机械设计英文版全册配套完整教学课件

机械设计(英文版）全册配套完整教学课件DesignofMachineElementsChapter1IntroductionTheMeaningofDesignEngineeringMechanicalEngineeringDesignMachinedesign

Engineeringisconcernedwiththeconversionofenergyfromoneformtoanotherwiththeobjectiveofextractingusefulwork.Mechanicalengineeringhasthemorespecificobjectiveofdesigningmachinerytothispurposeinthemostsuitablemannerpossible.“Engineering”

istheartof

directingthegreatsourcesofpowerinnaturefortheuseandconvenienceofhumanbeings

/utilizingtheexistingresourcesandnaturallawtobenefithumanityWaterwheelofYellowRiverPotentialenergytokineticenergyDieselgenerator

Compressionstroke压缩冲程(机械能-热能)Powerstroke做功冲程(化学能-机械能)Design

requiressignificantcreativity,practice,andvisiontobedonewell.

Machinedesign

isthefundamentalpracticeinengineering;isthetransformationofconceptsandideasintousefulmachinery.

Machinedesign

istheapplicationofscienceandtechnologytodeviseneworimprovedproductsforthepurposeofsatisfyinghumanneeds.

Designproblemsare,almostwithoutexception,open-endedproblemscombininghardscienceandcreativity.DefinitionofMachineComposition1.AboutMachine1)DefinitionofMachine

Amachineisacombinationofmechanismsandothercomponentsthattransforms,transmits,orusesenergy,load,ormotionforaspecificpurpose.

Amachineisacombinationofcomponentswhichcantransmitpowerinacontrolledmannerbyconvertingenergyfromoneformtoanotherandeventuallyproducingusefulwork.

Intermsofbasicmanufacturingunit,amachineiscomposedbymachineelements/components.Intermsofthefunctionsofthesub-systemofamachine,itiscomprisedofthesource-power,transmission,executionandcontrol/manipulationparts.Fortheconvenienceofkinematicordynamicanalysisofamachine,amachinecanbedecomposedintomechanisms.2)CompositionofMachine3)ClassificationofMachineElementsNormalloadtransmitter:slidingbearings滑动轴承,rolling-elementbearings滚动轴承Torquetransmitter:gears,tractiondrives牵引传动,chaindrives链传动,beltdrives带传动,powerscrews螺纹连接Energyabsorber:brakes制动器anddampers减震器Supportingelements:shafts,springs弹簧,seals密封Connectionandjoints:threadfasteners螺纹紧固件,keys键,couplings联轴器slidingbearingsrolling-elementbearingsOffshoredrillingplatformWheelshaftoflocomotivechaindrives链传动ApplicationBeltdrive带传动Automobileengine(multiwedgebelt)

汽车发动机（多楔带）Automobileengine(synchronousbelt)汽车发动机（同步带）powerscrews螺纹连接2.MachineDesign—ADiscipline-blendingHumanEndeavorMachinedesigninvolvesmanydecision-making,includingengineeringonesandnon-engineeringones.Engineeringdecisions:loading载荷,kinematics动力学,materials材料;strength强度,reliability可靠性,deformation变形,tribology摩擦学,weight重量,etc.Non-engineeringdecisions:cost成本,marketability可销售性,productliability产品责任,ethics伦理学,politics政治,etc.

FundamentalDesignConsiderations1Strength强度2Reliability3Thermaleffect4Corrosion腐蚀5Wear磨损6Friction摩擦7Processing8Utility9Cost10Safety11Weight12Noise13Styling造型14Shape形状15Size尺寸16Flexibility17Control18Stiffness19Surfacefinish20Lubrication21Maintenance22Volume3.BasicRequirementsfor

MachineDesignEconomicsandfunctionalityarealwayspressingconcerns,andgooddesigninherentlymeanssafe,economical,andfunctionaldesign.Aprimaryresponsibilityofanymechanicaldesigneristoensurethattheproposeddesignwillfunctionasintended,safelyandreliably,fortheprescribeddesignlifetimeand,atthesametime,competesuccessfullyinthemarketplace.1)FunctionalRequirements

