Among the many different escapement types, one in particular stands out, the detent escapement. Taking its name from détente, French for “trigger”, this system is often considered to be the purest type of escapement due to its efficiency and virtually lubrication-free operation.

Any sort of mechanical clock or watch requires an escapement, a mechanism that transfers torque from the power source to the regulating organ. The escapement bridges the steady rotation of the gears in the going train and the oscillation of the balance. As a result, the escapement fulfils a double function, both maintaining the swing of the balance and regulating the discharge speed of the mainspring and going train.

The detent escapement was invented in the second half of the 18th century,  somewhat simultaneously but independently by the English watchmakers Thomas Earnshaw (1749-1829) and John Arnold (1736-1799).

A somewhat similar escapement was prototyped by the Frenchman Pierre Le Roy (1717–1785) some three decades earlier, but the modern detent escapement is derived mostly from Earnshaw’s design. Initially, the escapement was conceived for use in marine chronometers, explaining why it is also known as a chronometer escapement.

The chronometer escapement also inspired many innovative subsequent escapements, like the Breguet’s natural escapement, the Robin escapement, and the Daniels co-axial.

Drawings of detent escapement types taken from ‘The Marine Chronometer: Its History and Development’ by R.T. Gould

How it works

At the time of the detent escapement’s invention, most existing escapements of the 18th century, like the virgule, duplex or cylinder, were frictional-rest escapements that lock directly on the balance arbour. This frictional rest acts very much like a brake on the balance, consuming its energy inconsistently, which results in gross timekeeping deviations. This  explains why even the finest watches with these early escapements keep time to within minutes a day at best.

In contrast, the detent is a detached, single impulse escapement. As the balance oscillates, the escapement only comes swiftly in contact with it periodically and only provides an impulse to the balance every two vibrations. As such, each oscillation of a detent escapement comprises of an active and a “dead” vibration. This contrasts with a conventional Swiss lever, which is a dual impulse escapement that provides impulse each vibration and entertains a longer contact with the balance.

The detent is constructed to allow the flat escape wheel to lock on a purpose-built jewel. During every complete oscillation, the locking stone is triggered by a dedicated discharge pallet on the balance, thus momentarily freeing the escape wheel. The escape wheel advances by one tooth spacing, coming in contact with the larger impulse pallet on the balance and restoring lost energy to the oscillating organ. The interaction between the balance and escape wheel is swift, with a lift angle of generally 36°, and happens close to the equilibrium point, leaving the balance to follow its supplementary arc with practically negligible disturbance. 

During the dead-beat vibration, the small pallet engages briefly with a passing spring, which is a slender, linear blade spring attached to the sprung detent. The minimal resistance of the passing spring lets the balance brush past it, leaving the detent unmoved.  

The Earnshaw chronometer escapement from ‘Britten’s Watch & Clock Makers Handbook, Dictionary, and Guide’ by F.J. Britten

The great advantage of the detent escapement lies in the way impulse is parted: the so-called tangential contact between escape wheel and pallet, which roll together akin to gears, means that there is little to no need for lubrication. In this regard, the chronometer escapement is vastly superior to the Swiss and English lever escapements, as they rely on a generous sliding action, thus require proper lubrication. A “dry” escapement is always preferable, as there is no risk of deterioration of lubricants affecting performance over time. 

Most classical of chronometer escapement constructions feature a spring-like detent, affixed to the mainplate. It works as a rudimentary compliant mechanism, arching when engaged to unlock the escape wheel then snapping back to its neutral position, locking on the next tooth. 

Another execution is the pivoted detent escapement, which as the name suggests features a pivoted lever linked to a spiral spring which brings it back in position after unlocking. This construction is considered more refined and complex, usually requiring a highly skilled watchmaker to make and set it.

The pivoted detent escapement as executed by Raúl Pagès

In wristwatches

The detent escapement was perfect for marine chronometers, which were not subject to sudden shocks and usually featured stable weighted balances and low frequencies, but not so much for wristwatches meant for daily wear. By construction, the detent escapement is susceptible to suffer from shocks or violent shakes, which can cause it to “trip”, or unlock prematurely, resulting in either timekeeping errors or the movement stopping altogether.

Safety measures to prevent that, the dart and roller system, are difficult to adapt in chronometer escapements and require heavy workarounds. The chronometer escapement also lacks a self-starting capability, meaning it often requires a gentle shake to start. All of these mean that detent escapements are rarely found in wristwatches, with only a handful of watchmakers having succeeded in implementing the escapement in a wrist-worn timepiece.

A prototype Urban Jürgensen with the P8 movement. Image – Phillips

Some of the notable attempts at making the detent escapement suitable for everyday wear in a wristwatch include the Urban Jürgensen P8 movement developed by Jean-Francois Mojon of Chronode and Kari Voutlainen. The UJ P8 calibre featured an updated take on the pivoted detent escapement, complete with a novel safety finger to overcome tripping issues. However, the production remained small due to the escapement’s finicky nature and complex construction, meaning that few  P8 examples were made.

A drawing of a traditional detent escapement (left), and the Raúl Pagès RP1 detent escapement

The detent escapement can also be found in some Christophe Claret movements that were similarly made in small numbers. Equally few in number but arguably more successful is the  Raúl Pagès Régulateur à détente RP1 that’s equipped with a pivoted detent escapement that’s been modified to be substantially shock resistant.