Interference fit 1

Interference fitAn interference fit, also known as a press fit or friction fit,[1] is a fastening between two parts which is achieved byfriction 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 ofone part against the other, which relies on the tensile and compressive strengths of the materials the parts are madefrom. Typical examples of interference fits are the press fitting of shafts into bearings or bearings into their housingsand the attachment of watertight connectors to cables. An interference fit also results when pipe fittings areassembled and tightened.

Introducing interference between partsAn interference fit is generally achieved by shaping the two mating parts so that one or the other, or both, slightlydeviate in size from the nominal dimension. The word interference refers to the fact that one part slightly interfereswith 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 passwith an interference fit) may be ground slightly undersized. When the shaft is pressed into the bearing, the two partsinterfere with each other's occupation of space. The result is that both parts elastically deform slightly to fit togethercreating an extremely high force which results in extremely high friction between the partsso high that even largeamounts of torque cannot turn one of them relative to the other; they are locked together and turn in unison.

Tightness of fitThe tightness of fit is controlled by amount of interference; the "allowance". Formulas exist to compute thisallowance (planned difference from nominal size) that will result in various strengths of fit such as loose fit, lightinterference fit, and interference fit. The value of the allowance depends on which material is being used, how bigthe parts are, and what degree of tightness is desired. Such values have already been worked out in the past for manystandard 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 10mm (unknown operator: u'strong'in) shaft made of 303 stainless steel, theengineer can look up the needed allowance in a reference book or computer program, rather than using a formula tocalculate it.

AssemblingThere are two basic methods for assembling an oversize shaft into an undersized hole, sometimes used incombination:1.1. force,2.2. thermal expansion or contraction.

ForceThere are at least three different terms used to describe an interference fit created via force: press fit, friction fit, andhydraulic dilation.[2][3]

Press fit is achieved with presses that can press the parts together with very large amounts of force. The presses aregenerally hydraulic, although small hand-operated presses (such as arbor presses) may operate by means of themechanical advantage supplied by a jackscrew or by a gear reduction driving a rack and pinion. The amount of forceapplied in hydraulic presses may be anything from a few pounds for the tiniest parts to hundreds of tons for thelargest parts.

Interference fit 2

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 occurgradually instead of all at once, thus helping the pressing operation to be smoother, to be more easily controlled, andto require less power (less force at any one instant of time), and to assist in aligning the shaft parallel with the hole itis being pressed into.

Thermal expansion or contractionMost materials expand when heated and shrink when cooled. Enveloping parts are heated (e.g., with torches or gasovens) and assembled into position while hot, then allowed to cool and contract back to their former size, except forthe compression that results from each interfering with the other. This is also referred to as shrink-fitting. Railroadaxles, wheels, and tires are typically assembled in this way. Alternatively, the enveloped part may be cooled beforeassembly such that it slides easily into its mating part. Upon warming, it expands and interferes. Cooling is oftenpreferable 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.

References[1] Alan O. Lebeck (1991). Principles and design of mechanical face seals (http:/ / books. google. com/ books?id=RnOZ4zl6CRMC&

pg=PA232& dq="Friction+ fit"+ "interference+ fit"+ "press+ fit"& lr=& ei=8YNkS4L2KJH2NPGp_IsO& cd=1#v=onepage& q="Friction fit""interference fit" "press fit"& f=false). Wiley-Interscience. p.232. ISBN978-0-471-51533-3. .

[2] Heinz P. Bloch (1998). Improving machinery reliability (http:/ / books. google. com/ books?id=pBhKQu8WwL8C& pg=PA216&dq="Friction+ fit"+ "interference+ fit"& lr=& ei=f4BkS4HmAabuNN3AvYoO& cd=2#v=onepage& q="Friction fit" "interference fit"&f=false) (3rd ed.). Gulf Professional Publishing. p.216. ISBN978-0-88415-661-1. .

[3] "Coupling Design and Selection" (http:/ / www. emerson-ept. com/ EPTRoot/ kopflex/ Engineered/ FAQ/ design. htm). . Retrieved2010-01-30.

External links Diagram of an interference fit (http:/ / engineeronadisk. com/ notes_manufact/ assemblya3. html) Interference fitting (http:/ / www. eminebea. com/ content/ html/ en/ engineering/ bearings/ shaftbrg_10. shtml) -

formulae for calculating clearance reductions when using interference fits for bearings on shafts and in housings

Article Sources and Contributors 3

Article Sources and ContributorsInterference fit Source: http://en.wikipedia.org/w/index.php?oldid=497185316 Contributors: A2Kafir, Anachron, AndrewDressel, BAxelrod, Balok, Bbaddorf, BenFrantzDale,Bigdumbdinosaur, Cstaffa, DerrickOswald, Dispenser, Evand, HPeugeot, Hashem62, Hooperbloob, Longhair, Lumbercutter, Otrfan, Peter Horn, Reelrt, Regofix, Rlsheehan, S Roper, Sameerpandya, SamuelRiv, Sole Soul, Sourgreentomatoes, Tagishsimon, Three-quarter-ten, VBGFscJUn3, Wizard191, Xchbla423, Ymulleneers, 26 anonymous edits

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Interference fitIntroducing interference between partsTightness of fitAssemblingForce Thermal expansion or contraction

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