Themachinetobedesignedshouldfulfillspecifiedfunctions.Thisrequires:Fundamentaldecisionsregardingloading,kinematics,andthechoiceofmaterialsmustbemadeproperlyduringthedesignofamachine.Besides,strength,reliability,deformation,tribology(friction,wear,andlubrication),alsoneedtobeconsidered.2)EconomicDemandsTheobjectiveistoproduceamachinethatnotonlyistofunctionproperly

forareasonabletimebutisalsoeconomicallyfeasible.Theeconomicrequirementsneedtobeaddressedovertheentirelifecircleofdesign,manufacturingandoperationoftheproduct.3)OtherDemands

Safetyandspacerequirements,convenienceintransportationalsoneedtobeconsidered.

Nonengineeringdecisionsregardingmarketability,productliability,ethics,politics,etc.,mustbeintegratedintothedesignprocessearly.

4.ContentsandPurposeoftheCourseFundamentaldesignprinciplesofvariousmachineelements.Eventhedesignofasingleboltorspringneedsthedesigner’sthoroughunderstandingoftheprinciplesandmethodsofmachinerydesignandmore.Masterthebasicknowledge,methodsandproceduresandgainthecompetenceinapplyingthemintopracticaldesign.OtherPurposes

DevelopcompetenceofcreativedesignandsolvingpracticalproblemMechanicalsystemdesignrequiresconsiderableflexibilityandcreativitytoobtaingoodsolutions.Creativityseemstobeaidedbyfamiliaritywithknownsuccessfuldesignsofrelatedsystems,componentsorelements.

GeneralProcedureofMachineDesign

1)Recognitionoftheneed,marketanalysis,specification;2)Definitionoftheproblem(originalconcept),conceptualdesign.3)Synthesis,designanalysisandoptimization,(detaildesign)4)Productiondrawings…5)EvaluationandpresentationTwoApproachesofMachineDesignConceptualdesignofconfiguration1)Suggestfeasibleconfigurationsandalternatives2)Analyzeandsynthesizethembydecomposingtheconsistingunits,evaluatetheirfunctionalparametersandotherrequirements.Detailedtechnicaldesign1)Completeanumberofassemblyandcomponentdrawingswhichreflectdetaileddesignbypresentingstructuraldetails,materials,geometric/dimensionaltolerances.2)Completedesigncalculationsanddetailedtechnicaldocuments.SuggestionsDesignengineerscannolongerworkaloneandmustparticipateingroupdiscussionsanddesignreviews.Theyneedgoodcommunicationsskills.Designengineerscannotmerelyfocusontheirdisciplineandrelyonexpertsfortherest.Theyneedtoknowotherdisciplines,atleastfromalinguisticsstandpoint.

Improveyourpresentationskills.1)Selectingasuitabletypeofmachineelementfromconsiderationofitsfunction2)Estimatingthesizeofthemachineelementthatislikelytobesatisfactory3)Evaluatingthemachineelement'sperformanceagainsttherequirements4)Andthenmodifyingthedesignandthedimensionsuntiltheperformanceisneartowhicheveroptimumisconsideredmostimportant5.DesignofMachineElement1)AbouttheFirstTwoStepsRequiresomecreativedecisions,representthemostdifficultpartofdesignAfterasuitabletypeofmachineelementhasbeenselectedfortherequiredfunction,thespecificmachineelementisdesignedbyanalyzingkinematics,load,andstress—coupledwithpropermaterialselection—enableastress-strain-strengthevaluationintermsofasafetyfactor2)FailurePrevention—theBasisforSuccessfulDesignofMachineElement

Designermusthaveagoodworkingknowledgeofanalyticaland/orempiricaltechniquesforpredictingpotentialfailuresatthedesignstage.Thesepredictionsmustthenbetransformedintoselectionofamaterial,determinationofashape,andestablishmentofthedimensionsforeachparttoensuresafe,reliableoperationthroughoutthedesignlifetime.

