Introduction to Fit

When two parts are to be assembled, the relationship resulting from the difference between their sizes before assembly is called a fit.

Types of Fits

Depending upon the actual limits of hole or shaft, the fit may be a clearance fit, a transition fit or an interference fit.

  1. Clearance Fit

  2. Interference Fit

  3. Transition Fit

  1. Clearance Fit : In this type of fit, the largest permitted shaft diameter is smaller than the diameter of the smallest hole, so that the shaft can rotate or slide through with different degrees of freedom according to the purpose of mating members.
    Clearance Fit Examples : sleeve shafts, clutches, movable gears in change gear trains

  2. Interference Fit : It is defined as the fit established when a negative clearance exist between the sizes of the holes and the shaft. In this type of fit, 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 members are intended to be attached permanently and used as a solid component.
    Interference Fit Examples : Bearing bushes, Small end in the connecting rod.

  3. 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 that the largest shaft, so that a small positive or negative clearance exists between the shaft and the hole.
    Transition Fit Examples : Coupling rings, spigot in mating holes.