HEART PIECE

The cam in the shape of a heart, or simply the heart piece, is the component that resets the counters of a chronograph. It is a cam whose profile, shaped like a stylized heart, has a radius that varies continuously between a maximum, at the pointed notch, and a minimum, at the opposite end.

Rotating in unison with the chronograph wheel or the counter it is fitted to — seconds, minutes, or hours — the heart piece receives, when the pusher reset, the impact of an articulated hammer. Regardless of the angular position of the heart piece at the moment of impact, the tip of the hammer slides along the profile toward the smallest radius and drives the wheel until it settles into the notch of the point, thereby invariably returning the hand to zero, no matter when the timing was stopped. A chronograph with multiple counters generally has a separate heart piece and hammer for each counter, actuated simultaneously by the same reset lever.