Interference fit

Interference fit

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.

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 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.

Tightness of fit is controlled by amount of interference ("allowance")

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 convert|10|mm|in|3|abbr=on 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.

Assembling an oversize shaft into an undersized hole

There are two basic methods for assembly, sometimes used in combination: Force, and changing the size of the parts by heating.

Force

This is achieved with presses that can press the parts together with very large amounts of force. (Hence the term "press fit.") 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 screw jack. The amount of force applied 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 (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).

Heating to temporarily remove the interference

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.

External links

* [http://claymore.engineer.gvsu.edu/~jackh/eod/manufact/manufact-155.html Diagram of an interference fit]
* [http://www.eminebea.com/content/html/en/engineering/bearings/fitting.shtml Interference fitting] - formulae for calculating clearance reductions when using interference fits for bearings on shafts and in housings
* [http://www.amerace.com/NewFiles/interfit5.html Interference fit watertight cable connector]


Wikimedia Foundation. 2010.

Игры ⚽ Нужен реферат?

Look at other dictionaries:

  • interference fit — noun (engineering) A fit between two parts which requires force on assembly to expand the outer or contract the inner • • • Main Entry: ↑interfere …   Useful english dictionary

  • interference fit — A condition of fit (contact) between two parts that requires pressure to force the parts together. Usually the shaft is slightly larger than the hole so that they must be pounded or forced or driven together. Also called force fit, press fit, or… …   Dictionary of automotive terms

  • interference fit — noun a fit in which the external dimension of one part exceeds the internal dimension of the part into which it has to fit …   English new terms dictionary

  • Interference (disambiguation) — Interference may refer to:;Sciences *Interference (physics), in physics, the superposition of two or more waves resulting in a new wave pattern. *Interference (communication), in telecommunication and electronics, anything which alters, modifies …   Wikipedia

  • fit — [1] Contact area between two parts. [2] The way in which two parts come together. [3] To attach or put into place. Also see clearance fit drive fit force fit force fit hubcentric fit interference fit press fit running fit …   Dictionary of automotive terms

  • Engineering fit — Fit refers to the mating of two mechanical components. Manufactured parts are very frequently required to mate with one another. They may be designed to slide freely against one another or they may be designed to bind together to form a single… …   Wikipedia

  • press fit — An interference fit. A fit so tight that an object must be removed with a puller or a press. See drive fit interference fit force fit …   Dictionary of automotive terms

  • Shrink fit — A shrink fit is an interference fit achieved by a relative size change after assembly. This is usually achieved by heating or cooling one component before assembly and allowing it to return to the ambient temperature after assembly. It may also… …   Wikipedia

  • press fit — noun : the fit of a shaft driven into a hole slightly smaller than itself and held tight and motionless compare loose fit * * * pressfit, adj. Mach. assembly of two tightly fitting parts, as a hub on a shaft, made by a press or the like. Also… …   Useful english dictionary

  • drive fit — A condition of fit (contact) between two parts that requires pressure to force the parts together. Usually the shaft is slightly larger than the hole so that they must be pounded or forced or driven together. Also called force fit, press fit, or… …   Dictionary of automotive terms

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”