Engineering fit

During assembly of two mating parts, there may be either tightness or looseness between them. The degree of tightness or looseness between them is termed as fit. 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 unit.

There are three general categories of fits:

Within each category of fit there are several classes ranging from high precision and narrow tolerance (allowance) to lower precision and wider tolerance. The choice of fit is dictated first by the use and secondly by the manufacturability of the parts. Fits may be specified according to the standards of geometric dimensioning and tolerancing.

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.

Force Fits are designed to maintain a controlled pressure between mating parts, and are used where forces or torques are being transmitted through the joining point. Like interference fits, force fits are achieved by applying a force during component assembly.

FN 1 to FN 5

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.

Location fits are for parts that do not normally move relative to each other.

LN 1 to LN 3 ( or LT 7 to LT 21? )

LT 1 to LT 6

LC 1 to LC 11

The smaller RC numbers have smaller clearances for tighter fits, the larger numbers have larger clearances for looser fits.

Fits of this kind are intended for the accurate location of parts which must assemble without noticeable play.

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 about the closest fits which can be expected to run freely. Precision fits are intended for precision work at low speed, low bearing pressures, and light journal pressures. RC3 is not suitable where noticeable temperature differences occur.

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Wikipedia