Machinery's Handbook, 31st Edition
British Standard Metric ISO Limits and Fits 675 apply equally well to other sections, and the general terms hole and shaft can be taken to mean the space contained by or containing two parallel faces or tangent planes of any part, such as the width of a slot or the thickness of a key. It is also strongly emphasized that the grades series of tolerances are intended for the most general application and should be used wherever possible whether the features of the component involved are members of a fit or not. Definitions.— The definitions given in the Standard include the following: Limits of Size: The maximum and minimum sizes permitted for a feature. Basic Size: The reference size to which the limits of size are fixed. The basic size is the same for both members of a fit. Upper Deviation: The algebraic difference between the maximum limit of size and the corresponding basic size. It is designated as “ES” for a hole and as “es” for a shaft, which stands for the French term écart supérieur. Lower Deviation: The algebraic difference between the minimum limit of size and the corresponding basic size. It is designated as “EI” for a hole and as “ei” for a shaft, which stands for the French term écart inférieur. Zero Line: In a graphical representation of limits and fits, the straight line to which the deviations are referred. The zero line is the line of zero deviation and represents the basic size. Tolerance: The difference between the maximum limit of size and the minimum limit of size. It is an absolute value without sign. Tolerance Zone: In a graphical representation of tolerances, the zone comprised between the two lines representing the limits of tolerance and defined by its magnitude (tolerance) and by its position in relation to the zero line. Fundamental Deviation: That one of the two deviations, being the one nearest to the zero line, which is conventionally chosen to define the position of the tolerance zone in relation to the zero line. Shaft-Basis System of Fits: A system of fits in which the different clearances and inter ferences are obtained by associating various holes with a single shaft. In the ISO system, the basic shaft is the shaft the upper deviation of which is zero. Hole-Basis System of Fits: A system of fits in which the different clearances and inter ferences are obtained by associating various shafts with a single hole. In the ISO system, the basic hole is the hole the lower deviation of which is zero. Selected Limits of Tolerance, and Fits.— The number of fit combinations that can be built up with the ISO system is very large. However, experience shows that the majority of fits required for usual engineering products can be provided by a limited selection of tolerances. Limits of tolerance for selected holes are shown in Table 1 and for shafts in Table 2 . Selected fits, based on combinations of the selected hole and shaft tolerances, are given in Table 3. Tolerances and Fundamental Deviations.— There are 18 tolerance grades intended to meet the requirements of different classes of work, and they are designated IT01, IT0, and IT1 to IT16. (IT stands for ISO series of tolerances.) Table 4 shows the standardized numerical values for the 18 tolerance grades, which are known as standard tolerances. The system provides 27 fundamental deviations for sizes up to and including 500 mm, and Table 5a and Table 5b contain the values for shafts and Table 6a and Table 6b for holes. Uppercase (capital) letters designate hole deviations, and the same letters in lower case designate shaft deviations. The deviation j s (J s for holes) is provided to meet the need for symmetrical bilateral tolerances. In this instance, there is no fundamental deviation, and the tolerance zone, of whatever magnitude, is equally disposed about the zero line. Calculated Limits of Tolerance.— The deviations and fundamental tolerances pro- vided by the ISO system can be combined in any way that appears necessary to give a required fit. Thus, for example, the deviations H (basic hole) and f (clearance shaft) could
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