PRODUCT DESIGN: The "shaft basis" and the "hole basis' system of fits

In all the examples given above, the discussion has been concerning 'shafts' and 'holes'. It should be remembered that this does not necessarily apply to shafts and holes. These are just generic termsthat mean anything that fits inside anything else. However, whatever the case, it is often the case that either the shaft or the hole is the easier to produce. For example, if they are cylindrical, the shaft will be the more easily produced in that one turning tool can produce an infinite number of shaft diameters. This is not the case with the cylindrical hole in that each hole size will be dependent on a single drill or reamer.

The right-hand diagram in Figure 5.10 shows the situation in which the shaft is the more difficult of the two to produce and this is referred to as the 'shaft basis' system of fits. In this case the system of fits is used in which the required clearances or interferences are obtained by associating holes of various tolerance classes with shafts of a single tolerance class. Alternatively if the shaft is the easier part to produce then the hole basis system of fits is used. This is a system of fits in which the required clearances and interferences are obtained by associating shafts of various tolerance classes with holes of a single tolerance class. In the case of the shaft basis system the shaft is kept constant and the interference or clearance functional situation is achieved by manipulating the hole. If the hole-based system is used, the opposite is the case. The appropriate use of each

system for functional performance situation is thus made easier for the manufacturer.