Machinery's Handbook, 31st Edition
Fits 643 Expansion Fits.— In assembling certain classes of work requiring a very tight fit, the inner member is contracted by sub-zero cooling to permit insertion into the outer member and a tight fit is obtained as the temperature rises and the inner part expands. To obtain the sub-zero temperature, solid carbon dioxide or “dry ice” has been used, but its temperature of about –109 ° F (–78 ° C) below zero will not contract some parts sufficiently to permit insertion in holes or recesses. Greater contraction may be obtained by using high purity liquid nitrogen which has a temperature of about –320 ° F (–196 ° C) below zero. During a temperature reduction from 75 ° F to –321 ° F (220 ° C difference), the shrinkage per inch of diameter varies from about 0.002 to 0.003 inch for steel; 0.0042 inch for aluminum alloys; 0.0046 inch for magnesium alloys; 0.0033 inch for copper alloys; 0.0023 inch for monel metal; and 0.0017 inch for cast iron (not alloyed). The cooling equipment may vary from an insulated bucket to a special automatic unit, depending upon the kind and quantity of work. One type of unit is so arranged that parts are precooled by vapors from the liquid nitrogen before immersion. With another type, cooling is entirely by the vapor method. Shrinkage Fits.— General practice seems to favor a smaller allowance for shrinkage fits than for forced fits, although in many shops the allowances are practically the same for each, and, for some classes of work, shrinkage allowances exceed those for forced fits. The shrinkage allowance also varies to a great extent with the form and construction of the part that has to be shrunk into place. The thickness or amount of metal around the hole is the most important factor. The way in which the metal is distributed also has an influence on the results. Shrinkage allowances for locomotive driving wheel tires adopted by the American Railway Master Mechanics Association are as follows: Center diameter, inches 38 44 50 56 62 66 Allowances, inches 0.040 0.047 0.053 0.060 0.066 0.070 Whether parts are to be assembled by forced or shrinkage fits depends upon conditions. For example, to press a tire over its wheel center, without heating, would ordinarily be a rather difficult job. On the other hand, pins, etc., are easily and quickly forced into place with a hydraulic press, and there is the additional advantage of knowing the exact pressure required in assembling, whereas there is more or less uncertainty connected with a shrinkage fit, unless the stresses are calculated. Tests to determine the difference in the quality of shrinkage and forced fits showed that the resistance of a shrinkage fit to slippage for an axial pull was 3.66 times greater than that of a forced fit, and, in rotation or torsion, 3.2 times greater. In each comparative test, dimensions and allowances were equal. Allowances for Shrinkage Fits.— The most important point to consider when calcu- lating shrinkage fits is the stress in the hub at the bore, which depends chiefly upon the shrinkage allowance. If the allowance is excessive, the elastic limit of the material will be exceeded, and permanent set will occur, or, in extreme conditions, the ultimate strength of the metal will be exceeded and the hub will burst. The intensity of the grip of the fit and the resistance to slippage depend mainly upon the thickness of the hub; the greater the thickness, the stronger the grip, and vice versa. Assuming the modulus of elasticity for steel to be 30,000,000 (206.8 × 10 –6 MPa) and for cast iron 15,000,000 (103.4 × 10 –6 MPa), the shrinkage allowance per inch (mm) of nominal diameter can be determined by the following formula, in which A = allowance per inch (mm) of diameter; T = true tangential tensile stress at inner surface of outer member, psi (MPa); C = factor taken from one of the accompanying Table 1, Table 2, and Table 3. For a cast-iron hub and steel shaft: (1a) (1b) , , A T C 30 000 000 2 US = + ^ h . . A T C 254 206843 × 10 2 metric 9 = + ^ h
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