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Preloading Ball Bearings Pt. III: Axial Adjustment and Duplex Bearings

Over the last two installments of the Bearings Blog,  we’ve been examining preloading; follow the links below to catch up:

We’ve examined two methods of preloading ball bearings so far in our preload series, and today we will look at two more: axial adjustment and duplex bearings.

Axial adjustment requires great care, accuracy, proper tooling and cleanliness during the assembly process to avoid excessive preloading and ensure correct conditions. First, two bearings are mounted with the desired axial offset. The shaft and/or housing are threaded. Finally, the internal clearance in the bearings is removed by installing precision ground washers and then tightening the nut or threaded collar.

Duplex bearings are matched pairs of bearings with “built-in” preload. The inner or outer ring faces have been ground to a precise dimension known as the preload offset. This offset corresponds to the rings axial movement when a specific axial preload is applied. When the bearings are clamped together at assembly the offset faces abut, establishing a permanent, rigid preload in the bearing set.

Duplex bearings have increased radial and axial rigidity. There are three common preload configurations. DB (back-to-back) and DF (face-to-face) can handle bi-directional thrust loads. DT (tandem) can handle very heavy unidirectional thrust loads. At higher speeds, these bearings can run hotter due to the rigid preload. These bearings are commonly used in machine tool and other spindle applications, due to their low deflection rate, minimal runout and ease of assembly.

Our series on preload will continue in our next scheduled post on April 29. In the meantime, if you would like to speak with a bearing expert about this or any bearing-related topic, please contact us here.