Style / World of Watches (WOW)

Force Majeure – Exploring the Constant Force Mechanism

The constant force mechanism experiences periodical resurgences in popularity, leading us to explore exactly why this fascinating device isn’t present in every brand that also boasts tourbillons, chronograph, perpetual calendars and repeaters.

Dec 13, 2019 | By Ashok Soman

A. Lange & Söhne Zeitwerk Decimal Strike Image by Ching/GreenPlasticSoldiers

Time is a mysterious force of nature. Notwithstanding this magazine’s obsession with the concept and the little machines that allow us to visualise and work with this “force,” spacetime is a real thing that we don’t have an intuitive sense about. Celestial and astronomical watches, clocks and orreries all address this directly, but these show us how time works as a measure of the motions of celestial bodies, including the earth. They don’t tell us about the fundamental forces at work in the heart of our watches.

When we think of amazing complications, the usual suspects come to mind, including those mentioned above. These are so pervasive that you will not need me to get further into it, so I will refrain. It is also helpful that most complications are easy to recognise – most chronographs, moon phase indicators, and perpetual calendar have easily identifiable features. Chiming watches are a notable exception to this, primarily because there’s no visual component; unlike other functions, there’s nothing to see here (a point H. Moser & Cie exploits to hilarious effect, as reported in issues #52 and #53).

All Zeitwerk models use constant force mechanisms, including this L043.8 calibre. You can spot it by looking for the blue hairspring.

When it comes to complications that do not have functions, such as tourbillons for example, the situation is, well, more complicated. Tourbillons, karussels and carrousels don’t need to be observed to work, basically. In fact, these mechanisms can be thought of as escapements, although Berner’s Illustrated Professional Dictionary of Horology advises against it, literally (“The tourbillon and karussel mechanisms are not escapements.”) For more details on this, see the first part of our story on the regulating organs of 21st century mechanical watches elsewhere in this issue. We also recommend the Dictionary of Horology, which is available as a free app for both iOS and Android.

The important thing to take away from this brief (I promise) but admittedly obtuse section is that we need to see watchmaking complications to appreciate them properly. The 21st century is an age without subtlety and nuance after all, where transparency is key to cutting through information overload. So, if a watchmaking brand says it has a novel way to improve the performance of its tickers, you expect to see it. Even when it comes to a rarity such as the constant force mechanism, visibility is key, as many of the watches shown here illustrate.

A Couple of Questions…

There are two big questions hovering over the constant force mechanism: what it is, and why is it so rare. In a world where tourbillons are uncommon, not rare, the second question is more difficult to address, so we’ll begin this story with the first part. Besides, knowing what we’re talking about would be useful.

Turning back to the Dictionary, it says “A constant force escapement is fitted with a device that always transmits the same amount of force to the escape wheel.” Believe it or not, there is a lot to unpack in that seemingly simple statement. First of all, the escape wheels of all mechanical movements require regularity and predictability. This poses a problem because no energy source is perfectly regular in all circumstances. In the case of the mechanical wristwatch movement, the issue comes down to the unwinding of the mainspring. When the mainspring is fully wound, it delivers a lot of power. As it winds down, this levels off until about two-thirds of the power reserve is used up. At that point, the force drops off sharply and comes to an end.

Zenith Defy El Primero Fusee Tourbillon

To do its job properly, the balance of a mechanical movement must be isochronous, which essentially means that oscillations must be uniform. “To convert that into practice – constant amplitude in equal periods – the balance needs nothing to change, especially not the energy it receives,” David Chokron writes for WatchAround. The implication here is that a watch will not run at the same rate for the duration of its power reserve.

There are three constant force mechanisms that have been implemented to address this problem, and this story will tackle each one in turn.

Never Break the Chain

One way to compensate for steadily decreasing power is to rachet up the torque. German watchmaker A. Lange & Sohne has probably done more over the last 20 years or so to build awareness of such constant force devices. It uses what’s known as the fusee-and-chain system across its entire range of “Pour le Merite” watches. When the barrel of a watch equipped with this system is fully wound, it pulls the chain off the top of the fusee cone. As the mainspring winds down, it takes the chain off the ever-increasing diameter of the cone, until it achieves maximum torque at the base. In this way, the fusee- and-chain system matches the diminishing power of the mainspring with the increasing torque of the fusee. The Dictionary defines the fusee like this: a “more or less conical part with a spiral groove on which is wound a chain or cord attached to the barrel.”

