Have you heard? The tourbillon is old news. The new kid on the block is the constant force escapement, that Holy Grail of watchmakers everywhere. The arcane mechanism is certainly not as sexy as Breguet’s “whirlwind” nor is its name, which makes up for its lack of poetry with its descriptiveness. But it may do more for a watch’s accuracy than the vaunted tourbillon, which was designed to combat the effects of gravity on a vertically-oriented pocketwatch.
Last week, Girard-Perregaux invited retailers and press to Zurich for the unveiling of its own constant force escapement, or, as G-P calls it, the “Constant Escapement” in a nod not only to the mechanism itself but to the company’s founder, Constant Girard. And while it remains to be seen whether this latest bit of horological wizardry lives up to the fanfare and proves to be more than a passing fancy, it has created quite a buzz in our little world of watch-watching.
Girard-Perregaux Constant Escapement: Movement under construction
A constant force escapement is designed to flatten out the amplitude curve of an escapement as the mainspring unwinds by providing a–big surprise–constant force to the balance as it oscillates. Standard Swiss lever escapements work fine when a mainspring is fully wound and delivering consistent torque to the balance. But as the mainspring unwinds, its torque delivery diminishes, causing the amplitude of the balance wheel to drop off over the duration of the power reserve. What is needed then, is an extra little “kick” to keep the balance wheel swinging the same amount each and every time, even as the mainspring loses power. How to do this?
The problem of getting unvarying energy from a coiled spring is as old as watchmaking itself. By the 15th century the fuseé and stackfreed (another constant force device which appears to have been invented in Germany) were in use. Abraham Louis Breguet patented a constant force escapement in 1798, while the English watchmaker Charles Haley was granted a patent in 1796.
A constant force escapement can be distinguished from another constant force deviceas well, which is the remontoire d’egalité. The latter consists of a spiral spring on the axis of one of the wheels in the going train, which is periodically re-armed by the mainspring. The first spring-powered remontoire d’egalité was developed by John Harrison, and used in his H4 marine chronometer; it is rewound by the mainspring once every 7 1/2 minutes. The constant force escapement, on the other hand, delivers impulse directly to the balance, and is not part of the going train. It is the habit of some today to refer to a remontoire d’egaliteé as a “constant force escapement” but though related, the constant force escapement and remontoire d’egalité are distinctly different devices –albeit with an identical goal.
Dr. George Daniels wrote about the constant force escapement on several occasions, and offered this definition in his Watchmaking:
“In the constant force escapement, the impulse to the oscillator is delivered by a separate, small detent, either spring or pivoted with spring. After each impulse to the balance the detent releases the escape wheel which re-sets it on its catch ready for the next impulse.”
Daniels seems to have been of several minds over the constant force escapement over the years. In Watchmaking he goes on to say “. . . such escapements do not fulfill their promise, and their rate of timekeeping is not better than a conventional detent escapement.” On the other hand, in The Art of Breguet, he writes, “Mechanisms of this type . . . are extremely delicate and difficult to adjust (but) once properly set up, they are very reliable and keep close . . . time . . . when properly adjusted and cleaned these escapements are capable of a performance superior to any other balance wheel timekeeper.”
Despite the theoretical superiority of the constant force escapement it seems to have been used seldom if at all for over a century after Mudge, Haley, and Breguet’s experiments. They were, for all their advantages, far too complex and difficult to adjust to be practical to make in any numbers, and seemed destined to remain a footnote in horological history –albeit a fascinating one.
And then, one day in 1996, a watchmaker working at Rolex named Nicolas Dehon had an idea.
As G-P R&D engineer, Stéphane Oes, explains it, Dehon was commuting by train when he had an epiphany while absentmindedly flicking his ticket between his fingers. A blade spring, kept slightly flexed between two fixed points, could like his train ticket be made to snap back and forth, delivering a predictable pulse of energy each time. If the bit of cardboard between his fingers could be transformed into a source of energy for the balance, it might be the answer to the constant force riddle. The process of development began, and though Dehon and Rolex were able to cobble together a prototype, and even took out a patent for this constant force escapement (granted in 1999) its design was still slightly ahead of its time. The materials needed to build a reliable and effective version were not available.
