What are fits and what are the type of fits in metrology?

Fits in metrology are the dimensional relation between the mating parts which are known as fits. It indicates that the tightness or the loosening of the mating parts. For engineering purposes, this is very important because it is used in real life applications for the satisfactory working of parts in movement such as a piston in a cylinder, journal in a bearing, gear on splines, a valve in a valve guide and wheel on a shaft.

There are very important terms related to a fit which is:

  • Clearance − Clearance in a fit is when the difference between the sizes of the hole and the shaft before being assembled is when the difference is positive. The clearance may be maximum clearance or minimum clearance.

  • Interference − Interference is the difference between the sizes of the hole and the shaft which is formed before assembly. And this is usually when the difference is negative. The interference may be maximum or minimum.

  • Transition − Transition can be defined as it is a fit that always ensures some interference between the hole and the shaft in the coupling. In transition, the upper limit size of the hole is smaller or at least equal to the amount of the lower limit size of the shaft.

The types of fits

The types of fits depend upon the actual limit of the hole or shaft which can be divided into three categories which are:

  • Clearance fit
  • Interference fit
  • Transition fit

Clearance Fit

  • Clearance fit is an air space or clearance which exists between the shaft and holes. Such clearance fits give loose joints.

  • The clearance fit has to be joined with other fits so that the clearance fit can have its space. These fits are guaranteed with clearance and are designed for movable couplings of parts in which pivots running and sliding fits of shapes, guiding bushings, sliding gears and clutch discs and pistons of hydraulic machines are used.

Interference Fit

  • The interference fit is also called as a press fit or friction fit. The interference fit is a fastening between two parts and generally achieved by friction after the combined parts are pushed together.

  • In these kinds of fits, the minimum permissible diameters of the shaft are larger than the maximum allowable diameter of the hole. If there is ever a difference between the sizes of the hole and shaft before being assembled then the negative fit is called the interference fit.

Transition Fit

  • Transition fit is a fit which may provide either a clearance or interference.

  • In transition fit the tolerance zones of the hole and shaft overlap with each other. Transition fit lies between clearance and interference fit.

  • These are used for applications where the accurate location is needed and it can be utilized with small clearance and hole or shaft should be precise.