How A ‘Fusee’ Watch Works

Part 1

Date: Thu, 23 Oct 97 17:06:34 -0700

From: Tuan Ngo <>

Subject: Fusee

Hello Richard,

Fusee (French word): means something conical (in fact fusee with an accent on the first e common meaning is used for calling a rocket, else in the old days I think it is a common name for the shuttle as used on the fabric weaving machine) More related to the world of time keeping device the fusee have been invented in the early 16th century: it is an assembly not unlike the main spring but with a difference, the purpose to deliver constant torque through pinions and wheel to the escape movement. The end result is constant time keeping when the main spring is fully wound or when it is at the end of the power reserve. Implementation: It is a spirally grooved pulley (in a shape of a cone or) placed between the main spring barrel and the the center pinion of a watch or clock to convert the varying force of the main spring barrel to a constant torque. A chain is coiled around the fusee spiral, the first coil is on the larger diameter (base) of the fusee and the last coil at the smaller end. As the main spring rundown, the barrel rotates and coils onto itself the chain from the fusee: when the main spring is fully wound it pulls on the smallest diameter of the fusee resulting in a large force X small diameter, but when the main spring rundown a smaller force X large diameter, if the spiral diameter is chosen properly then the torque (force X diameter) applied to the center pinion is constant. There are 2 types of fusee: the “going fusee” and “plain fusee”. The “going fusee” is designed to maintain power to the pinion when the main spring is being rewound.

The fusee seems to be an elegant solution to the main spring problem (not counting the volume required by the fusee and chain), but back to the fundamental accuracy problem if the spring has a high spring constant and the barrel has a high power reserve (for example 80 hrs in the Blancpain tourbillon or 7 days in the Hebdomas), after a period of 24 hours the change in torque delivered by the main spring should be negligible (compared to issues related to proper balance/hair spring adjustment in position and heat/cold) so that I believe the advantage of the fusee is negligible. On the other hand it is a feat to package the fusee in a wristwatch (Lange tourbillon?) and like the minute repeater the practicality of these complications are questionable, although from an engineering stand point it is a chef d’oeuvre to admire!