A cap jewel is a jewel with no hole drilled through it. It limits how far the balance staff can move along its axis. It works with an ordinary pierced bearing jewel, and together they keep friction as low as possible, both sideways and lengthwise.
How does an endstone bearing work?
An endstone bearing uses two stones. The first is an ordinary bearing jewel. It is pierced with a cylindrical hole, and the staff turns inside it. The second is the cap jewel, a flat stone with no hole. It limits how far the wheel can move up and down. The flat stone also helps hold the oil in place. This kind of bearing suits small pivots that do not carry heavy torque.
Where are cap jewels used in a watch?
A typical fully jewelled watch that shows only the time has 17 jewels. Two of them are cap jewels, and both serve the balance wheel. A classic watchmaking text holds that end stones on the escape wheel and lever are more a matter of taste than of real use. The balance, it says, must have them. The same text recommends a diamond end stone for the upper balance pivot. That pivot end can carry the whole weight of the balance.
How do cap jewels work with shock protection?
Cap jewels are normally used together with a pivot jewel. The pair is often fitted with spring shock protection, with one unit at each end of the balance staff. In a shock absorber, a spring holds the endstone to its jewel setting. The setting is called the chaton, and the spring is called the lyre.
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