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The Encyclopedia of Horology

Component record · Escapement

Pallet Stones (Pallet Jewels)

Verified

The two hard synthetic ruby (or, historically, natural ruby) stones set into the arms of the pallet fork, whose faces actually lock and unlock the escape wheel.

What it does

Provide the hardened locking and impulse faces that engage the escape-wheel teeth, transmitting impulse to the fork while withstanding constant repetitive contact wear.

How it works

Each pallet stone presents a locking face, which momentarily arrests the escape wheel, and an impulse face, angled so that as the wheel presses against it the fork receives a push that is passed on to the balance via the roller table.

The angle, polish and exact position of the pallet stones relative to the fork body and to the escape wheel are critical to correct 'lock' and 'draw', the geometry that keeps the escapement running safely; incorrect stone position or angle causes tripping, banking, or excessive draw.

Pallet stones are set into the fork body either by shellac cement (older and traditional practice, allowing the stone to be repositioned by heat) or by more modern mechanical staking or adhesive methods.

Evolution

From plain hardened steel pallets in cheap or early work, to jewelled pallets as a mark of quality, to today's near-universal synthetic ruby or sapphire pallet stones even in modest-grade watches.

Known variations

  • Shellac-set pallet stones (traditional, adjustable by heat)
  • Pinned or mechanically staked pallet stones
  • Steel (unjewelled) pallets in economy movements

Identification for collectors

  • Visible as two small red or clear inserts at the ends of the pallet fork's arms when the fork is examined under magnification

Common failure modes

  • Chipped or cracked stones from shock or mishandling during service
  • Stone shifted in its shellac setting, altering lock and draw and causing poor timekeeping or stopping
  • Wear on the impulse face after very long service

Related photographs

Chinese movement escapement and jewels
Chinese movement escapement and jewels — Hustvedt, CC BY-SA 3.0. Wikimedia Commons file page

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

  1. Gazeley, W. J., Clock and Watch Escapements
  2. de Carle, D., Practical Watch Repairing

Where sources disagree, the disagreement is stated rather than resolved silently.