Rotor

A rotor is the pivoted weight in an automatic watch that swings as your arm moves and winds the mainspring. It is usually a semicircular slab on a central bearing, weighted at the rim so that a small movement of the wrist produces useful turning. A train of reduction gears takes that swing down to the barrel, where a slipping bridle prevents the spring being wound past full.

Mass and materials

What a rotor harvests depends on how much mass sits how far from the pivot, so weight goes to the rim in the densest material the price allows. Brass is about 8.5 grams per cubic centimeter and tungsten heavy alloys roughly twice that, near 18, which is why a mid-priced automatic often has a brass rotor with a tungsten segment screwed to its edge. It is also why a skeletonized rotor gives up less winding than it appears to: the metal removed came from the middle, where it was doing least.

The bearing carries all of it. Cheaper movements run the rotor on a plain bushing pressed into a bridge; most Swiss automatics use a ball race, which has less friction and is usually quieter. Both are wear items.

Bidirectional and unidirectional winding

A rotor swings both ways, but not every system uses both. Bidirectional designs use reversing wheels, as in the familiar ETA arrangement, or a pawl system such as Seiko's Magic Lever, to convert swings in either direction into winding. Unidirectional systems wind one way and free-wheel the other. They are simpler and have fewer parts to wear, but harvest less per swing, which the designer offsets with rotor mass and gearing.

The Valjoux 7750 uses a single reversing wheel, so it winds in one direction only, and the Miyota 9015 is likewise a one-way winder with an audible rotor. Seiko's NH35 uses the Magic Lever, a two-pawl arrangement that takes both directions with very few parts, which is part of why that caliber is so cheap and so durable.

Types worth knowing

A full rotor is the common arrangement: a central weight covering most of the movement, efficient and cheap to make, but it hides everything behind it.

A micro-rotor is a small weight sunk into a cut-out so that it sits level with the bridges rather than above them. Piaget built a caliber this way in 1960 to make the thinnest automatic of its day. It leaves most of the movement visible and saves real height, at the cost of winding efficiency, since a smaller weight has less inertia.

A peripheral rotor is a ring running around the outside of the movement, which keeps the view clear and the watch thin but demands careful bearing design.

The idea is old. John Harwood patented a self-winding wristwatch in the 1920s using a weight that bumped between buffer springs, and Rolex introduced its Perpetual rotor, turning through a full circle, in 1931.

How much winding it actually does

An ordinary day of walking about winds a good automatic further than it spends. A day at a desk may not, because the wrist barely rotates, which is the usual explanation for a watch that runs down over a weekend of light wear. Winder makers state the requirement in turns per day, and most movements are quoted between roughly 650 and 1,000; our guide to winders and storage and the answer on whether you need one put that in context. If a watch is not worn enough to stay wound, 30 or 40 turns of the crown does the same job for nothing.

In practice

A rotor you can hear spinning is normal, and on some calibers it is loud. Grinding or rattling, particularly after a knock, can mean a worn bearing or a rotor touching the caseback, and scoring inside the caseback confirms it. Bearings are checked at every service.

Play is what a watchmaker tests. Held flat, a healthy rotor spins freely and coasts, and it should not rock vertically or scrape as it passes. Play you can feel means the bearing, and left alone it lets the rotor mark the caseback.

Terms it gets confused with

A rotor is not the movement, and a decorated rotor seen through an exhibition caseback is the least mechanically interesting part of what is behind it. Nor does rotor motion mean winding: past full wind the mainspring barrel's slipping bridle lets it all turn while nothing accumulates, which is why an automatic cannot be overwound. Shaking a watch to wind it is the related mistake: it loads the bearing sideways in a way it was not designed for, and the crown does the job better. Nor is a heavier rotor automatically a better one, since trading mass for thinness in a micro-rotor caliber is a considered choice.