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标题党了一把,其实这是个机械英语教学帖,当然,里面干货也不少。
今天这篇文章的主题是engineeringcalculation。是一个资深以色列工程师对于刚入行的年轻人在工程计算上的意见和建议。
老规矩,我先把原文贴出来,大家自由翻译文章,并把译文贴在回复中,可以是文章中的一句,一段,有空的同学翻译全文也是极好的,参与学习的社友都能得到金币奖励。
以下是作者简介。作者出生在以色列,毕业于以色列理工学院(传说中的中东MIT)机械工程专业,专注于机械设计超过50年,目前在以色列本古里安大学教授机械光学设计。
AdamRubinstein
BorninIsrael,studiedMechanicalengineeringintheTechnion,specializedinmechanicaldesignandparticularlymechano-optics.Over50yearsexperienceasadesignengineer,andabout24ofthemasanindependentconsultant.lately,partiallyretiredandteachingmechano-opticaldesignatBGUinBeer-Sheva,Israel.
下面是正文分隔线
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1、Don'tbelieveeverythingtheyteachyouatschool,reallifeisverydifferent!
2、Youshouldimmediatelydismissfromyourheadtheideathatthereisonlyonerightanswertoanydesignquestion.Keepyourmindopentoalloptions.
3、Designfailurescanhavemanydifferentformsandallshouldbeconsidered.
4、Whatcamefirst,thechickenortheegg?Theanalysisofahypotheticaldesignwillalwaysstartwitharangeofassumptionswhichcanbeadjusted.
Thedifferencebetweenschooland“reallife”problemsolving
Whenwepassfromhighschooltouniversitywebringwithustheideathatanswersareobtainedbyengineeringcalculations.Weuseanalyticalformulasandweareeducatedtobelievethattheresultsareeitherrightorwrong–thereisnoinbetweenandnogreyareas.Weareusedtohavingalloftherelevantdataassociatedwithaproblem.IevenremembercheckingmyanswersbyverifyingthatifIusedallthedatagiveninaproblem–dowealwaysneedtouseallofthedatatogettheanswer?
Thisisnotreallife.Dataisnever“given”,itisusuallyincomplete,andthereisnotnecessarilyasingle“right”formula.Ihaveseenmanystudentsaskmewhichoftwodifferentformulas(teethstrengthandsurfacefatigue)touseforcalculatinggearwheels.Itwasverydifficulttomakethemunderstandthatbothwere“right”andthattheyshouldusetheworstcase.Alwayserronthesideofcaution.
Therearemanywaystofail(butitonlytakesone)
WhileteachingstrengthofboltsIoncecalculatedaboltforanumberofdifferentpossiblefailuremodes.Icalculatedforthe“standard”,thepullstrengthofthecoreforthelesscommonshavingofthethreadsandalsoforshearingoftheheadandafewmorebizarrepossiblemodes.Ishowedthat,althoughallarepossible,thestandardboltswillnearlyalwaysfailatthecorefirst.However,undercertainconditions(toofewmatingthreads,aweakmatingmaterial,etc)thethreadsmightshavefirst.Confused?Letmeexplain….
Forgettingtheotherpossiblefailuremodesmaybeverydangerous.AsthedesignersofthefirstBritishjetairliner,theComet,discoveredatthehighcostoflostplanesandlivesintheearly1950’s.
Therisksofdesigningfortherealworld
Itisalwaysriskytodesignsomethingnew.Highflyingjetsandtheuseofaluminiumwereprettynewinthosedays.Britishengineersalsofailedtorealisetheeffectsofrepeatingdecompressionloadsandstressconcentrationsonthefatiguestrengthofthealuminiumairframe.Calculatingforstaticloadswasjustnotenough.
Howdidtheycalculatetheirairframethen?ComputerswerestillintheirinfancyandFEAprogramswerenotwrittenyet.Theyhadtousestressformulasandmanualstructuralanalysistools.Eventoday,withourpowerfulcomputersandprograms,itisnoteasytoanalyseastructureascomplexasanairframe.Wehavebetterresearchtoolsbuttheoldproblemsandchallengesarestillthere.
Skill,commonsenseandpatience
Whattheymusthavedonewastomakesimplifications,calculatingpartsofthestructureoneatatimeandperformingtestsforrealpartsandassemblies.Thiswasinfactacombinationofsomecalculationswithexperimentaltestingtoprovethedesign.Wemustadmitthatinspiteofthecrudetoolstheyhadtheydidagoodjobafterall.
Wewouldliketothinkthattodaywedomuchbetterinourdesignwork.Indeedourtoolsarefarbetterthantheirsbutinprinciplewhatwedoisverymuchthesame.WebuildourmodelsandtestthembyapplyingFEAprograms.Thefactthatwecanrunthetestonacomputermodelratherthanonahardwaremodelisonlyadifferenceoftimeandcost.Wemuststillhaveafinisheddesignbeforewecantestoranalyseit.
Whatcamefirst,thechickenortheegg?
Itisaviciouscircle:weneedacompletedesigninordertotestit,yetwecannotcompletethedesignwithoutcalculatingit.Howdowebreakthisviciouscircle?Byguessingourwaytothedesign!Byexperience,byrulesofthethumbandbyroughcalculationsbasedonpreliminarydesignideasandsketches!
Don’tforgetthattheseearlyassumptionscanberevisitedandrecalculatedbutatleasttheyallowustobegintheprocess. |
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