How does a mechanical watch work?

A mechanical watch has no battery, no circuit board and no quartz crystal. It runs on a coiled strip of metal, a handful of gears and a tiny wheel that swings back and forth several times a second. That's it. And yet a good one keeps time to within a few seconds a day.

Once you understand the five or six parts involved, every mechanical watch — from a vintage hand-wound dress watch to a modern automatic diver — makes sense. Here's the whole thing, in the order the energy flows.

The short version

A mechanical watch does three jobs, one after another:

1. Store energy — winding the crown coils a spring called the mainspring.
2. Pass it on — the spring slowly unwinds, turning a chain of gears called the gear train.
3. Let it out in equal slices — the escapement and balance wheel release the gears one tiny step at a time, at a steady rhythm. That rhythm is what makes it a clock rather than a spinning top.

The hands are just attached to gears that happen to turn once an hour and once every twelve hours.

1. The mainspring: the watch's battery

The mainspring is a long, thin ribbon of hardened alloy coiled inside a small drum called the barrel. When you turn the crown (or when the rotor of an automatic swings), you coil the spring tighter. As it slowly unwinds, it turns the barrel, and the barrel's toothed edge drives everything else.

How long a spring can run the watch is the power reserve. Most watches manage around 40 hours; some use a longer spring, or several barrels in a row, to run for eight days. A ratchet wheel and click stop the spring from simply unwinding backwards the moment you let go of the crown — that's the clicking you feel when winding.

2. The gear train: turning force into time

The barrel turns slowly and with a lot of force. The gear train — usually four wheels called the centre, third, fourth and escape wheels — steps that up into faster, gentler motion. Each wheel meshes with a small gear called a pinion on the next wheel, and the ratios are chosen so that:

• the centre wheel turns once an hour (it usually carries the minute hand),
• the fourth wheel turns once a minute (it often carries the seconds hand — which is why small-seconds subdials sit wherever that wheel happens to be),
• the escape wheel turns quickly, one tooth at a time.

Left alone, the spring would spin this train in a fraction of a second and run the watch down. Something has to hold it back.

3. The escapement: letting energy out in slices

The escapement is the clever bit. In most modern watches it's a Swiss lever escapement: an escape wheel with oddly shaped teeth and a small anchor-shaped pallet fork with two jewelled "pallet stones".

The fork rocks from side to side. Each time it rocks, one pallet stone lets an escape-wheel tooth slip past, and the other catches the next one. The gear train advances exactly one step, then locks again. That lock-and-release is the tick you hear — and the reason the seconds hand on a mechanical watch moves in tiny steps rather than a true smooth glide.

The escapement does a second job, too: as each tooth slides past, it gives the fork a tiny push, which the fork passes to the balance wheel. That push keeps the balance swinging.

4. The balance wheel and hairspring: the heartbeat

The balance wheel is a small weighted ring that swings back and forth on a pivot. A very fine spiral spring called the hairspring (or balance spring) pulls it back towards the centre each time, so it oscillates at a steady rate — like a pendulum, but one that works in any position.

Every swing of the balance flicks the pallet fork, which releases the escape wheel one tooth. So the balance sets the rhythm of the whole watch. Most modern watches beat 21,600 or 28,800 times an hour — that's 6 or 8 steps of the seconds hand per second. We go into what those numbers mean in watch beat rate explained.

Accuracy comes down to how consistently the balance swings. Temperature, magnetism, the watch's position and how much power is left in the spring all nudge it. A watchmaker "regulates" a movement by adjusting the effective length of the hairspring until the watch runs neither fast nor slow.

5. Jewels: why a watch has rubies inside

Where the gear pivots spin, and on the pallet stones, watches use tiny synthetic rubies. Ruby is extremely hard and smooth, so it wears slowly and needs very little oil. A basic hand-wound movement typically has 17 jewels — enough to cover every important friction point. Automatics often have 21 to 25, because the winding system adds more moving parts.

More jewels don't automatically mean a better watch. Past the functional set, extra jewels are mostly decoration.

6. Keyless works: the crown's job

The crown connects to a stem inside the watch, and a small set of levers and gears called the keyless works decides what turning the crown does:

• Pushed in — turning it winds the mainspring.
• Pulled out one step (on watches with a date) — turning it changes the date.
• Pulled out fully — turning it moves the hands to set the time.

Before about 1840, you needed a separate key to wind and set a watch — hence "keyless". More on this in how to set a mechanical watch.

7. The dial train: hours from minutes

The minute hand sits on the centre wheel's arbor, but the hour hand has to turn twelve times slower. A little set of gears under the dial — the motion works or dial train — divides the minute hand's rotation by twelve and drives a tube that carries the hour hand. Date wheels, day discs and moon phase discs take their motion from here too, usually via a wheel that turns once every 24 hours.

Where automatics differ

An automatic watch has everything above, plus a half-moon weight called the rotor. As your wrist moves, the rotor swings and, through a set of reversing wheels, winds the mainspring for you. A slipping bridle at the end of the spring stops it being overwound. See automatic vs manual watches for how that changes living with one.

Why a mechanical watch stops

A mechanical watch stops for one simple reason: the mainspring runs out of energy. When it's nearly unwound, it can no longer push the escape wheel hard enough to keep the balance swinging, and the whole train comes to rest. Wind it, and it starts again. That's not a fault — it's the deal you make with a mechanical watch.

See it working in Mecha Watch

Reading about a balance wheel is one thing. Watching one is another. In Mecha Watch you choose one of four movements — each ticking at its real beat rate — build a watch around it, then flip the watch over to see the movement through the caseback.

And because it behaves like the real thing, the energy has to come from you. Wind the crown and feel each ratchet click; walk and an automatic winds itself; ignore it and the mainspring runs down and the watch stops. It's the best way we know to understand a mechanical watch: make a watch — or, said quickly, Mecha Watch — and keep it alive.

Frequently asked questions

Does a mechanical watch need a battery?

No. A mechanical watch is powered entirely by a coiled mainspring, which you wind by turning the crown or, in an automatic, by moving your wrist.

What makes a mechanical watch tick?

The escapement. A pallet fork locks and releases the escape wheel one tooth at a time, in rhythm with the swinging balance wheel. Each release is one tick.

How accurate is a mechanical watch?

A typical mechanical watch gains or loses somewhere between a few seconds and around 20 seconds a day. Certified chronometers are held to tighter limits. Quartz watches are far more accurate.

Why do mechanical watches have jewels?

Synthetic rubies are used as bearings at high-friction points because they're hard and smooth, so parts wear slowly and need little oil.

What is the difference between the mainspring and the hairspring?

The mainspring stores energy and drives the watch. The hairspring is a much finer spring attached to the balance wheel that controls its swing and therefore the watch's timekeeping.