Crowned Races for Crossed Roller Bearings

NASA’s Jet Propulsion Laboratory

Crowned races have been proposed for crossed roller bearings. Crowning of the races is expected to reduce scuffing of the cylindrical rollers. Crowning of the races is expected to be especially beneficial in bearings made of polymers (instead of metals) to reduce weight.

Crowned Races, in contradistinction to conical races, would function with less scuffing in roller/race contact areas.

The crossed-roller bearing design is the roller equivalent of the x-type ball bearing design. In a crossed-roller bearing, the races are conical surfaces at angles of 45° to the axis of rotation. The advantages and disadvantages of the ball and crossed-roller designs are complementary: In comparison with ball bearings, roller bearings can withstand greater loads. On the other hand, roller bearings are more susceptible to scuffing, which is caused by a kinematic mismatch between tangential roller speeds at the inner and outer diameters of the races.

In a crossed-roller bearing with crowned races (see figure), the contact area, and thus the amount of scuffing, would be reduced (relative to that of conical races) to a value near that of ball bearings. The crown radius is a free design parameter that can be chosen, along with other parameters, in consideration of the bearing material(s) and the loads that must be borne in a given application.

One might ask why it would be preferable to crown the races instead of crowning the rollers. The reason is a practical one: Unlike in the case of steel bearing balls, it is difficult to fabricate polymeric bearing balls or polymeric crowned rollers with sufficient precision to ensure sharing of loads as needed for long bearing life. On the other hand, cylindrical crowned rollers of sufficient precision can be fabricated easily by center-less grinding in conventional machines.

This work was done by Donald Bickler of Caltech for NASA’s Jet Propulsion Laboratory.