A mechanical watch operates on the same premise as it did over 200 years ago — a curiously complete system of minute parts moving in perfect harmony. It begins with a coiled mainspring which, when wound tightly, causes a succession of toothed wheels to turn, powering the hands, while an escapement regulates the expenditure of power to ensure timekeeping is precise. Its heart is the balance wheel attached to a coiled hairspring that “breathes,” winding and unwinding. Known collectively as the regulator, they control the escapement, which creates the characteristic “tick tock” of a movement. This single, composite system is all that is necessary for a movement to come alive.
However, there are several instances when the design of a movement deviates from this basic anatomy, notably by doubling up on critical components such as the regulator to offer optimal timekeeping performance with respect to a manufacture’s motive. Although watches with double regulators offer a similar visual effect of symmetry across the board, they operate on starkly different principles and can be categorized into three main groups.
Two Independent Transmission Systems
The simplest in theory are watches with two independent and complete transmission systems — double barrels, gear trains and regulators — each dedicated to a different function. Essentially two separate movements in one watch, this format is often employed to reduce the load on the gear train. It has proven to be particularly beneficial for complications that otherwise require high inertia such as the chronograph.
The main objective for such a construction in a chronograph is to do away with coupling systems and their associated drawbacks. Be it vertical or horizontal clutches, a chronograph mechanism is, to a greater or lesser extent, an additional load driven off the fourth wheel in a movement — the fastest wheel in the gear train as well as the one with the least torque, being furthest from the mainspring. Isolating the chronograph from the timekeeping gear train completely will result in greater rate stability.
Often, to make the most out of this format, the second transmission system is used to power a high frequency balance so as to achieve a more precise reading beyond the standard one-sixth (3Hz) or one-eighth (4Hz) of a second.
In watches such as the Breguet Tradition Chronograph 7077, an independent transmission system was used to power a 5Hz chronograph while the timekeeping balance operates at a more sedentary 3Hz. A 5Hz balance, however, vibrates up to 172,800 times more than a 4Hz balance within a day, in other words, generating way more friction. Thus, the escapement utilizes a silicon pallet fork to reduce friction. The power source of the chronograph is a linear blade-like spring that buckles when the reset button is activated, powering the chronograph for 20 minutes when needed.
A more potent example that exploits the potential of a dual-train construction is the Zenith El Primero Defy 21. Like the Breguet Tradition Chronograph, it was designed with complete and separate transmissions systems for the chronograph and timekeeping function, but instead of a blade spring, it featured a proper mainspring barrel for the chronograph, delivering greater torque to achieve a higher frequency. While the timekeeping escapement runs at a frequency of 5Hz, the escapement for the chronograph operates at 50Hz, making it a true 1/100th of a second chronograph. To reduce inertia and wear, both pallet forks and escape wheels are made of silicon which offers excellent friction resistance and energy saving, both of which help deliver a higher frequency while minimizing the effects of wear.
Beyond chronographs, a watch that employed a dual-train construction in a particularly novel way was the Vacheron Constantin Traditionnelle Twin Beat Perpetual Calendar. It addresses the problem of having to reset a perpetual calendar that has lain dormant by prolonging mainspring torque. While the movement features double barrels, gear trains and oscillators, the barrels are coupled in series and unwind at two different speeds thanks to a differential.
Each barrel powers a balance wheel that runs at a frequency of 5Hz and 1.2Hz respectively for two different modes. In active mode, the watch runs at a frequency of 5Hz with a four-day power reserve, while in standby mode, the watch operates at a frequency of 1.2Hz, receiving just enough energy to keep the calendar indications running for up to 65 days.
The concept of having two independent transmission systems, however, is not to be confused with movements that incorporate twin barrels and gear trains that power a single escapement such as the Jaeger-LeCoultre Duomètre à Chronographe and the Grönefeld One Hertz, in which power is drawn from a separate source purely to reduce amplitude loss in a single 3Hz balance wheel. There are also the Charles Frodsham Double Impulse Chronometer and the Bernhard Lederer Central Impulse Chronometer, both of which feature such a dual-train architecture to eliminate play in the independent double-wheel escapement.
