Machinery's Handbook, 31st Edition
642 Fits The initial dimensions placed on component drawings should be the exact dimensions that would be used if it were possible to work without tolerances. Tolerances should be given in that direction in which variations will cause the least harm or danger. When a vari ation in either direction is equally dangerous, the tolerances should be of equal amount in both directions, or bilateral. The initial clearance, or allowance, between operating parts should be as small as the operation of the mechanism will permit. The maximum clearance should be as great as the proper functioning of the mechanism will permit. Direction of Tolerances on Gages.— The extreme sizes for all plain limit gages shall not exceed the extreme limits of the part to be gaged. All variations in the gages, whatever their cause or purpose, shall bring these gages within these extreme limits. The data for gage tolerances on page 673 cover gages to inspect workpieces held to tolerances in the American National Standard ANSI B4.4M-1981 (Withdrawn). Allowance for Forced Fits.— The allowance per inch of diameter usually ranges from 0.001 inch to 0.0025 inch (0.0254-0.0635 mm), 0.0015 inch (0.0381 mm) being a fair aver age. Ordinarily the allowance per inch decreases as the diameter increases; thus the total allowance for a diameter of 2 inches (50.8 mm) might be 0.004 inch (0.102 mm), whereas for a diameter of 8 inches (203.2 mm) the total allowance might not be over 0.009 or 0.010 inch (0.23 or 0.25 mm). The parts to be assembled by forced fits are usually made cylindrical, al - though sometimes they are slightly tapered. Advantages of the taper form are: the possibility of abrasion of the fitted surfaces is reduced; less pressure is required in assembling; and parts are more readily separated when renewal is required. On the other hand, the taper fit is less re - liable because, if it loosens, the entire fit is free with but little axial movement. Some lubricant, such as white lead and lard oil mixed to the consistency of paint, should be applied to the pin and bore before assembling, to reduce the tendency toward abrasion. Pressure for Forced Fits.— The pressure required for assembling cylindrical parts depends not only upon the allowance for the fit, but also upon the area of the fitted surfaces, the pres - sure increasing in proportion to the distance that the inner member is forced in. The approx- imate ultimate pressure in tons can be determined by the use of the following formula in conjunction with the accompanying table of Pressure Factors for Forced Fits . Assuming that A = area of surface in contact in “fit”; a = total allowance in inches; P = ultimate pressure required, in tons; F = pressure factor based upon assumption that the diameter of the hub is twice the diameter of the bore, that the shaft is of machine steel, and that the hub is of cast iron: P A a F 2 # # = Pressure Factors for Forced Fits Diameter, Inches Pressure Factor Diameter, Inches Pressure Factor Diameter, Inches Pressure Factor Diameter, Inches Pressure Factor Diameter, Inches Pressure Factor 1 500 3 1 ⁄ 2 132 6 75 9 48.7 14 30.5 1 1 ⁄ 4 395 3 3 ⁄ 4 123 6 1 ⁄ 4 72 9 1 ⁄ 2 46.0 14 1 ⁄ 2 29.4 1 1 ⁄ 2 325 4 115 6 1 ⁄ 2 69 10 43.5 15 28.3 1 3 ⁄ 4 276 4 1 ⁄ 4 108 6 3 ⁄ 4 66 10 1 ⁄ 2 41.3 15 1 ⁄ 2 27.4 2 240 4 1 ⁄ 2 101 7 64 11 39.3 16 26.5 2 1 ⁄ 4 212 4 3 ⁄ 4 96 7 1 ⁄ 4 61 11 1 ⁄ 2 37.5 16 1 ⁄ 2 25.6 2 1 ⁄ 2 189 5 91 7 1 ⁄ 2 59 12 35.9 17 24.8 2 3 ⁄ 4 171 5 1 ⁄ 4 86 7 3 ⁄ 4 57 12 1 ⁄ 2 34.4 17 1 ⁄ 2 24.1 3 156 5 1 ⁄ 2 82 8 55 13 33.0 18 23.4 3 1 ⁄ 4 143 5 3 ⁄ 4 78 8 1 ⁄ 2 52 13 1 ⁄ 2 31.7 … … Allowance for Given Pressure.— By transposing the preceding formula, the approximate allowance for a required ultimate tonnage can be determined. Thus, a AF P = 2 . The average ultimate pressure in tons commonly used ranges from 7 to 10 times the diameter in inches.
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