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An interference fit, also known as a press fit or friction 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. A press fit is also required to mount wheels on an axle to make a wheel set.
Introducing interference between parts
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, or an axle, may be ground slightly oversize and the hole in the bearing, or the wheel, (through which it is going to pass with an interference fit) may be ground slightly undersized. When the shaft is pressed into the bearing, or when the wheels and roller bearings are pressed onto an axle as in the case of a wheel set, the two parts interfere with each other's occupation of space. The result is that both parts elastically deform slightly to fit together creating an extremely high force which results in extremely high friction between the parts—so high that even large amounts of torque cannot turn one of them relative to the other; they are locked together and turn in unison. These fits, though applicable to shaft and hole assembly, are more often used for bearing-housing or bearing-shaft assembly.
Tightness of fit
The tightness of fit is controlled by amount of interference; the allowance (planned difference from nominal size). Formulas exist  to compute allowance 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.
As an example, a 10 mm (0.394 in) shaft made of 303 stainless steel will form a tight fit with allowance of 3–10 µm (0.0001–0.0003 in). A slip fit can be formed when the bore diameter is 12–20 µm (0.0005–0.0008 in) wider than the rod; or, if the rod is made 12–20 µm under the given bore diameter.
An example: The allowance per inch of diameter usually ranges from 0.001 inch to 0.0025 inch (0.1–0.25%), 0.0015 inch (0.15%) being a fair average. 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, 0.2%), 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, i.e. 0.11–0.12%). The parts to be assembled by forced fits are usually made cylindrical, although 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 reliable, 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.
There are two basic methods for assembling an oversize shaft into an undersized hole, sometimes used in combination:
- thermal expansion or contraction.
Press fit is achieved with presses that can press the parts together with very large amounts of force. The presses are generally hydraulic, although small hand-operated presses (such as arbor presses) may operate by means of the mechanical advantage supplied by a jackscrew or by a gear reduction driving a rack and pinion. The amount of force applied in hydraulic presses may be anything from a few pounds for the tiniest parts to hundreds of tons for the largest parts.
Often the edges of shafts and holes are chamfered (beveled). The chamfer forms a guide for the pressing movement, helping to distribute the force evenly around the circumference of the hole, 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), and to assist in aligning the shaft parallel with the hole it is being pressed into.
Thermal expansion or contraction
Most materials expand when heated and shrink when cooled. Enveloping parts are heated (e.g., 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. This is also referred to as shrink-fitting. 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 the risk exists of heating the gear too much and drawing its temper.
- Alan O. Lebeck (1991). Principles and design of mechanical face seals. Wiley-Interscience. p. 232. ISBN 978-0-471-51533-3.
- Press Fit Engineering and Design Calculator
- Heinz P. Bloch (1998). Improving machinery reliability (3rd ed.). Gulf Professional Publishing. p. 216. ISBN 978-0-88415-661-1.
- "Coupling Design and Selection". Retrieved 2010-01-30.[dead link]