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The Encyclopedia of Horology

Component record · Power

Ratchet Wheel

Verified

A coarse-toothed wheel mounted on the mainspring barrel arbor, engaged by a click to permit winding in one direction only.

What it does

Allows the mainspring to be wound by turning the arbor while a click prevents the arbor (and hence the spring) from running back in reverse.

How it works

The ratchet wheel's teeth are cut with one steep face and one shallow, angled face, so a spring-loaded click can ride freely over the teeth when the arbor is turned in the winding direction but drops into a tooth root and jams the wheel if it tries to turn the opposite way.

In hand-wound watches, the crown and stem turn the ratchet wheel via a winding pinion and crown wheel train; in automatic watches, a similar arrangement is fed by the oscillating weight's winding gear train instead of, or as well as, manual winding.

The ratchet wheel and click together form the essential one-way valve that lets energy be pumped into the mainspring without letting it escape except through the controlled release of the going train.

Evolution

Fundamentally unchanged in principle since verge and fusee days; refined in tooth geometry and materials but conceptually stable across centuries of watchmaking.

Known variations

  • Ratchet wheel used with a crown wheel in key-wind and keyless winding systems
  • Ratchet wheel integrated with automatic winding trains

Identification for collectors

  • Visible coarse-toothed wheel, often with a ratchet-wheel screw or ratchet-wheel cap securing it centrally over the barrel arbor
  • Clicking sound when the crown is wound is the click riding over ratchet teeth

Common failure modes

  • Worn or broken click spring causing the mainspring to unwind uncontrolled ('running down') when winding is attempted
  • Chipped ratchet teeth

Photographs

Longcase clock movement
Longcase clock movement — Edmund Beckett (Lord) Grimsthorpe, Public domain. Wikimedia Commons file page

Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.

Sources & references

  1. de Carle, D., Practical Watch Repairing

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