机械设计准则外文翻译

SomeRulesforMechanicalDesignDesigningstarswithaneed,realorimagined.Existingapparatusmayneedimprovementsindurability,efficiently,weight,speed,orcost.Newapparatusmaybeneededtoperformafunctionpreviouslydonebymen,suchascomputation,assembly,orservicing.Withtheobjectivewhollyorpartlydefined,thenextstopindesignistheconceptionofmechanismsandtheirarrangementsthatwillperformtheneededfunctions.Forthis,freehandsketchingisofgreatvalue,notonlyasarecordofone’sthoughtsandasanaidindiscussionwithothers,butparticularlyforcommunicationwithone’sownmind,asastimulantforcreativeideas.Whenthegeneralshapeandafewdimensionsoftheseveralcomponentsbecomeapparent,analysiscanbegininearnest.Theanalysiswillhaveasitsobjectivesatisfactoryorsuperiorperformance,plussafetyanddurabilitywithminimumweight,andacompetitivecost.Optimumproportionsanddimensionswillbesoughtforeachcriticallyloadedsection,togetherwithabalancebetweenthestrengthoftheseveralcomponents.Materialsandtheirtreatmentwillbechosen.Theseimportantobjectivescanbeattainedonlybyanalysisbasedupontheprinciplesofmechanics,suchasthoseofstaticsforreactionforcesandfortheoptimumutilizationoffriction;ofdynamicsofinertia,acceleration,andenergy;ofelasticityandstrengthofmaterialsforstressanddeflection;andoffluidmechanicsforlubricationandhydrodynamicdrives.Finally,adesignbaseduponfunctionandreliabilitywillbecompleted,andaprototypemaybebuilt.Ifitstestsaresatisfactory,andifthedeviceistobeproducedinquantity,theinitialdesignwillundergocertainmodificationsthatenableittobemanufacturedinquantityatalowercost.Duringsubsequentyearsofmanufactureandservice,thedesignislikelytoundergochangesasnewideasareconceivedorasfurtheranalysisbasedupontestsandexperienceindicatealterations.Salesappeal,customersatisfaction,andmanufacturecostareallrelatedtodesign,andabilityindesignisintimatelyinvolvedinthesuccessofanengineeringventure.Tostimulatecreativethought,thefollowingrulesaresuggestedforthedesigner.Applyingenuitytoutilizedesiredphysicalpropertiesandtocontrolundesiredones.Theperformancerequirementsofamachinearemetbyutilizinglawsofnatureorpropertiesofmatter(e.g.,flexibility,strength,gravity,inertia,buoyancy,centrifugalforce,principlesoftheleverandinclinedplane,friction,viscosity,fluidpressure,andthermalexpansion),alsothemanyelectrical,optical,thermal,andchemicalphenomena.However,whatmaybeusefulinoneapplicationmaybedetrimentalinthenext.Flexibilityisdesiredinvalvespringsbutnotinthevalvecamshaft;frictionisdesiredattheclutchfacebutnotintheclutchbearing.Ingenuityindesignshouldbeappliedtoutilizeandcontrolthephysicalpropertiesthataredesiredandtominimizethosethatarenotvideforfavorablestressdistributeandstiffnesswithminimumweight.Oncomponentssubjectedtofluctuatingstress,particularattentionisgiventoareductioninstressconcentration,andtoanincreaseofstrengthatfillets,threads,holes,andfits.Stressreductionaremadebymodificationinshape,andstrengtheningmaybedonebyprestressingtreatmentssuchassurfacerollingandshallowhardening.Hollowshaftsandtubing,andboxsectionsgiveafavorablestressdistribution,togetherwithstiffnessandminimumweight.Sufficientstiffnesstomaintainalignmentanduniformpressurebetweencontactingsurfacesshouldbeprovidedforcrank,cam,andgearshafts,andforenclosuresandframescontainingbearingsupports.Thestiffnessofshaftsandothercomponentsmustbesuitabletoavoidresonantvibrations.Usebasicequationstocalculateandoptimizedimensions.Thefundamentalequationsofmechanicsandtheothersciencesaretheacceptedbasesforcalculations.Theyaresometimesrearrangedinspecialformstofacilitatethedeterminationoroptimizationofdimensions,suchasthebeamandsurfacestressequationsfordetermininggear-toothsize.Factorsmaybeaddedtoafundamentalequationforconditionsnotanalyticallydeterminable,e.g.,onthinsteeltubes,anallowanceforcorrosionaddedtothethicknessbasedonpressure.Whenitisnecessarytoapplyafundamentalequationfortoshapes,materials,orconditionswhichonlyapproximatetheassumptionsforitsderivation,itisdoneinamannerwhichgivesresults”onthesafeside”.Insituationswheredataareincomplete,equationsofthesciencesmaybeusedasproportioningguidestoextendasatisfactorydesigntonewcapacities.choosematerialsforacombinationofproperties.Materialsshouldbechosenforacombinationofpertinentproperties,notonlyforstrengths,hardness,andweight,butsometimesforresistancetoimpact,corrosion,andloworhightemperatures.Costandfabricationpropertiesarefactors,suchasweldability,machinability,sensitivitytovariationinheat-treatingtemperatures,andrequiredcoating.selectcarefullybetweenstockandintegralcomponents.Apreviouslydevelopedcomponentsisfrequentlyselectedbyadesignerandhiscompanyfromthestocksofpartsmanufacturers,ifthecomponentmeettheperformanceandreliabilityrequirementsandisadaptablewithoutadditionaldevelopmentcoststotheparticularmachinebeingdesigned.However,itsselectionshouldbecarefullymadewithafullknowledgeofitsproperties,sincethereputationandliabilityofthecompanysufferifthereisafailureinanyoneofthemachine’sparts.Inothercasesthestrength,reliability,andcostrequirementsarebettermetifthedesignerofthemachinealsodesignsthecomponent,withtheparticularadvantageofcompactnessifitisdesignsintegralwithothercomponents,e.g.,gearstobeforgedinclustersorintegralwithashaft.Provideforaccuratelocationandnon-interferenceofpartsinassembly.Agooddesignprovidesforthecorrectlocatingofpartsandforeasyassemblyandrepair.Shouldersandpilotsurfacesgiveaccuratelocationwithoutmeasurementduringassembly.Shapescanbedesignedsothatpartscannotbeassembledbackwardsorinthewrongplace.Interferences,asbetweenscrewsintappedholes,andbetweenlinkagesmustbeforeseenandprevended.Inaccuratealignmentandpositioningbetweensuchassembliesmustbeavoided,orprovisionmustbemadetominimizeanyresultingdetrimentaldisplacementsandstresses.机械设计准则设计时从实际或者假想的需要开始的。对于现有的设备可能需要在耐用性、效率、重量、速度或成本等方面做进一些改进工作；也可能需要新的设备完成以前由人来做的工作，如计算或装配。当目标完全或部分被确定以后，下一个设计步骤是对能够完成所需要功能的机构及其布局进行总体设计。对于此项工作，徒手画的草图是很有价值的，它不仅可以记录下我们的想法，而且还有助于与别人进行讨论，特别是和自己的大脑进行交流，从而促进创新想法的产生。当一些零件的大致形状和几个尺寸被确定后，就可以开始认真的分析工作。分析工作的目的是要在重量最轻、成本最低的情况下，获得令人满意，即优良的工作性能，并且还要安全耐用。对于每个关键承载截面，应该寻求最佳的比例和尺寸，同时要对这几个零件的受理进行平衡。要对材料和处理方式进行选择。只有根据力学原理进行分析才能达到这些重要目的。这些分析包括根据静力学原理分析反作用力和充分利用摩擦力，根据动力学原理分析惯性、加速度和能量；根据弹性力学和材料力学分析应力和变形；根据流体力学来分析润滑和流体传动。最后，完成基于功能要求和可靠性所进行的设计，且要制作一台样机。如果试验结果令人满意，而且该装置将要进行批量生产，就应该对最初提出的设计方案做一些修改，使其能以较低的成本进行批量生产。在以后的制造和使用期内，如果产生了新的想法或者根据试验和经验所做的进一步分析结果表明，可以有更好的替代方案，则很可能对原设计方案进行修改。销售吸引力、客户的满意程度和制造成本均与设计有关，而设计能力则与工程创新的实现是密切相关的。为激发创造性思维，建议设计人员遵循下列准则。创造性地利用所需要的物理性能和控制不需要的物理性能。可以利用自然法则或物质的性能（如柔性，强度，重力，惯性，浮力，离心力；杠杆原理和斜面原理，摩擦，粘性，流体压力和热膨胀）和许多电学，光学和化学现象来满足一台机器的设计要求。一种性能在某种场合下可能是有用的，而在另外一种场合下则可能是有害的。阀门的弹簧应该有柔性，而阀门的凸轮轴就不需要柔性。离合器结合面上需要有摩擦，而离合器轴承却不需要摩擦。设计时，需要创造性地利用和控制所要的物理性能，将不需要的物理性能减至最小。在重量最轻的情况下，提供合理的应力分布和刚度。对于承受交变应力的零件，应该特别注意减轻应力集中和提高圆角，螺纹和配合处得强度。改变零件的形状，可以降低它所承受的应力，对零件施加预应力，如表面滚压和浅表面硬化，均可使其得到强化。空心轴和空心管道，箱形截面能获得有利的应力分布，同时具有强度高而重量轻的特点。曲轴，凸轮轴以及含有轴承支座的外壳和构架都应有足够的刚度以保证直线对中精度和接触表面之间的压力均匀分布。轴和其他零件须