The detent escapement: from marine chronometers to wristwatches

An escapement is a device in mechanical watches that transforms the energy from the unwinding of a coiled spring (the power source) into countable impulses.

In particular, each swing of the balance wheel releases a tooth of the escapement's escape wheel gear, allowing the clock's gear train to advance or "escape" by a fixed amount. This regular periodic advancement moves the clock's hands forward at a steady rate.

Without the escapement, the spring would unwind in a few seconds making the wheel turn too fast, just like a coil spring that, compressed between two fingers, is released suddenly.

The most popular escapement is the so-called in-line or Swiss lever escapement. Invented by British watchmaker Thomas Mudge (1715-1794) in 1750, it has been used in the vast majority of watches since the 19th century.

This type of escapement offers two main advantages: 1) the balance wheel is only in contact with the lever during the short impulse period (detached escapement) so reducing friction and improving accuracy, and 2) it is a self-starting escapement, so if the balance wheel stops, a delicate twist will be sufficient to restart it.

The rotation of the escape wheel is controlled by the pallets. The escape wheel has specially shaped teeth which interact with the two jewels called the entrance and exit pallets.

These pallets are attached solidly to the lever, which has at its end a fork to receive the ruby impulse pin of the balance roller which is fixed to the balance wheel shaft.

Animation of a Swiss lever escapement - Courtesy of Mark Headrick and abbeyclock.com

But there is an escapement that can guarantee a much higher precision.

This is the detent escapement. Originally invented by Pierre Le Roy (1717–1785) in 1748, in the following years it was modified and further developed by other watchmakers.

In the late 18th century British watchmakers John Arnold (1736-1799) and Thomas Earnshaw (1749-1829) developed their own versions of detent escapements.

Earnshaw's design was the simplest possible and demonstrated to be the most efficient and reliable establishing itself as the reference and most deployed detent escapement.

Like the lever escapement, the detent escapement is a detached escapement, allowing the balance wheel to swing undisturbed during most of its cycle, except for the brief impulse period. But while the lever escapement gives two impulses per cycle, the detent escapement gives just one impulse per cycle (every other swing) interfering with the balance as little as possible.

Animation of a spring detent escapement (or chronometer escapement) - Courtesy of Mark Headrick and abbeyclock.com

As shown in the picture below, the detent escapement uses a very thin blade spring with a locking pallet to hold the escape wheel in place.

When the impulse roller rotates counter clockwise, the discharging pallet lift up the spring while the impulse pallet contact the escape wheel allowing it to move forward.

The escape wheel is then locked again as the blade spring falls back into place.

Drawing from “The Marine Chronometer, its history and development” by Rupert Gould  (London, 1923)

Since the driving escape wheel tooth moves almost parallel to the pallet and in one direction only, the escapement has little friction and need no oiling.

For these reasons, the detent demonstrated to be the most accurate escapement for balance wheel timepieces keeping time to within 1 or 2 seconds per day, an accuracy which approaches that of a modern quartz watch.

Nonetheless, the detent also had a few important weaknesses.

First of all, it could be tripped by a shock, allowing the escape wheel to unlock when it shouldn't.

Moreover, it was not self-starting and, in some cases, the balance wheel could swing more than 360 degrees, causing a second impulse to throw the timing off.

For these reasons the detent was mostly used in marine chronometers where its vulnerabilities could be solved by mounting these timepieces on gimbals in wooden square boxes with the dials facing upwards to protect them from shocks.

A marine chronometer produced by Thomas Earnshaw and featuring a spring-detent escapement, circa 1800 - Courtesy of the National Maritime Museum, London

These limitations were a small price to pay compared to the exceptional precision that such timepieces could supply.

In fact, the detent escapement became the universal standard for marine chronometers and were used on ships for almost two centuries before being replaced by electronic clocks in the 1970s.

For centuries, the detent escapement could not be used for wearable timepieces, except for few rare high-precision pocket watches. The continuous changes of position, the exposure to shocks and to thermal changes appeared to be insurmountable issues that could not be solved in a wristwatch.

The smaller size of wearable timepieces represented another important issue.

It actually took more than two centuries before a detent escapement could be successfully implemented in a wristwatch.

The challenge was won at the beginning of the 21st century by Urban Jürgensen & Sønner, the Danish brand which, during the 18th and 19th centuries, created numerous chronometers of this type for the Danish Navy becoming the symbol of fine watchmaking in Denmark.

