mainsprings

Mainsprings are spiral torsion springs that store and release energy to power mechanical timekeeping devices, most notably watches and clocks. They are typically a flat strip of tempered steel wound into a tight spiral within a circular barrel. The inner end of the spring is attached to the barrel arbor, while the outer end engages the gear train.

When wound, the mainspring stores potential energy. As it unwinds, it applies torque to the gear train, which drives the escapement and hands. The torque is not constant, being strongest when the spring is fully wound and decreasing as it unwinds. To compensate, some movements use a fusee and chain or a constant-force mechanism to flatten the torque curve.

Most modern portable timepieces wind by hand or automatically (via a rotor). The material and treatment of mainsprings vary; early mainsprings were plain carbon steel; contemporary springs may use tempered steel alloys with coatings to resist corrosion and minimize friction. Tolerances are tight, as even small variations affect timekeeping.

Reliability depends on proper lubrication, avoidance of corrosion, and safe winding; over- or under-winding can cause failure. Mainsprings can degrade with age and may eventually need replacement. In addition to clocks and watches, mainsprings have been used in wind-up toys, cameras, and other mechanical devices.

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