機械設計英文版全冊配套完整教學課件

機械設計(英文版）全冊配套完整教學課件DesignofMachineElementsChapter1IntroductionTheMeaningofDesignEngineeringMechanicalEngineeringDesignMachinedesign

Engineeringisconcernedwiththeconversionofenergyfromoneformtoanotherwiththeobjectiveofextractingusefulwork.Mechanicalengineeringhasthemorespecificobjectiveofdesigningmachinerytothispurposeinthemostsuitablemannerpossible.“Engineering”

istheartof

directingthegreatsourcesofpowerinnaturefortheuseandconvenienceofhumanbeings

/utilizingtheexistingresourcesandnaturallawtobenefithumanityWaterwheelofYellowRiverPotentialenergytokineticenergyDieselgenerator

Compressionstroke壓縮沖程(機械能-熱能)Powerstroke做功沖程(化學能-機械能)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滑動軸承,rolling-elementbearings滾動軸承Torquetransmitter:gears,tractiondrives牽引傳動,chaindrives鏈傳動,beltdrives帶傳動,powerscrews螺紋連接Energyabsorber:brakes制動器anddampers減震器Supportingelements:shafts,springs彈簧,seals密封Connectionandjoints:threadfasteners螺紋緊固件,keys鍵,couplings聯(lián)軸器slidingbearingsrolling-elementbearingsOffshoredrillingplatformWheelshaftoflocomotivechaindrives鏈傳動ApplicationBeltdrive帶傳動Automobileengine(multiwedgebelt)

汽車發(fā)動機（多楔帶）Automobileengine(synchronousbelt)汽車發(fā)動機（同步帶）powerscrews螺紋連接2.MachineDesign—ADiscipline-blendingHumanEndeavorMachinedesigninvolvesmanydecision-making,includingengineeringonesandnon-engineeringones.Engineeringdecisions:loading載荷,kinematics動力學,materials材料;strength強度,reliability可靠性,deformation變形,tribology摩擦學,weight重量,etc.Non-engineeringdecisions:cost成本,marketability可銷售性,productliability產(chǎn)品責任,ethics倫理學,politics政治,etc.

FundamentalDesignConsiderations1Strength強度2Reliability3Thermaleffect4Corrosion腐蝕5Wear磨損6Friction摩擦7Processing8Utility9Cost10Safety11Weight12Noise13Styling造型14Shape形狀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脆性斷裂;fatiguefracture疲勞斷裂Surfacefailure表面實效—Wear磨損,Fatigue疲勞,ductilerupture韌性斷裂,Creep蠕變,Corrosion腐蝕,spalling剝落Violationofitsintendedfunction—overallslipofbelt,Bucklingofspring

彈簧屈曲蠕變：固體材料在保持應力不變的條件下，應變隨時間延長而增加的現(xiàn)象。它與塑性變形不同，塑性變形通常在應力超過彈性極限之后才出現(xiàn)，而蠕變只要應力的作用時間相當長，它在應力小于彈性極限施加的力時也能出現(xiàn)。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（屈服強度）,theultimatestrength（極限強度）,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應力集中σL實σL名MM應力集中區(qū)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.曲軸疲勞斷裂斷口特征cracksourcezone裂紋源區(qū)Fatigueextendedarea疲勞擴展區(qū)finaltransientfaultzone最終瞬斷區(qū)內(nèi)部缺陷引起疲勞斷軸：

初始裂紋起源于軸內(nèi)部，裂紋由內(nèi)至外擴展

裂紋源區(qū)cracksourcezone擴展區(qū)expansionarea瞬斷區(qū)Transientfaultzone裂紋擴展方向Directionofcrackextension鍵槽應力集中導致齒輪軸疲勞斷裂鍵槽初始裂紋源initialcracksource裂紋擴展區(qū)Crackpropagationregion最終瞬斷區(qū)finaltransientfaultzoneMethodstoExtendFatigueLife1.Byminimizinginitialflaws,especiallysurfaceflaws,throughprocesses,suchasgrindingorpolishing,thatleaveexceptionallysmoothsurfaces.2.Bymaximizinginitiationtime.

