Detached escapement

Detent (chronometer) escapement

Verified

A spring or pivoted detent locks the escape wheel; the balance unlocks it once per revolution and receives a single direct impulse, remaining otherwise entirely free.

Schematic arrangement

How it is laid out

Outline schematic only, drawn to show the characteristic arrangement of wheel, locking member and oscillator. It is not a dimensioned working drawing and should not be used for setting up or repairing a movement.

Advantages

Finest rate of the classical escapements
Minimal interference with the balance

Drawbacks

Not self-starting
Vulnerable to shock — unsuited to pocket use

Attribution and period

Late eighteenth century, associated with Earnshaw and Arnold

Horological firsts are frequently contested. Where an invention is attributed to a named maker, that attribution is recorded as such rather than treated as settled fact.

How to recognise it

A long, delicate spring detent (or, less commonly, a pivoted detent) rather than a lever.
A discharging (passing) spring on the detent, and a separate impulse roller on the balance staff.
Usually found in a gimballed box movement — a marine chronometer — or in a deck watch.

Servicing and conservation

The detent's passing spring is extremely fine and is easily bent beyond recovery; chronometers should be handled only with the balance secured and never carried while running out of their gimbals. Cleaning and re-setting a detent escapement is chronometer-specialist work.

Work on an antique escapement by a qualified watchmaker or clockmaker. Conservation of original parts is preferred to replacement wherever the object will still run.

Bibliography

Gould, R. T., The Marine Chronometer: Its History and Development
Gazeley, W. J., Clock and Watch Escapements

Description of the Le Roy detent escapement mechanism 14 August 1748 — Academie des Sciences, France 14 August 1748, Public domain. Wikimedia Commons file page