unit 7 Tolerances and Fits

1、UNIT 6Tolerances and Fits第第1010讲讲 1Introduction 2Tolerances3Fits 4ISO System of Limits and Fits Any manufacturer should have a thorough knowledge of the tolerances to increase the quality and reliability of a manufactured part with the least expense. Deviations of actual surfaces are divided into fo

2、ur groups to enable assessment, prescription and checking of the permitted inaccuracy during production: 1. Dimensional deviationsdi:viein;（尺寸偏差）（尺寸偏差） 2. Shape deviations;（形状偏差）（形状偏差） 3. Position deviations;（位置偏差）（位置偏差） 4. Surface roughness deviations;（表面粗糙度偏差）（表面粗糙度偏差）7.1 Introduction It is quite

3、sufficient that the actual dimension of the part is found between two limit dimensions and permissible deviation is kept with production to ensure correct functioning of engineering products. 在很多情况下，零件的实际尺寸介于两个极限尺寸之间在很多情况下，零件的实际尺寸介于两个极限尺寸之间，在制造的过程中一直维持一个允许偏差以保证工程零，在制造的过程中一直维持一个允许偏差以保证工程零件所具有的功能。件所具有

4、的功能。 Unnecessarily tight tolerances lead to high cost of manufacture resulting from more expensive manufacturing methods and higher reject rates. 不必要的紧公差会导致由于昂贵的制造方法和较高的废不必要的紧公差会导致由于昂贵的制造方法和较高的废品率所带来的高的制造成本。品率所带来的高的制造成本。7.1 Introduction Definitions. Tolerance is the difference between the maximum an

5、d minimum size limits of a part. Size(尺寸尺寸). The size of an object or its mate is known as nominal, basic, or design size. Allowance(允差允差). The difference between the largest allowable shaft size and the smallest hole diameter is termed as the allowance. The quality of the fit is characterized by th

6、e allowance value. Fit (配合配合). Clearance, interference, or transition fit refer to how the object fits an assembly.7.2 TolerancesNominal Size (basic size)(名义尺寸，基本尺寸名义尺寸，基本尺寸). It is the designation used for general identification and usually expressed in common fractions. It is an exact theoretical

7、size of a part from which limit dimensions are computed. The nominal size of a part should be selected from the preferred dimension series indicated by the national standard.是用于识别的一个表示，通常用简分数的形式给出。它表示一个是用于识别的一个表示，通常用简分数的形式给出。它表示一个零件的精确的理论尺寸，根据名义尺寸可以计算出极限尺寸。零零件的精确的理论尺寸，根据名义尺寸可以计算出极限尺寸。零件的名义尺寸应该从国家标准规

8、定的优先尺寸系列中选取。件的名义尺寸应该从国家标准规定的优先尺寸系列中选取。Limit size(极限尺寸极限尺寸). It includes upper limit size and lower limit size which are allowable extreme size of a part.Actual size(实际尺寸实际尺寸). It is the measured size of the finished part.7.2 Tolerances Deviations(偏差偏差). The upper and lower deviations obtained by sub

9、tracting basic size from limit size. Allowance(允差允差). The intentional difference between the maximum material limits of mating parts. This is a minimum clearance (positive allowance) or maximum interference (negative allowance) between mating parts. 配合零件的最大极限尺寸间的差值，它是配合零件之间配合零件的最大极限尺寸间的差值，它是配合零件之间的最

10、小间隙量（正允差）或最大过盈量（负偏差）。的最小间隙量（正允差）或最大过盈量（负偏差）。7.2 Tolerances Bilateral Tolerances(双边公差双边公差). A bilateral tolerance is a tolerance in which variation is permitted in both directions from a specified dimension. 双边公差允许相对于基本尺寸的沿两个方向变动。双边公差允许相对于基本尺寸的沿两个方向变动。 The actual size of the object may be larger or s

11、maller than the stated size limitation if there can be equal variation in both directions. The plus and minus limitations combine to form a single value. 如果在两个方向上具有相同的变动量，零件的实际尺寸如果在两个方向上具有相同的变动量，零件的实际尺寸就有可能大于或小于标注的尺寸极限。正负极限就构成就有可能大于或小于标注的尺寸极限。正负极限就构成一个数值。一个数值。7.2 Tolerances Unilateral Tolerances (单边

12、公差单边公差). A unilateral tolerance is a tolerance in which variation is permitted only in one directions from a specified dimension. 单边公差是指相对于名义尺寸只能沿一个方向变动。单边公差是指相对于名义尺寸只能沿一个方向变动。 Unilateral tolerance expression has the advantage that they are easier to check on drawings and that the change in the tole

13、rance can be made with the least disturbance to other dimensions. 单边公差表达方式的优点是用它来检查图纸比较方便。单边公差表达方式的优点是用它来检查图纸比较方便。7.2 Tolerances Clearance fit. it is a fit enabling a clearance between the hole and shaft in the coupling. The lower limit size of the hole is greater or at least equal to the upper limi