Surfaceresidualstressesareimpartedorrelievedthroughmanufacturingprocesses,suchasshotpeeningorburnishing,orbyanumberofsurfacetreatments.3.Bymaximizingpropagationtime.Usingamaterialthatdoesnotpresentelongatedgrainsinthedirectionoffatiguecrackgrowthcanextendfatiguelife(e.g.,byusingcold-workedcomponentsinsteadofcastings).4.Bymaximizingthecriticalcracklength.Fracturetoughnessisanessentialproperty.Shotpeening拋丸硬化2)CyclicStress循環(huán)應力Cyclicstress,alsocalledfluctuatingoralternatingstress,isafunctionoftime,butthevariationissuchthatthestresssequencerepeatsitself.Fatiguefailureoccursatrelativelylowstresslevelstoacomponentorstructuresubjectedtofluctuatingorcyclicstresses.ParametersusedtocharacterizefluctuatingcyclicstressMeanstressStressrangeStressamplitudeStressratioAmplituderatioFigure2.1Thecyclicvariationofnonzeromeanstresswithtime.Fourpatternsofconstant-amplitudecyclicstressCompletelyreversed()orzero-meancyclicstress對稱循環(huán)變應力σtσm=0；σmax=σa=

-σminr=σmin/σmax=-1Fourpatternsofconstant-amplitudecyclicstressNonzeromean(-1≤r≤+1)非對稱循環(huán)變應力σtσmaxσminσmσa應力幅：σa=(σmax-σmin)/2平均應力：σm=(σmax+σmin)/2應力循環(huán)特性：r=σmin/σmax最大應力：σmax

最小應力：σminFourpatternsofconstant-amplitudecyclicstressReleasedtension()脈動循環(huán)變應力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微動磨損Ductilerupture韌性斷裂Brittlefracture脆性斷裂Fatigue疲勞Corrosion腐蝕Wear磨損Impact沖擊Frettingcorrosion摩擦腐蝕Creep蠕變Thermalrelaxationorstressrelaxation熱或應力松弛Thermalshock熱擊Spalling剝落Buckling屈曲Failure失效Fractureofgearshaft齒輪軸斷裂fractureofthewholecrankshaft曲軸整體斷裂Fractureamplification斷口放大Twofractures斷口對Thebrokentoothofthespiralscrewinrollingmachine軋鋼機壓下螺旋絲杠斷牙絲杠斷牙部位絲杠斷牙局部放大

螺栓聯(lián)接滑移

boltslip

被聯(lián)接件斷裂

thefractureofconnectedpart被聯(lián)件相對滑移被聯(lián)件拉斷Toothwear輪齒磨損Coldbonding/welding冷膠合：

Underthelowspeedandheavyload,contactpartsareextrudedandadhered,andtherelativemovementisprocessedandcontactpartsaretorn.低速重載，接觸零件擠壓粘著，相對運動撕裂。Thermalbonding熱膠合：潤滑不良引起的齒面Pittingcorrosion點蝕Plasticdeformationofgeartooth輪齒塑性變形整體塑變overallplasticdeformationGeneralfailure綜合失效：toothwear

toothfracture,shaftbrokenduetofatigue齒面磨損、斷齒、疲勞斷軸5)AboutDesignAnalysisDesignanalysisattemptstopredictthestrengthordeformationofamachineelementsothatitcansafelycarrytheimposedloadsforaslongasrequired.Ananalysisbyitselfshouldnotbelookedonasanabsoluteandfinaltruth.

Ananalysisislimitedbytheassumptionsimposedandbyitsrangeofapplicability.Rigidity剛度RigidityRigidityisthecapacityofcomponentstoresistelasticdeformationswhensubjecttoforces,is,togetherwithstrength,oneofthemostimportantcriteriaoftheworkingcapacityofmachines.RigidityThepermissibledeflection,anglesofinclinationandtorsionaredeterminedbyempiricalformulas,oronthebasisofcalculationcharacterizingrigidityrequirements,i.e.,Rigidityhomeworkstrengthcalculationofmachinepartsundervariablestressinalternatingunsymmetricalcycle非對稱循環(huán)變應力下零件強度計算

handoutPPTPresentationTwoopportunitiesforpresentationsin5minutes非對稱循環(huán)變應力下零件強度計算

(Strengthcalculationofmachinepartsundervariablestressinalternatingunsymmetricalcycle)一、極限應力線圖(Curveoffatiguelimit)1、材料的極限應力線圖同種材料、r不同時，σr在σm–σa

坐標系下的關(guān)系曲線

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—實驗線圖ACED—簡化線圖A點代表對稱循環(huán)疲勞極限B點代表抗拉強度極限C點代表脈動循環(huán)疲勞極限工作應力點位于曲線ACB以內(nèi)時，材料不發(fā)生破壞曲線ACB是材料發(fā)生破壞的臨界狀態(tài)§1.6非對稱循環(huán)變應力下零件強度計算一、極限應力線圖

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)2、簡化極限應力線圖—謝林森折線ACEDAE:DE:A(0,

