21 aug. 2019
 10 minutes

Technical Guide: Constant Force Mechanisms in Watchmaking

By Tim Breining
CAM-666-Constant-force_2_1

Technical Guide: Constant Force Mechanisms in Watchmaking

It’s no secret that mechanical watches can’t hold a flame to their quartz counterparts when it comes to accuracy, but that doesn’t detract from the fascinating technological complexity and finely finished components of a mechanical timepiece.

Nevertheless, many manufacturers are committed to building the most precise mechanical watch possible to reinforce their watchmaking prowess. There are a number of complications that boost a watch’s level of precision. Watch fans and collectors will have heard of the more common complications, but others very seldom appear in haute horlogerie. Constant force mechanisms fall into the latter category.

An Exotic Complication

Whether constant force mechanisms can truly be considered complications is a matter of opinion. True, they don’t give the watch any added functionality, but the same can be said of the tourbillon. They do, however, occupy a niche market compared to tourbillons and other major complications. This is in large part due to the fact that prices for the most affordable tourbillons pale in comparison to those of new watches with a constant force mechanism. Moreover, the way a constant force mechanism works isn’t exactly straightforward. The use of constant force mechanisms has, thus, generally been isolated to small high-end manufacturers who are well supported by a group of passionate collectors. Let’s take a closer look at this rare complication.

 

Ferdinand Berthoud Chronomètre 1.2-1
Ferdinand Berthoud Chronomètre 1.2-1Image: Bert Buijsrogge

What does constant force mean?

Mechanical watches are powered by a mainspring. If you’ve ever wound a watch by hand, you’ve likely noticed that the resistance increases as you wind, which is an indication that the spring doesn’t always deliver an even amount of force to the movement. When the watch is fully wound, the force exerted is at its highest level, and it gradually decreases as the watch runs. Thus, the energy transferred to the escape wheel, which drives each alternation, is also not uniform in nature.

This “disrupts” the balance wheel because its frequency varies depending on the deflection angle and the amount of force exerted, meaning the watch runs at different speeds depending on the degree to which it is wound. That being the case, how could you ever determine a reliable level of accuracy for a watch? It’s simple: Manufacturers typically list a watch’s level of precision when it’s fully wound. Strictly speaking, every watch varies in its accuracy over time, unless it’s a constant force watch, of course.

An important term to know when speaking about mechanical watches is isochronism. A watch, or its escapement, is considered isochronous when its frequency is independent of the balance wheel’s amplitude. Dutch scholar Christiaan Hyugens proved the isochronous ability of the balance wheel and balance spring on paper in the mid-17th century. However, theory rarely aligns with reality. Other factors like friction, temperature fluctuations, air pressure, magnetism, and more all contribute to watches being far from isochronous.

Zenith Academy Georges Favre-Jacot
Zenith Academy Georges Favre-JacotImage: Bert Buijsrogge

It’s, therefore, easy to understand why the constant force mechanism came to be. If it’s impossible to make the frequency of the balance wheel independent of the force exerted, then why not make the force constant?!

Before we dive into fascinating constant force designs by contemporary brands like A. Lange & Söhne, Romain Gauthier, Grönefeld, and Oscillon, let’s look at the techniques employed by historical watchmakers.

The History of Constant Force in Watchmaking

Before the balance spring became standard, the foliot was the regulator of choice. This horizontal bar has moveable weights at each end. In the absence of a balance spring, pocket watches were incredibly sensitive to fluctuations in force; they were nearly unusable without a constant force mechanism.

At the time, the most common solution was a fusee, a cone-shaped pulley system with a small chain that was connected to the barrel. As the watch ran, the chain slowly unwound. The cone shape ensured that the varying axis of rotation compensated for the changing force coming from the mainspring, keeping the drive torque constant. Thus, when the watch was fully wound, the maximum amount of force was paired with the smallest fusee radius, and near the end of the mainspring’s power, there was less force paired with the maximum turning moment. By carefully designing the fusee, watchmakers could guarantee a near-constant drive force whenever the watch was running.

Version 1: Fusee in Wristwatches

Fitting a fusee into a petite pocket watch is no small task, but modifying and shrinking the system so it fits inside a wristwatch is a completely different story.

