Historically, the lever escapement has proven to be robust, reliable and comparatively easy to manufacture, making it the de facto escapement in wristwatches today. However, the lever escapement is not perfect, especially with the discrepancies in accuracy caused by frictional changes at its contact points. Decades of research have attempted to improve its efficiency while others have refocused their efforts on promising alternatives.
One of the most seductive and elaborate developments has been the independent double-wheel escapement. Like the co-axial, it was invented by the late George Daniels for the express purpose of combining the dual impulse and self-starting capability of the lever escapement and the frictionless, direct impulse system of the detent escapement, while eliminating both their shortcomings. Beyond its superior functional benefits, the double-wheel escapement arguably offers the most visual appeal, as having a pair of counter-rotating wheels that impulse the balance alternately and directly creates an incredibly elegant motion.
In 2020, German independent watchmaker Bernhard Lederer presented a prototype of his Central Impulse Chronometer which took the escapement to a new height by improving its geometries and incorporating a pair of remontoirs. The watch is now in its final form and represents the first in a series of watches dedicated to innovative escapements.
A watchmaker of immense technical and mathematical talent, Lederer is a founding member of the AHCI (Académie Horlogère des Créateurs Indépendants) whose career, in many ways, represents a long examination into the heart of a mechanical watch, from the gravity escapement in his first timepiece — a table clock with a 1,000-year perpetual calendar, sidereal time and high-accuracy moonphase — to the innovative tourbillons produced under BLU (Bernhard Lederer Universe). Based in St. Blaise near Neuchâtel, his company MHM (Manufacture de Haute Horlogerie et Micromécanique) has been a go-to specialist for movement construction and parts production for many esteemed brands.
Like many of his peers who have shown a similar predilection for escapements, he was deeply inspired by the work of George Daniels. “When I bought my ‘bible,’ the book Watchmaking by George Daniels, I learned about the independent double-wheel escapement and it was clear that the time will come when I feel strong and good enough to fulfil George Daniels’ dream to incorporate it in a wristwatch,” Lederer says. “He had always been convinced that it is better than the co-axial, but it demands extreme tolerances, and it is difficult to ensure stability for the abrupt movements that a wristwatch faces.”
Building on the Past
Conceived by Daniels in the mid-1970s, the independent double wheel escapement was in turn founded on the principles of Breguet’s natural escapement, so called because the impulses are delivered directly to the balance axis by each escape wheel, eliminating the need for lubrication.
In Breguet’s construction, the first escape wheel, which is driven by the fourth wheel, drives a second escape wheel directly. However, due to the technological limitations that were tethered to his time, primarily manufacturing tolerances that would result in play between the escape wheels, he eventually abandoned the idea and focused instead on improving the lever escapement.
Breguet’s idea has since spawned three distinct trajectories of development that differ in the manner they circumvented the issues he faced. The first branch has greatly benefitted from modern materials such as silicon and nickel phosphorous, which can be fabricated with extreme precision, thereby managing to achieve a construction that is most similar to Breguet’s early design, eliminating the issue of backlash caused by play between the escape wheels. Some examples include the Ulysse Nardin dual-direct escapement as well as the Laurent Ferrier natural escapement.
The second adopts a pair of auxiliary wheels below to drive the escape wheels above, resulting in a rather power-hungry construction that totals four wheels being driven by a single gear train. This is the basis of the F.P. Journe bi-axial high-performance escapement as well as the Voutilainen double direct impulse escapement.
Lastly, the third and most concrete solution, as pioneered by Daniels, was to provide each escape wheel with its own gear train and mainspring, enabling each of them to operate independently, eliminating the geared connection in the previous two approaches.
Although Daniels had looked upon the double wheel escapement with great favor, incorporating it in a total of six watches including the Space Travellers, he had identified several inherent disadvantages in this setup. The double unlocking at each vibration consumes balance energy, and above all, he believed that its complex construction could not be sufficiently miniaturized in a wristwatch.
Lederer’s ambition of adapting it for a wristwatch ran parallel to that of English watchmaker Charles Frodsham, who presented the first independent double wheel escapement in a wristwatch in 2018. The Bernhard Lederer Central Impulse Chronometer represents an evolution that differs markedly in several ways.
Perfecting the Escapement
First, it bears noting that the independent double wheel escapement is itself of superior efficiency. As with other direct or radial impulse escapements, it is far more efficient at transmitting energy from the escape wheel to the balance than a lever. But where it differs from most modern escapements including the co-axial is that both impulses delivered to the balance are direct. One of the key challenges is its power supply as torque delivered to the respective escape wheels has to be equal; if one depletes faster than the other, the escapement simply will not work.
