How a mechanical watch works

The problem a watch actually solves

Storing energy is easy. A coiled spring does it, and if you let one unwind freely it releases everything it has in a fraction of a second. That is the entire difficulty of watchmaking: not storing power, but releasing it at a rate so regular that you can count on it.

Every part in the diagram above exists to solve that one problem. The mainspring stores. The gear train transmits and divides. The escapement releases, one tooth at a time. And the balance wheel decides when each release happens, because it swings at a rate determined by its own physical properties rather than by how much power is pushing on it.

That last point is the elegant part. A balance wheel and hairspring behave like a pendulum: the period depends on the mass and the spring stiffness, not on the size of the push. So a nearly-unwound mainspring pushes more weakly but the balance keeps roughly the same rhythm, which is why a watch stays approximately accurate as it winds down.

Why the gear train has so many wheels

The barrel turns very slowly, roughly once every few hours. The escape wheel needs to turn many times faster. The wheels between them exist to trade torque for speed, each pair stepping the rate up while reducing the force available.

Those ratios are also where the hands come from. The wheel driving the minute hand is geared to complete exactly one turn an hour, and the seconds hand sits on a wheel turning once a minute. The hour hand is driven through a further reduction, which is why setting the time moves the hands together in the correct relationship.

Friction at each pivot is the enemy, which is why movements list a jewel count. Those jewels are synthetic rubies pressed into the plates, acting as bearings. Ruby is used because it is extremely hard, wears slowly and holds oil well, not because it is precious.

What automatic winding adds, and what it does not

An automatic watch is a manual watch with one component added: a weighted rotor that swings as your wrist moves, winding the mainspring through a reduction gear. Everything downstream of the barrel is identical.

This is why the accuracy of an automatic is no better than a hand-wound movement of the same design. The rotor solves convenience, not precision. It also introduces a slipping clutch in the barrel so that continued winding cannot overstress the spring, which is why you cannot overwind a modern automatic.

The trade-off is thickness and one more thing to service. A hand-wound movement is thinner and simpler, which is why dress watches frequently skip the rotor entirely and why the Speedmaster Professional is still hand-wound sixty years on.

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