Mainspring

The mainspring is the coiled spring inside the barrel of a mechanical movement that stores the energy released to drive the watch.

It's wound by the crown or, in an automatic, by the rotor. Over time a mainspring can weaken, which shortens the power reserve and can affect timekeeping, and it's commonly replaced during a service.

A watch that won't run through a full day after winding may need a new mainspring, a relatively routine part of a service.

In older watches, an original mainspring may have lost strength over the years, so a service that replaces it can noticeably improve how long the watch runs.