14、t size of the shaft. Interference fit. It is a fit always ensuring interference between the hole and shaft in the coupling. The upper limit size of the hole is smaller or at least equal to the lower limit size of the shaft. Transition fit. It is a fit where (depending on the actual sizes of the hole

15、 and shaft) both clearance and interference may occur in the coupling. Tolerance zones of the hole and shaft partly or completely interfere.7.3 Fits How mating parts or assemblies fit together with component parts is referred to as fit, which includes clearance fit, interference fit, or transition f

16、it, see Fig.7.3. Fit is general range of tightness resulting from the application of a specific combination of allowance and tolerances in the design of mating parts.The types of fittingClearance fit 间隙配合间隙配合Interference fit 过盈配合过盈配合Transition fit 过渡配合过渡配合7.3 Fits下偏差下偏差公差带公差带+0.008-0.008+0.008+0.024

17、-0.006-0.022公差带图：公差带图：作用：直观地表示出了公差的大小及公差带相作用：直观地表示出了公差的大小及公差带相对于零线的位置。对于零线的位置。50+0.024+0.00850-0.006-0.02200基本尺寸基本尺寸 50例例: 500.008上偏差上偏差7.3 FitsClearance fitFit with clearance (including minimum clearance zero).Maximum clearanceMinimum limit of sizeMaximum limit of sizeMinimum clearanceMaximum clear

18、anceThe tolerance zone of hole is above that of the shaft.Minimum limit of sizeMaximum limit of sizeInterference fitMinimum interferenceMaximum interferenceThe tolerance zone of hole is below that of the shaft. Fit with interference (including minimum Interference zero)Maximum limit of sizeMinimum l

19、imit of sizeMaximum interference第第11讲讲Transition fitFit having clearance of interference .The tolerance zone of hole overlaps that of the shaft. The ISO system of tolerances and fits can be applied in tolerances and deviations of smooth parts and for fits created by their coupling. It is used partic

20、ularly for cylindrical parts with round sections. Tolerances and deviations in this standard can also be applied in smooth parts of other sections. ISOISO公差和配合体系可以用于光滑零件的公差和偏差以及连公差和配合体系可以用于光滑零件的公差和偏差以及连接件的配合的确定，特别用于圆形截面的圆柱形零件。该体接件的配合的确定，特别用于圆形截面的圆柱形零件。该体系中的公差和偏差也可以用于其它形状截面的零件。系中的公差和偏差也可以用于其它形状截面的零件。

21、7.4 ISO System of Limits and Fits Similarly, the system can be used for coupling (fits) of cylindrical parts and fits with parts having two parallel surface (e.g. fits of keys in grooves). The term “shaft”, used in this standard has wide meaning and serves for specification of all outer elements of

22、the part, including those elements which do not have cylindrical shapes. Also, the term “hole” can be used for specification of all inner elements regardless of their shape. 同样地，该标准还可以应用于圆柱形零件的连接（配合）及具有同样地，该标准还可以应用于圆柱形零件的连接（配合）及具有两平行表面的零件的配合（如键和键槽的配合）。标准中采用的两平行表面的零件的配合（如键和键槽的配合）。标准中采用的“轴轴”这一词有广泛的含义，

23、它用于说明零件的所有外部元素，这一词有广泛的含义，它用于说明零件的所有外部元素，包括那些不是圆柱形表面的元素。而包括那些不是圆柱形表面的元素。而“孔孔”这个词则可以指不考这个词则可以指不考虑形状的零件的所有内部元素。虑形状的零件的所有内部元素。7.4 ISO System of Limits and Fits In order to meet the requirements of various production branches for accuracy of the product, the system ISO implements 20 grades of accuracy. E

24、ach of the tolerances of this system is marked “IT” with attached grade of accuracy (IT01, IT0, IT1 IT18). Note: IT is an acronym for International Tolerance. 为了满足不同生产部门对产品精度的需求，为了满足不同生产部门对产品精度的需求，ISO标准规定标准规定了了20级精度等级。级精度等级。7.4 ISO System of Limits and Fits The tolerance zone is defined as annular z

25、one（环形（环形区域）区域） limited by the upper and lower limit dimensions of the parts. The tolerance zone is therefore determined by the amount of the tolerance and its position related to the basic size. The position of the tolerance zone, related to the basic size (zero line), is determined in the ISO syst

26、em by a so-called basic deviation. 公差带是上极限尺寸和下极限尺寸间的环形区域。因此，公差带是上极限尺寸和下极限尺寸间的环形区域。因此，公差带由公差值及其与基本尺寸的相对位置所决定的。公差带由公差值及其与基本尺寸的相对位置所决定的。ISO标准中公差带相对于基本尺寸（零线）的位置是由基标准中公差带相对于基本尺寸（零线）的位置是由基本偏差决定的。本偏差决定的。7.4 ISO System of Limits and Fits The system ISO defined 28 classes of basic deviations for holes. These

