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The Encyclopedia of Horology

Component record · Train

Pinions

Verified

Small-diameter, few-toothed ('leaved') gears, typically the driven member in a wheel-and-pinion pair within the going train.

What it does

Mesh with the larger wheel of the preceding train stage to receive rotational drive, mounted on the same arbor as the next wheel to pass motion onward through the train.

How it works

A typical train stage pairs a larger wheel (many teeth) with a smaller pinion (few 'leaves', the pinion's equivalent of teeth) mounted on the same arbor as the next wheel; because the wheel is large and the pinion small, each stage multiplies rotational speed while dividing torque, which is how a watch train steps a slow-turning barrel up to a fast-beating escapement.

Pinions are made of hardened steel rather than the wheel's brass, since a given pinion leaf engages far more times per revolution than a given wheel tooth does, making wear resistance critical for the pinion.

Pinion leaf counts and profiles (e.g. lantern pinions historically, and cycloidal or involute-form leaves in precision work) affect meshing smoothness and are a specialised area of horological gear design.

Evolution

Pinion-cutting precision improved dramatically from hand-filed lantern pinions in early clockwork to precisely hobbed, hardened steel pinions in industrial watch and clock manufacture.

Known variations

  • Lantern pinion (older, simpler construction using round steel pins as leaves)
  • Solid cut-steel pinion (standard in fine watchmaking)

Identification for collectors

  • Pinion wear typically shows as a 'hooked' or asymmetrically worn leaf profile, causing rough or noisy meshing with its wheel

Common failure modes

  • Leaf wear from insufficient lubrication or contaminated oil
  • Chipped or broken leaves from shock or foreign debris in the train
  • Worn pivots at either end of the pinion's arbor

Photographs

Pocketwatch movement
Pocketwatch movement — Park Benjamin, 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. Daniels, G., Watchmaking

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