamidentifiesadditionalconstraintsthatarecommontotheirfirstandsecondconcepts.Forinstance,thestudentsagreethatthevehicleshouldbedesignedsothatthedrivewheelsdonotspinandslipasthevehicleaccelerates.Otherwise,someportionofthelimitedenergythatisavailablefromthemousetrapspringwouldbewasted.Topreventslippage,weightcouldbeaddedtothevehicleinordertoimprovecontactbetweenthedrivewheelsandtheground.Ontheotherhand,aheaviervehiclewouldtravelslowerasthepotentialenergyofthemousetrapspringisconvertedintothevehicle'skineticenergy.Astheyinvestigatetheprojectinmoredetail,thestudentsseethatthetechnicalissuesathandareinterrelated.Eveninthecontextofthisseeminglystraightforwardexercise,nottomentionsuchchallengesasthemechanicalengineeringprofession'stoptenachievements（Section1.3）,designersmustgrapplewithcompetingconstraintsandspecifications.

ThirdConcept:

Sector-ShapedGear

Theteam'sthirdconceptcombinesandextendscertainideasthataroseduringtheirdiscussionsoftheearlierconcepts.ThedesignofFigure8.6incorporatesageartrainbetweenthemousetrapandthedrivewheels,butitenablesthevehicletocoastoncethetraphasclosed.Thestudentsenvisionsuchavehicleasacceleratingquicklyoverthefirstfewmetersoftheracecourse,reachingpeakvelocity,andthencoastingovertheremainingdistance.Intheirconcept,asector-shapedgear,insteadofafullcircularone,isattachedtothesnaparmofthemousetrap.Asmallnotchatoneendofthegearenablesthemousetraptodisengagefromthesimplegeartrainoncethesnaparmhasclosed,asshownintheFigure8.6（c）.Thesectorgearservesastheinputtothegeartrain,andtheoutputgearisdirectlyattachedtothefrontdriveaxle.Theidlergearisincludedtoincreasetheoffsetbetweenthemousetrapandthefrontaxle.

Withseveralconceptsinmind,thestudentsarenowatapointtobeginmakingtradeoffs,narrowingdowntheiroptions,andexperimentingwithprototypes.Inaddition,theyconsidervariousmaterialsthatareavailableanddesirableforconstruction.Althoughitisstillprematuretoselectspecificcomponents,thestudentsusetheirimaginationstolistsomeoptions:

foamcoreposterboard,balsaandpoplarwood,aluminumandbrasstubing,threadedrod,plexiglass,ballbearings,oilandgraphitelubricants,wire,andepoxy.Afteraddressingsomeofthetechnicalissuesthathavebeenraised,andperformingback-of-the-envelopecalculationsandorder-of-magnitudeapproximations,theteammightnextdecidetobuildandtestafewprototypesbeforeselectingonetoberefinedindetail.

8.4CASESTUDYINCOMPUTER-AIDEDDESIGN:

NONINVASIVEMEDICALIMAGING

InChapterl,weintroducedcomputer-aidedengineeringanddescribeditsimpactasoneofmechanicalengineering'stoptenachievements:

Engineersdesign,analyze,simulate,andmanufacture:

products:

withthe.aidof:

special-purposecomputersoft