-1)

m

-1

0/2D(

S,0)C(

0/2,

0/2)45°O

0/2E

S45°

a§1.6非對稱循環(huán)變應力下零件強度計算一、極限應力線圖

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)3、零件的極限應力線圖—A’C’E’D

試件線圖ACED—強度影響因素修正Kσ—零件線圖A’C’E’D

mD45°O

0/2

S45°

a§1.6非對稱循環(huán)變應力下零件強度計算二、非對稱循環(huán)變應力下零件疲勞強度計算

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)二、非對稱循環(huán)變應力下零件的疲勞強度計算（r=C）

mO45°

a疲勞強度區(qū)OA’E’靜強度區(qū)OE’D（1）工作應力點--由載荷、結(jié)構(gòu)求得1、極限應力線圖分析（2）極限應力點由工作應力點及對應的加載規(guī)律確定§1.6非對稱循環(huán)變應力下零件強度計算二、非對稱循環(huán)變應力下零件疲勞強度計算

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)2、r=C簡單加載分析加載曲線OMM’為直線，疲勞強度區(qū)OA’E’、靜強度區(qū)OE’DM’點坐標：3、無限壽命疲勞強度計算

mO

aD在曲線OA’E’內(nèi)的點發(fā)生疲勞破壞，在曲線OE’D內(nèi)的點發(fā)生靜強度破壞§1.6非對稱循環(huán)變應力下零件強度計算二、非對稱循環(huán)變應力下零件疲勞強度計算

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)對應工作應力點M的極限應力：無限壽命疲勞強度計算安全系數(shù)§1.6非對稱循環(huán)變應力下零件強度計算二、非對稱循環(huán)變應力下零件疲勞強度計算

(Strengthcalculationofmachinepartsundervariablestressinanalternatingsymmetricalcycle)4、靜強度區(qū)安全系數(shù)計算靜強度計算安全系數(shù):當工作應力點N位于OE’D靜強度區(qū)，極限應力點為OM與DE’的交點N’,極限應力為：工作應力最大值:

mO

aFourpatternsofconstant-amplitudecyclicstressReleasedcompression()FatiguefailureexperimentConstantlife

fatiguecurve

等壽命疲勞曲線Constantlife

fatiguecurve等壽命疲勞曲線謝林森折線第二章機械零件的計算準則及強度計算沈陽農(nóng)業(yè)大學工程學院機械設計教研室張祖立基本要求1、掌握載荷和應力的分類、含義及其確定方法2、掌握靜應力下零件的強度計算判據(jù)，計算應力，許用應力和安全系數(shù)的確定方法3、了解疲勞現(xiàn)象和疲勞曲線的來源、意義和用途4、了解疲勞損傷積累的概念、意義及其應用5、了解疲勞極限線圖的來源、意義和用途，能根據(jù)材料的極限應力繪制簡化疲勞極限線圖6、掌握變應力下機械零件的疲勞強度安全系數(shù)校核計算方法7、了解接觸疲勞強度的概念和接觸應力的計算方法

重點內(nèi)容1、機械零件的失效分析2、靜應力下機械零件強度計算準則、計算應力、材料極限應力和安全系數(shù)的確定3、疲勞現(xiàn)象及其斷口特征、疲勞曲線及其表達式4、線性疲勞損傷積累理論及其表達式5、材料極限線圖的功用、常用的簡化疲勞極限線圖的繪制及其數(shù)學表達式6、變應力下機械零件的疲勞強度計算第一節(jié)機械零件的

主要失效形式及計算準則

一、機械零件的主要失效形式失效的概念

機械零件在規(guī)定的使用期間內(nèi)，在規(guī)定的條件下，不能完成規(guī)定的功能而喪失工作能力時機械零件常見的失效形式:

1．整體斷裂

靜強度斷裂

——

靜應力過大產(chǎn)生的

疲勞斷裂

——

變應力的反復作用下產(chǎn)生的

機械零件整體斷裂中，80%屬于疲勞斷裂2．表面破壞

表面磨粒磨損、膠合、疲勞點蝕、腐蝕磨損、表面壓潰、表面塑性流動等3．變形量過大

彈性變形塑性變形

4．破壞正常工作條件引起的失效

有些零件只有在一定的工作條件下才能正常地工作。如帶傳動和摩擦輪傳動，高速轉(zhuǎn)動的零件同一種零件發(fā)生失效的形式可能有數(shù)種

齒輪的失效形式有：輪齒折斷、齒面點蝕、齒面膠合、齒面磨損、齒面或齒體塑性變形、齒輪其他部分的破壞主要失效形式將由零件的材料、具體的結(jié)構(gòu)及工作條件等決定工作能力