To get a better sense of what this entails, you should know that a simple watch movement can have as many as 50 components, though most have over 100. The chain in a fusee system can have over 600 individual pieces alone! Thus, it should come as no surprise that the assembly time required is significant to say the least.

As if that wasn’t enough, to protect the delicate chain, you have to decouple it from the movement when the watch is fully wound. Otherwise, a strong twist of the crown could break the chain. The useable region of the mainspring must also limited. This range guarantees near-constant force. However, the supply of force drops more rapidly when the spring is close to fully wound or close to fully relaxed. It is, therefore, necessary to limit the mainspring to some degree.

Zenith Constant Force
Image: Bert Buijsrogge

If the watch is close to running out of power, an additional mechanism stops it to avoid using the more “extreme” regions of the mainspring. This limits the dimensions of the fusee, which would otherwise need to compensate for a much larger area, but it also has some disadvantages. Limiting the range of the mainspring reduces the watch’s power reserve. The A. Lange & Söhne Richard Lange “Pour Le Mérite”, for example, only has a power reserve of 36 hours, and the Zenith Defy Fusee Tourbillon has a 50-hour power reserve.

The real challenge, however, resides elsewhere: In conventional mechanical watches, the continuously rotating barrel ensures that the power supply to the movement is not interrupted, even when the watch is being wound. This is not the case with a fusee because the chain needs to be wound on and off the barrel. This means the barrel’s direction of rotation changes. This problem is typically resolved by relying on a planetary winding mechanism, which works using the principle of differential gears, ensuring a seamless power supply.

Fusee 2.0

The Logical One watch by the young manufacturer Romain Gauthier combines a number of innovations that make the historical fusee fit for the 21st century. It is the perfect example of a modern constant force watch and one of my personal favorites to date.

Gauthier, an engineer by trade, reexamined the traditional mechanism, not from a place of watchmaking nostalgia but from a rational, technical perspective. He identified the weakness of many historical fusee systems as the chain. Most models alternated single and double links so that the chain would be thin enough to coil around the cone-shaped pulley. The single links, however, were noticeably weaker and the chain had a tendency to rub against the barrel when it was winding.

With these issues in mind, Romain Gauthier came up with a design inspired by modern chains, such as those used in bicycles. The chain features alternating pairs of inner and outer links, thus avoiding any weak spots. To reduce friction, Gauthier fixed small rollers made of synthetic rubies to each chain pin. The attachment to the pulley was also revised in order to distribute the burden of contact across several links at once.

Romain Gauthier Logical One
Romain Gauthier Logical OneImage: Romain Gauthier

Rather than using a traditional vertical cone, the Logical One relies on a two-dimensional disc cam. The 360° snail-shaped disc performs the same function as the fusee but prevents the chain from rubbing against the barrel since it no longer has to slip as it rotates. Moreover, there are extra gears between the disc cam and barrel, as well as the disc cam and pointer mechanism, that take advantage of the full rotation of the barrel, thus maximizing the power reserve. The chain has a total of 136 components, which is four to five times less than any competitor. Romain Gauthier isn’t concerned with the number of movement components or blind watchmaking nostalgia. He builds systems that make sense; hence the name Logical One.

Due to the complexity of the entire mechanism, this watch isn’t offered with a traditional hand-winding system. The Logical One gets its energy from a patented winding push-piece that winds the watch by a set amount each time the button is pressed.

The Logical One's robust chain
The Logical One’s robust chainImage: Romain Gauthier

 

Version 2: The Remontoire

A remontoire, from the French “remonter” meaning “to wind,” follows an entirely different concept than the fusee. Movements that feature a remontoire do not have a continuous flow of energy from the barrel to the escapement. Instead, the escapement is powered by an intermediate spring that looks quite similar to the balance spring. The spring is repeatedly rewound so it continuously drives the escapement.

Energy passes from the barrel to this intermediate spring in defined intervals, re-tensing the spring by the exact amount it relaxes over the interval. The force thus arrives at the escapement in an even, saw-toothed pattern over time. The drive force sinks ever so slightly during an interval but returns to the original level once the spring has been wound. This process repeats itself again and again until the watch runs out of power. The drive force received by the escapement remains constant on average across the entire run time. When the barrel no longer has enough torque to wind the spring, the watch stops running.

The intervals between re-tightenings can last anywhere from a second to a minute. If the former is the case, the complication can also function as a “jumping seconds” assuming the second hand is attached accordingly.

