How an Automatic Watch Winds Itself

An automatic watch, also called self-winding, is a mechanical watch that charges its own mainspring from the motion of your wrist. You never need to wind it daily, though the crown still allows hand winding.

The key part is the rotor, a semicircular weight that pivots freely on the back of the movement. When your arm moves, gravity swings the rotor around its axis. The rotor is often made of a dense metal such as tungsten so it carries more momentum in a small space.

The rotor's motion passes through reduction gears and a reverser, a mechanism that lets the spring wind whichever way the rotor turns. Some designs wind in both directions and others in one, and bidirectional winding generally charges the watch faster for the same amount of movement.

To stop the spring being over-wound, the mainspring is attached with a slipping bridle or clutch. Once the spring is fully wound, the end of it slips against the barrel wall, so the rotor keeps turning freely without straining anything.

How well a watch stays charged depends on how much you move. An active wearer usually keeps it fully wound, while a desk worker may find it stops after a weekend off the wrist. Typical power reserves run from around 38 hours to several days, so check the spec sheet.

If your automatic has stopped, give the crown a few turns before setting the time. Wearing it daily is the best way to keep it running well, and a watch winder can help if you rotate through several pieces.