Assembling ball bearings into plastic housings can be tricky because housings can be slightly out of round, and at high temperatures they may expand faster than the bearing. The bearing outer race may spin, or vibrate, shortening brush life. When the bearing inner race is already pressed onto the shaft, the other race must be a slip fit, and may require a preload device to minimize noise.
Messy adhesives or elastomer boots are conventional solutions, but reliable devices that handle these issues are from USA Tolerance Rings. The USA Tolerance Rings are corrugated steel fasteners, which act like elastic shims because of the waves on the ring are radial springs.
Tolerance rings work according to the physics principles of springs and friction. Each corrugation acts like a stiff radial spring. The spring rate can be calculated by specifying the deflection and the coefficient of friction. Assembly force calculations are determined by the material’s yield limit along the torque and radial load capacities. The rings compensate for thermal growth in motor assemblies. Such growth makes the bearing outer race rotate or vibrate at high RPMs, limiting brush life.
As printed in Machine Design Magazine July 8, 1999. Copyright 2001 by Penton Media., Cleveland, Ohio 44114