How does an automatic watch store energy?

An automatic watch winds itself as you wear it. Behind this convenience is a clever mechanism.

The rotor

A semicircular weight, the rotor, is mounted on the movement so that it can turn freely. Every movement of your wrist makes it rotate – gravity does the work.

From rotation to tension

A series of wheels, often with reversing wheels, converts the rotor's movement into a slow, one-directional rotation that tightens the mainspring. Many movements wind in both rotor directions; some in only one.

Protection against overwinding

Because the rotor never stops, the mainspring would eventually be overwound. That is why automatic watches use a slipping bridle: when the spring is fully wound, its end slips along the barrel wall instead of tightening further.

Energy storage

The energy is stored in the mainspring and released evenly to the gear train. How long a fully wound watch runs is its power reserve.

Practical tips

• If you wear the watch only occasionally or are not very active, a few turns of the crown before putting it on help.
• A watch winder can keep a rarely worn automatic running – but is not strictly necessary.
• If the watch stops overnight despite being worn daily, a service check is worthwhile.