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Every Mechanism

Home How an Automatic Watch Works

Inside a self-winding watch

How an automatic watch works

No battery. The swing of your wrist winds a spring, a row of gears turns the hands, and a tiny balance wheel lets the energy out one tick at a time, eight ticks a second.

Hear it tick
28 800
beats an hour
45 h
of running on one full wind
12×
slower: the hour hand
0
batteries
An automatic watch movement seen from the back: the gold half-moon rotor, rhodium bridges with blue screws and red jewels, the balance wheel with its hairspring, and the crown on its winding stem.
3D model · 360° · the back of the movement

Wind it, watch it tick

The movement seen through its back, the bridges see-through so you can watch the wheels. Drag the gold rotor to swing it, or press Swing your wrist, and watch the mainspring wind. Drag anywhere else to turn the view, scroll or pinch to zoom, and click a part to use it.

Try itDrag the gold rotor, or press Swing your wrist. Whichever way it swings, the ratchet wheel only ever turns one way.
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Pace
Rotor
Bridges

Scroll to take it apart

Under the caseback

Keep scrolling: the movement comes out of its case, then it comes apart in the order a watchmaker would take it down, and every part lands face up on the bench.

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  1. Step 1Open the backThe caseback and the case come away, and the bare movement is left.
  2. Step 2Self-winding worksThe rotor lifts off its ball bearing, then the automatic bridge and the two reversing wheels under it.
  3. Step 3Ratchet and bridgesThe balance cock, the ratchet wheel and its click, then the bridges with their jewels and blue screws.
  4. Step 4Escape­ment and trainThe balance, the pallet fork, the escape, fourth, third and centre wheels, then the barrel and its lid.
  5. Step 5The dial sideThe crown and its stem slide out; turned face up, the hands, the dial and the motion works.

Meet the parts

Laid out on the bench

The whole watch taken down and laid face up beside its main plate: the self-winding works along the top, the gear train along the bottom, the dial turned over on the left. Tap any part of the picture or pick a name below.

The parts of the automatic watch laid out face up: the gold rotor, the automatic bridge, the reversing wheels, the balance cock and the bridges along the top; the blue dial, the main plate, the balance wheel, the pallet fork and the crown in the middle; the hands, the motion works, the barrel and its lid, the ratchet wheel and click, and the centre, third, fourth and escape wheels along the bottom.

Hover or tap the picture. Gold rings mark every part.

Parts diagram

Follow the energy

The movement from the back, with the rotor and the bridges shown see-through. Numbers 1 to 5 wind the spring, 6 to 11 carry its energy down the gear train, 12 to 14 let it out in even steps, and 15 to 17 hold it all together.

The watch movement from the back with the rotor and bridges see-through, showing the reversing wheels, the ratchet wheel and click, the barrel, the gear train, the pallet fork and the balance, with seventeen numbered parts.
    How it works, step by step

    From your wrist to the hands

    The same twenty steps as the video. Each picture is a moment from it; the strip shows which link of the energy’s path you are on. Press Show me in 3D to set the model up for that step.

        The watch in its cradle on the bench.
        Step 1 of 20

        No battery, just your wrist

          Demo · winding

          Wind it with your wrist

          Every move of your arm makes the heavy rotor swing. Two reversing wheels turn both directions of swing into one, and a ratchet with its click stores the energy in the mainspring. The watch spends it steadily, day and night.

          Drag the watch to turn it, or pick how you wear it below. The rotor hangs like a pendulum: when the watch turns, it lags behind, and that swing between the rotor and the movement is what winds. Watch the power reserve fill, then keep going past full.

          • Both ways wind: whichever way the rotor swings, the ratchet wheel turns the same way.
          • Full: the slipping bridle lets the spring slide round the barrel instead of winding tighter.
          Wearing it
          –power reserve
          –spring wound
          0rotor turns, both ways
          0ratchet teeth past the click

          Demo · the gear train

          Faster at every step

          Each wheel turns its neighbour’s small pinion, so each turns faster than the one before it, with less force. Four meshes take the barrel’s slow seven-hour turn and speed it up 5 040 times, to the escape wheel’s twelve turns a minute.

          10:09:35on the dial
          ×1faster than real time
          1 han hour passes in
          12escape wheel, turns a minute

          Demo · the escapement

          Tick-tock

          Every tick is a tooth of the escape wheel landing on a ruby pallet. Between ticks the wheel is locked. The swinging balance unlocks it, the tooth gives the fork a push, and the fork passes the push on to the balance.

