How an Automatic Watch Works: One Spring, Slowed Down
Everything in an automatic watch exists to slow one spring down
A fully wound automatic watch is holding about forty-two hours of running energy in a coiled strip of alloy, and if nothing stood in its way, that strip would spend it all in a few seconds. The hands would whip round the dial, the train would spin to a blur, and the watch would be flat before you finished looking at it. Watchmakers treat a wound mainspring as a hazard for exactly this reason: before taking a movement apart, they let the power down slowly, because a spring released all at once can strip the teeth off its own gears.
So one part of an automatic movement exists to wind the spring back up. Everything else exists to slow the spring down. Hold that frame and the machine stops being mysterious.
The movement has five jobs. It stores energy in a spring. It lets that energy out in small, identical doses. It uses a swinging wheel to decide how often a dose goes out. It counts the doses and turns the count into moving hands. And it tops the store back up from the motion of your wrist. Every automatic on earth, the £400 microbrand and the Submariner north of £9,000, is those five jobs in a stack of plates and wheels about the diameter of a two-pence piece.
The calibre I will use for this post is the Miyota 9039, because it is the movement inside a large share of the watches that cross my desk: 24 jewels, 28,800 beats per hour, roughly forty-two hours of power reserve, 3.9mm thick, hand-winding, hacking seconds. If you have bought a microbrand three-hander in the last five years, there is a decent chance this exact machine is behind the dial.
The spring
Take the caseback off and under the rotor sits the barrel: a flat drum with gear teeth around its rim. Inside is the mainspring, a strip of hardened alloy about the length of your forearm, coiled around a central arbor. Winding the watch turns the arbor and coils the strip tighter. That is the entire power supply. There is no battery anywhere and there never will be.
Two things about this spring matter more than the rest. The first is that you CANNOT overwind an automatic. The spring's outer end is not fixed to the barrel wall; it grips it through a slipping bridle, and once the spring is fully coiled, further winding just drags the bridle around the wall. The rotor can spin all day and nothing breaks. The overwound-automatic myth should have died decades ago (forcing a hand-wound watch past its hard stop is a different matter) and it refuses to.
The second is that the spring does not push evenly. Fresh off a full wind it delivers its strongest torque, and the push fades across the hours that follow. A fading push means the balance swings through a shorter arc, and a shorter arc lets the rate drift. Miyota's own accuracy figure for the 9039 (minus ten to plus thirty seconds a day) is measured within the first hour after a full wind, dial up. That is not the company hiding anything; it is the honest physics of springs. A watch running on the last few hours of its reserve keeps worse time than one worn all day, and now you know why.
The train
The barrel's teeth mesh with the first wheel of the gear train, and the train is nothing more exotic than a chain of gears in which each wheel turns faster than the one before it, with less force behind it. One wheel in the chain rotates exactly once an hour, and the minute hand is driven from it. Further along, a wheel turns once a minute and carries the seconds hand. At the far end, turning fastest of all, sits the escape wheel, and everything interesting in watchmaking happens there.
The 24 jewels on the spec sheet live mostly along this chain. They are synthetic rubies, drilled and polished into bearings for the fastest pivots, because ruby is hard enough not to wear and slick enough to run for years on a trace of oil. They are not decoration. They are worth nothing as stones, and the word on the dial is left over from the era when they had to be cut from real ones.
On its own, the train has no speed. Connect a wound barrel to a free train and the watch does not tick; it unwinds in a scream. Something has to stand at the end of the chain and refuse to let it run.
The brake
That something is the escapement, and it is two parts. The escape wheel carries angled teeth. The pallet fork is a small lever with a ruby stone at each end of one arm and a slotted horn at the other. At rest, one stone sits locked against a wheel tooth, and the entire train (barrel, gears, the whole force of the mainspring) stands still against that single stone.
Then the balance wheel swings past. A pin on the balance flicks the fork across, the locked stone lifts, and the wheel gets free, but only for an instant. As it turns, a tooth slides down the stone's angled face and gives the fork a push, and the fork passes that push on to the balance to keep it swinging. Before the wheel can go any further, the fork's second stone drops into its path and locks it again. The wheel gets free, pays for its freedom with a push, and is caught. The exchange takes a few thousandths of a second and happens eight times every second. The tick of a watch is the sound of it. Each time the fork catches, the seconds hand has moved one small step, an eighth of a second at a time.
This is where the title happens, literally. The escapement's whole purpose is to spend the mainspring's energy at a controlled rate: to lose over forty-two hours a fight it could lose in seconds. Everything else exists to slow the spring down, and this is the room where it is done.