The second group of watches was designed with twin escapements and regulators that are driven by a differential to mechanically average out their errors in rate. The first such wristwatch was the Philippe Dufour Duality launched in 1996. Dufour was inspired by a 1930s double regulator pocket watch produced by the students at the watchmaking school in his hometown of Le Sentier. The differential gear, on the other hand, has a history dating back to 1827. It was invented by French watchmaker and engineer Onésiphore Pecqueur and proved vital to the development of the automobile.
In the Duality, a differential on the fourth wheel averages out the difference in rate between the two balances and produces a single output for the time display. Thus, if one, for instance, beats at a rate of +2s and the other at −2s, the watch achieves a perfect zero deviation. The two escapements in this approach serve a single gear train, which is not to be confused with a resonance watch in which two individual movements linked by a common main plate achieves a more stable rate through the principle of resonance (more on that below).
The Duality eventually became the inspiration for the MB&F Legacy Machine 2 in 2013. The latter featured two balance wheels over the dial, with part of the differential exposed at six o’clock. This construction differs slightly; the differential is located on the second wheel of the movement and divides power to two separate trains. The exposed wheel drives the first third-wheel pinion on the right while the other wheel, positioned on the hind side of the differential, drives the second third-wheel pinion.
It bears noting that it is the differential that drives the escapements, and not the inverse. The differential splits the power from the mainspring, apportioning the energy to each escapement, while averaging their respective errors to produce a single output. This format of increasing precision lends itself particularly well to visual spectacle as the reliance on a single gear train leaves room for multiple oscillators or tourbillons. Some examples include the Antoine Preziuso Tourbillon of Tourbillons as well as the Roger Dubuis Double Flying Tourbillon with Differential and the Excalibur Quatuor.
Notably, the Excalibur Quatuor features four balances, each inclined at a 45-degree angle at the edge of the movement to compensate for rate variations caused by positional changes. To achieve a single output, a total of three differentials was necessary to link the balances in the geartrain. The pair of balances at the top are powered by a differential on the third wheel while a second differential drives the pair at the bottom. The errors of these two differentials are in turn averaged out by a third differential on the second wheel of the movement.
One watch, however, that combined the differential with other exotic solutions to rate stability was the Greubel Forsey Double Balancier in which the differential that drives both escapements doubles as a constant force mechanism.
The differential acts as a remontoir d’égalité which ensures consistent power transmission to the respective escape wheels regardless of the movement’s state of wind. This is achieved with an additional spring attached to the set of gears that make up the differential. The spring is rewound once every four minutes, serving as a secondary store and source of power. In addition, the double oscillators are inclined at a 30-degree angle to the main plate to avoid the most extreme gravitational errors in both horizontal and vertical positions. They are also positioned in symmetry, in other words, inclined to each other so that the positional errors in one balance in any given position is compensated by those experienced by the other via the differential.
The third variety of watches incorporates two independent transmission systems that achieves greater rate stability via the principle of resonance thanks to a shared main plate or bridge. The first wristwatch to achieve a true resonance effect was the F.P. Journe Chronomètre à Résonance, launched in 2000. It was inspired by Breguet’s No. 3177, a resonance clock François-Paul Journe had encountered during his time as a restorer.
The phenomenon of resonance, however, was first observed by Dutch scientist Christiaan Huygens who discovered that two of his pendulum clocks, which were suspended from a common wooden beam, tended to display a sympathetic motion. This phenomenon was later researched and built upon by both Antide Janvier and Abraham-Louis Breguet, who were noted for having made clocks with double pendulum systems, each driven independently. The basis for resonance in a clock is enhanced precision as any deviations in rate in one pendulum tended to be canceled out by the other.
