Rotor
A rotor is the half-moon weight inside an automatic watch. It swings with the motion of your wrist, and that swing winds the mainspring. You can usually see it spinning through a display caseback. Its free swing is what makes an automatic watch self-winding.
At a glance
- Job
- Winds the mainspring from wrist motion
- Shape
- Half-moon weight
- Designs
- Central rotor, micro-rotor
In an automatic watch, the winding work is done by the motion of your wrist instead of your fingers, and the part that makes that possible is the rotor. The freer and more efficient its swing, the more winding you get from ordinary movement through the day.
How it works
The rotor is a half-moon weight fixed to a central axis. Every time your wrist turns, gravity pulls its mass downward, it rotates, and that rotation tightens the mainspring through a train of gears.
- Central rotor: sweeps a full circle over the movement, the most common design
- Micro-rotor: a smaller, slimmer weight sunk into the movement itself
Why it matters
The free swing of the rotor is what keeps an automatic watch running without hand winding. It is also the most visible detail of a mechanical watch in daily use, since you often watch it spin through a display caseback. For the wider movement family see the movement page, and for a real example read our guide to the best Japanese watches.
Examples
On an automatic watch with a display caseback you can see the rotor at work, watching the half-moon weight spin and wind the mainspring as you turn your wrist.
View this watch
Comparison
The two rotor designs do the same job with a different layout.
| Option A | Option B | Notes |
|---|---|---|
| Central rotor | Micro-rotor | A central rotor sweeps a full circle over the movement; a micro-rotor sits sunk into the movement, runs thinner, and leaves more of the movement on view. |
Related terms
Watches that show this
Frequently asked questions
Is the rotor what makes a watch self-winding?
Yes. The free swing of the rotor is what makes an automatic watch self-winding. As your wrist moves, the rotor rotates and winds the mainspring, so you do not need to hand wind it every day.
What is the difference between a central rotor and a micro-rotor?
A central rotor is a large weight that sweeps a full circle over the movement, and it is the most common design. A micro-rotor is a smaller weight sunk into the movement itself, so it runs thinner and leaves more of the movement on view. Both do the same job.
Is the rotor what makes a watch self-winding?
Yes. The free swing of the rotor is what makes an automatic watch self-winding. As your wrist moves, the rotor rotates and winds the mainspring, so you do not need to hand wind it every day.