Component record · Train
Third Wheel
VerifiedThe wheel positioned between the centre wheel and the fourth wheel in the going train.
What it does
Continues the transmission of power and speed increase from the centre wheel toward the escapement.
How it works
Each wheel-and-pinion stage in the train increases rotational speed while reducing torque, since a large wheel drives a small pinion on the following arbor. The third wheel is simply the intermediate stage that bridges the once-per-hour centre wheel to the once-per-minute (typically) fourth wheel.
In most watch calibres the third wheel carries no display function and exists purely to fill the necessary gear ratio between centre and fourth wheels; in some layouts it may be positioned off-centre to help fit components around the movement.
Evolution
Stable in function since early train architecture; position and exact ratio vary by calibre design.
Known variations
- Off-set third wheel positions to accommodate different bridge layouts
Identification for collectors
- Identified by its position in the train between centre and fourth wheels; carries no hand or display function in most designs
Common failure modes
- Worn pivots and teeth as with other train wheels
- Depthing errors causing rough running if disturbed during service
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.