Improperfunctioning

ofamachineormachinepartconstitutesfailure.WhenitbecomescompletelyinoperableWhenitisstilloperablebutisunabletoperformitsintendedfunctionsatisfactorilyWhenseriousdeteriorationhasmadeitunreliableorunsafeforcontinueduse.3)FailureofMachineElements4)FailureModesFailureofamachinepartmightbebroughtaboutbyanyoneoracombinationofmanydifferentresponsestoloadsandenvironmentswhileinservice.Force—and/ortemperature—inducedelasticdeformationYielding屈服—plasticdeformationBrittlefracture脆性断裂;fatiguefracture疲劳断裂Surfacefailure表面实效—Wear磨损,Fatigue疲劳,ductilerupture韧性断裂,Creep蠕变,Corrosion腐蚀,spalling剥落Violationofitsintendedfunction—overallslipofbelt,Bucklingofspring

弹簧屈曲蠕变：固体材料在保持应力不变的条件下，应变随时间延长而增加的现象。它与塑性变形不同，塑性变形通常在应力超过弹性极限之后才出现，而蠕变只要应力的作用时间相当长，它在应力小于弹性极限施加的力时也能出现。FailureModesofaShaftFatiguefracture/forcommonshaftExcessiveelasticdeformation/forprecisionshaftResonantvibration/forrotatingshaftathighspeed5)AboutDesignAnalysisDesignanalysisattemptstopredictthestrengthordeformationofamachineelementsothatitcansafelycarrytheimposedloadsforaslongasrequired.Ananalysisbyitselfshouldnotbelookedonasanabsoluteandfinaltruth.

Ananalysisislimitedbytheassumptionsimposedandbyitsrangeofapplicability.6.SomeTerminologyConcerningMachineDesign1)CodesandStandards2)Reliability3)SafetyandProductLiability1)CodesandStandardsAstandardisasetofspecificationsforparts,materials,orprocessesintendedtoachieveuniformity,efficiency,andaspecifiedquality.Acodeisasetofspecificationsfortheanalysis,design,manufacture,andconstructionofsomething.

2)ReliabilityThestatisticalmeasureoftheprobabilitythatamechanicalelementwillnotfailinuseiscalledthereliabilityofthatelement.ThereliabilityRcanbemeasuredbyanumberhavingtherange(1-1)AreliabilityofR=0.90meansthatthereisa90percentchancethatthepartwillperformitsproperfunctionwithoutfailure.

3)SafetyandProductLiability

Liabilityconceptstatesthatthemanufacturerofanarticleisliableforanydamageorharmthatresultsbecauseofadefect.Thebestapproachestothepreventionofproductliabilityaregoodengineeringinallanalysisanddesign,qualitycontrol,andcomprehensivetestingprocedures.

Chapter2FundamentalsforStrengthDesign2.1DesignforStaticStrength2.2FatigueandCyclicStresses2.3DesignforCyclicLoading2.4FailureCriteriaandModesofMechanicalFailure

2.1DesignforStaticStrengthStrengthisaninherentpropertyofanelement,dependingonthechoice,thetreatmentandtheprocessingofthematerial.Astaticloadmeansaloadwhichhasanunchangingmagnitude,unchangingpointorpointsofapplication,andanunchangingdirection.Astaticloadcanbeaxialtensionorcompression,ashearload,abendingload,atorsionalload,oranycombinationofthese.Stressandstrainyieldstrength

tensilestrengthneckingAbouttheDesignDataAdesignerneedmanydatasuchastheyieldstrength（屈服强度）,theultimatestrength（极限强度）,etc.,whichcanbeprovidedbytests.Thesetestsshouldhavebeenmadeonspecimenshavingthesameheattreatment,surfacefinish,andsizeastheelementtheengineerproposestodesign,butthesetestsarecostly.Nearlyallofthenumericaldatathatweuseindesignhavesomeuncertainties,concerningthestrengthsaswellastheloads.1)FactorofSafetyToaccountforalltheuncertaintiesindesignRelatetheloadsactingonamechanicalpart,orthestressesresultingfromthoseloads,tothestrengthofthepartDeveloptherelationsbetweenstrengthandloadstoachieveoptimumcomponentdimensions.Factorofsafetyisusedbyengineerstoaccountseparatelyfortheuncertaintiesthatmayoccurinthestrengthofapartandtheuncertaintiesthatmayoccurwiththeloadsactingonthepart.Twofactorsofsafety:ns

isusedtoaccountfortheuncertaintiesinthestrength;nl,accountsfortheuncertaintieswithregardtotheload.Totalfactorofsafety:

n=nsnl

ThecommoncaseTheentirefactorofsafetyisappliedtothestrength:Stressesandarecalledthesafe,ortheallowablestresses.

