On the basis of Indian standards fits can mainly be categorized into three groups:

> Clearance Fit: These types of fits are characterized by the occurrence of a clearance between the two mating parts. The difference between the minimum size of the hole and the maximum size of the shaft is called the minimum clearance, the difference between the maximum size of the hole and the minimum size of the shaft is known as maximum clearance.

> Interference Fit: In these types of fits the size of the mating parts are predefined so that interference between them always occurs. The tolerance zone of the hole is completely below the tolerance zone of the shaft.

> Transition Fit: As the name suggests these type of fit has its mating parts sized limited to allow either clearance or interference. The tolerance zone of the hole and the shaft overlaps in case of such fits.

For a shaft designated as 40 H8/f7, calculate the tolerances.

Given: Shaft designation = 40 H8/f7

The shaft designation 40 H8/f 7 means that the basic size is 40 mm and the tolerance grade for

the hole is 8 ( i. e. I T 8) and for the shaft is 7 ( i. e. I T 7).

Since 40 mm lies in the diameter steps of 30 to 50 mm, therefore the geometric mean diameter,

D = Square root of (30 x 50) = 38.73 mm

We know that standard tolerance unit,

i = 0.45 x Cube root of (D) + 0.001 D

i = 0.45 × 3.38 + 0.03873 = 1.559 73 or 1.56 microns

i = 1.56 × 0.001 = 0.001 56 mm ...(1 micron = 0.001 mm)

The standard tolerance for the hole of grade 8 (IT8)

= 25 i = 25 × 0.001 56 = 0.039 mm

The standard tolerance for the shaft of grade 7 (IT7)

= 16 i = 16 × 0.001 56 = 0.025 mm