Component record · Power
Mainspring Hook
VerifiedThe small formed hook, tab, or slot at the outer end of a mainspring that anchors it to the inside of the barrel wall.
What it does
Secures the spring's outer coil to the barrel so that torque generated by the coiled spring is transmitted to the barrel itself rather than allowing the spring to spin freely inside it.
How it works
Traditional hooks are simply a bent or punched tab formed in the spring's own material that catches on a peg or notch in the barrel wall.
Some modern barrels instead use a slipping bridle at the outer end rather than a fixed hook, so that a fully wound spring cannot be forced further and instead slips harmlessly against the barrel wall, protecting the train from shock loading.
At the arbor end a separate hook or slot performs the equivalent function, anchoring the spring's inner coil so the arbor can coil and uncoil it as it is wound and as it drives the train.
Evolution
The shift from a hard barrel-wall hook to a slipping bridle attachment was an important refinement that reduced mainspring and train damage from overwinding or shock.
Known variations
- Fixed hook-and-notch attachment
- Slipping bridle (self-limiting) attachment
- Riveted steel hook piece versus a hook formed in the spring itself
Identification for collectors
- Examine the outer coil of a removed mainspring for a hook, tab, or bridle detail matching the barrel wall's corresponding feature
Common failure modes
- Hook breakage, particularly in brittle old steel springs
- Bridle attachment wearing smooth and slipping prematurely, giving artificially reduced power reserve
Related photographs
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
- Daniels, G., Watchmaking
Where sources disagree, the disagreement is stated rather than resolved silently.