Fusee-and-chain assembly at A. Lange & Söhne; it is a painstaking manual process.

Basically a gear-shifting mechanism – akin to what you’d find in a typical bicycle – the fusee-and-chain system is actually quite old. The earliest evidence for the fusee (it acquired its features progressively) is a 1430 clock owned by Philippe the Good, Duke of Burgundy; this conflicts with other sources that report the fusee being the creation of none other than Leonardo da Vinci in 1540. Whatever the case, all sources agree that the fusee was soon to be connected to the barrel with catgut, which was replaced by 1660 with a metal chain. Obviously, catgut was far from ideal, and the metal chain that replaced it was reportedly not much better, at least initially. This speaks to the quality of the mainsprings in use at the time because all high-end timepieces dating to the 16th and 17th centuries are fusee-equipped.

Today, the chain is much improved, and the skill-level required to produce it is not inconsiderable; chain links are less than 0.5mm thick and any given chain might have hundreds of links. At the same time, the quality of mainsprings has improved tremendously, making the fusee-and-chain system largely irrelevant. There are niche uses for it, of course, including managing and controlling the power reserve to an extreme degree, which is not unheard of in haute horlogerie. Brands that have deployed the fusee-and-chain system include Romain Gauthier, Breguet and of course the aforementioned scions of Glashutte. For an example of a fusee-and-chain this year, look no further than Zenith, which we covered in issue #53. We shall reserve A. Lange & Sohne for our next section.

The Great Equaliser

Aside from having an amazing name (say it three times fast and see if you don’t love it), the remontoir d’egalite is also known as a constant force escapement. Positioned near the escape wheel, it consists of a blade spring or coiled spring with a lock and release mechanism; it stocks up power from the mainspring and uses it to maintain the balance at a constant amplitude. Christophe Claret explains that the higher the frequency at which the remontoir spring is re armed, the more constant the torque. This is evident in the brand’s Kantharos chronograph of 2013.

Interestingly, this device has an interesting history too, preceding the invention of the hairspring escapement (see aforementioned story on escapements this issue). Francois-Paul Journe notes that a 15th century watchmaker named Jost Burgi came up with the following idea: an extra mechanism representing an independent system that would be wound in spurts by the
mainspring. That sounds eerily like the contemporary description of a remontoir d’egalite. Journe adds that the constant force mechanism found its largest application in clocks intended for buildings in the 19th century. The idea here was to isolate the timekeeping mechanism from the hands outside – apparently the strong winds were found to actually interfere with the smooth running of the clock. This secondary function of the remontoir d’egalite echoes into the age of the mechanical wristwatch, as we will see.

FP Journe Tourbillon Souverain Vertical

Still, returning to the matter of pushing the frequency of the remontoir d’egalite, one can’t load up the frequency willy nilly. “It’s a question of the energy used to operate the remontoir,” Stephen Forsey (he of Greubel Forsey fame) told Chokron. “Every complex system produces unwanted side-effects.” The normal course of action seems to be to recharge the remontoir every second, which has the interesting side-effect of making the seconds hand jump rather than glide around the dial. This can be seen in the FP Journe Tourbillon Souverain Vertical, but is absent in this issue’s cover watch, the Jaeger-LeCoultre Master Grande Tradition Gyrotourbillon Westminster Perpetuel. The Le Sentier manufacture opted to have its remontoir recharge every minute, meaning the minute hand jumps (this watch does not have a seconds hand anyway).

