机械精度设计与检测技术 (3).pdf

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1、LIMIT AND TOLERANCESECTION 2.1Basic Terminology Nominal size(D,d):Also called basic size,refers to the size of a feature of perfectformasdefinedbythedrawingspecification.Actual size(Da,da):refers to size of theassociated integral feature.It is obtainedby measurement.D(d)DadaBasic Terminology Deviati

2、on(Ea,ea):refers to value ofactual size minus its nominal size.for hole:Ea=Da-Dfor shaft:ea=da-dLimit deviation(ES/EI,es/ei):refers toalgebraic different value between limitsof size and nominal size.for hole:ES=Dmax-DEI=Dmin-Dfor shaft:es=dmax-dei=dmin-dDadaEaeaD(d)Nominal sizeeiTolerance interval o

3、f holeTolerance interval of shaftEIESD(d)Nominal sizeDminDmaxdmaxdminesDiagram of limits and fitsBasic Terminology Size tolerance(Th,Ts):refers to the difference between the upperlimit of size and the lower limit of size.for hole:Th=Dmax-Dminfor shaft:Ts=dmax-dmin=ES-EI=es-eieiTolerance interval of

4、holeTolerance interval of shaftEIESD(d)Nominal sizeDminDmaxdmaxdminesThTsIllustration of limits and fitsIllustration of Limits and FitsThe concepts of limits and fits can be illustrated in the figurebellow.Zero line:the nominal size,to which the deviations and tolerancesare referred.Size tolerance i

5、nterval:contained between two lines representingthe upper and lower deviations，defined by the magnitude andposition of the size tolerance interval.Example 1：Calculate the Limit Deviation and Size ToleranceQuestion:Calculate the limit deviations and size tolerance.thenominal size of hole is 30mm,=30.

6、021mm,=30mm.Answer:Upper limit deviation of hole:=30.021 30=+0.021Lower limit deviation of hole:=30 30=0The size tolerance of hole:=30.021 30=0.021030+-ES=+0.021EI=0HoleIllustration of size tolerance intervalStandard tolerance gradesNominal size rangeStandard ToleranceStandard tolerance(IT):refers t

7、o any tolerance belonging to theISO code system for tolerances on linear sizes.Standard tolerance grade:It is designated by the letters ITfollowed by the grade number(e.g.IT7).There are 20 standardtolerance grades designated IT01,IT0,and IT118.Nominal size range:The standard tolerances are not defin

8、edindividually for each specific nominal size,but for nominal sizerange.Calculation of Standard Tolerance ValueFor each nominal size range,geometrical mean(D)of the extremesize(D1and D2)is calculated as:=1 2（1）Standard tolerance factor:=0.453+0.001（2）IF nominal size range is from 0 to 500mm,IT grade

9、 range is fromIT5 to IT18,the value of standard tolerance is calculated as:Nominal size(mm)IT5Standard tolerance gradeIT6IT7IT8IT9IT10IT17IT18107Up to and including500above-Standard tolerance value()Standard tolerance valueExample 2：Calculate the Standard Tolerance ValueQuestion:Calculate the value

10、of standard tolerance IT7 ofdiameter 100 mm of the shaft.Answer:Step1.Calculate the geometrical mean.The shaft diameter,100 lies in the nominal size range,80to 120mm and the geometrical mean is=80 120=98Step2.Calculate standard tolerance factor,=0.45398+0.001 98=2.172Step3.Calculate the value of tol

11、erance IT7,7=16=16 2.172=35Fundamental DeviationThe fundamental deviation is that limitdeviation,which defines that limit of size,which is the nearest to the nominal size.The fundamental deviations are identifiedand controlled by:upper case letter(s)for holes(A.ZC)lower case letter(s)for shafts(a.zc

12、)Fundamental Deviation ValueGenerally,the fundamental deviation value can be identified intables below.Example 3：Calculate the Fundamental DeviationsQuestion:Calculate the fundamental deviations for the hole sizes90F7.Answer:90is the nominal size 90mm;Fis the fundamental deviation identifier for a hole;7is the standard tolerance grade number;According to table 1,IT7=35.According to table 2,Lower limit deviation =+36Thus,Upper limit deviation=+=35+36=71From that follows:907=900.0360.071