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The Encyclopedia of Horology

Glossary · Mechanism

Rotor

The oscillating weight of an automatic movement that rewinds the mainspring via a reduction and reversing gear train.

Explanation

The rotor is the semicircular weighted mass in an automatic watch that swings with wrist motion to wind the mainspring, through reduction gears and reversers that let it wind in one or both directions. Worn rotor bearings make a scraping noise and can let the rotor rub the movement — a common service item. Heavy modern rotors wind efficiently but add thickness.

In the object catalogue

Catalogued objects whose own source record mentions “Rotor”. The quoted line shows where; it is the seller’s or museum’s wording, not independent verification.