PLAIN BEARINGS
Figure 1: Plan of a simple bearing
Figure 2: Section view of the second wheel’s simple bearing
Most of the wheels in a movement use plain bearings (Figure 1), each consisting of a single jewel, synthetic corundum, per pivot. The jewel features a precisely calibrated hole through which the pivot runs. A circular recess, concentric with the hole, forms an oil sink that holds the lubricant necessary for smooth rotation and reduced friction (Figure 2).
Typically, a bearing jewel is pressed directly into the mainplate or a bridge. In high-grade watchmaking, however, jewels may be set in a gold setting. In such cases, the setting is fitted into a cylindrical recess in the plate or bridge and secured with screws (usually three).
While most plain bearings used in wristwatches and pocket watches are made of synthetic ruby, wheels with larger pivots or high torque (the barrel or centre wheel) may instead run in metal bushings (typically beryllium-bronze alloys). Given the dimensions and forces specific to watchmaking, the combination of hardened steel pivots and ruby bearings offers an optimal reduction of friction and wear.