Derek Pratt Derek Pratt by Luca Soprana, Remontoir d’Egalité, Stainless Steel

£140,000
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This is one of seven examples of the collaboration between Luca Soprana’s Atelier 7h38 and the UAE-based Perpétuel Gallery. At its heart is the calibre DP07 which features Derek Pratt’s Reuleaux triangle remontoire*. It’s 41.6mm stainless steel case holds within it a pleasing salmon dial with eastern Arabic numerals for hour markers.

The Reuleaux triangle

In 1981, Derek Pratt set to work on solving a problem that Breguet was not able to remedy – providing constant force to a tourbillon. As Andrew Crisford an expert on British watchmaking and Breguet, points out in his memorial in the British Horological Institute book, Breguet had attempted this in a couple of watches in which he had engraved “Echappement à force-constanté”, yet there is only one of these watches surviving, and it has since had its escapement replaced “due to the unsuitability of Breguet’s constant-force escapement for use in watches”.

Read DEREK PRATT: THE FORGOTTEN WATCHMAKER | A Collected Man Journal
Read DEREK PRATT: THE FORGOTTEN WATCHMAKER | A Collected Man Journal

Pratt’s solution was to incorporate a remontoire inside the tourbillon. This remontoire is what drives the tourbillon and is directly driven by the gear train. This means that the escape pinion that normally drives the tourbillon around the fixed fourth gear is replaced by the pinion attached to the remontoire. This one-second remontoire is attached to the escape wheel via the remontoire spring, which, as the name suggests, winds and releases every second, providing an impulse to the escape wheel.

The component of this construction that stands out more than most is the Reuleaux triangle. This curved equilateral triangle acts as a cam here, controlling the movement of the pallets that stop the remontoire wheel. Attributed to 19th-century German mechanical engineer Franz Reuleaux, the Reuleaux triangle is part of the Reuleaux polygons which all share the unique quality of having a constant diameter. This allows the tines of the fork in which it sits to remain in constant contact with the cam as it rotates and shifts the pallets with each rotation.