7750: Zero-Reset Hand Alignment

Casing up a chronograph is significantly more complicated than a two or three-hand watch, not just because of the extra hands, but because they have very specific alignment requirements.

Most chronographs use the same system to reset their chronograph hands: reset hammers strike "heart cams" on the chronograph wheels, and the cams' geometry brings the hand back to zero regardless of where it was pointing.

Chronographs using a classic design, like the Lemania 1873, use a solid reset hammer that requires filing for adjustments.

The 7750, by contrast, uses a self-centering reset hammer, which pivots at the head to apply equal pressure to the second and minute counter wheels. This design doesn't require filing, but it does make the minute counter adjustment necessary. Each has its advantages and disadvantages.

Either way, the reset hammers work the same—the lowest point on the cam is between the two "top" heart lobes, so the hammer's pressure will force the wheel to turn until the hammer face is resting in that low spot.

The hammers strike with a tremendous amount of force, and the wheels themselves have significant momentum from the strike. In this slow-mo, you can see how far the seconds and minute hands overshoot their marks before bouncing back and forth to zero. It all happens in a split second (this video is 1/32 speed), and puts a lot of force on the hands themselves.

Because of these forces, chronograph hands must be installed extremely tightly, lest they wander around during repeated reset maneuvers. The hands also must line up perfectly with their zero marks every time, which can make the hand-setting process a long one. You have to test the hands from every position, and make sure that the heart cams are lubricated evenly. Unfortunately, parallax error makes the chronograph second hand seem to be a little left-of-center on reset in the photos in this article, so I'll have to get better shots at some point!