nincludeallowancesfortheuncertaintiesinstrengthandtheuncertaintiesinloads.CriteriaofStrengthDesignCalculationstressshouldbesmallerthanallowablestressCalculationshearstressshouldbesmallerthanallowableshearstressFactorofsafetyshouldbelargerthanallowablefactorofsafety2)StressConcentrationStressconcentrationiscausedbydiscontinuityinamachineelement.Discontinuitiesincludechangesinthecrosssectionoftheparts,holes,grooves,notches,etc..Suchdiscontinuitiesarecalledstressraisers.Stressconcentration应力集中σL实σL名MM应力集中区Stress-concentrationFactorAtheoretical,orgeometric,stress-concentrationfactorKt

orKts

isusedtorelatetheactualmaximumstressatthediscontinuitytothenominalstress.Methodstodeterminethevaluesofstress-concentrationfactors:1)Byusingthetheoryofelasticity;2)Photo-elasticity;3)Finite-ElementTechniques4)ExperimentalmethodsPhoto-elasticityFinite-ElementTechniques2.2FatigueandCyclicStresses1)FatigueFatigueisacomplexphenomenonwhencyclicstressesarepresent.Appearingascrackpropagation,initiallyonamicroscaleandthenextremelyrapidasthefatiguecrackreachesacriticallength.Thetotalfatiguelifeisthetimeittakesacracktobeginplusthetimeitneedstopropagatethroughthecrosssection.曲轴疲劳断裂断口特征cracksourcezone裂纹源区Fatigueextendedarea疲劳扩展区finaltransientfaultzone最终瞬断区内部缺陷引起疲劳断轴：

初始裂纹起源于轴内部，裂纹由内至外扩展

裂纹源区cracksourcezone扩展区expansionarea瞬断区Transientfaultzone裂纹扩展方向Directionofcrackextension键槽应力集中导致齿轮轴疲劳断裂键槽初始裂纹源initialcracksource裂纹扩展区Crackpropagationregion最终瞬断区finaltransientfaultzoneMethodstoExtendFatigueLife1.Byminimizinginitialflaws,especiallysurfaceflaws,throughprocesses,suchasgrindingorpolishing,thatleaveexceptionallysmoothsurfaces.2.Bymaximizinginitiationtime.

Surfaceresidualstressesareimpartedorrelievedthroughmanufacturingprocesses,suchasshotpeeningorburnishing,orbyanumberofsurfacetreatments.3.Bymaximizingpropagationtime.Usingamaterialthatdoesnotpresentelongatedgrainsinthedirectionoffatiguecrackgrowthcanextendfatiguelife(e.g.,byusingcold-workedcomponentsinsteadofcastings).4.Bymaximizingthecriticalcracklength.Fracturetoughnessisanessentialproperty.Shotpeening抛丸硬化2)CyclicStress循环应力Cyclicstress,alsocalledfluctuatingoralternatingstress,isafunctionoftime,butthevariationissuchthatthestresssequencerepeatsitself.Fatiguefailureoccursatrelativelylowstresslevelstoacomponentorstructuresubjectedtofluctuatingorcyclicstresses.ParametersusedtocharacterizefluctuatingcyclicstressMeanstressStressrangeStressamplitudeStressratioAmplituderatioFigure2.1Thecyclicvariationofnonzeromeanstresswithtime.Fourpatternsofconstant-amplitudecyclicstressCompletelyreversed()orzero-meancyclicstress对称循环变应力σtσm=0；σmax=σa=

