One-directional thrust bearings can support heavy thrust loads in one direction, combined with moderate radial loads. High shoulders on the inner and outer rings provide steep contact angles for high thrust capacity and axial rigidity. Bearings with contact angles from 15 to 40° are available, but contact angles above 30° are not recommended for high-speed applications.
Duplex bearings are supplied in matched pairs and should never be separated or used with single bearings from other pairs. They are used because most applications require support for bidirectional thrust loads. Duplex bearings are match-ground, so their inner and outer ring faces have identical stickout under a nominal thrust load.
When a duplexed pair is mounted either back-to-back or face-to-face, the bearings become preloaded. The preload depends on original bearing stickout and load-deflection characteristics.
Preloading provides a stiffer bearing system with precise shaft location. On the other hand, after application of a thrust load, the working bearing must carry a higher load. In applications with high-acceleration starts, bearings must be preloaded to prevent ball skidding. As a rule of thumb, use a preload equal to about half the applied thrust load to avoid unloading the reverse-thrust bearing.
Duplex back-to-back and face-to-face mounts are functionally identical, except for their resistance to moment loading. Back-to-back arrangements are more resistant to moment loading and shaft bending because the lines of contact intersect outside the bearing envelope.
Duplex bearings are also used in tandem arrangements for very high unidirectional thrust loads. Stackups of three or more bearings are used when a simple arrangement is impossible.
Double-row, angular-contact bearings are the equivalent of two single-opposed angular-contact bearings. They have lines of contact converging outside the bearing envelope (as in a back-to-back duplex pair) or inside the envelope (as in a face-to-face pair). Bearings with externally converging lines of contact are much more resistant to overturning moments.
Thrust capacity of the double-row maximum-capacity bearing is somewhat limited in one direction because of the filling notch.