Urban Jorgensen marine chronometer with pivoted detent escapement and gimbal suspension in a mahogany box, circa 1848 - Courtesy of the Designmuseum Copenhagen, Denmark

Established in 1773, the brand founded by Jurgen Jurgensen and then developed by his son Urban innovated watchmaking become renowned for the exceptional precision of its timepieces.

Unfortunately, after the Great Depression of the 1930s the brand went through several changes of ownership which limited long-term planning and strategy definition.

Things changed in 1979 when Peter Baumberger, a passionate watchmaker and collector, started the acquisition of Urban Jürgensen & Sonner for the production of limited series of complicated wristwatches and pocket watches with a small but highly skilled team of master watchmakers and multi-awarded Derek Pratt, one of the most respected watchmakers of his generation, as technical director.

Baumberger’s main goal was very ambitious: the development of the world's first wristwatch for series production to feature a detent escapement within it.

At the beginning of the new century, plans were defined. The development was a co-operation between several competent people. Derek Pratt proposed a solution to make a pivoted detent escapement usable in a wristwatch and he constructed a prototype.

To avoid the risk that the escape wheel could be tripped in case of shocks, Pratt designed a special detent with a perfectly balanced counterweight to the pallet jewel at the other end of the lever and a safety roller limiting the movement of the detent in case of strong impacts.

3D drawings of the detent escapement in the UJS-P8 movement. The upper drawing is the top view.  In the lower drawing the escapement is viewed from the bottom. Courtesy from the book “The Jurgensen Dynasty, four centuries of watchmaking in two countries” by John M. R. Knudsen

The results were very promising and Jean-François Mojon, a well-known movement specialist, refined some of these ideas further to make it suitable for series production optimizing all components.

In 2008 Peter Baumberger and Jean-François Mojon were granted a world-wide patent for their pivoted detent escapement.

Unfortunately, Derek Pratt died in 2009 and Mr. Baumberger in 2010 so they could not see the new calibre on the market but for sure they had realized that the challenge was won.

Finnish born Kari Voutilainen, another master horologist, assisted in developing UJS-P8 prototypes and was responsible for assembling, finishing and regulating the movements.

In 2012, Urban Jorgensen was finally ready to present the new in-house calibre, the automatic UJS-P8 which successfully passed the Chronofiable test - one of the toughest standards in the Swiss Horological Industry - certifying not only its precision but also its reliability. During the test the movement was subjected to more than 20,000 shocks in two weeks. In terms of pure chronometry, the precision of the watch was of the order of −1/+1 seconds per day.

The Urban Jurgensen P8 Chronometer Reference 11C, with its typical hand engine-turned silver dial and teardrop lugs, was the first model to house and successfully implement a detent escapement.

A clue to the presence of a special escapement is provided by the seconds hand which moves three times per second even if the frequency of the movement is 3 Hz or 21,600 vibration per hour (at this frequency with a standard Swiss lever escapement the second hand moves six times per second).

In 2014 the Urban Jürgensen 1142C CS chronometer model featuring the patented detent escapement was awarded as the best watch in the 'Men’s Watches' category at the Grand Prix d’Horlogerie de Genêve.

Developing a wristwatch with a detent escapement remains one of the most difficult challenge that only few watchmakers have successfully mastered but, with its UJS-P8 calibre, Urban Jürgensen paved the way for its future evolution.

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Time and Watches | The watch blog: The detent escapement: from marine chronometers to wristwatches
The detent escapement: from marine chronometers to wristwatches
The detent escapement: from marine chronometers to wristwatches. An escapement is a device in mechanical watches that transforms the energy from the unwinding of a coiled spring (the power source) into countable impulses. There is an escapement that can guarantee a much higher precision than the Swiss lever escapement. This is the detent escapement also known as the chronometer escapement. Originally invented by Pierre Le Roy (1717–1785) in 1748, in the following years it was modified and further developed by other watchmakers. In the late 18th century British watchmakers John Arnold (1736-1799) and Thomas Earnshaw (1749-1829) developed their own versions of detent escapements. It actually took more than two centuries before a detent escapement could be successfully implemented in a wristwatch. The challenge was won at the beginning of the 21st century by Urban Jürgensen & Sinner. Urban Jorgensen. The Urban Jurgensen P8 Chronometer Reference 11C was the first model to house and successfully implement a detent escapement.
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