Component record · Train
Fourth Wheel
VerifiedThe train wheel that typically rotates once per minute and commonly carries the seconds hand.
What it does
Drives the escape wheel pinion and, where fitted, the seconds indication.
How it works
The fourth wheel is geared so that, in most conventional trains, it completes one revolution per minute, making its arbor a natural point from which to take a seconds hand via a subsidiary dial or, with additional gearing, a centre-mounted sweep seconds hand.
It meshes with, and drives, the escape wheel's pinion, delivering the escapement the final stage of torque before impulse is passed to the pallets and balance.
Evolution
Its role as the seconds-carrying wheel became widespread with the popularity of subsidiary seconds dials in the 18th and 19th centuries and remains standard practice today.
Known variations
- Subsidiary (small) seconds via direct fourth-wheel arbor
- Centre seconds via an indirect sweep-seconds gear train taken off the fourth wheel or another point in the train
Identification for collectors
- Identified as the wheel meshing directly with the escape-wheel pinion; carries the seconds hand in most conventional layouts
Common failure modes
- Pivot wear affecting seconds-hand steadiness
- Bent pinion leaves
Photographs
Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.
Sources & references
- Daniels, G., Watchmaking
Where sources disagree, the disagreement is stated rather than resolved silently.