The wheel that keeps the time
Notice what the escapement cannot do: it cannot decide when to let go. It waits for the balance. The balance wheel and its hairspring are the only parts of the movement that keep time. Everything else is either feeding them or counting them.
The balance is a weighted wheel on pivots about the thickness of a human hair, with a coiled hairspring finer still fixed to its axis. Turn the wheel one way and the spring pulls it back; it overshoots, and the spring pulls it back again. It is a pendulum that works in any position, which is the entire reason clocks left the mantelpiece and got onto wrists. In the 9039 it swings at four hertz: eight beats a second, 28,800 an hour, 691,200 a day. Wear the watch for a year and that small wheel reverses direction roughly a quarter of a billion times, dry of any electricity, on two specks of oil.
The swing's period is set by the wheel's inertia and the spring's stiffness, and a regulator arm pinches the hairspring to change its working length. Shorten the spring and the watch gains; lengthen it and it loses. That single adjustment is most of what a watchmaker means by regulating. The balance is also where the outside world gets in. Gravity drags on those hair-thin pivots differently dial-up than crown-down (Miyota quotes a posture difference of up to thirty seconds a day for the 9039), and temperature and magnetism both work on the hairspring itself.
That is where the money goes. The current no-date Submariner runs Rolex's calibre 3230, and in architecture it is the same machine as the 9039: a barrel, a train, a lever escapement, a balance, three hands and nothing else. What Rolex sells is execution. The escapement is reworked, with a skeletonised escape wheel and revised lever geometry that Rolex says make it about fifteen per cent more efficient. That, together with a longer spring in a thinner-walled barrel, is where its seventy-hour reserve comes from. The hairspring is Parachrom, an alloy that shrugs off the magnets and temperature swings that send an ordinary spring wandering. The finished watch is regulated and tested to within two seconds a day, against the 9039's minus ten to plus thirty. Those are real differences, and every one of them is the same five jobs done to tighter numbers. Nothing in the Submariner does anything the £400 watch does not also do.
The automatic part
None of the above is what makes the watch automatic. The self-winding is one assembly on top of everything else: a half-moon weight riding on a ball bearing, swinging whenever your wrist moves. The 9039's rotor winds in one direction only, clockwise, and freewheels back the other way, and a set of reduction gears turns many lazy swings of the weight into a few hard turns of the barrel arbor, through a ratchet so the spring cannot push back. A normal day on the wrist keeps the spring near the top of its wind. Going back to the torque curve, that is also where the watch keeps its best time.
Leave it on the desk for two days and it stops, and nothing whatsoever is wrong with it. About forty turns of the crown fills it back up. Pull the crown out and a small brake touches the rim of the balance and holds it still. That is hacking, and it lets you set the seconds against a time signal and start the watch on the mark. The smooth sweep that gets called the signature of a mechanical watch is not smooth at all. It is eight small steps a second, too quick for your eye to separate, where quartz takes one. The glide is a flicker you cannot see.
What quartz did
In December 1969, Seiko's Astron put a quartz crystal where the balance wheel goes. Its crystal vibrated 8,192 times a second, and the industry later settled on 32,768. Either figure is a long way from the 9039's eight. A circuit counts the vibrations, and a small motor steps the hands once a second. An ordinary quartz movement today holds to around fifteen seconds a month, runs for years on a battery, does not care what position it is lying in, and costs a few pounds to make. On every axis you can measure, quartz wins. It is not close, and I am not going to pretend it is.
The price only makes sense after that concession. When you buy an automatic you are not buying the best way to know the time; your phone settled that contest years ago. You are buying the mechanism itself: a stored spring, a brake, and a swinging wheel, made finely enough to run a quarter of a billion beats a year on the movement of your arm. Nobody needs it. That is close to the point.
Two working notes to finish. The oils in these movements dry out, and the trade's rule of thumb is a service every five to seven years. On a 9039 the arithmetic is blunt: a brand-new movement sells for about a hundred dollars, which is less than the labour on a proper service, so the right fix for a tired one is usually a fresh movement. A dealer who quotes you a full strip-down without saying so is not doing you a favour. On the Rolex, the service bill is real and worth paying, because the movement is the thing you bought.
Most of what I sell runs a 9039 or a movement drawn on the same plan, and my listings give it a single line of spec. This piece is the rest of that line. Take an automatic off tonight, give the crown twenty turns, and hold it up to your ear. You will hear eight beats a second, which is the fork catching the wheel and letting it go. One part wound that spring this morning. Everything else exists to slow the spring down.