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 unit. The most common fit found in the machine shop is that of a shaft in a hole.

There are three general categories of fits:

1) Clearance fits for when it may be desirable for the shaft to rotate or slide freely within the hole.

2) Transition fits for when it is desirable that the shaft to be held precisely, yet not so tightly that it cannot be disassembled, this is usually referred to as a Location or Transition fit.

3) Interference fits, for when it is desirable for the shaft to be securely held within the hole and it is acceptable that some force be necessary for assembly.

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.

Fit Classes

Interference fits

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.

Force fits

FN 1 to FN 5

Shrink fits

When the parts can only be easily put together when one or the other is expanded by heating or contracted by cooling.

Location fits

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

Location iNterference fit

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

Location Transition fit

LT 1 to LT 6

Location Clearance fit

LC 1 to LC 11

RC fits

The smaller RC numbers have smaller clearances for tighter fits, the larger numbers have larger clearances for looser fits.^[1]

RC1: Close Sliding Fits

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

RC2: Sliding Fits

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.

RC3: Precision Running Fits

Fits of this kind are about the closest fits which can be expected to run freely. Precision running 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.

RC4: Close Running Fits

Fits of this kind are mostly for running fits on accurate machinery with moderate surface speed, bearing pressures, and journal pressures where accurate location and minimum play are desired. Fits of this kind also can be described as smaller clearances with higher requirements for precision fit.

RC5 and R6: Medium Running Fits

Fits of this kind are designed for machines running at higher running speeds, considerable bearing pressures, and heavy journal pressure. Fits of this kind also can be described with greater clearances with common requirements for fit precision.

RC7: Free Running Fits

Fits of this kind are intended to use where accuracy is not essential. It is suitable for great temperature variations. This fit is suitable to use without any special requirements for precise guiding of shafts.

RC8 and RC9: Loose Running Fits

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

ISO Metric fits

ISO-R286 and ANSI B4.2-1978

Fitting

The task of fitting is making skilled cuts to parts, on a cut-and-try basis (cut, try; cut more, try again), so that they will fit together with the desired degree of fit. Prior to the advent of interchangeable manufacture, fitting was the only way to create precise assemblies, such as the locks of firearms, and it was done manually, one assembly at a time, with tools such as files and laps. The person who did the fitting could be a craftsman such as a gunsmith, or a factory worker at a bench (called a fitter), or a toolmaker or toolfitter. When interchangeable manufacture began, its first form was one that did not obviate fitting, but simply shifted its locus, from the fitting of mating parts to each other, to the fitting of identical parts to a jig, template, or gauge ("filing to jig", "filing to gauge"). Gradually, manufacturing technology transitioned ever more toward processes in which parts were machined in such a precise, repeatable way that they required no fitting at all. For example, a pistol's hammer could be milled with a milling machine directly to gauge size, rather than being rough-milled to nearly that size followed by hand filing to gauge.

See also

• Interchangeable parts
• Statistical interference
• Coiled spring pins

References