Ultimately, the key element for resonance to occur is its coupling structure. Breguet was one of the first watchmakers to have successfully achieved the phenomenon of resonance in pocket watches. Having tested and seen that resonance could occur in a vacuum, he realized that the effect relied on torsional resonance — the transfer of vibration from the balance cocks to the shared main plate.
In a wristwatch, the same logic applies; there is a tendency for the vibrations of two balance wheels to sync up, thereby achieving a better rate stability than either balance can achieve in isolation. The caveat to this is that the coupling forces are inherently weak. The two balances must be adjusted so that their rates are as close to each other as possible while being free sprung, as having an index regulator with two pins reduces the effect of the vibration transmitted from the hairspring to the cock and main plate.
The Chronomètre à Résonance remains the most faithful interpretation of Breguet’s resonance pocket watches; however, Journe has since developed a new resonance movement with several technical upgrades. The twin gear trains are now driven by a single barrel, thanks to a differential which divides power to each train. Additionally, each gear train is now equipped with its own remontoir d’égalité to address the problem of unequal torque delivered to the balance as the mainspring unwinds. As a result, the balance achieves more constant amplitude and consequently a higher degree of rate stability.
Several others have built upon Breguet’s findings, most notably Armin Strom, which has to date, introduced a total of four resonance watches including a minute repeater. Its method of attaining resonance is one of the most convincingly reliable and efficient. To strengthen the coupling force between the two balance wheels, the outer terminal curves of the hairsprings are connected directly to each other via a patented clutch spring.
This physical connection forces both balances to beat in sympathy, in other words, ensuring that resonance occurs more readily. In addition, the dual train architecture in a resonance watch is a convenient canvas for additional indications and complications. In the Masterpiece 1 Dual Time Resonance GMT, for instance, the brand made the most out of the construction by having each gear train drive a different time zone indication.
An even more exotic and elaborate development was the Haldimann H2 Flying Resonance. In contrast to the aforementioned resonance watches, in which the barrel, gear train and balance are duplicated, arriving at two independent systems, the H2 Flying Resonance relies on a single gear train to power a tourbillon carriage that houses two balance wheels and escapements. One escape wheel is equipped with a remontoir d’égalité that serves to provide a steady amount of torque to the balance, which then influences the second balance through the principle of resonance. At the same time, the remontoir also allows the escape wheels to decouple so as to prevent the escapement from locking up as this format employs a single gear train and would typically require a differential to split power to both escapements. Additionally, the two balance wheels in the H2 Flying Resonance are physically linked by a straight spring which once again helps to strengthen its coupling force.
The most extravagant take on the principle of resonance, however, is the Vianney Halter Deep Space Tourbillon Resonance. A single mainspring drives the tourbillon (triple-axis in this case) which houses a pair of balance wheels and escapements. Power is distributed to each set of gears and escapement via a differential, ensuring both systems are operating independently from each other in order for the vibrational effects of resonance to occur purely via a shared cage with a pillar-type construction.
The innermost cage rotates on the first axis once a minute. This cage sits in a traversal structure that rotates around a horizontal axis in six minutes and the entire structure is in turn mounted on a cradle which competes one revolution on its vertical axis in 30 minutes.
Beyond the three main types of watches with double oscillators, there is one particular watch that achieves greater rate stability with a completely different approach — the Audemars Piguet Royal Oak Double Balance Wheel.
As with a standard movement, it features a single barrel, gear train and escapement, which impulses the balance wheel. This balance wheel, however, is mounted on the same balance staff as a second balance wheel. As such, the two balance wheels beat in sync, producing a single rate.
Timekeeping in general benefits from a balance with high inertia, as opposed to the escapement where low inertia is favored. This patented configuration essentially allows each balance to maintain its diameter while increasing rotational inertia, resulting in greater stability in the face of shocks as compared to a single balance wheel. In addition, the hairsprings have opposite pinning points, each breathing inversely to the other, averaging out errors while keeping the balance wheels centered regardless of position.