BEARINGS (frequently referred to as rubies or jewels)
The use of synthetic ruby jewels in a movement is primarily intended to minimise friction between moving components. The bearings are placed at the points at which the wheels and pinions rotate, generating significant friction, which leads to a loss of energy efficiency. To counter this, the use of synthetic ruby bearings has become standard practice throughout modern watchmaking. Depending on the wheel assembly, its function, size, and inertia, one of the three main types of bearings described in this section is used.
PLAIN BEARINGS
Most of the wheel mobiles and pinions in a movement require plain bearings, each consisting of a single jewel per pivot. The jewel features a precisely calibrated hole through which the pivot runs. A circular recess, concentric with the jewel hole, serves as an oil sink, holding the lubricant necessary to ensure smooth operation and minimise friction at the bearing.
CAP JEWEL BEARINGS
Pivots of small diameter and subjected to moderate torque are well-suited to this type of bearing. A cap-jewel system uses two jewels. The first functions as a plain bearing jewel, pierced to accommodate the pivot. The second, an unpierced cap jewel, restricts the vertical play of the wheel. This configuration requires conical pivots and offers the significant advantage of minimising both radial and axial friction, thereby improving efficiency and durability.
SHOCK PROTECTION SYSTEMS
This type of system is used primarily for balancing staff bearings, and more rarely for those of the escape wheel. The balance staff pivots are extremely fine to minimise friction, while the balance wheel itself has significant inertia. This combination makes the pivots particularly vulnerable to shocks. In a shock protection system, the cap jewel is held within a jewel setting by a spring (called the lyre spring). During an impact, the cap jewel can move slightly, allowing the spring to absorb the shock and thereby prevent the pivot from breaking.
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