27、 classes are marked by capital letters (A, B, C ZC). The tolerance zone for the specified dimensions is prescribed in the drawing by a tolerance mark, which consists of a letter marking of the basic deviation and a numerical marking of the tolerance grade (e.g. H7, H8, D5, etc.). 对于孔，对于孔，ISO标准定义了标准定

28、义了28类基本偏差，由大写字母类基本偏差，由大写字母(A, B, C )表示。在图样中规定尺寸（设计尺寸）的公差带是通过公差表示。在图样中规定尺寸（设计尺寸）的公差带是通过公差代号表示的，它由表示基本偏差的字母和表示公差等级的数字代号表示的，它由表示基本偏差的字母和表示公差等级的数字组成。组成。Though the general sets of basic deviations and tolerance grades can be used for prescriptions of hole tolerance zone by their mutual combinations, in p

29、ractice only a limited range of tolerance ones is used.7.4 ISO System of Limits and Fits Though the general sets of basic deviations and tolerance grades can be used for prescriptions of h o l e t o l e r a n c e z o n e b y t h e i r m u t u a l combinations, in practice only a limited range of tol

30、erance ones is used. 尽管基本偏差和公差等级的尽管基本偏差和公差等级的所有集合所有集合可以通过相互组可以通过相互组合确定孔的公差带，但是在实际应用中，只采用公差合确定孔的公差带，但是在实际应用中，只采用公差带中的一小部分。带中的一小部分。7.4 ISO System of Limits and Fits Basic deviationRS TPCDEFFGCDEHYABGJSKM NUVXZZAZBZCFBasic sizezero line0+-HoleJ0+rafgdefcdbhjsjkmnps tuxzzazbzcgecfvyBasic sizezero line0-

31、Shaft07.4 ISO System of Limits and Fits Although there can be generally coupled parts without any tolerance zones, only two methods of coupling of holes and shafts, namely basic hole system and basic shaft system, are recommended due to constructional, technological and economic reasons.尽管存在没有公差带的一般

32、装配零件，基于结构、技术和经济尽管存在没有公差带的一般装配零件，基于结构、技术和经济的原因，只推荐两种孔和轴的配合方法，即基孔制和基轴制。的原因，只推荐两种孔和轴的配合方法，即基孔制和基轴制。Basic hole system (BHS): A system of fits in which the design size of the hole is the basic size and the allowance, if any ,is applied to the shaft.Basic shaft system (BSS): A system of fits in which the

33、design size of the shaft is the basic size and the allowance, if any, is applied to the hole.7.4 ISO System of Limits and Fits BHS: When specifying the tolerance for a hole and cylinder and determining their dimensions, you should begin calculating by assuming either the minimum hole or maximum shaf

34、t size if they are to fit together well. 当给一个孔或圆柱面指定公差和决定尺寸时，需要假设最当给一个孔或圆柱面指定公差和决定尺寸时，需要假设最小尺寸的孔或最大尺寸的轴配合良好，并进行计算。小尺寸的孔或最大尺寸的轴配合良好，并进行计算。 The desired clearances and interferences in the fit are achieved by combinations of various shaft tolerance zones with the hole tolerance zone “H”. 配合中所需要的间隙和过盈量可

35、以通过不同的轴的公差带配合中所需要的间隙和过盈量可以通过不同的轴的公差带与代号为与代号为“H”的孔的公差带组合来实现。的孔的公差带组合来实现。7.4 ISO System of Limits and Fits basic size25 H7/g6basic dev.for the holeIT gradefor the holebasic dev.for the shaftIT gradefor the shafttolerance zoneBasic. Dev.BHS7.4 ISO System of Limits and Fits 第第12讲讲 allowance: 0.003the mi

36、nimum hole size (basic size) : 1.500tolerance: 0.002What is the maximum diameter of the shaft? (clearance fit or interference fit)holeCminshaftAllowance of the fit corresponds to maximum material condition (Cmin).holeshaftmaxIAllowance of the fit corresponds to maximum material condition (Imax).7.4

37、ISO System of Limits and Fits To calculate the maximum diameter of the shaft, assume an allowance of 0.003 and subtract that from the basic hole size. Arbitrarily selecting a tolerance of 0.002 for both the hole and shaft gives a maximum hole (1.502) and minimum shaft (1.495). The minimum clearance

38、fit is the difference between the smallest hole (1.500) and the largest shaft (1.497) or 0.003. By adding the allowance to the basic hole size, the maximum shaft size of an interference fit is obtained. To convert basic hole size to basic shaft size, we subtract the allowance for a clearance fit or

39、add it for an interference fit.7.4 ISO System of Limits and Fits tolerance zoneBasic. Dev.Basic. Dev.BHSBSSbasic sizebasic sizeholetolerance zoneshaftholeshafttolerance zone7.4 ISO System of Limits and Fits Any manufacture should have thorough knowledge of the tolerances and fits to increase the quality and reliability of a manufactured part with the least expense. The increase compatibility（互换性）（互换性） of a certain product with mating parts by difference manufacturers makes it more viable in market competition. Moreover, a knowledge of tolerances