-σminr=σmin/σmax=-1Fourpatternsofconstant-amplitudecyclicstressNonzeromean(-1≤r≤+1)非对称循环变应力σtσmaxσminσmσa应力幅：σa=(σmax-σmin)/2平均应力：σm=(σmax+σmin)/2应力循环特性：r=σmin/σmax最大应力：σmax

最小应力：σminFourpatternsofconstant-amplitudecyclicstressReleasedtension()脉动循环变应力σtσmin=0；σm=σar=σmin/σmax=0Example1Forarotationshaft,theaveragestressonthedangerouscrosssectionis20MPa,thestressamplitudeis30MPa,calculatingthemaximumstress,theminimumstressandthestressratio.Nonzeromean2.3DesignforCyclicLoadingFatigueorendurancelimit:whenthestressisbelowit,thepartwillnotsufferfromfatigue.Thevalueofendurancelimitofanelementisdependentonmanyfactors,suchasthesize,shape,materialcomposition,heattreatment,stressconcentration,residualstress,corrosion,typeofstress,etc..Mostmachineelementsaredesignedonthebasisoffinitelife,ratherthaninfinitelife.2.4FailureCriteriaandModesofMechanicalFailure

Improperfunctioning

ofamachineormachinepartconstitutesfailure,i.e.,failurecriteriaisitsincapabilityofperformingitsintendedfunction.

Becauseofthechangeinthesize,shape,ormaterialpropertiesofthepart,resultingfromanyoneoracombinationofmanydifferentresponsestoloadsandenvironmentswhileinservice.FailuremodesForce-and/ortemperature-inducedelasticdeformationYielding屈服Brinnelling微动磨损Ductilerupture韧性断裂Brittlefracture脆性断裂Fatigue疲劳Corrosion腐蚀Wear磨损Impact冲击Frettingcorrosion摩擦腐蚀Creep蠕变Thermalrelaxationorstressrelaxation热或应力松弛Thermalshock热击Spalling剥落Buckling屈曲Failure失效Fractureofgearshaft齿轮轴断裂fractureofthewholecrankshaft曲轴整体断裂Fractureamplification断口放大Twofractures断口对Thebrokentoothofthespiralscrewinrollingmachine轧钢机压下螺旋丝杠断牙丝杠断牙部位丝杠断牙局部放大

螺栓联接滑移

boltslip

被联接件断裂

thefractureofconnectedpart被联件相对滑移被联件拉断Toothwear轮齿磨损Coldbonding/welding冷胶合：

Underthelowspeedandheavyload,contactpartsareextrudedandadhered,andtherelativemovementisprocessedandcontactpartsaretorn.低速重载，接触零件挤压粘着，相对运动撕裂。Thermalbonding热胶合：润滑不良引起的齿面Pittingcorrosion点蚀Plasticdeformationofgeartooth轮齿塑性变形整体塑变overallplasticdeformationGeneralfailure综合失效：toothwear

toothfracture,shaftbrokenduetofatigue齿面磨损、断齿、疲劳断轴5)AboutDesignAnalysisDesignanalysisattemptstopredictthestrengthordeformationofamachineelementsothatitcansafelycarrytheimposedloadsforaslongasrequired.Ananalysisbyitselfshouldnotbelookedonasanabsoluteandfinaltruth.

Ananalysisislimitedbytheassumptionsimposedandbyitsrangeofapplicability.Rigidity刚度RigidityRigidityisthecapacityofcomponentstoresistelasticdeformationswhensubjecttoforces,is,togetherwithstrength,oneofthemostimportantcriteriaoftheworkingcapacityofmachines.RigidityThepermissibledeflection,anglesofinclinationandtorsionaredeterminedbyempiricalformulas,oronthebasisofcalculationcharacterizingrigidityrequirements,i.e.,Rigidityhomeworkstrengthcalculationofmachinepartsundervariablestressinalternatingunsymmetricalcycle非对称循环变应力下零件强度计算

handoutPPTPresentationTwoopportunitiesforpresentationsin5minutes非对称循环变应力下零件强度计算

(Strengthcalculationofmachinepartsundervariablestressinalternatingunsymmetricalcycle)一、极限应力线图(Curveoffatiguelimit)1、材料的极限应力线图同种材料、r不同时，σr在σm–σa

