A horizontal clutch is a chronograph coupling that links the timekeeping train to the chronograph by moving a wheel sideways until its teeth mesh with another wheel. It is the older design. Because the teeth must line up, the seconds hand may twitch on starting, and running the chronograph adds drag to the going train.
How does a horizontal clutch work?
A wheel sits on the pivot of the fourth wheel and turns with it. That wheel drives an intermediate wheel, called the clutch wheel. When the chronograph is started, a lever moves the clutch wheel sideways. Its teeth then engage the chronograph seconds wheel, and the chronograph runs from the timekeeping train. The two wheels meet side by side, in the same plane. That is where the name comes from, and why it is also called a lateral clutch. The Valjoux 7750 uses this kind of clutch, together with cam switching.
What are the drawbacks of a horizontal clutch?
Gear teeth have to line up as they engage, so the chronograph seconds hand may twitch slightly when started. While the chronograph runs, the lateral clutch puts extra drag on the going train. Engaging the chronograph can also briefly lower the balance amplitude. None of this stops the design from being used. Horizontal clutches came first, and a few brands still choose them on purpose.
Not to be confused with
A vertical clutch does the same job in a different way. It grips two wheels stacked on one axis by friction, instead of meshing teeth side by side. With no teeth to align, it starts the seconds hand without the jump a horizontal clutch can produce. The oscillating pinion is another variant. It links the fourth wheel directly to the central chronograph seconds wheel.
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