Escapement

The escapement stands between the gear train and the balance wheel, and it performs two jobs at once. It locks the train so that the stored energy of the mainspring is released in equal, countable steps rather than spinning away in seconds. And at each release it delivers a small push, an impulse, to the balance so that the oscillation never dies. Everything else in the movement is either supplying energy to this point or counting what comes out of it.

How the Swiss lever works

In the near-universal Swiss lever escapement, the last wheel of the train is the escape wheel, held by a pallet fork carrying two jeweled pallet stones. With one stone locking a tooth, the whole train is frozen. As the balance swings back it strikes the fork and knocks it aside; one tooth escapes, the fork swings across, and the next tooth is caught by the other stone. During that transit the escaping tooth pushes the pallet stone, the fork pushes the balance's impulse pin, and the balance gets its kick.

The Swiss lever, refined from Thomas Mudge's design of about 1754, has dominated since the 19th century because it is robust, self-starting, forgiving of shocks and manufacturable at scale. Its weakness is that impulse involves sliding contact, so it needs lubrication, and lubrication ages.

Draw, and the parts that stop it misfiring

The reason a lever watch survives being knocked is easy to miss. The locking faces of the pallet stones are not radial but angled slightly, so the escape wheel's pressure on a locked stone pulls the fork harder against its banking pin instead of pushing it away. This is draw, and it means any accidental displacement of the fork has to fight the mainspring itself.

Two safety features back it up. A guard pin on the fork's tail rides just clear of a circular safety roller on the balance staff, so a fork that moves at the wrong moment is stopped dead. A crescent notch in that roller clears the guard pin only at the instant the impulse pin enters the fork, the one moment the fork is meant to move.

The numbers that describe one

The escape wheel advances one tooth per complete swing of the balance, so at 28,800 vibrations per hour, or 4 Hz, it steps forward four times a second and unlocks eight times.

Two other figures follow the escapement around. Lift angle is the part of the swing during which impulse is delivered, usually about 52 degrees but ranging from the high forties to the high fifties, and a timing machine needs the right value for that caliber before its amplitude reading means anything. Amplitude at full wind, dial up, normally sits around 270 to 310 degrees; below roughly 200 the escapement wants attention, and above about 320 the impulse pin can strike the outside of a fork horn, a fault called knocking. Our timegrapher reading explainer covers those traces.

Alternatives worth knowing

The co-axial escapement, designed by George Daniels and put into series production by Omega from 1999, separates locking from impulse across two levels of escape wheel to reduce sliding friction at the impulse surfaces. Detent escapements, brought to practical form by John Arnold and Thomas Earnshaw in the 1780s and used in marine chronometers, impulse the balance in one direction only and are exquisitely efficient but fragile. Silicon components, used for escape wheels and pallet forks by several manufacturers, are light, non-magnetic and can run with little or no oil. Our guide to escapements explained covers the full history.

Lubrication and service

Almost every escapement problem an owner meets is an oil problem. The pallet stones carry a thixotropic grease in quantities measured in fractions of a microliter, placed on the impulse faces rather than the wheel teeth so each tooth picks up its share in passing. As that lubricant thickens or migrates, amplitude sags, positional variation widens and the rate drifts over months. That is why a watch needs stripping and re-oiling every few years, as our guide to watch servicing and care explains.

Why it matters

The escapement is where efficiency and wear concentrate. Amplitude, the figure a watchmaker reads first, is largely a report on how well it is doing its job.

Common confusions

The word is often used for the whole regulating organ. Strictly the escapement is the escape wheel, pallet fork and roller; the balance and hairspring are the oscillator it drives.

A tourbillon is not an escapement: it is a rotating carriage that carries an ordinary lever escapement around with it, changing how positional errors average out and nothing about how the escapement works. A co-axial escapement is not oil-free either, since less sliding friction at the impulse faces buys longer service intervals but the movement is still lubricated. Spring Drive has no mechanical escapement at all, its train being braked electromagnetically rather than locked and released.