零件不發(fā)生失效時的安全工作的限度同一種零件可能有數(shù)種不同的失效形式，顯然，起決定作用的將是承載能力中的較小值二、機械零件的計算準則計算準則——用于計算并確定零件基本尺寸的主要依據(jù)常用的計算準則有：

1．強度準則

強度是零件在載荷作用下抵抗整體斷裂、表面接觸疲勞及塑性變形的能力

2．剛度準則

剛度是指零件在載荷作用下抵抗彈性變形的能力

3．壽命準則

影響零件壽命的主要失效形式：腐蝕、磨損、疲勞

腐蝕壽命、磨損壽命沒有提出實用有效的或通行的定量計算的方法

疲勞壽命計算通常是求出使用壽命時的疲勞極限來作為計算的依據(jù)

4．耐磨性準則

耐磨性是指磨損過程中材料抵抗脫落的能力

——

采用條件性計算滑動速度低，載荷大時

可只限制工作表面的壓強p

——

防止過快磨損滑動速度u

較高時

還要限制摩擦功耗

——

防止加劇磨損或膠合高速時

還要限制滑動速度u

——

防止加速磨損

5．振動穩(wěn)定性準則

失穩(wěn)

零件的自振頻率f與激振源的激振頻率fp相等或相接近時，零件發(fā)生共振的現(xiàn)象，即喪失振動穩(wěn)定性振動穩(wěn)定性準則

使機器中各零件的自振頻率與激振源的激振頻率錯開

6．可靠性準則設一批相同零件的件數(shù)為N0，如在t時間后仍有N件在正常地工作，則此零件在工作時間t的可靠度R

零件的可靠度是時間的函數(shù)

如果時間t到t+dt的間隔中，又有dN件零件發(fā)生失效，則在此時間間隔內(nèi)失效的比率式中：l（t）稱為失效率，負號表示dN的增大將使N減小分離變量并積分，得即浴盆曲線零件或部件的失效率l（t）與時間t的關(guān)系，一般是用試驗的方法求得該曲線分為三段：

第Ⅰ段：早期失效階段

失效率由開始的很高的數(shù)值急劇地下降到某一穩(wěn)定的數(shù)值原因是零、部件中所存在的初始缺陷第Ⅱ段：正常使用階段

失效的發(fā)生是隨機性的，失效率則表現(xiàn)為一常數(shù)

第Ⅲ段：損壞階段

由于長時間的使用而使零件發(fā)生磨損、疲勞等原因，使失效率急劇增加第二節(jié)靜應力下機械零件的強度計算一、載荷及應力的分類

1．載荷的分類靜載荷

大小和方向不隨時間變化或變化緩慢的載荷

變載荷

隨時間作周期性變化或非周期性變化的載荷

名義載荷

根據(jù)機器原動機的額定功率或穩(wěn)定和理想工作條件下的工作阻力，用力學公式計算出作用在零件上的載荷

計算載荷

載荷系數(shù)K與名義載荷的乘積。

如FC=KF，PC=KP，TC=KT載荷系數(shù)K（或工作情況系數(shù)）

概略估計實際載荷隨時間作用的不均勻性、載荷在零件上分布的不均勻性及其他因素的綜合影響

2．應力的分類靜應力

不隨時間變化或變化緩慢的應力，它只能在靜載荷下產(chǎn)生

變應力

隨時間變化的應力，它可由變載荷產(chǎn)生，也可由靜載荷產(chǎn)生變應力穩(wěn)定變應力非穩(wěn)定變應力非對稱循環(huán)變應力脈動循環(huán)變應力對稱循環(huán)變應力規(guī)律性非穩(wěn)定變應力無規(guī)律性非穩(wěn)定變應力（隨機變應力）1）變應力參數(shù)最大應力：σmax

最小應力：σmin應力循環(huán)特性

用來表示應力的變化情況

平均應力：應力幅：σmaxσmσminσaσatσ2）典型變應力及應力循環(huán)特性ra）靜應力：r=+1

變應力特例b）非對稱循環(huán)變應力r

在（+1～-1）間變化σmaxσmσminσaσatσσtσ=常數(shù)c）對稱循環(huán)變應力r

=-1σtσaσmaxσmind）脈動循環(huán)變應力r

=0σtσaσaσmaxσm二、機械零件的強度判據(jù)機械零件的強度判據(jù)的兩種表達方式

1.危險截面處的最大應力小于或等于許用應力

2.危險截面處的實際安全系數(shù)大于或等于許用安全系數(shù)

