The balance wheel is the beating heart of every mechanical timepiece. This is most likely not news to you. Along with its audible pulse and sweeping seconds hand (as opposed to the ticking seconds hand in quartz watches), the oscillating balance wheel is one of the clear signs you’re dealing with a mechanical watch. You can often witness it in action thanks to a see-through case back or cutout in the dial.
Then there are the watches with not only one, but two, or even four (!) balance wheels. And not just in haute horlogerie models, but affordable timepieces as well. What exactly is going on inside these movements that have more than the one mandatory escapement built into them? Are there specific reasons for them, or is this perhaps a case of “more is more?” Just how can an automatic watch that costs only a few hundred dollars feature a double escapement when, to be honest, we would expect this more from a five- or even six-figure, highly sought-after luxury timepiece?
We’ll be diving into all of these questions by looking at specific models that feature these technologies. There are a number of different constructions and designs behind what appear to be similar watches. After a first passing glance at their outer appearances, however, keen observers will discover that these timepieces really couldn’t be more different from one another when we start poking around inside.
The MB&F HM9: Two Balance Wheels Compensated By a Central Differential
Let’s take a quick detour to get a better understanding of a watch with two balance wheels. Bear with me for just a moment – it’ll be worth it, I swear!
Whether in your car or a Lego set you put together way back when, most folks have heard of a differential. It’s an essential component in most vehicles that enables the wheels at each end of a drive axle to rotate at different speeds while being powered by the same motor and, for example, turning or taking a curve. Here, the radius travelled by one wheel varies in distance compared to the wheel on the other end of the axle, meaning it has to turn slower or faster than its counterpart. The differential placed in the middle of the axle does exactly what its name says: it compensates for the difference between the rotational speed of each wheel. This same technological phenomenon is also invaluable when it comes to watchmaking.
A common application of a differential in watches is seen with a power reserve indicator, i.e., the indicator showing how much “wind” remains in the mainspring that sits in its barrel. In the movement, a train of gears connected to the ratchet wheel or barrel arbor drives the power reserve indicator. The barrel’s teeth are connected to the other side of the differential train used in this process. As the mainspring’s power is used, the power reserve indicator moves in the opposite direction. Depending on the amount of remaining power reserve, the needle attached to the differential displays the difference between the turning gears. The needle thus describes a specific, limited angle, intuitively interpreted as the amount of power reserve.
So, how is this principle applied to watches with more than one balance wheel?
As luck would have it, the various MB&F HM9 models use a differential to unify two seemingly independent movements to work together within one timepiece.
The complex, three-dimensional sapphire crystal housing of this watch gives the wearer an unobstructed view of the two balance wheels as they oscillate back and forth. The time itself is displayed on a dial positioned at a 90-degree angle to the balance wheel axes.
The differential on this watch is also very visible, located at a central position under MB&F’s typical ax-shaped bridge. This so-called planetary gear differential, as opposed to the differential in a car, has co-axially positioned gears that rotate on the same axis. It’s aligned in a way that lets a drive gear of the differential correspond to exactly the average rotational speed of the two driven gears. Because their speed is determined by the independent balance wheels, the watch’s dial displays a time that is the average output of both escapements. Both of the HM9’s balance wheels are powered by a single spring barrel, which by the way is not an absolute mechanical must; other designs are possible that use two barrels.
The question, of course, is why all of this is even necessary in the first place. From a purely scientific perspective, and putting aside its usefulness compared to the effort put into creating this technology, the answer is simple: No single escapement is ever perfectly regulated, especially when it’s impacted by a watch’s position as well as by other environmental factors that affect how accurately a watch runs. No matter how much time and effort are put into constructing a watch, even the best watch brands only guarantee a certain range of accuracy (fast or slow). Plenty of timepieces from a single batch of watches will operate in the middle of this given range, while the rest will run at the upper or lower end of it.
This is precisely the reason why utilizing two escapements and a differential to average their output holds significant importance in terms of precision: The likelihood of one escapement not running accurately is moderate, whereas the likelihood of two escapements not running accurately is significantly less. And the more, the merrier…
The merrier, because with the Quatuor, Roger Dubuis has truly pushed the envelope: This watch has four escapements, with an impressive five differentials to average their rotational speed. This watch has achieved a constructional complexity that will be difficult for future timepieces to top anytime soon.
There’s another relatively unrelated, albeit equally important reason for making a watch with more than one escapement: gravitational force, and/or a watch’s position.
A perfect example of a timepiece that works against these influences is the Greubel Forsey Double Balancier. It, too, uses a differential to average the movement of two independent balance wheels. In a significant departure from the HM9, however, the two balance wheels aren’t mounted on the same plane inside this watch, but rather set at a relative angle to each other.
The idea behind a tourbillon is that the escapement and balance wheel are mounted in a rotating cage, which helps eliminate the impact of gravity. With the positioning of its two balance wheels’ axes, Greubel Forsey also aims to achieve an averaging: if gravity affects one of the balance wheels in its position, the second balance wheel in its respective, different position will minimize this impact, or cancel it out altogether.
Obviously, the HM9 doesn’t have this trick up its sleeve, but despite all this ambitious tech aimed at improving watch accuracy, the question remains: Are these highly complex, limited watch series actually achieving the desired results? Perhaps these timepieces should instead be considered a kind of “proof of concept,” demonstrating the watchmakers’ unwillingness to shy away from a design challenge, theoretical or otherwise. Hence, the fascination with and, yes, sometimes exorbitantly high prices for these timepieces.
