OSCILLATING WEIGHT

The oscillating weight, sometimes called a rotor, is an essential component of self-winding mechanical watches. It is shaped like a half-disc (sometimes slightly less) that pivots freely around its axis. On the periphery of the oscillating weight is a segment of heavy metal (often gold) to increase its moment of inertia.

During the wearer’s movements, the oscillating weight is drawn towards the ground by gravity. The arbor of the oscillating weight carries a pinion that drives the automatic winding mechanism as the weight moves. The pivot of the oscillating weight is generally positioned at the centre of the movement. In this case, the oscillating weight covers the surface of the movement over which it moves, and the automatic mechanism adds extra thickness to the movement.

An alternative construction is to recess part of the movement to accommodate the automatic mechanism. In this case, the oscillating weight occupies only a small area and avoids adding thickness to the construction. When such small, off-centred oscillating weights are used, they are commonly referred to as micro-rotors.