A heart-shaped cam used in chronograph mechanisms to reset the central chronograph hand instantly to zero. Distinct from the spiral-profiled snail cam used in striking mechanisms.
The heart cam, often called a coeur (French for heart), sits at the centre of a chronograph's reset mechanism. When the reset pusher is pressed, a hammer snaps against the cam's pointed edge and slides down its curved flank until it settles at the narrowest point, instantly returning the chronograph hand to zero regardless of where it was mid-sweep.
A chronograph typically uses one heart cam per chronograph hand being reset, so a full chronograph with running seconds, minute, and hour counters may use three. It's worth keeping distinct from the snail cam, a spiral-profiled component used in striking mechanisms like minute repeaters to count out chimes, a completely different function despite both being cam-based components.
It resets a chronograph hand to zero. When the reset pusher is pressed, a hammer snaps against the cam's point and slides down its curved edge until it settles at the narrowest point, snapping the hand instantly back to its zero position.
The heart shape gives the cam a continuously narrowing profile from its widest point to its point, which is exactly what allows the hammer to always slide down to the same zero position regardless of where the hand was when reset.
No. A heart cam resets a chronograph hand to zero. A snail cam has a spiral profile and is used in striking mechanisms like minute repeaters to count out a number of chimes. They serve entirely different functions.