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The Fusee Mechanism Described
Single Fusee Movement
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The fusee (pronounced fu-say, from the French fusele, which means tapered or spindle shaped) was probably invented around1450, though some feel it was earlier. As the mainspring of a watch unwinds, it delivers progressively less torque to the wheel train, which, all else being equal, will make the watch run slower.

The fusee is a device for converting the varying torque of the mainspring as it unwinds into a more or less constant torque at the center (or minute) wheel pinion. The fusee is very similar, in principle, to an automobile transmission, although the fusee is a continuously variable gear (as opposed to the discreet 4, 5, or 6 gears of most autos).

The fusee is a tapered, spiraled, grooved pulley which is placed between the mainspring barrel and pinion of the center wheel (like the auto transmission, between engine and drive wheels). A very fine chain (or, earlier, cat gut string), is wound around the fussee. (Envision a cone, pointed end up, with a groove starting at the bottom and spiraling up to the top). The first coil is at the larger end (bottom) of the cone (where the chain is attached), the last coil at the smaller end (top) and running from there to the spring barrel. As the mainspring unwinds, the chain is pulled off the fusee (from the top, small end) and wrapped around spring barrel. With the spring fully wound, torque is reduced because the barrel is much larger than the small fusee coil (gear ratio). As the spring unwinds, the chain "moves down" the fusee and operates on progressively larger spirals of the fusee. This increases the torque being put out by the fusee to the center wheel pinion, because the effective fusee diameter is becoming progressively larger in relation to the diameter of the barrel. The result is an even output of torque from the fusee to the center wheel of the train.

When the fusee movement is wound, it is the fusee that is turned by the key or stem winding mechanism: the chain is wound off the mainspring barrel (turning the barrel) and on to the fusee. For this reason, a fusee driven watch normally stops during winding because the fusee is turning backwards (from its normal running rotation). Many later fusee movements use a "maintaining power" mechanism (usually a planetary gear mechanism in the base of the fusee "cone") which keeps the watch running during winding. Such a fusee movement is called a "going fusee."

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