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Engineering fit


Engineering fits are generally used as part of geometric dimensioning and tolerancing when a part or assembly is designed. In engineering terms, the "fit" is the clearance between two mating parts, and the size of this clearance determines whether the parts can, at one end of the spectrum, move or rotate independently from each other or, at the other end, are temporarily or permanently joined together. Engineering fits are generally described as a "shaft and hole" pairing, but are not necessarily limited to just round components. ISO is the internationally accepted standard for defining engineering fits, but ANSI is often still used in North America.

ISO and ANSI both group fits into three categories: clearance, location or transition, and interference. Within each category are several codes to define the size limits of the hole or shaft - the combination of which determines the type of fit. A fit is usually selected at the design stage according to whether the mating parts need to be accurately located, free to slide or rotate, separated easily, or resist separation. Cost is also a major factor in selecting a fit, as more accurate fits will be more expensive to produce, and tighter fits will be more expensive to assemble.

The International Organisation for Standardization system splits the three main categories into several individual fits based on the allowable limits for hole and shaft size. Each fit is allocated a code, made up of a number and a letter, which is used on engineering drawings in place of upper & lower size limits to reduce clutter in detailed areas.

A fit is either specified as shaft-basis or hole-basis, depending on which part has its size controlled to determine the fit. To elaborate: in a hole-basis system, the size of the hole remains constant and it is the diameter of the shaft that is varied to determine the fit; conversely in a shaft-basis system, the size of shaft remains constant and the hole diameter is varied to determine the fit.

The ISO system uses an alpha-numeric code to illustrate the tolerance ranges for the fit, with the upper-case representing the hole tolerance and lower-case representing the shaft. For example, in H7/h6 (a commonly-used fit) H7 represents the tolerance range of the hole and h6 represents the tolerance range of the shaft. These codes can be used by machinists or engineers to quickly identify the upper and lower size limits for either the hole or shaft. The potential range of clearance or interference can be found by subtracting the smallest shaft diameter from the largest hole, and largest shaft from the smallest hole.

Interference fits, also known as press fits or friction fits, are fastenings between two parts in which the inner component is larger than the outer component. Achieving an interference fit requires applying force during assembly. After the parts are joined, the mating surfaces will feel pressure due to friction, and deformation of the completed assembly will be observed.

Shrink fits serve the same purpose as force fits, but are achieved by heating one member to expand it while the other remains cool. The parts can then be easily put together with little applied force, but after cooling and contraction, the same dimensional interference exists as for a force fit. Like force fits, shrink fits range from FN 1 to FN 5.

Fits of this kind are intended for the accurate location but with greater maximum clearance than class RC1. Parts made to this fit turn and move easily. This type is not designed for free run. Sliding fits in larger sizes may seize with small temperature changes due to little allowance for thermal expansion or contraction.

Fits of this kind are intended for use where wide commercial tolerances may be required on the shaft. With these fits, the parts with great clearances with having great tolerances. Loose running fits may be exposed to effects of corrosion, contamination by dust, and thermal or mechanical deformations.