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, 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.
The classification of fits is given in Fig. 
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. 
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 the hole and the size of the shaft is defined as clearance. Clearance fits have limits of the 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 fits.
Examples of clearance fit are door hinges, wheel, and axle, shaft and bearing, etc. used in the assembly of parts. Clearance fit can be sub-classified as follows : Loose Fit It is used between those mating parts where no precision is required. It provides minimum allowance and is used on loose pulleys, agricultural machineries, etc. Running Fit For a running fit, the dimension of the shaft should be smaller enough to maintain a film of oil for lubrication. It is used in bearing pair etc. An allowance 0.025 mm per 25 mm of diameter of bearing may be used.
It is used on those mating parts where great precision is required. It provides medium allowance and is used in tool slides, slide valves, automobile parts, etc.
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 are 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 the shaft and the size of the 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 are of two kinds namely driving or press-fit and shrink or force fit. To assemble, parts are usually pressed together using an arbor press
