Fusee (horology) - How IT Works

How It Works

The mainspring is coiled around a stationary axle (arbor), inside a cylindrical box, the barrel. The force of the spring turns the barrel. In a fusee clock, the barrel turns the fusee by pulling on the chain, and the fusee turns the clock's gears.

1. When the mainspring is wound up (Fig. 1), all the chain is wrapped around the fusee from bottom to top, and the end going to the barrel comes off the narrow top end of the fusee. So the strong pull of the wound up mainspring is applied to the small end of the fusee, and the torque on the fusee is reduced by the small lever arm of the fusee radius.
2. As the clock runs, the chain is unwound from the fusee from top to bottom and wound on the barrel.
3. As the mainspring runs down (Fig. 2), more of the chain is wrapped on the barrel, and the chain going to the barrel comes off the wide bottom grooves of the fusee. Now the weaker pull of the mainspring is applied to the larger radius of the bottom of the fusee. The greater turning moment provided by the larger radius at the fusee compensates for the weaker force of the spring, keeping the drive torque constant.
4. To wind the clock up again, a key is fitted to the protruding squared off axle (winding arbor) of the fusee and the fusee is turned. The pull of the fusee unwinds the chain off the barrel and back onto the fusee, winding the mainspring. The presence of the fusee means that the force required to wind up the mainspring is constant; it does not increase as the mainspring tightens.

The gear on the fusee drives the movement's wheel train, usually the center wheel. There is a ratchet between the fusee and its gear (not visible, inside the fusee) which prevents the fusee from turning the clock's wheel train backwards while it is being wound up. In quality watches and many later fusee movements there is also a maintaining power spring, to provide temporary force to keep the movement going while it is being wound. This type is called a going fusee. It is usually a planetary gear mechanism (epicyclic gearing) in the base of the fusee "cone") which then provides turning power in the opposite direction to the 'winding up' direction therefore keeping the watch or clock running during winding.

Most fusee clocks include a 'winding stop' mechanism to prevent the mainspring and fusee from being wound up too far, possibly breaking the chain. As it is wound, the fusee chain rises toward the top of the fusee. When it reaches the top, it presses against a lever, which moves a metal blade into the path of a projection sticking out from the edge of the fusee. As the fusee turns, the projection catches on the blade, preventing further winding.