Types Of Fits

FIT

A fit may be defined as the relative motion which can exist between a shaft and hole (as defined above) resulting form the final sizes which achieved in their manufacture. or in other words When two parts are to be assembled the relation resulting from the difference between their sizes before assembly is called a fit. The fit signifies the range of tightness or looseness which may result from the application of a specific combination of allowances and tolerances in the design of mating parts.

There are three classes of fit in common use :

1. clearance

2. transition

3. interference.

CLEARANCE FIT

This fit results when the shaft size is always less than the hole size for all possible combinations within their tolerance ranges. Relative motion between shaft and hole is always possible. The minimum clearance occurs at the maximum shaft size and the minimum hole size. The maximum clearance occurs at the minimum shaft size and the maximum hole size. Clearance fits range from coarse or very loose to close precision and location.

CLEARANCE FIT

TRANSITION FIT

A pure transition fit occurs when the shaft and hole are exactly the same size. This fit is theoretically the boundary between clearance and interference and is practically impossible to achieve, but by selective assembly or careful machining methods, it can be approached within very fine limits.

TRANSITION FIT

Practical transition fits result when the tolerance are such that the largest hole is greater than the smallest shaft and the largest shaft is greater than the  smallest hole. Two transition fits are given on the data sheet.

Relative motion between shaft and hole is possible when clearance exists but impossible when interference exists.