Fusee movements are rugged in construction, and their success rests with the fusee, a conical-shaped component (or more accurately a hyperboloid) that evenly regulates the power output of the spring.
Most fusee movements had anchor escapements, though some had lever escapements or deadbeat escapements. Earlier fusee movements had decorative sculpted pillars and pins to hold the plates whereas later models had plain pillars with screws. It is one way to tell how old the clock is.
Jacob Zech is credited for inventing the fusee in 1525 although drawings of a fusee appear in Leonardo De Vinci’s notebooks.
Parts of a fusee movement
a) Spring drum
b) Winding arbour
d) Main wheel
e) Chain (or gut or wire)
Of course, the above diagram does not show the wheels above the main wheel and merely illustrates the essential components of the fusee section.
A chain-driven fusee is the norm although wire and gut are common. Gut was used at first but around 1650 chains began to be used, which lasted longer.
So, how does it work?
The spring drum’s axis is parallel to the axis of the fusee. Around the drum is wound a chain that is attached to the fusee at its largest diameter. An attached mainspring is coiled inside the drum that sits next to the fusee. The run-down condition of the fusee movement is seen in the diagram and the photo above it.
When the key is inserted into the arbour of the fusee it pulls the chain causing the drum to rotate. Because the inner end of the spring is attached to the drum, the drum rotates until the spring is completely coiled. The spring is then at its maximum tension. At full winding, the chain is completely wound onto the fusee.
As the spring begins to uncoil the chain is on the smallest diameter of the fusee. The pull on the spring drum becomes less as the diameter of the fusee increases. The diameter of the fusee becomes larger giving greater leverage.
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 and that is why fusee clocks are better timekeepers than conventional spring clocks.
Fusee movements are generally more expensive than conventional spring drive movements. Double and triple fusees add to the cost and complexity.
Disadvantages? They are difficult to adjust. If the fusee chain broke, the force of the mainspring sends the end whipping about the inside of the clock, potentially causing damage. Collateral damage (if any) must be addressed as part of the repair. Should a replacement mainspring be required it must be exact otherwise the fusee had to be adjusted to the new spring.
Despite their complexity and challenge to service Fusee clocks have retained their value over the years and are sought after by serious collectors.