F.P. Journe Chronomètre à Résonance: The Myth of Resonance?
F.P. Journe’s Chronomètre à Résonance is a classic timepiece, with a legendary watchmaking phenomenon shrouded in myth: resonance.
If you’re wondering what the physics definition of resonance is, it’s what occurs when an object is subjected to an external force/vibration that matches its natural frequency. Maybe you’ve seen that old video of the Tacoma Narrows Bridge reeling in the wind, achieving resonance, and crashing into the river below. When it comes to clocks and watches, resonance is the phenomenon where two pendulums or balance wheels that are seemingly not mechanically connected to one another start to swing or beat at the same frequency after a time.
Like the other watches discussed above, the Chronomètre à Résonance has two separate balance wheels, i.e., two entirely separate escapements. It has a dual display on its dial, but no differential to average the two escapements. Here, the differential’s job is to direct the power flow from the watch’s single spring barrel to each of the separate escapements. One real gem in this timepiece is that both gear trains are equipped with a separate Remontoir, a constant-force escapement that guarantees a continuous and even drive force of energy for the entire duration of the mainspring unwinding, with stable accuracy as the result. Be sure to check out my Technical Guide: Constant Force Mechanisms in Watchmaking to find out more.
Just What Exactly Is Resonance?
The resonance phenomenon has been known about for centuries. Legendary watchmakers such as Breguet experimented with the concept. He designed resonance pocket watches with two escapements and tested them extensively, including inside a vacuum chamber. Breguet hypothesized that the air displaced by the vibrations and resulting turbulence would lead to a reciprocal influence of each independent balance wheel on the other. His experiments inside the vacuum chamber, as well as with thin steel guards around the balance wheels to eliminate air turbulence, proved that this factor was in fact insignificant.
Breguet also adjusted the distance between the balance wheels to test the impact of this parameter, proving it to be equally unimportant.
The logical conclusion to this work was that what was happening inside the caliber was the result of static components such as the movement’s plates or bridges. Tiny, virtually undetectable vibrations occurring in them and traversing to the spring mountings had to be the cause of the synchronization between the two escapements.
F.P. Journe put this concept to the test around the turn of the century, inside a compact wristwatch: the various iterations of the Chronomètre à Résonance have been the brand’s prime pièces de résistance ever since. The model range’s dual display lets its wearer see exactly how synchronously both escapements are working.
Nowadays, there are countless watches with two escapements that, at first glance, could be easily mistaken for “genuine” resonance timepieces. Inside most of these models, however, there’s a direct mechanical link, like a differential, as discussed above. But there’s also another resonance principle at work in some exclusive timepieces that I’ll discuss below.
Armin Strom Resonance: Two Balance Wheels and a Patented Spring
As a watch tech heavyweight whose timepieces foreground innovative solutions, Armin Strom has created its own unique version of the resonance watch.
As we’ve seen, the resonance principle is not based on mysterious, inexplicable phenomena, but on the transfer of oscillations – even if they are almost undetectable. Instead of leaving the transfer of oscillation from one balance wheel to another up to plates and static components, Armin Strom has developed a complex, patented spring for this purpose.
The outermost mounting points of the free-spinning, counter-rotating balance wheels on these watches are mounted to this spring. This connection is far more effective than a rigid mount to a plate, and enables a synchronization of the balance wheels, which would otherwise deviate from one another by a few minutes per day. F.P. Journe’s technology achieves a balance wheel regulation that allows for a fast, stable resonance state, which can be seen in action on many of their open dial models. They also deliver a permitted tolerance of just five seconds of deviation per day!
But the idea of directly connecting the balance wheel mounting points to a spring is not entirely a new concept. Beat Haldimann’s did this with their impressive Beat Haldimann H2 Flying Resonance in 2005. However, except for the concept itself, this timepiece has little in common with Armin Strom’s futuristic, unconventional designs.
The counter-rotating balance wheels keep the clutch spring remarkably stable, and the wearer can observe this process in action. Clutch springs and balance wheels tend to take up a significant amount of space on a watch dial. The watches in this extensive collection feature varying levels of visibility of these components, which means the line can cater to the various tastes of individual owners.
Along with Pure Resonance, Armin Strom also offers the Mirrored Force and Zeitgeist models, which feature twin counter-rotating seconds fly-back dials, another indication of the remarkable synchronicity of the balance wheels in this timepiece. The Armin Strom Masterpieces collection is home to the Dual Time Resonance with two displays on its oblong case. The Minute Repeater Resonance pushes the envelope even further, with an additional minute repeater complication.
Discover more about Armin Strom in our brand portrait.
Summary
As you can see, not every watch that proudly boasts two balance wheels is powered by the same technology. There are countless possibilities, physical principles, and mechanical laws that can be combined to achieve fascinating movements. The development and finishing found in these watches can also vary widely from brand to brand, which you’ll of course notice is reflected in their prices. The watches I discussed here today are all on the high end of the price spectrum.
In case you’re wondering if there are any dual escapement watches that set a record for affordability, you’ll be interested to know that you can purchase a watch with two working escapements visible on/through their dials for a low three-figure amount. How to explain the unbelievably low price? Well, it’s pretty simple: Only one of the escapements actually powers the watch; the other escapement doesn’t do anything other than move to delight the wearer.
