Rotor
The semicircular weighted piece in an automatic watch that spins with wrist motion to wind the mainspring.
What is a Rotor?
The rotor — sometimes called the oscillating weight — is the component that makes an automatic (self-winding) watch work. It is a semicircular piece of metal, typically tungsten or gold alloy for maximum weight, that pivots freely on the center of the movement. As your wrist moves throughout the day, the rotor swings under gravity, and that motion is converted through a series of gears into winding tension on the mainspring.
How It Winds the Watch
Unlike a hand-wound watch where you wind directly by turning the crown, an automatic's rotor spins in both directions. A mechanism called the reversing wheel (or reverser) converts both clockwise and counterclockwise rotation into a single winding direction so the mainspring winds regardless of which way the rotor swings.
Rotor Problems
Common rotor issues include:
- Worn rotor bearing — the rotor develops play or a grinding sensation; addressed during service by cleaning or replacing the bearing
- Rotor contact — a bent rotor scrapes the movement or case back, producing a scratching noise
- Worn reversing wheels — the watch winds in only one direction or stops winding altogether
If your automatic watch is consistently undercharged despite regular wear, the rotor system is the first place a watchmaker will investigate.