An automatic watch is a mechanical watch that winds itself as you wear it, with no battery. A weighted rotor spins with the movement of your wrist and winds a mainspring housed in a barrel. The spring slowly releases its energy through a gear train, and the escapement lets it out in small, regular impulses timed by the balance wheel. Take it off and it keeps running until its power reserve is used up, then stops.
How do automatic watches work, step by step?
An automatic watch is a mechanical watch with a self-winding system added. It works as a simple chain, from the energy source to the hands.
| 1. Rotor | picks up the movement of your wrist |
| 2. Mainspring and barrel | store the energy |
| 3. Gear train | transmits and gears down that energy |
| 4. Escapement | releases it in impulses |
| 5. Balance and hairspring | set the rhythm, and therefore the accuracy |
The rotor, or oscillating weight
This is the half-moon part you can see through a display caseback. Weighted at its edge, it pivots freely around a central axis every time your arm moves. A system of reverser wheels or pawls turns these swings, in one direction or both depending on the calibre, into rotation that winds the mainspring.
Some movements wind when the rotor turns either way, others in one direction only. That is why a watch winder can be set to clockwise, anticlockwise or bidirectional.
The mainspring and barrel
The mainspring is a long strip of steel coiled inside a drum called the barrel. The more it is wound, the more energy it holds. In an automatic watch, its outer end is held by a slipping bridle: once the spring is fully wound, the bridle slips, so the watch cannot be overwound.
What about the crown?
Most automatic watches can also be wound by hand: turning the crown winds the same mainspring by a different route from the rotor. It is useful for restarting a watch that has stopped. A few entry-level calibres, such as the Seiko 7S26, do not allow it. For the right technique, see our guide on how to wind an automatic watch.
The gear train
A series of toothed wheels carries the force of the barrel to the escapement, while driving the hour, minute and seconds hands at the right speeds.
The escapement and balance
Without a brake, the mainspring would unwind in seconds. The escapement, made up of an escape wheel and a pallet fork, locks and releases the gear train at regular intervals. It is what makes the tick.
The rhythm comes from the balance wheel, a small wheel that swings back and forth, returned by a very fine hairspring. Most modern calibres beat at 21,600 or 28,800 vibrations per hour, that is 3 or 4 hertz. This is what gives the seconds hand its smooth sweep in small, rapid steps.
What is automatic winding?
Automatic winding, also called self-winding, simply means the mainspring is wound by the rotor rather than by hand. Every movement of your wrist adds a little energy; a normal day's wear is generally enough to keep the watch running. The term describes the winding system, not a separate type of movement: underneath, an automatic is a mechanical watch like any other.
Power reserve: how long does it run off the wrist?
The power reserve is how long the watch keeps running once removed from your wrist with a full mainspring. On common calibres it ranges from around 38 hours, like the ETA 2824-2, to around 80 hours, like the Tissot Powermatic 80.
In practice, a watch with 38 or 48 hours of reserve taken off on Friday evening will have stopped before Monday morning. We list figures by calibre in our guide to automatic watch power reserve.
Do automatic watches need batteries?
No. An automatic watch is entirely mechanical: all its energy comes from the mainspring, wound by your movements or the crown. There is no battery to replace, although it still needs a periodic service, because oils age and parts wear.
One exception causes confusion: a few so-called automatic quartz watches, such as Seiko Kinetic, use an oscillating weight to generate electricity, stored in a rechargeable cell that powers a quartz movement. These are quartz watches, not mechanical automatics in the watchmaking sense.
Automatic vs quartz vs manual
| Automatic | mechanical, wound by the wrist (and usually the crown), no battery |
| Manual wind | mechanical, wound only by the crown, no rotor |
| Quartz | electronic, battery-powered, timed by a quartz crystal |
A quartz watch is more accurate and does not stop when you take it off. An automatic appeals through its mechanics, its longevity and the craftsmanship behind it. A mechanical watch certified as a chronometer by COSC must stay within −4 to +6 seconds a day, whereas an ordinary quartz watch drifts by a few seconds a month. We compare all three in automatic vs manual vs quartz watch.
A little history
The oscillating weight goes back to pocket watches of the late 18th century, notably the work of Abraham-Louis Perrelet. For the wristwatch, John Harwood patented a self-winding system in the early 1920s, and in 1931 Rolex introduced its full-rotation rotor in the Oyster Perpetual, a principle still used today.
Why it stops when you do not wear it
Without movement, the rotor stops turning and the mainspring is no longer wound. The watch uses up its reserve, loses accuracy in the last hours, then stops. That is not a fault: it is how it works, and a stopped watch is not damaged. You simply wind it and set the time again.
There are three ways to keep an automatic watch running when not worn:
- wear it regularly, if you only own one;
- wind it by the crown before wearing it, around 20 to 40 turns, then set the time;
- place it on a watch winder, which turns it gently to keep the mainspring wound.
If you rotate several watches, or yours has a date, a calendar or a moon phase, a watch winder saves you resetting everything each time. For the right direction and turns per day, see our watch winder settings by calibre.
FAQ
Can an automatic watch run without being worn?
Yes, for the length of its power reserve, generally 38 to 80 hours depending on the calibre. After that it stops until it is wound again.
Do you need to wind an automatic watch by hand?
Only if it has stopped or you rarely wear it. Day to day, the movement of your wrist is enough.
Are automatic watches less accurate than quartz?
Generally, yes: a few seconds a day against a few seconds a month. That is the trade-off for a fully mechanical movement.
How long does an automatic watch last?
Well looked after and serviced regularly, an automatic watch can run for generations: none of its parts is designed to be thrown away.