Setting Mainsprings

The mainsprings provides the power than drives the watch. When a watch is wound, the mainspring, a flat ribbon of coiled steel, is coiled up around a center arbor. The force of the spring wanting to uncoil turns the mainspring barrel providing the initial motion of the watch's wheel (gears in other words) train. Were it not for the escapement, the watch train would rapidly run, spinning the hands, as the mainspring unwinds and turns the barel.

Over time, a couple of years of continuous use perhaps, this compression/expansion cycles cause the mainspring to give up some of it's force of wanting to return to it's original shape. Such a spring is said to be "set." In the most extreme cases , a spring can actually suddenly set into a fully coiled tight state. Although this is rare, some setting is notable in a mainspring even fairly quickly as it is in use. Watches in daily use need a new mainspring on a regular basis.

Shown here is an old spring and a new spring for an 18 size Elgin pocket watch. Note that the new spring turns back on itself at the outside end. This curve compensates for the fact that the force provided by the mainspring varies from when it is fully wound to nearly unwound. This variation can impact timekeeping over the watch's run-time.