F.P. Journe Tourbillon Souverain
This Tourbillon Souverain by F.P. Journe features a tourbillon alongside a remontoire and jumping seconds.

An excellent example of a remontoire is the F.P. Journe Tourbillon Souverain Remontoir d’Égalité. This constant force mechanism includes a jumping seconds, also known as a dead seconds. F.P. Journe also adds a tourbillon to the movement, which likewise benefits from the constant force system.

The 1941 Remontoire by Grönefeld features an eight-second remontoire, while A. Lange & Söhne’s Lange 31 boasts a particularly long one-minute interval and unbelievable 31-day power reserve. The latter is delivered with a separate key that winds the sizeable barrel and protects the spring from accidental over-winding.

While the fusee is certainly a treat to watch in action, remontoires are typically only marked by a subtle imprint on the watch’s dial. As a side note, it was watchmaker George Daniels’ preferred constant force solution. He was fully aware of the complexity and costs involved and commented that the mechanism was “quite unnecessary” and, therefore, a charming addition for collectors.

Version 3: Constant Force Escapement

The third category of constant force mechanisms is a bit more difficult to summarize. Strictly speaking, remontoires and constant force escapements work in the same way. The main difference is that in the latter, the function is completely integrated into the escapement.

A rare example of this is the Girard Perregaux Constant Escapement. This movement passes its force through a 14-micrometer blade spring within a complex silicon component. It’s best to watch an animation or video of this movement at work to get a sense of how it all works, but the most important thing to note is that it also has an interruption of power between the barrel and escapement. However, the winding of the silicon blade spring, which drives the rotor and thus the balance wheel, takes place with each alternation of the balance wheel. In the case of the Girard Perregaux Constant, which beats at a frequency of 3 Hz, this happens six times per second.

Girard Perregaux Constant
Girard Perregaux ConstantImage: Bert Buijsrogge

The Ulysse Nardin Ulysse Anchor Escapement features a tourbillon and constant force escapement that follows a similar concept, including the use of a silicon blade spring. You’d expect nothing less from the brand that pioneered the use of silicon in watchmaking.

Version 4: Constant Force Spring

The final watch that deserves mentioning resolves the constant force issue in an apparently simple way, but that doesn’t make it any less impressive. We’re talking about the L’instant de vérité watch from the boutique Swiss manufacturer Oscillon. “Watchmaking” and “handmade” are terms that are used quite liberally these days, but they ring 100% true when it comes to Oscillon. This brand makes their watches entirely by hand with manually-operated tools and without the assistance of CNC machines. The masterminds behind this brand are watchmakers Cyrano Devanthey and Dominique Buser.

Oscillon l’instant de vérité
Oscillon l’instant de véritéImage: Oscillon

The subdial on the L’instant de vérité reads “Constant Torque Spring,” a reference to the constant force spring within. The mechanism has two drums around which a spring is wound in opposite directions, resulting in a near-constant output of force. The system makes the dream of supplying constant force to the barrel a reality, but it isn’t a new invention. You can find this type of spring in everyday items like vacuum cleaners and seat belts. In these cases, the constant force spring is used to ensure even force when an object is being wound. Oscillon’s version isn’t quite so simple. Similar to the way a fusee works, this watch relies on differential gears to prevent any interruption when the watch is being wound.

A constant force spring and its two drums.
A constant force spring and its two drums.Bild: Oscillon

As you can see, there is no single way to address the issue of constant force in mechanical movements. You can take a more traditional approach, as taken by A. Lange & Söhne and Zenith, or reinterpret a historical method like Romain Gauthier. Girard Perregaux, on the other hand, has turned to high-tech materials, paving the way forward, while Grönefeld relies on putting time-tested solutions into watches that are anything but conventional. Finally, you could go the way of Oscillon, which has gathered inspiration from beyond the world of watchmaking and adapted it for use in timepieces – the choice is yours!

Read more

The Race to Make the Thinnest Watch

50 Years of Automatic Chronographs – Vintage Treasures and Modern Interpretations

The Development of the Automatic Movement – Part 1


About the Author

Tim Breining

My interest in watches first emerged in 2014 while I was studying engineering in Karlsruhe, Germany. My initial curiosity quickly evolved into a full-blown passion. Since …

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