坐标系下的关系曲线

a

B

m

-1

0/2DCA45°O

0/2EB

S45°A:σm=0,r=-1C:σm=σa,r=0B:σa=0,r=+1σr=σm+σa(-1≤r≤+1)ACB—实验线图ACED—简化线图A点代表对称循环疲劳极限B点代表抗拉强度极限C点代表脉动循环疲劳极限工作应力点位于曲线ACB以内时，材料不发生破坏曲线ACB是材料发生破坏的临界状态§1.6非对称循环变应力下零件强度计算一、极限应力线图

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)2、简化极限应力线图—谢林森折线ACEDAE:DE:A(0,

-1)

m

-1

0/2D(

S,0)C(

0/2,

0/2)45°O

0/2E

S45°

a§1.6非对称循环变应力下零件强度计算一、极限应力线图

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)3、零件的极限应力线图—A’C’E’D

试件线图ACED—强度影响因素修正Kσ—零件线图A’C’E’D

mD45°O

0/2

S45°

a§1.6非对称循环变应力下零件强度计算二、非对称循环变应力下零件疲劳强度计算

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)二、非对称循环变应力下零件的疲劳强度计算（r=C）

mO45°

a疲劳强度区OA’E’静强度区OE’D（1）工作应力点--由载荷、结构求得1、极限应力线图分析（2）极限应力点由工作应力点及对应的加载规律确定§1.6非对称循环变应力下零件强度计算二、非对称循环变应力下零件疲劳强度计算

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)2、r=C简单加载分析加载曲线OMM’为直线，疲劳强度区OA’E’、静强度区OE’DM’点坐标：3、无限寿命疲劳强度计算

mO

aD在曲线OA’E’内的点发生疲劳破坏，在曲线OE’D内的点发生静强度破坏§1.6非对称循环变应力下零件强度计算二、非对称循环变应力下零件疲劳强度计算

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)对应工作应力点M的极限应力：无限寿命疲劳强度计算安全系数§1.6非对称循环变应力下零件强度计算二、非对称循环变应力下零件疲劳强度计算

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)4、静强度区安全系数计算静强度计算安全系数:当工作应力点N位于OE’D静强度区，极限应力点为OM与DE’的交点N’,极限应力为：工作应力最大值:

mO

aFourpatternsofconstant-amplitudecyclicstressReleasedcompression()FatiguefailureexperimentConstantlife

fatiguecurve

等寿命疲劳曲线Constantlife

fatiguecurve等寿命疲劳曲线谢林森折线第二章机械零件的计算准则及强度计算沈阳农业大学工程学院机械设计教研室张祖立基本要求1、掌握载荷和应力的分类、含义及其确定方法2、掌握静应力下零件的强度计算判据，计算应力，许用应力和安全系数的确定方法3、了解疲劳现象和疲劳曲线的来源、意义和用途4、了解疲劳损伤积累的概念、意义及其应用5、了解疲劳极限线图的来源、意义和用途，能根据材料的极限应力绘制简化疲劳极限线图6、掌握变应力下机械零件的疲劳强度安全系数校核计算方法7、了解接触疲劳强度的概念和接触应力的计算方法

重点内容1、机械零件的失效分析2、静应力下机械零件强度计算准则、计算应力、材料极限应力和安全系数的确定3、疲劳现象及其断口特征、疲劳曲线及其表达式4、线性疲劳损伤积累理论及其表达式5、材料极限线图的功用、常用的简化疲劳极限线图的绘制及其数学表达式6、变应力下机械零件的疲劳强度计算第一节机械零件的

主要失效形式及计算准则

一、机械零件的主要失效形式失效的概念

机械零件在规定的使用期间内，在规定的条件下，不能完成规定的功能而丧失工作能力时机械零件常见的失效形式:

