Component record · Power

Mainspring

Verified

A long, coiled ribbon of tempered spring steel (or, from the mid-twentieth century, a cobalt-nickel alloy) that stores mechanical energy when wound.

What it does

Provides the energy that drives the entire going train, from barrel to escapement, for the watch's power reserve.

How it works

Winding (by hand via the crown and stem, by key in earlier watches, or automatically via a rotor) turns the barrel arbor, which coils the spring tighter against the fixed or slipping attachment at the barrel wall. As the spring unwinds, it exerts torque on the barrel, which is geared to the centre wheel and so drives the rest of the train.

Torque delivered by a spring is not constant: it is highest fully wound and falls as the spring relaxes. Early watchmakers addressed this uneven delivery with the fusee and chain; the going barrel, which drives directly and relies on a long spring worked over a modest fraction of its range plus an escapement tolerant of torque variation, became the practical standard from the eighteenth century onward, especially once the lever escapement made rate largely independent of drive force.

Modern mainsprings typically use a self-compensating alloy that resists set (permanent bending) and are manufactured with a slipping bridle at the barrel wall so that a fully wound spring cannot be forcibly overwound, protecting the train from shock.

Evolution

Steel mainsprings were prone to breakage, rust and set; alloy springs developed in the twentieth century (e.g. Nivaflex and similar proprietary alloys) greatly reduced breakage and improved rate stability, and remain standard in mechanical watchmaking today.

Known variations

Self-compensating alloy spring with slipping bridle (most modern watches)
Traditional steel spring with a hooked end, common in antique watches and clocks
Going-barrel spring versus fusee-driven spring, differing in working length and taper of torque delivery

Identification for collectors

A broken mainspring typically causes the watch to run briefly then stop, or fail to run at all despite winding
A set (permanently bent, no longer tightly coiled) spring gives short, erratic running and weak amplitude
Barrel should feel smooth resistance when wound; grinding or slipping too easily on a full wind suggests bridle or spring problems

Common failure modes

Breakage, especially in old steel springs with hardened, brittle sections
Set (loss of elasticity) reducing power reserve and amplitude
Rust and corrosion from moisture ingress
Bridle slipping prematurely, giving an artificially short power reserve

Clock Mainspring — Dionysius Lardner, Public domain. Wikimedia Commons file page

Watch mainsprings — Clock Mainspring — Dionysius Lardner, Public domain. Wikimedia Commons file page

Sources & references

Daniels, G., Watchmaking
de Carle, D., Practical Watch Repairing

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