Chronometry: new precision solutions (part 2)

Par David Chokron

Refocusing the spring
The movement of the spring is, by its very nature, harmful to chronometry. The spring is coiled in on itself and opens and closes constantly. In doing so, its centre of gravity is forever moving around its central axis. Due to this non-concentric movement, it pushes the balance wheel against the openings in which it rotates. This friction varies according to position, slows the balance wheel and pumps energy. It is an unavoidable scourge that undermines the basic goal of the balance wheel: its isochronism. And as a result, this concentricity has occupied watchmaking research for a few centuries.
The terminal curves of the springs were devised in such a way as to enable it to beat closer to its centre of gravity, but this is not the ideal solution. Recent research, such as that of DeBethune, who developed a flat terminal curve, contrary to the Guillaume curve, which rises above the spring, is one solution that has been surprisingly neglected. Three brands have decided to remedy the problem through addition and subtraction. Audemars Piguet (cal. 2908), H. Moser & Cie (cal. HMC 324.607 or 343.506) and Laurent Ferrier (cal. FBN 916.01) use two virtually identical springs. Moser belongs to the same entity as Precision Engineering (which supplies Ferrier), which manufactures springs and sells pairs issuing from the same batch. They are baked and wound together.
The twin parts, which have similar properties, are mounted head-to-tail on the same balance wheel. When one spring dilates with offset, the other spring does so with similar offset in the opposite direction. The sideways movements of one are compensated for by those of the other, with the result that the balance wheel rotates around its axis.
Moser alleges its models run at an average of 4 seconds better than the COSC Swiss standard for chronometers.

ulysse nardin

Controlling the spring
Escapement work is a foundation for chronometry. For the sake of greater precision and prestige, it has been the subject of progress or rework. That progress basically takes the form of the double impulse and the direct impulse, and sometimes a combination of the two.
In the Audemars Piguet escapement, the impulse is delivered directly by a plate with two pallet stones, not by the pallets. The first advances a pallet lever, the other advances the escapement wheel. On the return of the balance wheel, the pallets are returned to their initial position and the escapement wheel finishes its course against it.
At Ulysse Nardin, the information transmitted from the balance wheel to the train wheel passes directly through the balance wheel axis. This impulse is delivered to two distinct escapement wheels, each with its own unique teeth profile. In both cases, there are fewer points of impact in the information/energy transmission chain than with the Swiss club-tooth lever escapement, and far fewer movements, which thus minimises the force of the impact, a major loss of energy and a source of vibrations. In principle, these escapements resemble the coaxial invented by Georges Daniels and taken over by Omega. However, their sophistication comes from their limited movement.
Moreover, rework consists in improving the most precise escapement of all, the pivoted detent escapement. Most marine chronometers, instruments upon which men depend for their survival at sea, were regulated by detent escapements. Simple, reliable, they had only two faults. They were difficult to miniaturise and jumped out of alignment when subjected to shock. Not a problem when the timepiece is gimballed and lying on a table, but far more bothersome on the wrist, where resistance to shock is but limited.
The two-man team comprising Jean-François Mojon and Kari Voutilainen fitted just such an escapement in the UJS 8 chronometer by Urban Jürgensen & Sonner. With this exception, that the balance wheel was fitted with a limiting plate. This limits movement in the detent and ensures it is locked against the escapement wheel. The brand announced that its average operational stability was 40% higher than that of a sample of 300 COSC-certified chronometer watches. It was also subjected to Chronofiable tests to prove the solidity of its detent.

detente jurgensen
échappement à détente Jurgensen

Fuelling the spring
At a time when springs were rudimentary, magnetisable and sensitive to absolutely every influence, some watchmakers considered fuelling it with constant energy to ensure that its oscillations were as identical as possible. There are three solutions for achieving this.
The first is to smooth out the energy delivery to the gear train at source. This is typically achieved using a fusée, which, like a bike cassette, regulates the mainspring torque. This solution belongs very much to the 18th century and is in use at Breguet (cal. 569), A. Lange & Söhne (cal L044.1 or L072.1) and Cabestan (cal. EC101).
The second consists in interposing an intermediate wheel immediately before the escapement to act as an energy buffer, but which still has the same force transmitted to power the balance wheel. This is the so-called remontoir d’égalité as used by F.P. Journe (cal. 1403) or DeWitt (cal. DW8050).
The constant force escapement is otherwise more sophisticated since it guarantees that the energy is regulated in the escapement. The last watchmaker to have opted for this solution was Karsten Frässdorf and his new brand, Heritage Watch Manufactory. His constant force escapement is on two levels. The upper level connects with the balance wheel. It is connected to the lower level through a second spring, which ensures that the upper escapement wheel always delivers the same energy at the same frequency as the lower escapement wheel, which engages with the gear train. IWC, with the continuing help of Jean-François Mojon, presented a constant force tourbillon in its Portuguese Sidérale Scafusia. The beneficial effects of having the same top-quality energy at all times on the escapement are increased by that of the tourbillon.

Secondary qualities
The topic of research into materials has already been broached (in french), as has the issue of higher frequencies (in french). The work on teeth profiles is also important and of a very special technical complexity. On the other hand, mechanical/magnetic hybridisation has thus far only been used in Seiko’s Spring Drive system. Although being extremely intelligent, it has the double inherent defect of being neither totally mechanical, nor Swiss. Despite all the efforts on the part of rare, precious and expensive brands, precision remains of secondary importance. However, those who consider it the cardinal virtue of a watch may rejoice! It has been a good hundred years since such a profusion of offerings saw the light of day.