• Graham Powis Member of the British Horological

Fusee Clock Service & Repair

Updated: Feb 18

The first mechanical clocks appeared in the 13th century, these early mechanical clocks were weight driven. Clocks that are weight driven have the advantage that the power supply to the escapement is constant throughout the winding cycle, the main drawbacks are the clock's size and non portability as a result. It soon became apparent that clocks needed to occupy a smaller space and have a degree of portability.

It's not known exactly when the first spring driven clocks appeared but it was probably the early part of the 15th century. The immediate problem was the constantly changing force applied to the escapement by the spring as it wound down within the barrel, invariably the clock would run fast at the beginning of the wind and then slower as the spring was loosing power towards the end of the wind. No one can be exactly sure who invented the Fusee but the invention is attributed to Jacob Zech of Prague around 1525.

Fusee Mechanism

Shown above is a line drawing of the Fusee mechanism. Basically a key is inserted at the squared end at the top of the Fusee cone and the chain or line is wound clockwise around the Fusee cone, the operation is checked by the Fusee stop work which prevents the chain or line being wound off the end of the cone thus jamming the works. The barrel which contains the mainspring exerts a force and winds the drive line back around the barrel.

The clever part is that as it does so the line is pulled from an increasing radius on the Fusee cone as the line is wound back around the perimeter of the spring barrel. While this is happening the spring is loosing power but because the line is being wound back from an increasing radius the lesser spring power is compensated by the larger radius from which the line is being wound back around the barrel. To summarise, at full wind the line or chain is at the top of the cone {thinner end} and at the broad end when the spring has almost wound down. Thus, the force is somewhat constant throughout the winding cycle.

In the name of brevity it isn't possible to delve into the complex physics set up by the spring acting on the Fusee cone or indeed the Fusee stop work. But now lets have a look at the reinstatement of a twin Fusee clock movement dating from the later Georgian era.

Fusee Clock Movement

Here is the Fusee Movement as removed from the clock case. It has been reported that the clock isn't releasing the strike at the hour? After having a good look over the clock it was noted that the wheel pivot oil sinks had blackened. This is a sign that the clocks lubrication has dried up and needs replacing.

There are some' repairers' that will add some fresh oil to your clock DONT BE TEMPTED TO ALLOW THEM! the new oil will mix with the old oil creating a 'gunge' which will wear moving parts in double quick time and flow here it's not wanted. There is only one way to lubricate a clock and that's to thoroughly dismantle and clean it first; see our blog "Longcase Clock repair", a conservation approach.

So now the clock has been dismantled into its component parts, from this more problems have been noted.. The same nasty cheap oil has been used all the way through the clock even on the mainsprings, On a Fusee almost more than any other clock movement the correct viscosity of oils in the correct place is paramount. A molybedenum mainsping grease for the springs up to a light Microgliss oil nearer the top end of the gear trains.

The clock mainsprings of a Fusee are big and powerful, the correct method to remove them must be employed. Ignoring this will damage the spring and could cause imjury to the person removing the spring.

Having now removed the springs and measured them unfortunately they are certainly too big for the spring barrels in which they were fitted..I wound one back in to make sure but when the spring coils were wound up they formed a hump in the barrel, there is only one thing for it, replacements will have to be sought. Pictured below are the springs as removed from the barrels.

Clock Springs

As you can see the springs fitted to this clock are quite large, there's a lot of power stored up when the coils are wound tight..It is such a shame that the wrong springs have been used here, these spring aren't worn out and ordinarily would have been cleaned, re-greased and replaced in the spring barrels.

The clock will be cleaned using the conservation approach mention in another blog and Replacement springs will be fitted to the mainspring barrels.

Clock springs

This picture shows the new springs fitted into the barrels, these springs are 2.5 metres long yet the barrel in which they fit is about 80 millimetres in diameter. These new springs fit the barrel properly.Previously the inner and outer hookings didn't align on the old springs thus when the spring was tightened the coils of the spring would start to rise from the centre, absolutely no good at all..

Fusee line and barrel

The Fusee drive lines have now been attached at both ends and the covers for the barrels snapped back into place. Drive lines are always replaced on a service, lines made of phosphor bronze are favoured over ones made of steel, a phosphor bronze line doesn't mark the barrel unlike a steel one which all too often leaves a mark on the barrel perimeter. The bronze lines are more supple but still have the necessary strength required for the job.

Having prepared all the parts for reassembly we can now consider putting the clock back together. This part is very important, absolute cleanliness is essential. The new drive lines will have to wound neatly back in place, if there was any signs of a kink in the line the clock movement would have to be dismantled and the process started over.

Fusee clock movement

So! The clock mechanism is now partially reassembled, as mentioned earllier there is only one chance at getting the lines wound on correctly; if the lines twist or tangle whilst being installed the clock movement will have to come apart again.

Note that this clock movement has five pillars, one at each corner and one above the spring barrels. This gives the extra rigidity needed, these springs are very strong and the extra pillar makes all the difference to the rigidity of the movement.

Pictured above the lines have now been installed on to the Fusee cones. Shown here is the assembled clock which is fully wound, note the drive line pushing against the stop piece at the bottom right of the picture. It's vital that the stop work is operating correctly, here the stop piece has engaged into the notch at the top of the Fusee cone due to the drive line pushing it towards the locking notch position now the clock cannot be wound any further due to the locking action of the stop piece.

Some of the purists may say: "The drive lines should be gut, these are what would have been fitted originally. Our Answer "They break suddenly without warning"

The purists also say " well alright use nylon, it looks like gut but is less prone to break". Our Answer. "Nylon stretches under load thus you are defeating the object of the Fusee, at least the gut doesn't stretch.

Any restoration carried out on an artefact must be reversible, if it is decided that gut drive lines must be fitted for originality then it's only a case of dismantling the clock. As this particular clock is going to be used as a house clock then phosphor bronze is the best option.

This clock is now working perfectly. The final task is to set the preload on the mainsprings, this is done in order that the pull on the Fusee lines is the same {or as near as can be} throughout the clock's weekly winding cycle. In this particular case the clock will be tested through two complete winding cycles because new mainsprings have been fitted, this is so the new lines and springs can be checked for correct preload when the clock has wound down.. This Clock is now sitting on it's bracket once again keeping time in the Customer's home,


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