1．整体断裂

静强度断裂

——

静应力过大产生的

疲劳断裂

——

变应力的反复作用下产生的

机械零件整体断裂中，80%属于疲劳断裂2．表面破坏

表面磨粒磨损、胶合、疲劳点蚀、腐蚀磨损、表面压溃、表面塑性流动等3．变形量过大

弹性变形塑性变形

4．破坏正常工作条件引起的失效

有些零件只有在一定的工作条件下才能正常地工作。如带传动和摩擦轮传动，高速转动的零件同一种零件发生失效的形式可能有数种

齿轮的失效形式有：轮齿折断、齿面点蚀、齿面胶合、齿面磨损、齿面或齿体塑性变形、齿轮其他部分的破坏主要失效形式将由零件的材料、具体的结构及工作条件等决定工作能力

零件不发生失效时的安全工作的限度同一种零件可能有数种不同的失效形式，显然，起决定作用的将是承载能力中的较小值二、机械零件的计算准则计算准则——用于计算并确定零件基本尺寸的主要依据常用的计算准则有：

1．强度准则

强度是零件在载荷作用下抵抗整体断裂、表面接触疲劳及塑性变形的能力

2．刚度准则

刚度是指零件在载荷作用下抵抗弹性变形的能力

3．寿命准则

影响零件寿命的主要失效形式：腐蚀、磨损、疲劳

腐蚀寿命、磨损寿命没有提出实用有效的或通行的定量计算的方法

疲劳寿命计算通常是求出使用寿命时的疲劳极限来作为计算的依据

4．耐磨性准则

耐磨性是指磨损过程中材料抵抗脱落的能力

——

采用条件性计算滑动速度低，载荷大时

可只限制工作表面的压强p

——

防止过快磨损滑动速度u

较高时

还要限制摩擦功耗

——

防止加剧磨损或胶合高速时

还要限制滑动速度u

——

防止加速磨损

5．振动稳定性准则

失稳

零件的自振频率f与激振源的激振频率fp相等或相接近时，零件发生共振的现象，即丧失振动稳定性振动稳定性准则

使机器中各零件的自振频率与激振源的激振频率错开

6．可靠性准则设一批相同零件的件数为N0，如在t时间后仍有N件在正常地工作，则此零件在工作时间t的可靠度R

零件的可靠度是时间的函数

如果时间t到t+dt的间隔中，又有dN件零件发生失效，则在此时间间隔内失效的比率式中：l（t）称为失效率，负号表示dN的增大将使N减小分离变量并积分，得即浴盆曲线零件或部件的失效率l（t）与时间t的关系，一般是用试验的方法求得该曲线分为三段：

第Ⅰ段：早期失效阶段

失效率由开始的很高的数值急剧地下降到某一稳定的数值原因是零、部件中所存在的初始缺陷第Ⅱ段：正常使用阶段

失效的发生是随机性的，失效率则表现为一常数

第Ⅲ段：损坏阶段

由于长时间的使用而使零件发生磨损、疲劳等原因，使失效率急剧增加第二节静应力下机械零件的强度计算一、载荷及应力的分类

1．载荷的分类静载荷

大小和方向不随时间变化或变化缓慢的载荷

变载荷

随时间作周期性变化或非周期性变化的载荷

名义载荷

根据机器原动机的额定功率或稳定和理想工作条件下的工作阻力，用力学公式计算出作用在零件上的载荷

计算载荷

载荷系数K与名义载荷的乘积。

如FC=KF，PC=KP，TC=KT载荷系数K（或工作情况系数）

概略估计实际载荷随时间作用的不均匀性、载荷在零件上分布的不均匀性及其他因素的综合影响

2．应力的分类静应力

不随时间变化或变化缓慢的应力，它只能在静载荷下产生

变应力

随时间变化的应力，它可由变载荷产生，也可由静载荷产生变应力稳定变应力非稳定变应力非对称循环变应力脉动循环变应力对称循环变应力规律性非稳定变应力无规律性非稳定变应力（随机变应力）1）变应力参数最大应力：σmax

