Technical: Constant Force Escapement
Imagine you're pushing a child on a swing for an indeterminate period of time. After a while, you become tired and the amount of force you can exert weakens. Each swing reduces in height and speed, until eventually the child comes to a complete stop. This is a problem that has confounded watchmakers for centuries: no, not crying children who want to go higher, but the application of constant force. So how exactly do you program a complex mechanical device to provide the same amount of power time after time, beat after beat? Girard-Perregaux has the answer.
The root of the constant force problem lies in every mechanical movement's use of a mainspring for its power source. Springs by their very nature provide inconstant force; as they gradually unwind, their power dwindles, or more specifically the torque, the twisting force they impart. While improvements in materials and manufacturing have minimised the impact of the problem, the fact remains that springs are a mechanical movement’s achilles heel. Fusée and chain watches (which we’ve featured in TWM before) have been the best countermeasure to this problem for a long time, but there’s never really been a proper solution.
Until now, that is. The story goes that a young Rolex watchmaker, Nicolas Déhon, was fiddling with a train ticket while waiting at a station one day. He noticed, as he pressed the ticket between his thumb and forefinger, that it bent into a C-shape that could be popped back and forth with a small but consistent application of pressure. He realised this could be a way of delivering discrete units of uniform power, constantly. Déhon set about designing an escapement based on this idea, but soon came to realise that the necessary precision and materials didn't yet exist to build the components he required.
Years later, when Déhon was working at Girard-Perregaux, the brand began experimentation with silicon technology. Silicon has a very high elasticity limit and does not fatigue like other materials; more importantly, it can be manufactured to very small tolerances. Finally, the missing piece in the puzzle had arrived and Déhon was able to realise his design. The fact that the name of Girard-Perregaux's founder is Constant Girard makes it seem like destiny.
The genius of Déhon's design is that it works within the escapement; previous attempts, using remontoires—a secondary, smaller spring or weight—had been located outside of it and were not as effective. At the centre of this new mechanism—dubbed the ‘butterfly’ because of its organic shape—floats a single strand of impossibly thin silicon called the ‘blade’. At fourteen microns thick, it's a sixth of a human hair. The workings of the system share the same principle as Déhon’s train ticket: the silicon blade, deformed into an S-shape, requires exactly the same amount of force each time to pop it into its reverse form. So as the mainspring winds down and its torque reduces, the blade acts as a buffer, storing the energy provided by the mainspring until—pop!—there’s enough to deform it, nudging the escapement along another beat. The idea is simple and elegant, just like the material used to make it all happen.
After showing a prototype of the design in 2008, Girard-Perregaux spent five years in development before finally unveiling the Constant Force watch. The revolutionary escapement system sits in the lower half of the dial, while the double-barrelled mainspring, made for a week-long power reserve to highlight the efficiency of the new design, sits beneath the hours and minutes display. The watch represents an achievement of historic proportions: the first to truly eliminate the hitherto inherent irregularities of spring-powered mechanical movements. It’s no wonder it was awarded the Aiguille D’Or, the highest prize in watchmaking.
