Constant force describes an escapement that gives the balance and hairspring impulses of the same strength, whatever the state of wind of the mainspring. It is a precision device. The push on the balance stays the same whether the mainspring is fully wound or nearly run down.
How does a constant-force escapement work?
The force a mainspring gives changes with its state of wind. A constant-force escapement places an elastic part, usually a flexible blade, between the escape wheel and the balance. In the Girard-Perregaux Constant Escapement, the blade is buckled into an S shape. It is elastic and has two stable positions. As the balance swings, it makes the blade flip from one curve to the opposite one. Each snap of the blade gives the balance an impulse of the same strength. The even force therefore comes with every oscillation, not over a longer stretch of time.
Where it comes from
The idea is old. The English horologist Thomas Mudge (1715–1794) designed constant-force mechanisms in the 18th century. Charles Haley was granted a British patent for a constant-force escapement in 1796. Breguet patented one for clocks in 1798. Much later, in 1996, Nicolas Dehon, then a watchmaker at Rolex, came up with the idea behind the Girard-Perregaux design. Watchmakers have made parts from silicon since the 2000s. Girard-Perregaux’s 2008 version made the frame, lever and blade spring as a single silicon part, using photolithography.
Not to be confused with
A remontoir also evens out force, but it does so further up the gear train, before the escapement. It uses a secondary spring to power the escapement. It evens the force over a stretch of time, not at each oscillation. The fusée is an older way to reach the same result. It was used in clocks and pocket watches from the 15th century. Both are constant-force devices in the broad sense. Neither is a constant-force escapement, which works at the escapement itself, between the escape wheel and the balance.
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