· By Buffy
Drop, Lock, and Lift
The geometry of the Swiss lever in three numbers watchmakers obsess over: drop (lost motion), lock (safety margin), and lift (the impulse gift).
This is the deep-dive companion to The Swiss Lever Escapement.
Ask a watchmaker how the escapement is adjusted and you will hear three words: drop, lock, and lift. They are angles and clearances measured in hundredths of a millimeter, and they decide whether a watch runs for decades or stops by lunch.
Drop
Drop is the free fall of a tooth from one pallet stone to the next after impulse — plus the corresponding drop across the fork at the balance end. Drop must exist (otherwise the tooth would drag on the stone) but every bit of it is wasted motion: the tooth falls and knocks, dissipating energy as sound and heat.
- Too much drop: audible knocking, lost impulse, low amplitude.
- Too little drop: the tooth lands on the corner and drags — friction, then stoppage.
Watchmakers measure drop as the angle the escape wheel moves between “tooth just leaving” and “tooth just landing.” Healthy drop is a sliver.
Lock
Lock is how deeply a tooth rests on a pallet stone when the escapement is at rest. Lock serves the safety action: it guarantees the fork stays pinned against its banking while the balance is detached.
- Lock depth is set by the pallet stones’ radial position and the fork’s horn.
- Draw — the locking face’s slight angle, plus the fork’s banking spring bias — pulls the fork against the banking pin instead of letting it float.
- Insufficient lock + no draw = the classic “trip”: a jar throws the fork to mid-stroke and the watch double-unlocks, momentarily running wild.
- Excessive lock = heavy unlocking friction, low amplitude, rate sensitivity to position.
Lock is checked with a special gauge or by eye and feel at the bench — the tooth must rest on the face, not at its edge.
Lift
Lift is the impulse gift: the combined angles of the pallet stone’s impulse plane and the fork slot / impulse jewel geometry that translate the train’s torque into the balance’s swing. The lift angle (typically 45–52° total, movement-specific) is what a timegrapher needs as an input to compute amplitude correctly.
Lift trades against safety:
| More lift | Less lift |
|---|---|
| bigger impulse, higher amplitude | less unlocking friction |
| more sliding friction per impulse | lower amplitude, more positional variance |
| faster pallet wear | less pallet wear |
There is no free optimum; there is only the movement’s design point — and oil. Impulse surfaces are oiled with microscopic amounts of light oil (and epilame-treated so it stays), because sliding friction is only tolerable when cushioned. When the oil migrates away — see Friction and Lubrication — the rate sags and the watch joins the service queue.
The three numbers together
A healthy escapement: drop barely audible, lock firm but not deep, lift angle as designed, amplitude 260–310° in dial-up. That signature on a timegrapher — two clean, parallel lines — is the sound of three hundred years of geometry behaving itself.