By 2008, Dehon had taken his idea to Girard-Perregaux who enthusiastically embraced it and, under the leadership of Stéphane Oes, finally saw the Constant Escapement through to fruition. The key to its success was the introduction of silicon, that hard, fatigue-resistant material used by more and more watch companies, mainly for balance springs. The 1999 prototype used a two-piece metal alloy design, which was its fatal flaw. The 2008 G-P version used a one-piece frame, lever and blade spring made from silicon by means of photolithography. Dehon remained at G-P long enough to see his inspiration tick to life and Oes then spent the next four years with his team refining it and making it fit into a watch. That watch is the Constant Escapement.
To understand how it works, let’s look at Girard-Perregaux’s animation of the escapement in action.
Though it may seem confusing at first, the basic principle, is simple. As the balance swings, it passes through the a tiny fork which is attached to the impulse lever. In doing so it causes the S-shaped blade spring to “snap” from one S-curve to its opposite. As it does so, it delivers energy to the balance. Once the balance has cleared the impulse lever, one of the curved faces of one of the two escape wheels makes contact with a jewel on the impulse lever, and the curve begins to push the S-curve back towards the previous shape –but before it can actually snap back, the lever is arrested, as one of its locking jewels makes contact with one of the escape wheel teeth. The blade spring is now armed and ready to snap to its opposite configuration as the impulse jewel trips the lever.
The Constant (Force) Escapement
While in an ordinary watch, the escape wheel passes energy directly to the balance via the lever, in the Constant Escapement the purpose of the escape wheels is to put the S-shaped blade spring in a configuration where it requires the least amount of energy for the balance to unlock it, while still delivering enough energy to impulse the balance. It’s moving the blade spring from its stable shape into a quasi-stable shape that requires energy from the going train and “arms” the blade spring. Both escape wheels –which rotate in opposite directions –are driven by the fifth wheel of the going train. The escape wheels and lever are pure nickel; the fork of the impulse lever is silicon, and of a single piece with the blade spring, its butterfly shaped mounting, and the semi-circular banking for the impulse lever. Impulse is given in both directions –unlike Breguet’s design, in which impulse is given, as is also true of the chronometer detent escapement, in one direction only.
Escape wheel, showing the sliding inclines for re-arming the blade spring, ending in locking teeth
Housed in a 48x14mm white gold case, the Constant Escapement is a gorgeous machine. Of course the escapement take center stage, occupying the entire lower half of the open dial. Its blue-hued silicon frame and dual escape wheels ticking at 3 Hz are mesmerizing, unlike anything anyone has seen before in a watch. The top half of the dial is comprised of a simple hour and minute counter while a full-sized red-tipped seconds hand sweeps across it all. A small linear power reserve sits unobstrusively to the left side, counting down the seven days of power in the two dual-spring barrels. This extraordinary power reserve is relevant to this watch as a showcase of how the escapement will maintain its flat power curve over the entire duration, which is also why G-P chose to fit this escapement in a handwound timepiece, which won’t be continually wound by wrist movement.
In a lovely homage to Girard Perregaux’s history, the bridges for the Constant Escapement watch are the double-arrowhead shape familiar from the Three Golden Bridges tourbillon.
Early reports on the Constant Escapement’s performance show an impressive two-second daily variability across the watch’s power reserve with consistent balance amplitudes that hover between 220-225 degrees. G-P is proud of this watch, and rightfully so. In an industry so bound by tradition but so obsessed with new ways of achieving better accuracy, the venerable company from La Chaux-de-Fonds has broken new ground. G-P will produce 10 of these watches in time for BaselWorld, each of which will sell for around $100,000 according to G-P CEO, Michele Sofisti.
Silicon monobloc blade spring and mount, with integrated impulse lever and banking
The construction of a true constant force escapement, which runs reliably, is robust enough for a portable timepiece, and can be produced in series, is a goal that’s eluded horologists for centuries. It’s no exaggeration to say that in bringing Nicolas Dehon’s conception to a finalized, functional realization, Girar- Perregaux has made watchmaking history.
The Girard-Perregaux Constant Escapement is offered in a 46mm white gold case. Movement: manual-wind Girard-Perregaux calibre MVT-009 100-0007, seven day power reserve, with Girard-Perregaux “Constant Escapement” in silicon and pure nickel; 21,600 vph (3 hertz.) 30 meters water resistant. The Constant Escapement watch will be offered in Girard-Perregaux’s Haute Horlogerie collection (of course!) For final pricing and availability, contact Girard-Perregaux.