          Slow it right down to see each phase: locked, unlocking, impulse, drop. Then change the beat. Watches beat from 18 000 to 36 000 times an hour: a faster beat makes a smoother seconds hand and keeps better time when you move, but it wears the parts more.

          • Tick: a tooth drops onto one pallet; tock: onto the other.
          • Half a tooth per beat: 20 teeth make 40 beats a turn.
          Beat
          Lockedright now
          28 800beats an hour
          8ticks a second
          ⅛ seach step of the seconds hand

          Demo · what happens if

          Take one part away

          Every part has one job. Take one away, or push the watch past its limits, and see what goes wrong.

          The picture follows the energy from the spring in its barrel, down the train to the escapement and the balance, with the dial’s hands beside it. Pick a case; press Again to replay it.

          –power reserve
          –escape wheel, turns a minute
          –swing of the balance
          –the watch

          More to know

          Kinds, history and care

          Six ways to keep a watch going

          Central rotorThe one on this page: a half-moon weight over the whole movement, winding both ways through two reversing wheels.
          One-way windingSome movements wind only when the rotor swings one way and let it spin freely the other. Simpler, but slower to wind.
          Magic LeverSeiko’s answer from 1959: a little lever with two hooks that push and pull a wheel round on every swing of the rotor, winding both ways with very few parts.
          Micro-rotorA small rotor set into the movement beside the barrel instead of over it, so the whole watch can be much thinner.
          Hand-woundNo rotor at all. You give the crown a few dozen turns every day or two; everything from the barrel on works the same way.
          Rotor and quartzSeiko’s 1988 automatic generating system uses the swinging weight to run a tiny generator for a quartz watch. Spring Drive (1999) keeps the mainspring but lets a quartz crystal set the pace, so the seconds hand sweeps without steps.

          From pocket watches to your wrist

          1. Christiaan Huygens builds a balance with a spiral spring (Robert Hooke said the idea was his). Watches go from losing hours a day to minutes.
          2. Nicolas Fatio de Duillier and Peter and Jacob Debaufre patent jewel bearings in England.
          3. Thomas Mudge invents the lever escapement, the ancestor of the one in nearly every mechanical watch today.
          4. Abraham-Louis Perrelet makes a self-winding pocket watch; a quarter of an hour’s walk was said to wind it fully.
          5. John Harwood patents a self-winding wristwatch. It goes on sale in 1928.
          6. Rolex brings out its Perpetual rotor: a half-moon weight that swings all the way round, as on this watch.
          7. Eterna mounts the rotor on ball bearings, the way most rotors still turn.
          8. On Christmas Day, Seiko’s Quartz Astron, the first quartz wristwatch, goes on sale. Mechanical watches carry on as fine craft.

          Keeping it ticking

          • Wear it, or wind it. If it has stopped, give the crown 20 to 40 turns before you put it on, then set the time.
          • Wind it off your wrist. Turning the crown while the watch is on your wrist pushes the winding stem sideways.
          • Keep it away from magnets. Speakers, phone cases and bag clasps can magnetize the hairspring: its coils cling together and the watch runs fast. A watchmaker can demagnetize it in seconds.
          • Mind the knocks. The balance turns on pivots finer than a pin. Shock settings protect them, but a hard knock can still bend one.
          • Water and the crown. If the crown screws down, screw it home before the watch gets wet.
          • Have it serviced every five to ten years, as most makers suggest: the oils at the pivots dry out and wear begins.

          Things worth knowing

          691 200 beats a dayTwo beats for every swing of the balance, there and back, four swings a second: about 252 million beats a year.
          Eight steps a secondThe seconds hand moves on with every beat, eight tiny steps a second, so it seems to sweep. Most quartz watches step once a second.
          Rubies made in a labSince Auguste Verneuil’s process of 1902, watch jewels have been synthetic ruby. A fully jewelled watch has about 17; automatics often have more.
          720 to 1In this movement the escape wheel turns 720 times for each turn of the centre wheel. The minute hand is a very slow copy of the tick.
          Two days in a springA full mainspring stores enough for roughly two days, so the watch keeps running through the night on your bedside table. This one runs about 45 hours.
          ChronometerTo be certified a Swiss chronometer, a movement must keep within four seconds slow and six seconds fast a day on average, through 15 days of tests.
          Quick check

          Four questions