Component record · Regulation
Endstone (Cap Jewel)
VerifiedA flat jewel (commonly synthetic ruby or sapphire) fitted over the end of a pivot hole to close its end and provide a smooth bearing surface against the flat end of a pivot.
What it does
Reduces friction and end-shake at the ends of high-speed pivots, particularly the balance staff and, in higher-grade movements, the escape wheel and pallet staff, by providing a hard, polished surface for the pivot's rounded end to bear against.
How it works
The pivot's rounded end bears directly against the flat, polished face of the endstone rather than against the raw metal of the plate or a jewel hole's own end, dramatically reducing friction at what would otherwise be a point of high wear given the pivot's speed of rotation (especially on the balance staff, which oscillates several times per second).
A tiny reservoir of oil is normally retained between the pivot end and the endstone by capillary action, and correct end-shake (a small, controlled amount of axial play) must be preserved so the pivot neither binds against the stone nor rattles loosely.
Endstones are typically used in pairs with a corresponding jewel hole (hole jewel) so that the pivot is supported radially by the hole and axially by the endstone.
Evolution
Higher jewel counts in twentieth-century mass production extended endstone use from the balance alone to the escapement's other high-speed pivots in better-grade movements, though many everyday movements still use endstones only on the balance.
Known variations
- Chaton-set endstone (jewel held in a separate metal collar, often screwed or friction-fit, common in higher-grade or serviceable designs)
- Friction-fit endstone set directly into the plate or cock (common in mass production)
- Shock-protected endstone assemblies (see Shock Protection) that allow the whole jewel unit to yield under impact
Identification for collectors
- A small, flat, transparent or reddish stone visible on the balance cock directly above the balance staff's upper pivot, and its counterpart beneath
- Missing or cracked endstones are a common finding when a balance runs with poor amplitude or excessive noise
Common failure modes
- Cracked or chipped endstone from shock, often accompanying a bent or broken balance staff pivot in the same event
- Dried-out or missing oil at the pivot/endstone interface causing increased friction and reduced amplitude
- Incorrect end-shake set during servicing, causing binding (too tight) or positional rate variation (too loose)
Related photographs
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No confidently matched photograph of this subject has been found yet. These photographs show a closely related record, and are not examples of the type described on this page.
Sources & references
- de Carle, D., Practical Watch Repairing
- Daniels, G., Watchmaking
Where sources disagree, the disagreement is stated rather than resolved silently.