Fits & their Types (metrology)


The degree of Tightness or looseness between two mating parts is called as Fit.

Fits are classified into 3 types
  1. Clearance fit
    a) Sliding Clearance fit
    b) Running Clearance fit
  2. Interference fit
    a) Longitudinal press
    b) Transverse press
  3. Transition fit
    a) Push fit
    b) Wring fir

1.Clearance Fit: when the difference between size of hole and size of shaft to be assembled is positive, then the fit is called as clearance fit. 
(ie., the size of shaft is smaller than the size of hole)

Ex. Max. size of hole is 50mm & max size of shaft is 49.85mm

Ex. Piston of hydraulic machine. 
       Shaft & bearing       



Minimum clearance: It is the difference between minimum size of hole and max. size of shaft.
Maximum clearance: It is the difference between minimum size of shaft and max. size of hole.


Sliding Clearance: when the mating parts are required to move slowly one over another in regular passion with a small gap between them is called sliding clearance.
Ex. Tail stock movement on lathe.
       Spindle of drilling machine
Running Clearance: When intentionally small clearance is required between two mating parts which are generally at lower or moderate speed. Then this clearance is called as running clearance.
Ex. Gearbox bearing
       Shaft carrying pullies.





2. Interference Fit: If the difference between size of hole and size of shaft before assembling is Negative, then it is called as interference fit


(ie., Size of shaft is greater than the size of hole)


Ex. Max. size of hole is 49.85mm and max. size of shaft is 50.01mm

Longitudinal Press: It is the forcible press of shaft into the hole under pressure.

Transverse Press:  It is non forcible inserting of part. Either hole or shaft is heated before inserting.




3. Transition Fit: A fit which may provide either Clearance or Interference fit is called as Transition Fit.
Push Fit: Consider the example like changing gears, slipping, bushing, whose sub components are disassembled during operation of the machine. It requires small clearance.
Wringing Fit: In case of reusable or repairable parts, the sub parts must be replaced without any difficult. In this case, assembling is done employing a wringing fit.