mainspring

The mainspring is a spiral torsion spring that stores elastic energy to power mechanical timekeeping devices, most notably watches and clocks. It is typically a thin flat coil housed inside a round barrel; when wound, the coil stores energy that is gradually released to drive the gear train through the barrel arbor and escapement.

In operation, the mainspring is made from high-strength steel or specialty alloys. Modern watches employ materials designed for stability under temperature changes and magnetic exposure, such as Nivarox. As the spring unwinds, its torque decreases, so the resulting force on the gear train is not constant. The design of the gear train and escapement, and in some cases devices like a fusee in older timepieces, compensate for this variation to maintain accuracy.

Winding brings energy into the system. Hand-wound watches require periodic crown winding to tighten the mainspring, while automatic (self-winding) watches use a rotor that winds the spring as the wearer moves. The amount of energy stored by the mainspring determines the power reserve—the length of time the watch will run between winds.

Materials, lubrication, and aging affect performance. Springs can fatigue or lose elasticity over time, and improper lubrication can increase wear. Overwinding is generally prevented by the spring’s geometry and the winding mechanism in modern watches.

Beyond watches, mainsprings have been used in various mechanical devices that require compact, reliable energy storage. The term is most commonly associated with horology, where ongoing improvements aim to reduce variability in torque and extend service life.