Systems Of Fit (Hole And Shaft Basis)

SYSTEMS OF FIT

A fit system is the systems of standard allowance to suit specific range of basic size. If these standard allowances are selected properly and assigned in  mating parts ensures specific classes of fit.

There are two systems of fit for obtaining clearance, interference or transition fit. These are :
(i) Hole basis system

(ii) Shaft basis system

Hole Basis System

In the hole basis system, the size of the hole is kept constant and shaft sizes are varied to obtain various types of fits. In this system, lower deviation of hole is  zero, i.e. the low limit of hole is same as basic size. The high limit of the hole and the two limits of size for the shaft are then varied to give desired type of fit.

The hole basis system is commonly used because it is more convenient to make correct holes of fixed sizes, since the standard drills, taps, reamers and  branches etc. are available for producing holes and their sizes are not adjustable. On the other hand, size of the shaft produced by turning, grinding, etc. can  be very easily varied.

Shaft Basis System

In the shaft basis system, the size of the shaft is kept constant and different fits are obtained by varying the size of the hole. Shaft basis system is used when the ground bars or drawn bars are readily available. These bars do not require further machining and fits are obtained by varying the sizes of the hole.

In this system, the upper deviation (fundamental deviation) of shaft is zero, i.e. the high limit of the shaft is same as basic size and the various fits are obtained  by varying the low limit of shaft and both the limits of the hole.