最小应力：σmin应力循环特性

用来表示应力的变化情况

平均应力：应力幅：σmaxσmσminσaσatσ2）典型变应力及应力循环特性ra）静应力：r=+1

变应力特例b）非对称循环变应力r

在（+1～-1）间变化σmaxσmσminσaσatσσtσ=常数c）对称循环变应力r

=-1σtσaσmaxσmind）脉动循环变应力r

=0σtσaσaσmaxσm二、机械零件的强度判据机械零件的强度判据的两种表达方式

1.危险截面处的最大应力小于或等于许用应力

2.危险截面处的实际安全系数大于或等于许用安全系数

三、静应力下机械零件的强度静应力下，零件的强度失效：塑性变形或断裂

1．塑性材料制成的零件强度失效：

塑性变形极限应力应取为材料的屈服极限，即

slim=s

S，t

lim=t

S复合应力时——弯曲正应力sb和扭转切应力tT

根据第三或第四强度理论来确定其强度条件按第三强度理论计算时近似取按第四强度理论计算时近似取，可得

或

其中2．脆性材料和低塑性材料的零件强度失效：脆性断裂极限应力应取为材料的强度极限，即

s

lim=s

B，t

lim=t

B复合应力时根据第一或第二强度理论来确定其强度条件组织不均匀的脆性材料（如灰铸铁），不考虑应力集中组织均匀的低塑性材料（如低温回火的高强度钢），应考虑应力集中四、许用安全系数与许用应力许用安全系数的选取原则：

在保证机器安全可靠的前提下，尽可能选用较小的许用安全系数选择许用安全系数要考虑的因素：

1）载荷和应力的性质及计算的准确性

2）材料的性质和材质的不均匀性

3）零件的重要程度

4）工艺质量和探伤水平

5）运行条件（平稳、冲击）

6）环境状况（腐蚀、温度）第三节对称循环稳定变应力下

机械零件的疲劳强度计算

一、疲劳断裂特征强度失效：疲劳断裂疲劳断裂的过程：

第一阶段形成疲劳源第二阶段裂纹扩展第三阶段发生瞬断

截面呈现两个区域：

光滑的疲劳区粗糙的脆性断裂区疲劳破坏的特点：

1）在循环变应力多次反复作用下产生

2）不存在宏观的、明显的塑性变形迹象

3）循环变应力远小于材料的静强度极限

4）对材料的组成、零件的形状、尺寸、表面状态、使用条件和外界环境等都非常敏感

疲劳破坏的突发生、高度局部性、对各种缺陷的敏感性，因而具有更大的危险性

二、疲劳曲线及疲劳极限疲劳极限s

rN

或t

rN

在循环特性r下的变应力，经过N次循环后，材料不发生疲劳破坏的应力最大值

疲劳曲线（s—N或t—N曲线）表示循环次数N与疲劳极限之间的关系曲线

分成两个区域：N<N0为有限寿命区N≥N0为无限寿命区N0为循环基数

1．有限寿命区N<103（104）

——低周循环疲劳

疲劳极限较高，接近屈服极限，疲劳极限几乎与循环次数的变化无关

低周循环疲劳的零件，一般可按静强度计算

N≥103（104）——高周循环疲劳

其中：103（104）≤N<N0，疲劳极限随循环次数增加而降低有限寿命设计

2．无限寿命区N≥N0时，疲劳曲线为水平线N0次循环时的疲劳极限：s

r、t

r

对称循环时为s

-1、t-1，脉动循环时为s0、t

0

无限寿命设计

有色金属和高强度合金钢没有无限寿命区

3．循环次数为N时的疲劳极限

疲劳曲线方程式

循环次数为N时的疲劳极限寿命系数

4．几个问题的说明（1）循环基数N0

及循环次数N材料性质不同，N0值也不同。钢的硬度（强度）愈高，N0值愈大按硬度粗略分：

≤350HBS的钢，N0≈106~107

>350HBS的钢，N0≈10×107~25×107

有色金属N0≈25×107通常疲劳极限在107循环次数下试验得来计算kN时：取N0=107

≤350HBS的钢：若N>107，取N=N0=107

，kN=1

>350HBS的钢：若N>25×107，取N=25×107

有色金属：当N>25×107时，取N=25×107（2）材料常数mm与应力状态、材料性质和热处理方法有关m值最好根据具体零件材料的疲劳曲线来确定

m的平均值为一般计算：对