ith his Central Impulse Chronometer, Bernhard Lederer revisits and refines past inventions from a contemporary perspective, with particular emphasis on maintaining a regular and constant flow of energy through the movement.

He openly draws on the ingenuity and perseverance of both Abraham-Louis Breguet and George Daniels, in their respective battle to overcome the physical and chemical factors that interfere with a mechanical watch’s functioning, and their pursuit of ever greater chronometric precision.

Already, in the eighteenth century, Breguet built a “natural” escapement, with two escape wheels, that functioned without oil: the great watchmaker was fully aware of the negative effect which the natural deterioration of lubricant has on the balance’s amplitude.

Remontoir d’égalité

The remontoir d’égalité, or constant-force mechanism as it’s also known, is a solution to an age-old problem. As the mainspring unwinds, the force transmitted through the gear train to the escapement varies, causing more or less noticeable variations in amplitude.

Already fitted into tower clocks, the remontoir d’égalité was reinvented in the eighteenth century, a golden age for advances in chronometry, by the French watchmaker Pierre-Basile Lepaute. His device was used in numerous marine chronometers, perfected by Ferdinand Berthoud and John Harrison, among others, and equipped the Paris Observatory’s reference clock.

At the heart of a remontoir d’égalité is a spring, which smooths the flow of power that is delivered directly to the escapement. This spring is recharged at regular intervals. These intervals vary, depending on the complications in the movement.

A remontoir d’égalité can also be used for certain complications such as jumping hours and minutes that require a sudden burst of energy, without affecting amplitude.

Technically speaking, the force released by the mainspring as it unwinds is not linear, being strongest when the mainspring is fully wound and weakest towards the end of the power reserve. These differences are more marked in movements with a large power reserve, such as ten or 15 days, where the mainspring is longer and often thicker.

The remontoir in Bernhard Lederer’s Central Impulse Chronometer stores and releases energy at ten-second intervals in the CIC 44 and fifteen-second intervals in the CIC 39. It releases the same amount of energy regardless of the state of the mainspring, and continues to do so until the mainspring no longer has sufficient force to wind the remontoir’s spring. Theoretically, amplitude and rate remain stable from full wind to empty.

Two independent gear trains

Building on George Daniels’ independent double wheel escapement, and to achieve better energy efficiency than its illustrious predecessor, Breguet’s natural escapement, Lederer equipped his Central Impulse Chronometer with two independent gear trains, each connected to its own barrel, remontoir d’égalité and escape wheel. Both barrels contain a slipping mainspring to prevent excess tension and are wound by the same (and only) crown.

Optimised energy distribution

Breguet fitted his natural escapement with two escape wheels for a single gear train, thereby exponentially increasing energy consumption: a single train driving two wheels quadruples the energy required for the same acceleration. The torque (also referred to as the moment of force) required to rotate a wheel around its axis depends on the radius of the wheel and the force applied to the tooth.

Working from this paradigm, Bernhard Lederer came up with the idea of a double gear train.

Revised calculations and geometry

The tension angle and rearming interval of a remontoir d’égalité can vary according to the movement’s complications. In the Lange & Söhne Zeitwerk, for example, the remontoir spring is rearmed every minute and has a 60-degree tension angle. This is required to release, almost instantaneously, sufficient power to drive one disc as it jumps and three discs simultaneously at the turn of the hour.

In Bernhard Lederer’s system, the tension angle for the remontoir spring was calculated at 6.7 degrees. This corresponds to the force to be transferred to the escapement and, at the same time, to minimise energy used by the barrel, knowing that one wind of the mainspring equals an angle of 360 degrees.

So as to reduce energy lost as the gears mesh, the geometry of certain wheels was reworked. Also, some wheels are in titanium, which is almost half the weight of steel.

Gafner system

Energy for the remontoir comes from the main gear train and must be managed in a way that doesn’t affect the escapement impulses, to maintain isochronism. This mechanism, developed by Robert Gafner and improved by Derek Pratt, uses a small spring that is periodically recharged. A triangular cam precisely controls the energy released with a minimum of friction, thanks to a system guided by a jewelled fork. However, this system has a drawback: when mainspring torque is high, it increases internal friction. A lot of energy is lost because of this, resulting in less efficient transfer of constant force to the escapement.

Position of the remontoir d’égalité

Depending on the functions, required force and rearming intervals, the remontoir d’égalité can sit on different wheels; often the fourth wheel, nearest the escapement. Bernhard Lederer positions his remontoir further “upstream”, on the third wheel, to obtain a more linear and consistent supply of energy.

The single balance wheel oscillates at 3 Hz (21,600 vibrations/hour) and has eight weights: four to poise the balance and four for micro-adjustments. The latter rotate inwards or outwards to adjust the balance’s inertia.

A single central pallet, dubbed the Metronome, alternately controls the Central Impulse Chronometer’s two escape wheels to give impulses to the balance. This Metronome is fitted with two pallet jewels plus a central jewel with a concave surface, for direct impulses and reduced friction. Each phase in a standard escapement’s functioning has been optimised, without energy loss or friction, to obtain perfect and stable isochronism.

This is sophistication at the highest level, rethinking and recalculating previous optimisations, and taking advantage of modern lightweight alloys.

Bernhard Lederer does more than revisit historical or more recent inventions. He goes deep into calculations and devises new and practical solutions.

True to his philosophy and the DNA of his brand, he has created an escapement at the peak of performance, for chronometry that borders on perfection, without a tourbillon!