"Fit is a term used in enginering to describe the condition of tightness or looseness between a hole and a shaft when assembled together."

Note: Any external feature of a manufacture part is called as a shaft. Any internal feature of a manufacture part is called as a hole.

In order to learn about fits, first and foremost let u know about allowance.
So what exactly is allowance?

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 indicates 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, concerning 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 the full assembly, the mating surfaces of different components are joined together for the proper functional requirement. One of them may fit into the other in the form of joint or fit. The fit may be with a suitable degree of tightness and freedom for required relative movement between mating parts for specific functional requirements of the fit.Clerance fit, transition fit and interference fit these are the three fits in engineering. Fits are very important for proper assembly of a product.

When a hole and shaft is assembled together, they may fit loosely or tightly or perfectly.Thus there are three types of fits namely :

1. Clearence fit When the maximum permissible shaft size is smaller than the smallest permissible hole size then the fit is said ti be clearence fit. Examples of clearance fit are door hinges, wheel, and axle, shaft and bearing, etc. used in the assembly of parts.

2.Interference fit When the maximum permissible hole size is smaller than the smallest permissilbe shaft size then the fit is known as interference fit . 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.

3. Transition fit- When a fit provides either clearence or interference it is known as transition fit. In this fit clearence or interference depends upon the actual size of the shaft and hole. And the fit can be identified only during its assembly. Spigot in mating holes, coupling rings and recesses are the examples of transition fit.

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