Stackfreed

A stackfreed is a simple spring-loaded cam mechanism used in some of the earliest antique spring-driven clocks and watches to even out the force of the mainspring, to improve timekeeping accuracy.

[7] The force of the mainspring, transmitted through the clock's gears, gives pushes to the oscillating balance wheel which keeps time.

The primitive verge and foliot movement used in all early timepieces was very sensitive to the amount of force applied to it, particularly before the balance spring was added in 1658; the weaker the drive force applied by the mainspring, the slower the balance wheel would oscillate back and forth.

Drawings of stackfreeds appear in Leonardo da Vinci's Codex 1 (1492-1497) and M3 (1497-1499); possibly the device was brought to his attention by his German assistant Giulio.

As the clock runs and the mainspring unwinds, the cam rotates and the spring bears against the narrower parts of the cam, reducing the retarding force gradually, to compensate for the declining force of the mainspring.

[7] This restricted the mainspring to the center part of its range, further reducing force variation.