The Dominique Renaud Pulse60 Resurfaces A Long Forgotten Moment In Horological History

Dominique Renaud is one of the most respected names in complicated watchmaking, having been the cofounder of Renaud & Papi, the complications specialist where some of watchmaking’s biggest names worked – among them, Robert Greubel, Stephen Forsey, Andreas Strehler, Carole Forestier-Kasapi, Bart and Tim Grönefeld, Peter Speake-Marin, and Stepan Sarpaneva. Renaud, after the sale of Renaud & Papi to Audemars Piguet in 2000 – the company is now Audemars Piguet (Renaud & Papi) SA, or simply Audemars Piguet Manufacture Le Locle –finally introduced a watch under his own name in 2017, which is the DR01 (Monochrome was able to go hands on with the only working model). This is an extremely exotic horological experiment, with a blade shaped oscillator controlled by a blade spring, which vibrates at 12Hz, and with a very low amplitude of just 30º. The project originally was slated to produce 12 watches, priced at CHF 1,000,000, but only one prototype was produced. This was followed by a collaboration: the Renaud Tixier “Monday,” with an unusual high efficiency microrotor, and the two also collaborated on the Furlan Mari Secular Calendar for Only Watch.

However, earlier this month, Dominique Renaud’s eponymous company launched a new watch almost diametrically opposed to the DR01: the Pulse60, which has an enormous balance beating at just 1Hz, or one full oscillation per second.

Unlike the DR01, which in prototype form is 45.17mm wide, 27.5mm thick, and 56.47mm long, and whose design meant that it wasn’t really practical to wear except as an occasional conversation piece, the Pulse60 is 40mm x 44mm, with a lugless design that lets the down-curving rubber strap conform to the wrist.

The balance, at 33mm is as big or bigger than a lot of watches, with the flat mainspring coming in at 15mm. The balance spring has a dog-leg outer terminal curve – in this respect it’s like most other Nivarox type flat balance springs – on which there is a regulator with two curb pins; this functions as it would in a conventional watch, which is to say, you can use it to adjust rate. The balance also has four timing weights, which might seem redundant to the regulator but in fact, historically this is a combination frequently seen in high grade watches and I assume the advantages here are the same – the watch is brought to time with the weights, and final fine adjustments are done with the regulator, whose curb pins can also be used to adjust for variations in rate in positions.

The regulator can be seen through the left side aperture on the dial, and on the right, there’s a torque indicator, which shows the amount of energy available to the going train (as opposed to a conventional power reserve display, which shows the remaining running time, but not the amount of torque per se). Hours and minutes are shown on the subdial at 12:00.

The 1Hz beat rate is what gives the watch its name – 60 beats per minute is roughly the human resting heart rate, although it can be more or less in any given person depending on physical fitness.

In addition to the very slow frequency, the balance can also do something which is generally impossible for modern lever watches: oscillate through more than 360º of arc. Normally, the amplitude of a balance paired with a lever escapement must avoid running at that amplitude or higher (in practice, to maintain a safety margin, the amplitude is usually lower; 270º to as much as 310º are considered reasonable. The reason for avoiding higher amplitudes is that if the balance comes too far around, the impulse roller strikes the lever (which is locked by the escape wheel against its bankings) and bounces back sharply, causing the watch to gain on its rate.

The Pulse60 has a lever escapement, albeit an unusual one, so the logical question is, how did Renaud get around this problem? And, connected to this question, why would you want to run a balance at a higher amplitude than 360º in the first place?

The back of the watch shows the very unusual solutions used in the Pulse60 and gives a partial answer as well to the question of why you’d want such a high amplitude; a complete answer includes the technical challenges of running a balance at such a low frequency, as well as how the problem was approached in the history of watchmaking.

You’ll notice first of all the quite robust looking crown wheel, and the even more robust looking ratchet wheel and mainspring barrel; the upper pivot is quite large and runs in a jeweled bearing designed to bear the considerable side load of the mainspring. The movement consists of just two bridges, which divide the movement into two offset hemispheres and as you can see, the mainspring takes up more than half the diameter of the watch. The balance is so large that keeping it vibrating – especially at anywhere near its theoretical maximum of 700º (that’s right, seven hundred degrees) requires a considerable kick during the impulse phase of the operation of the escapement. The escape wheel appears to be LIGA-fabricated, and skeletonizing escape wheels is often done by modern brands (like Grand Seiko, for instance, in its Hi-Beat watches) in order to lower inertia.

The answer to the question of how the balance can oscillate at amplitudes higher than 360º lies in the relationship of the lever to the balance itself. Here’s a diagram of a conventional lever escapement:

In the diagram (courtesy Wikipedia) the escape wheel is rotating clockwise and the impulse face of the left pallet jewel is about to slide across the impulse face of the escape wheel tooth, causing the lever to rotate clockwise. As it does so, the fork of the lever pushes the roller jewel on the balance’s impulse roller counterclockwise, giving impulse to the balance. Since the lever will then come to rest on the right banking pin and lock there, the roller jewel cannot come around a full 360º without hitting the back of the lever. The only solution would be to somehow set up gears that would allow the lever to impulse the balance indirectly.