三、靜應力下機械零件的強度靜應力下，零件的強度失效：塑性變形或斷裂

1．塑性材料制成的零件強度失效：

塑性變形極限應力應取為材料的屈服極限，即

slim=s

S，t

lim=t

S復合應力時——彎曲正應力sb和扭轉(zhuǎn)切應力tT

根據(jù)第三或第四強度理論來確定其強度條件按第三強度理論計算時近似取按第四強度理論計算時近似取，可得

或

其中2．脆性材料和低塑性材料的零件強度失效：脆性斷裂極限應力應取為材料的強度極限，即

s

lim=s

B，t

lim=t

B復合應力時根據(jù)第一或第二強度理論來確定其強度條件組織不均勻的脆性材料（如灰鑄鐵），不考慮應力集中組織均勻的低塑性材料（如低溫回火的高強度鋼），應考慮應力集中四、許用安全系數(shù)與許用應力許用安全系數(shù)的選取原則：

在保證機器安全可靠的前提下，盡可能選用較小的許用安全系數(shù)選擇許用安全系數(shù)要考慮的因素：

1）載荷和應力的性質(zhì)及計算的準確性

2）材料的性質(zhì)和材質(zhì)的不均勻性

3）零件的重要程度

4）工藝質(zhì)量和探傷水平

5）運行條件（平穩(wěn)、沖擊）

6）環(huán)境狀況（腐蝕、溫度）第三節(jié)對稱循環(huán)穩(wěn)定變應力下

機械零件的疲勞強度計算

一、疲勞斷裂特征強度失效：疲勞斷裂疲勞斷裂的過程：

第一階段形成疲勞源第二階段裂紋擴展第三階段發(fā)生瞬斷

截面呈現(xiàn)兩個區(qū)域：

光滑的疲勞區(qū)粗糙的脆性斷裂區(qū)疲勞破壞的特點：

1）在循環(huán)變應力多次反復作用下產(chǎn)生

2）不存在宏觀的、明顯的塑性變形跡象

3）循環(huán)變應力遠小于材料的靜強度極限

4）對材料的組成、零件的形狀、尺寸、表面狀態(tài)、使用條件和外界環(huán)境等都非常敏感

疲勞破壞的突發(fā)生、高度局部性、對各種缺陷的敏感性，因而具有更大的危險性

二、疲勞曲線及疲勞極限疲勞極限s

rN

或t

rN

在循環(huán)特性r下的變應力，經(jīng)過N次循環(huán)后，材料不發(fā)生疲勞破壞的應力最大值

疲勞曲線（s—N或t—N曲線）表示循環(huán)次數(shù)N與疲勞極限之間的關(guān)系曲線

分成兩個區(qū)域：N<N0為有限壽命區(qū)N≥N0為無限壽命區(qū)N0為循環(huán)基數(shù)

1．有限壽命區(qū)N<103（104）

——低周循環(huán)疲勞

疲勞極限較高，接近屈服極限，疲勞極限幾乎與循環(huán)次數(shù)的變化無關(guān)

低周循環(huán)疲勞的零件，一般可按靜強度計算

N≥103（104）——高周循環(huán)疲勞

其中：103（104）≤N<N0，疲勞極限隨循環(huán)次數(shù)增加而降低有限壽命設計

2．無限壽命區(qū)N≥N0時，疲勞曲線為水平線N0次循環(huán)時的疲勞極限：s

r、t

r

對稱循環(huán)時為s

-1、t-1，脈動循環(huán)時為s0、t

0

無限壽命設計

有色金屬和高強度合金鋼沒有無限壽命區(qū)

3．循環(huán)次數(shù)為N時的疲勞極限

疲勞曲線方程式

循環(huán)次數(shù)為N時的疲勞極限壽命系數(shù)

4．幾個問題的說明（1）循環(huán)基數(shù)N0

及循環(huán)次數(shù)N材料性質(zhì)不同，N0值也不同。鋼的硬度（強度）愈高，N0值愈大按硬度粗略分：

≤350HBS的鋼，N0≈106~107

>350HBS的鋼，N0≈10×107~25×107

有色金屬N0≈25×107通常疲勞極限在107循環(huán)次數(shù)下試驗得來計算kN時：取N0=107

≤350HBS的鋼：若N>107，取N=N0=107

，kN=1

>350HBS的鋼：若N>25×107，取N=25×107

有色金屬：當N>25×107時，取N=25×107（2）材料常數(shù)mm與應力狀態(tài)、材料性質(zhì)和熱處理方法有關(guān)m值最好根據(jù)具體零件材料的疲勞曲線來確定

m的平均值為一般計算：對