Engineering Fits – Clearance, Transition , Interference Fit

Introduction To Engineering Fit:

In a specific type of fit, the difference between hole size and shaft size is called allowance. Allowance is used to explain the difference between clearance fit and interference fit. Positive allowance specifies the clearance fit whereas negative allowance in a fit specifies the interference or force fit. The relationship existing between two parts, shaft and hole, which are to be assembled, with respect to the difference in their sizes before assembly is called a fit. When the parts are assembled into sub-assembly units and sub assembly units are assembled into full assembly, the mating surfaces of different components are joined together for proper functional requirement. One of them may fit into the other in form of joint or fit. The fit may be with suitable degree of tightness and freedom for required relative movement between mating parts for specific functional requirements of the fit. 

The classification of fits is given in Fig.

Clearance Fit-Loose Fit, Running Fit, Slide Fit
Clearance Fit-Loose Fit, Running Fit, Slide Fit

Components are assembled to perform a specific function. The characteristic of the assembly is determined by the fit. Fit is the general term used to signify the relative degree of tightness or looseness of assembled parts, which decides the relative movement between mating parts. Particular type of fit results from the difference in the sizes of mating parts. Fig. shows the various types of fits. Two parts can fit each other in three ways, viz.

Example Of Engineering Fits
Example Of Engineering Fits

Clearance Fit

Clearance fit is one in which two assembled parts are always free to move relative to each other in the assembly. In the clearance fit, the largest permitted shaft diameter is smaller than the diameter of the smallest hole. The difference between the size of hole and the size of shaft is defined as clearance. Clearance fits have limits of size prescribed so that a clearance always results in a positive allowance, or air space is left between mating parts. The parts can be assembled by hand. Clearance fit is of two kinds namely sliding and running
Examples of clearance fit are door hinges, wheel and axle, shaft and bearing, etc. used in the assembly of parts.

Interference Fit

In the interference fit, mating parts in sub assembly or main assembly are joined tightly together and no relative motion is possible. The minimum permitted diameter of the shaft is larger than the maximum allowable diameter of the hole. In this case, the shaft and the hole member in any sub assembly or main assembly is to be attached permanently and used as a solid component but according to the application of this combination, this type of fit can be varied. Interference fit is used for permanent or semi-permanent assembly of parts, which require rigidity and alignment and no relative motion, such as dowel pins and bearings in casting. The difference between the size of shaft and the size of hole in any sub assembly or main assembly is called interference or negative clearance or negative allowance. Interference fit has limits of size so prescribed that interference always results when mating parts are assembled. Transition fits is of two kinds namely driving or press fit and shrink or force fit. To assemble, parts are usually pressed together using an arbor press

Transition Fit

In this type of fit, the diameter of the largest allowable hole is greater than that of the smallest shaft, but the smallest hole is smaller than the largest shaft, so that small positive or negative clearance between the shaft and hole member is employable. Thus, transition fit has limits of size of hole and shaft such that either a clearance or an interference fit may result when two specific parts from the lot are assembled. Here the tolerance zones of the hole and shaft overlap. Transition fits are a compromise between clearance and interference fit where a small amount of either clearance or interference is permissible. Transition fit is of two kinds namely push fit and light keying fits. That is, sometimes shaft is slightly larger than the hole and sometimes slightly smaller than the hole. Spigot in mating holes, coupling rings and recesses are the examples of transition fit.

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