This is exactly the solution used in the Pulse60. Here’s a short video showing the action of the balance and escapement:

and here’s a diagram of the going train:

As you can see, the fork of the lever (which thanks to its size, shares the buttressed structure of the escape wheel, for lower inertia and better rigidity) doesn’t interact with the impulse roller of the balance directly. In fact, the balance has no conventional impulse roller at all. Instead, the balance staff has gear teeth which cause a secondary gear, which does have an impulse surface, to rotate back and forth in synchrony with the balance. This impulse gear, like the balance, is free to rotate and only has contact with the lever during the unlocking and impulse phases of the escapement’s operating cycle.

You can just see the beginning of the unlocking and impulse cycle in the video, but it’s blink and you miss it; the above is a screencap.

The above image is from an earlier patent filed by Dominique Renaud and shows a system different in key respects from the Pulse60, but it gives you a basic idea of the relationship between the balance and the actual impulse gear system.

The Balance: The Reason For Going Big And Going Slow

The practical rationale for this design, has to do with how you get a balance in a watch or clock to have a stable rate. The tendency in modern watchmaking is for balances to become relatively smaller, but run at higher frequencies, as at higher frequencies, the chances of a balance receiving a physical disturbance at anything near its natural frequency is much lower. For this reason, modern movements usually run at 28,800 vph or higher. The only modern 1Hz/3600 vph watch I’ve ever seen is Martin Braun’s Antoine Martin Slow Runner, which came out in 2013 and which had a 24mm balance, and a 96 hour power reserve, though it was not a commercial success. Pocket watches were made with one second balances, in the 18th and 19th centuries (here’s one example Sotheby’s had in 2020) but such watches are highly vulnerable to rate disturbances and even paired with a spiral balance spring, are not practical for portable timekeepers.

The verge escapement typically works best with an amplitude of around 100º for the balance, but early horologists recognized that both improving amplitude and increasing frequency would improve rate stability; Harrison’s H4, with its unusual crown wheel and diamond pallet escapement, could reach amplitudes of around 124º and beat at 2.5Hz/18,000 vph. However, larger increases in balance amplitude might, it was thought, also improve rate stability; the higher the amplitude for a given frequency, the greater the angular momentum and therefore, the greater resistance to external disturbances.

The solution to this problem was the so-called “pirouette” balance. Mark Kauzlarich mentioned the pirouette balance last week in his launch coverage of the Pulse60 and noted then that this was a very old mechanical solution – one source I have found says the pirouette balance, as a method for dramatically increasing balance amplitude in a verge watch, may go back as far as Christian Huygens (source, in French, via Google Translate). There is one very beautiful example in the collection of the Worshipful Company of Clockmakers, in the London Science Museum, made by the London watchmaker Peter Debauffre in 1700, with the pirouette mechanism a likely later addition; the Museum’s catalog notes say in part:

“A ‘Pirouette’ is a device in which the balance is not on the same axis as the verge, but on a separate arbor and geared to the verge, so that it turns much more rapidly. In this case a wheel of 60 teeth on the verge-staff gears with a pinion of 6 leaves on the balance-staff which thus has an arc of about 2½ turns. Although the watch is signed Debaufre and made around 1700, the beautiful verge escapement with a pirouette was undoubtedly the late 18th century work of the clockmakers Alexander Cumming.”

Debauffre watch with ‘pirouette,’ Science Museum

The British Antique Dealer’s Association also had listed a French-made watch with a double wheel Debauffre escapement with pirouette  which it describes as “very rare” and indeed this seems to be nothing but the truth.

Now the reason that you don’t see 1Hz balances nowadays is because purely from a practical standpoint, they are, well, not very practical; they are easily put off their rate, although this can to some extent be at least partly addressed by dramatically increasing the balance amplitude, which was the purpose of the pirouette mechanism and which is the reason behind the use of a balance with a theoretical maximum amplitude of 700º (which is fairly close to the “arc of two and a half turns” mentioned by the Science Museum). The larger balance and stronger balance spring require more energy per oscillation, albeit this is considerably mitigated by the slow beat rate, and the sheer size necessary for a 1Hz balance makes such a thing generally impractical.

However, seeing the Pulse60 as a resurrection of a technical solution to what is today a nonexistent problem, is perhaps unfair. Dominique Renaud is obviously interested in exploring the limits of practical escapement and oscillator design at least as much as he is interested in practical modern horological engineering and he’s clearly capable of pursuing both. The DR01 was not a practical watch per se – neither in its design nor in its price – but it is a kind of mechanical mediation on the limits of high frequency mechanical oscillators. The Pulse60 explores the limits of oscillator engineering from a different perspective, but the spirit of curiosity and experimentation behind it, is the same.

Find out more about the Dominique Renaud Pulse60 at DominiqueRenaud.com.