Jaeger-LeCoultre explains that its remontoir d’egalite allows the motion of its minute hand to better match the chiming of its minute repeater mechanism. The manufacture also tells us that the remontoir helps to keep the massive energy of the mainspring from perturbing the multi-axis gyrotourbilon. This is another way that a constant force escapement can be useful. Several years ago, when A. Lange & Sohne introduced the Lange 31, it too deployed a remontoir d’egalite, largely to keep the raw power of its prodigious mainspring (the 31 in the name refers to the 31-day power reserve) from breaking the escapement. This year, the Glashutte firm deploys a constant force escapement in its Zeitwerk model, as it usually does. In this family of watches, the constant force escapement serves to instantly move the minute disc, recharging as it does every 60 seconds.

A. Lange & Söhne Lange 31

In any case, if you have been following along, you might be wondering if A. Lange & Sohne is the only watchmaking firm to deploy both the fusee-and-chain system and the remontoir d’egalite. As far as we can tell, the answer is yes, but not thus far in the same watch.

All Things Being Equal

Finally, there is a system that supplies precisely the right amount of energy to the balance at every impulse, rather than every second, or minute. This is a truly constant escapement and to date, there is only one solution available on the market. This part of the story serves as the cap for the hairsprings feature this issue, even though it doesn’t close out this story. It is actually a fabled sort of escapement, like the natural escapement, but one brand has made it a reality, and another seems to have made something like it, at the very least, but one that demonstrates an entirely different approach to horology.

Girard-Perregaux Echappement Constant watch Photographed by Adison Rutsameeronchai

First up is the Girard-Perregaux Echappement Constant, which was introduced in 2009 at the SIHH, and then presented in working form at BaselWorld 2013. At that time, this escapement was the most advanced and innovative solution. As far as constant force mechanisms go, it still is. Basically, what happens here is that two torsion blades within a large butterfly-shaped component in silicon provide resistance to the force delivered through the going train. When the force reaches a peak, both blades bend for a fraction of a second – less than the blink of the eye – to allow the escape wheel to advance. In this way, energy and the rate are both kept constant at each impulse.

Zenith Defy Inventor

The example of Girard-Perregaux reminds us that there are non-traditional escapements that handle the issue of isochronism and amplitude in imaginative ways. In recent years, the Zenith Oscillator is definitely the most inventive regulator in mechanical watchmaking. The 18 Hz of this escapement is the fastest in serial production, hence it is also the most power-hungry; the next fastest-beat in series production is the 10 Hz Breguet calibre 589 F. At this sort of dizzying speed, mechanical wear-and-tear is a major concern but Zenith reports that its new oscillator boasts incredible tribological properties. By extension, it is also clear that the mass of the escapement is greatly reduced thanks to the new materials and structures of this new oscillator. This could mean that Zenith does not require any sort of constant force mechanism to manage the energy flow. Indeed, the manufacture informs us that its escapement is not a constant force device of any sort.

Simpler Solutions

Contemporary watches do have other means of keeping the flow of power constant (though crucially, not to keep the power of a muscular mainspring in check). Indeed, the simplest answer to keeping the torque line flat is to keep any given watch at full wind. With an automatic watch, this is certainly possible, as Richard Mille demonstrates in an extremely low-key fashion with what might be the watch trade’s most audaciously complicated automatic winding system.

The firm uses winding rotors equipped with variable inertia vanes to manage the energy generated by the motion of the wearer. Some models have an automatic clutch that disengages the rotor for x-amount of hours once the power reserve reaches a fixed level. In so doing, the movement avoids peak power, staying in its average range. This has the entirely intuitive effect of keeping the aforementioned torque line relatively flat. In modern and contemporary movements, this is good enough to replicate the effects of a constant force mechanism, without adding more complexity, friction and mass to the heart of the movement.

Given that such a simple and elegant solution is available, it is even more evident why constant force mechanisms are so rare. Observant readers will have noted though that the prominent examples listed in this article are largely manual-winding… Not for nothing, the function of the remontoir d’egalite to protect the escapement from very powerful mainsprings would still be useful if the watches in question were automatics. In a nutshell though, all this serves to explain why constant force mechanisms are pretty rare, and are only deployed in very specific circumstances. In a world where even chiming functions are accessible to all manner of brands, yet claimed to be rare treasures, the actual scarcity of constant force mechanisms is worthy of consideration.

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