The Role of Jewels in a Watch Movement
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A figure such as 17 jewels refers to all the jewel components inside a movement. The total includes bearings for the wheel pivots, the pallet stones, the balance’s impulse jewel and any cap jewels. Understanding that number means looking at how the stones are shaped, where they are fitted and what they do.
The examples here are time-only, hand-wound movements with 15, 17 and 21 jewels.
Materials and settings
Synthetic watch jewels are made from corundum, or crystallised aluminium oxide, the same base material as sapphire. Chromium gives ruby its red colour. The examples examined also include colourless jewels.
Jewels are commonly press-fitted into the mainplate. In more elaborate settings, they sit inside chatons: small brass or gold frames. These may be press-fitted into the plate or, in high-end watchmaking, held in place with screws.
In the 15-jewel FHF 76, the gold-coloured surrounds around the jewels are part of the mainplate, shaped to imitate separate chatons. The jewels themselves are press-fitted directly into the plate.
Hole jewels and lubrication
A conventional train jewel has a central hole that receives the wheel pivot. Its face towards the wheel is flat, while the outer face has a hemispherical recess that holds lubricant. The bore may have a curved profile to minimise the area of contact with the pivot.
The pivot’s fine tip enters the hole, while a flat shoulder meets the jewel’s inner face. This shoulder limits the wheel’s endshake, or axial play. A thin film of oil separates the shoulder from the jewel and the pivot from the bore, protecting the surfaces in contact.
Why the balance uses cap jewels
The balance staff uses two pairs of jewels, one at each end. Each pair comprises a hole jewel and a flat, unpierced cap jewel. The hole jewel’s recessed face points inwards towards the balance; its other face is domed.
The balance pivot has a tapered form rather than the flat shoulder used by the train pivots described above. Its rounded tip meets the cap jewel, which limits the staff’s axial play. Contact is ideally concentrated at a point, reducing the contact area.
The cap jewel also acts as a cover, helping to protect the lubricant between the two stones from dust and external contamination.
Counting the jewels in the FHF 76
The 15-jewel FHF 76 provides a clear example of how the total is calculated.
| Component | Jewels |
|---|---|
| Balance assembly | 5 |
| Pallet stones | 2 |
| Pallet fork bearings | 2 |
| Escape wheel bearings | 2 |
| Seconds wheel bearings | 2 |
| Intermediate wheel bearings | 2 |
| Total | 15 |
Four of the five jewels associated with the balance are the hole and cap jewels supporting its staff. The fifth is the impulse jewel, which moves within the fork and helps reduce wear at that contact.
The centre wheel runs in brass bushings. Its slower rotation makes the benefit of jewel bearings less significant here than at the other train wheels.
The jewels occur in pairs apart from the balance’s impulse jewel. This arrangement explains why jewel counts are often, though not always, odd.
Where the additional jewels are fitted
The 17-jewel FHF 76 adds a jewel for the centre wheel on the bridge side. The seventeenth jewel is a cap jewel for the escape wheel on the dial side, adopting the paired arrangement used for the balance. The centre wheel still has no jewel on its dial side in this version.
The 21-jewel AS 1130 has jewels at both ends of the centre wheel, a cap jewel for the escape wheel on the dial side and three cap jewels over the train pivots on the bridge side. These are functional stones that support or cap the pivots.
What additional cap jewels contribute
For these time-only, hand-wound examples, the gains in timekeeping and friction reduction from the additional cap jewels are very small when condition and lubrication are comparable. The arrangement is particularly useful at the rapidly oscillating balance and potentially at the escape wheel, with much less need at the other train wheels.
Additional cap jewels can nevertheless help protect the hole jewels and their lubricant from dirt and external contamination. They act as covers, providing a benefit in terms of cleanliness.