Lederer followed through with a rigorous approach that was three-pronged: 1) designing all components with low inertia in mind, 2) optimizing the geometry of the escapement to eliminate recoil or “draw,” 3) incorporating a pair of remontoirs to ensure consistent power transmission to the respective escape wheels.
It was essential to Lederer to increase the beat frequency of the balance wheel from 2.5Hz adopted in Daniels’ pocket watches to 3Hz because a higher frequency improves stability of the balance motion in the face of shocks, which a wristwatch is more susceptible to. With an increased frequency, the efficiency of energy transmission in the escapement in turn declines. Thus, the escapement had to be optimized for the lowest inertia to ensure the fastest rotational acceleration of the parts. The twin escape wheels as well as detent lever in Lederer’s escapement are fashioned from titanium, which is an ideal material due to its lightness and strength.
The geometry of the escapement also had to be refined. “The major issue with Daniels’ construction is that at lower amplitudes, the escape wheel has a tendency to slip with more than one tooth in front of the impulse pallet, resulting in an inaccurate time indication,” says Lederer. “All locking stones in his design have a lifting angle that causes the escape wheel to recoil slightly as the escape wheel is liberated from the principal locking pallet.”
Thus, the locking beaks on the principal pallet were modified into small sloped surfaces. This prevents the escape wheel from tripping or whizzing past the impulse pallet. At low amplitude, the tooth that was on the nib of the principal locking stone slides along this surface and pushes the detent to the side, causing the roller to rotate, or in other words, conveying an indirect impulse to the balance. This ensures that the impulse pallet is safely in front of the tooth that is ready to carry out the direct impulse transmission. “As soon as the balance has exceeded an amplitude of approximately 80°, the detent is so fast in its rotational movement that the tooth that was lying on the central pallet no longer reaches this lateral auxiliary surface” Lederer explains. This unique pallet profile is the subject of a pending patent.
Ensuring Constant Torque
The next order of business was to perfect the consistent transmission of power from the wound mainspring to the escapement with the use of a constant force mechanism in each wheel train. While the basis for a remontoir arises from the basic principles of a mechanical movement in that the energy produced by the mainspring varies across its state of wind, it is a mechanism that is particularly beneficial for an escapement that is reliant on the parity of two separate gear trains.
In the construction, the remontoir is located on the third wheel of each wheel train. Theoretically, it can be located on any wheel on the gear train and in some extreme cases, on the escape wheel, but as Lederer explains, “the question of where to use a remontoir opens up a wide range of arguments, because the closer it is to the escapement, the fewer intermediate axles I have, which is good and desirable, but at the same time all the disturbances are brought closer to the escapement, which should be avoided.”
The remontoir is managed by the fourth wheel via a Wankel disk, characterized by a Reuleaux triangle that releases and winds the remontoir every 10 seconds. The remontoir spring is tensioned by an angle of a mere 6.7°, ensuring a stable flow of power to the escapement. In addition, the timing of the remontoirs is staggered, releasing in alternating cycles every five seconds.
As the remontoir spring releases a fixed dose of energy each time, the balance achieves constant amplitude, resulting in an especially high degree of rate accuracy. The movement is also equipped with a free-sprung balance that is attached to an overcoil hairspring. It is no surprise that the watch is certified as a chronometer by the COSC.
The Watch in its Final Form
The beauty of a dual-train construction is its symmetrical architecture. The components are separately secured by slim, individual bridges, offering an expansive view of the transmission systems. Beyond the technical highlights, the rest of the movement is equally nuanced with many appealing details such as the triple winding click, the unique spokes of the gears as well as the design of the free-sprung balance.
All that unrestrained horological eye candy is housed in a white gold case that is expectedly large with a diameter of 44mm wide, but relatively flat at just 12.2 mm high. In comparison to the complexity of the movement, the dials have hewed to a pleasingly austere aesthetic. Available in either blue or silver, it features a pair of openworked seconds counters that reveal the twin escape wheels as well as the Wankel disk. The seconds hands rotate in opposite directions, offering a hint, at a glance, that it’s no an ordinary time-only watch.
In all, the Central Impulse Chronometer represents not just an obsessive exercise in chronometry, but also a direct connection to the work of the most remarkable figures in the history of precision timekeeping. With the exception of the dial, case, crystals and springs, all components of the watch were designed and manufactured in Lederer’s atelier in St. Blaise. It is available in a limited edition of 25 pieces for each dial colour.
Bernhard Lederer Central Impulse Chronometer
Movement: Manual winding caliber 9012; 42-hour power reserve
Functions: Hours, minutes and seconds
Case: 44mm; 18K white gold; water resistant to 30m
Strap: Alligator leather with gold pin buckle
Price: CHF 128,000 (approx. USD 140,000)
Availability: Limited to 25 pieces in each dial color