Constant force escapements are designed to maintain a consistent level of mainspring force at the point of impulse between the escapement and the balance. Maintaining even force application from a full wind to a nearly discharged mainspring is an eternal challenge to clockmakers, and it transferred its burden to watchmakers early in the pocket watch era.
Maintaining Even Force
During the late twentieth century, revived widespread interest in mechanical wristwatches inspired watchmakers to revisit the concept of the constant-force escapement.
Fusee systems, such as those used by pocket watch constructors of the seventeenth and eighteenth centuries, are not new, but their application in earnest to wristwatches is a recent novelty. Zenith‘s Georges Farve-Jacot family of fusee watches takes this, the most traditional route to constant force.
As the mainspring barrel discharges, a chain transfers from the fusee to the barrel, and the diameter of each cone-shaped (and grooved) structure is inverse; when the mainspring is largely discharged, the chain is riding the smallest diameter grove of the barrel.
At the same time, the remaining length of chain is riding the largest groove of the fusee. Relationship between the fusee, which is linked to the drivetrain of the watch, and the barrel, which turns the fusee, works exactly like the gears of a bicycle to maintain a constant force to the escapement.
Remontoir systems, such as those employed in several models by F.P. Journe, employ a second spring downstream of the mainspring barrel – the power supply. For historical perspective, Journe was the first manufacturer to apply a remontoir to a tourbillon wristwatch.
The additional remontoir spring, which may be a linear spring against an index wheel or a hairspring that buffers two separate wheels of the drivetrain, absorbs the energy of the mainspring and releases it to the second downstream elements of the drivetrain including the escapement and balance.
A remontoir spring is designed to absorb a set amount of mainspring energy before each transfer to the second portion of the drivetrain. In doing so, the remontoir acts like a small quantized power reserve that provides consistent packets of energy even as the mainspring is wound completely or nearly discharged.
The final and newest form of constant force system is the elastic/silicon system as used by Ulysse Nardin in its “Anchor Escapement” and Girard-Perregaux in its “Constant Force” escapement. In each case, the inherent flexibility of a synthetic band is the basis for the transfer of energy from the mainspring to the escapement.
Up to a certain point, the silicon band will resist deformation under pressure from the mainspring’s drive train; at a specific loading, the elastic band, which acts as a 21st-century remontoir of sorts, will snap forward and convey an impulse the balance. The mainspring never impulses the balance directly because the synthetic spring always acts as a buffer, conveys its quantized impulse energy, and snaps back into position to receive the next discrete packet of mainspring energy.
This system, which was introduced first to a production wristwatch by Girard-Perregaux, won GP the 2013 Auguille d’Or at that year’s Grand Prix d’Horlogerie de Genève – the Oscars of watchmaking.