Allowances

An allowance is a difference in dimension that is necessary to give a particular ‘class of fit’ between two parts. For example, if a shaft were required to locate with a corresponding hole in a component, then either the hole or the shaft is made as accurately as possible to the nominal size and an allowance is applied to the associated item in order to assist in the economy of manufacture. The term ‘shaft’ also includes bolts and pins.

If the shaft is constant and the hole varies in size, then the system used is said to be ‘shaft-based’. If the hole is constant and the shaft varies in size, then the system is ‘hole-based’. The hole-based system is the one in more general use.

The item dimensioned to include the allowance also has high and low limits and therefore, a tolerance. The correct allowance would be the difference between the high limit of the shaft and the low limit of the hole.