An interference fit, also known as a press fit, is a fastening between two parts which is achieved by friction after the parts are pushed together, rather than by any other means of fastening. For metal parts in particular, the friction that holds the parts together is often greatly increased by compression of one part against the other, which relies on the tensile and compressive strengths of the materials the parts are made from. Typical examples of interference fits are the press fitting of shafts into bearings or bearings into their housings and the attachment of watertight connectors to cables. An interference fit also results when pipe fittings are assembled and tightened.
An interference fit is generally achieved by shaping the two mating parts so that one or the other (or both) slightly deviate in size from the nominal dimension. The word interference refers to the fact that one part slightly interferes with the space that the other is taking up. For example: A shaft may be ground slightly oversize, and the hole in the bearing (through which it is going to pass with an interference fit) may be ground slightly undersized. When the shaft is pressed into the bearing, the two parts interfere with each others occupation of space; the result is that they plastically deform slightly, each being compressed, and the interface between them is one of extremely high friction—so high that even large amounts of torque cannot turn one of them relative to the other; they are locked together and they turn in unison.
Formulas exist to compute the "allowance" (planned difference from nominal size) that will result in various strengths of fit such as loose fit, light interference fit, and interference fit. The value of the allowance depends on which material is being used, how big the parts are, and what degree of tightness is desired. Such values have already been worked out in the past for many standard applications, and they are available to engineers in the form of tables, obviating the need for re-derivation. Thus if a loose fit is desired for a shaft made of 303 stainless steel, the engineer can look up the needed allowance in a reference book or computer program, rather than "reinventing the wheel" by using a formula to calculate it.
There are two basic methods for assembly, sometimes used in combination: Force, and changing the size of the parts by heating.
Often the edges of shafts and holes are chamfered (beveled). The chamfer forms a guide for the pressing movement, helping (a) to distribute the force evenly around the circumference of the hole, and (b) to allow the compression to occur gradually instead of all at once, thus helping the pressing operation to be smoother, to be more easily controlled, and to require less power (less force at any one instant of time).
Most materials expand when heated and shrink when cooled. Enveloping parts are heated (e.g., such as with torches or gas ovens) and assembled into position while hot, then allowed to cool and contract back to their former size, except for the compression that results from each interfering with the other. Railroad axles, wheels, and tires are typically assembled in this way. Alternatively, the enveloped part may be cooled before assembly such that it slides easily into its mating part. Upon warming, it expands and interferes. Cooling is often preferable as it is less likely than heating to change material properties, e.g. assembling a hardened gear onto a shaft, where heating the gear would alter its hardness.
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