Limits and fits

This chapter focuses on the system of engineering limits and fits. Engineering fits between two mating parts can be divided into three types, which include clearance fit, interference fit, and transition fit. In unilateral limits, the maximum and minimum limits of size are disposed on the same side of the basic size. The two bases of a system of limits and fits are the hole basis and the shaft basis. In the hole basis system, the basic diameter of the hole is constant while the shaft size varies according to the type of fit. This system leads to greater economy of production, as a single drill or reamer size can be used to produce a variety of fits by merely altering the shaft limits. In the shaft basis system, the hole size varies to produce the required class of fit with a basic-size shaft. As a series of drills and reamers are required for this system, therefore it tends to be costly. BS EN 20286 Part 1 describes the ISO system of limits and fits, which provides the basis of tolerances, deviations, and fits. BS EN 20286 Part 2 provides tables of standard tolerance grades and limit deviations for holes and shafts. The ISO system provides various hole and shaft tolerances to cater to a wide range of conditions.