Component record · Power
Mainspring Bridle (Slipping Bridle)
VerifiedA separate outer band, riveted or otherwise attached to the outer coil of a mainspring, designed to grip the inside of the barrel wall only up to a controlled maximum torque.
What it does
Allows the mainspring to slip against the barrel wall once fully wound, preventing overwinding damage while still transmitting normal running torque, in place of a fixed outer hook.
How it works
Instead of the outer coil hooking rigidly onto a fixed point on the barrel wall, the bridle is shaped and tensioned to grip the barrel wall by friction alone once the spring reaches full wind, so that continued winding simply causes the bridle to slip round the inside of the barrel rather than straining the spring or the winding mechanism.
This self-limiting behaviour is essential in automatic watches, where the rotor can otherwise attempt to wind the mainspring continuously through normal wrist motion; without a slipping bridle, a fully wound mainspring would risk damage or would need a separate mechanical slipping clutch elsewhere in the winding train.
Correct bridle tension is a deliberate design and manufacturing tolerance: too loose, and the spring slips under normal running load, causing poor amplitude and timekeeping; too tight, and it fails to protect against overwinding.
Evolution
Developed and refined specifically to solve the overwinding problem created by automatic winding systems, becoming a standard fitting in automatic movements through the second half of the 20th century, though also used in some hand-wound movements as an overwind safeguard.
Known variations
- Riveted bridle spring (separate component attached to the mainspring's outer coil)
- Integral slipping outer coil, cut and tensioned as part of the mainspring itself
Identification for collectors
- A mainspring that can be wound with the crown seemingly indefinitely without a hard stop suggests a slipping bridle system rather than a fixed-hook mainspring
Common failure modes
- Bridle grip too weak (loose in the barrel), causing the spring to slip under normal running torque and reducing amplitude and rate stability
- Bridle grip too strong or seized with old grease, defeating its overwind protection
- Bridle detached from the coil
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
- de Carle, D., Practical Watch Repairing
- Daniels, G., Watchmaking
Where sources disagree, the disagreement is stated rather than resolved silently.