CAP JEWEL BEARINGS

Description

Pivots of small diameter and subject to limited torque are suited to this type of bearing. Cap-jewel systems require two jewels: the first resembles a standard plain bearing jewel, while the second, flat and unpierced and called the cap jewel, limits the vertical endshake of the wheel (Figure 1).

Construction

Two methods of construction exist for cap-jewel bearing systems:

In the first, and most common today, both jewels are fitted into a single setting, which is itself pressed or screwed into the mainplate or bridge (Figure 2, lower pivot).
The second, older method is still used in traditional high-end watchmaking. In this case, the bearing jewel is fitted directly, or via a setting, into the mainplate or bridge. The cap jewel is fitted into a separate cap plate (usually steel), which is screwed over the bearing jewel on the bridge or plate (Figure 2, lower pivot).

Regardless of the construction, this system requires conical pivots. Its advantage lies in minimising both radial and vertical friction. In the horizontal position, only a minute area of the rounded pivot tip is in contact with the jewel. This system also allows for more precise control and limitation of the wheel’s endshake.

Lubrication

In the setting construction, oil is applied to the flat surface of the cap jewel using an automatic oiler (as the system cannot be opened).

In the cap plate construction, the system can be opened, allowing oil to be placed at the centre of the cap jewel using a manual oiler before carefully closing the assembly.

In both cases, the correct quantity and centring of the oil must be meticulously checked.

Applications

Cap-jewel bearing systems are used exclusively for the escape wheels and, historically, for balance staffs. Since the introduction of shock protection systems (cap-jewel bearings with shock absorbers), these have become standard for balance staffs and are now used less frequently for escape wheels.