ABSTRACT

Nearly all individual piece parts are manufactured to be assembled with other piece parts built to match them. For example, a shaft is made for an assembly within a bearing. It must be possible for the shaft to go into the bearing. That is, assuming perfectly round pieces, x1, the inside diameter of the bearing, must be larger than x2, the outside diameter of the shaft in order to permit assembly. On the other hand, the clearance in diameters, x1 – x2, should not be too large or the fit will be too loose. Therefore, we have a problem as to what tolerances to set for the bearing and the shaft to permit assembly in a very high percentage of cases, but to avoid fits which are too loose. The objective is to accomplish this with random assembly which means picking at random the bearing and shaft to be assembled. One other way sometimes used is to measure all bearing and all shafts and selectively assemble the large diameter bearings and shafts together and the small diameter shafts and bearing together. This has disadvantages: (1) it is expensive and time

consuming, and (2) when we need to replace a bearing we have trouble.