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THE CONSTANT-FORCE ESCAPEMENT

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1. Definition and Principle

The constant-force escapement is a precision mechanism designed to deliver to the balance-hairspring rigorously identical energy impulses, regardless of the tension level of the barrel. It belongs to the category of complications that improve precision. Its purpose is to isolate the regulating organ from the fluctuations of the driving force inherent to the progressive unwinding of the mainspring. To do this, it inserts between the train and the balance an intermediate elastic organ — generally a flexible blade or a small spring — which is recharged at constant tension with each beat and then delivers an impulse that is always identical. This principle differs from the remontoir, which acts upstream of the escapement within the train. The constant-force escapement, on the other hand, integrates energy regulation within the escapement mechanism itself.

2. The problem of variable driving force

Every mainspring of the Barrel develops a force that varies according to its state of tension. A fully wound spring exerts a maximum torque on the train. This torque decreases as the spring unwinds. In a typical movement, the difference between the force transmitted to the Balance at the beginning and at the end of the power reserve can be significant. This variation disrupts the Isochronism of the balance-spring, since the amplitude of the oscillations depends directly on the energy received at each impulse. A higher amplitude generates a different rate error than a lower amplitude, due to isochronism defects specific to the Hairspring (balance spring). The greater the force variation, the more the Amplitude varies, and the more irregular the running becomes depending on the winding state. It is to correct this fundamental flaw that horologists (watchmakers) developed constant-force mechanisms.

3. Distinction from the Remontoir

Two major families of mechanisms seek to equalize the driving force transmitted to the Regulating organ. The Remontoir is a device placed in the Train (wheel train), between the Barrel and the Escapement. It accumulates the energy from the Barrel and releases it at regular intervals, in equal amounts, regardless of the mainspring’s tension. The Constant-force escapement, on the other hand, integrates energy regulation directly into the Escapement mechanism itself. The Blade or intermediate spring is recharged to a set tension at each beat by the Train (wheel train), then releases to transmit its Impulse to the Balance. The distinction between the two families is not always absolute in horological literature, and some designs combine both approaches. The constant-force escapement is nevertheless characterized by the fact that the equalization of energy occurs at the very level of the Impulse transmitted to the Balance, rather than upstream of the Escapement.

4. The constant-force blade: operating principle

The central component of the constant-force escapement is generally a flexible blade or a laminar spring, called a constant-force blade. This blade is positioned between the Escape wheel and the Balance. At each beat, the Escape wheel, driven by the Train (wheel train), comes to re-arm the Blade to a position defined by a stop or a calibrated release point. This re-arming position is identical at each cycle. The Blade, thus tensioned to the same value each time, then transmits its energy to the Balance in the form of an Impulse of constant Amplitude. The Key to the mechanism is that the tension of the Blade at the moment of Impulse does not depend on the energy supplied by the Barrel, but solely on the geometry of the re-arming system. As long as the Barrel has enough energy to re-arm the Blade to its reference position, the Impulse delivered to the Balance remains identical.

5. The Impulse and re-arming cycle

The operation of the Constant-force escapement takes place in two distinct and alternating phases. During the Impulse phase, the constant-force Blade relaxes from its re-arming position and transmits its energy to the Balance via a pallet stone or an impulse pin. This energy is constant and calculated to maintain the Balance’s Amplitude within an optimal range. During the rewinding phase, the Escape wheel advances one step under the effect of the Train and brings the Blade back to the defined tension position. This rewinding occurs during the half-Oscillation of the Balance that does not receive an Impulse, so that the mechanism does not interfere with the free movement of the Regulating organ. The entire cycle must be designed so that the rewinding is always complete, even at the end of the Power reserve, in order to guarantee the constancy of the energy until the reserve is exhausted.

6. Types of designs

Several geometric approaches allow for the creation of a Constant-force escapement. The most common uses a flexible Blade made of Steel or Silicon, stretched between two fixed points and rewound by a beak or a pin attached to the Escape wheel. A variant uses a miniature spiral spring acting as the intermediate part. Some designs place the Blade in a straight line within the plane of the Movement, while others adopt an L-shaped or arc-shaped configuration. The Silicon, used since the 2000s for the manufacture of horological components, offers notable advantages for constant-force Blades: its high Elasticity, absence of internal friction, and insensitivity to Temperature variations make it a Material of choice. Some exceptional achievements employ a remontoir interposed between the center wheel and the Escape wheel, thus integrating the principles of the Remontoir and the Constant-force escapement into a single mechanism.

7. Historical achievements

The idea of delivering a constant force to the Regulating organ dates back to the 17th century, when Horologists (watchmakers) sought to eliminate irregularities in the driving force in precision instruments. In England, the Horologist (watchmaker) Thomas Mudge (1715–1794) designed in the 18th century constant-force mechanisms applied to precision timekeepers. John Harrison, in his work on marine timekeepers, also developed original solutions to regulate the force transmitted to the Regulating organ. In Switzerland, in the 19th century, precision Horologists (watchmakers) designed constant-force watches intended for observatory competitions, where the regularity of running throughout the entire Power reserve was rigorously evaluated. These instruments, produced in very small numbers, aimed to eliminate rate variations linked to the winding state. Their complexity limited their production to a few exceptional pieces, generally made for observatories or scientific institutions.

8. Design Difficulties

Designing a constant-force escapement raises several interdependent technical challenges. The first is the precise definition of the resetting tension: any variation in the resetting position of the blade directly affects the energy of the impulse and cancels out the benefit of the mechanism. The second challenge is managing friction within the resetting mechanism. Variable friction between the resetting beak and the blade introduces uncertainty in the final tension of the blade, a source of timekeeping error. The third challenge is robustness at the end of the power reserve: when the barrel is nearly unwound, the energy available to reset the blade may become insufficient, causing an abrupt stop rather than a gradual slowdown. The use of silicon for the constant-force blade has significantly reduced friction and improved the repeatability of resetting. Today, the constant-force escapement remains a rare complication, requiring design and adjustment expertise that few workshops fully master.