Mainspring
A coiled spring that stores the energy used to power a mechanical watch movement.
Also known as: main spring, watch mainspring, power spring
Definition
The mainspring is the long, flat spring that stores the energy of a mechanical watch. It is coiled inside the barrel around an arbor; winding tightens the spring, and its controlled unwinding turns the barrel and feeds torque into the gear train. The torque available is not perfectly constant from fully wound to nearly discharged, so movement designers manage that changing force through spring design, barrel geometry and sometimes constant-force mechanisms. Automatic movements commonly use a bridle that slips against the barrel wall when fully wound.
Why it matters
Mainspring design influences power reserve, torque delivery and movement packaging. A damaged, fatigued or contaminated mainspring can affect running performance and is a normal consideration during mechanical-watch servicing.