Fusee Clocks – what and why

Ok I did an opinion article and I owe the blog a good tech article for the clock enthusiast.

Spring driven clocks are not accurate without a fusee movement. Thats a massive controvercial statement in clock terms. It means that your grannys 1960 mantle clock probably tells the times accurately twice a day. You dont notice because you get the time from your phone or your watch or the TV. But its true.

To qualify this I am making this statement only in the context of pendulum clocks with a short (say 6 inch max) pendulum. The almost direct opposite applies to long case clocks with long pendulums.

A pendlum clock can be simplified into three elements. 1. Power 2. Gearing 3. Regulation. This is the device that divides up time. It could be a pendulum or a balance wheel or a verge escapement or and atomic decay cycle etc. Notice that I have not included any category for hands and face because I clock does not have to show you its working to be accurate. Displays vary. Mostly they are dials but then you get barometric devices that output a line to a graph paper on a drum that rotates on a weekly basis. Its still a clock in essence.

Now, given that you have these three element the most important thing is that everything continues to whizz and whirr at EXACTLY the same rate. This is achieved with item 3, Regulation. Regulation in a pendulum clock is pereformed by…the pendulum.

A pendulum with a fixed centre of gravity will continue to swing at the same rate if no friction is applied. The friction is the bend in the spring at the top of the pendulum, the light touch on the crutch fork on each swing and air resistance. A pendulum of about 1 metre will swing at 1 second per swing. It will only slow down because of the friction wind/string/crutch friction. Forget the crutch thing – just consider wind and the bendyness of your pendulum as the things which cause friction.

Ok so you swing the pendulum with pallets. Describing this is going to be a challenge. So…

The spring is the power plant that turns/pushes the cogs leading up to the top of the clock. Its essentailly a mini engine connected to a gearbox of gears. By the time the spin has been passed from cog to cog – usually about 3 “wheels” (cogs) the last cog is spinning freely at hundreds of revolutions per minute vs the very slow crawl of the power plant at the start of the gearing down process.

So what you end up with is a turning wheel at the end of all that. The wheel is cut so that it looks rather like a cog but is nothing of the sort. Its rather like a cog but each tooth is a sharks fin. Theres a reason for that.

You see that top sharks tooth wheel is the thing that makes the pendulum swing. It does it by getting each tooth, in turn, to push a pallet. A pallet is a bit of flat metal connected pretty directly to the pendulum. So as the sharks teeth pass the pallet it nudges the pendulum one way and by the time the pendulum swings back it blocks the path of the pallet again and the process repeats.

Even if you didnt really understand that what may have occured to you is that the push from the sharks tooth has to be hard enough to counter the friction from from the air and spring friction.

If the sharks tooth is pushing hard enough the clock keeps going becuase the pendulum keeps swining, and if its stops then it means the pendulum is not being pushed hard enough by the pallets due to dirty gearing and no oil.

That was so hard to explain in a few paragraphs. If you didnt understand it then you may be suffering from “brain cavities”. I think I may have brain cavities because Ive just read it back and, bearing in mind I wrote it, I can barely understand it. Right. Whats a fussee.

I said your grannys clock isnt accurate. Then I explained how a clock works. The reason I did this is that if you get the first bit about a clock being a simple device to measure how many times a pendulum swings a minute, then the fusee is an easy extension of this.

The whole point of having a spring and a gearbox in a clock is to push just enough energy into the swing of a pendulum to counter the friction forces. Its a tiny amount of power to add to the each swing. Imagine swinging a your friend on a playground swing and keeping them going at the same rate by breathing in their direction. If you wanted to keep your friend swinging you just covid cough in her direction and off they would swing. Interestingly they would swing a bit further if you coughed hard and less so if you just coughed normally.

Now, a spring fully wound will cough harder than one at the end of its infection. This means that Newtons laws of physics are violated by the input of extra power to the system thereby creating an artificial, and therefore intrinsically innacurate method of regulation should the extra power going into the system reduce. Which it does.

The reason I just scientifically upped the stakes with my last sentence was that your so far through this that Im betting you will carry on….maybe. I will never know to be honest as Im too busy to check the website statistics which all appear to be in Greek – Its all Greek to me.

So the fusee is an ice cream cone shaped cotton reel, or larger. that acts as a variable ration gearbox in the cog sequence in which it is second. The first is the spring. The spring drum and the fusee ice cream cone are connected via a chain or wire which wraps around one or the other. If its wrapped round the spring drum the clock is unwound. If its wrapped in ever increasing diameter loops round the ice cream cone then the clock is fully wound.

[When the clock is fully wond it has a stop system inside that wil not allow you to move the key further. Its solid and distinctly different from the normal wind pressure. The reason its there is so that you dont give it the beans and snap the fusee wire – these clocks have and easy but long wind to accomodate the “gearing”.]

Fusee movement were invented……ages ago. I have to idea but Ive seen them on clocks to 1700 and I think 1690 (im not sure it – was an expensive bracket clock) so quite early on. I think the reason it did not become a standard feature were multiple

  1. Difficult to engineer the ice cream cone drum on which to wind the wire from the spring drum. This is not at easy bit to make. Firstly you have the problem that it cant be lathed without a lathe capable of operating from a copy or being programmed. Imagine having to make a perfectly accurate helter skelter from brass. In 1680. Evidence of Aliens as far as Im concerned.
  2. Maintenance. This system is more complicated. It has less set pieces you could stamp out of a sheet or cut with a jig. The implications of the string snapping on a full wind (which is obviously the point at which its most likely to snap) are slightly horrific. All the energy from a 55mm x .5. x 100mm spring releases into the clock putting immense recoil pressure on the first wheel. This means the gearing on the ice cream cone goes flat. You loose a tooth or two. This is SPECTACTULARLY expensive to get repaired. I wont do one. Affordably.
  3. Its all very well having these brilliant methods of transferring torque and mixing things up a bit to get what you need. It is. You see the majority of fusee clocks we see are not 17th century guilt portable chronometers (pffft – accurate to a minute a day if its working), they were school and station and company clocks. Utilitarian devices. They were designed simply, and oddly, to a build profile common to all manufacturers, and they HAD to show the right time clearly, hence the tendency for mid sized (12″ white face) to large clocks . You cant miss your train because the clock is worng. You cant be late for work and argue your clock is more accurate than your employers. These clocks, these municipal and railway and council office clocks are BRILLAINT. They say so much more about the leap of our society where you didnt turn up and 8.47am and leave at 5.25. You worked 9 to 5. Hard. The other delightful thing about them is that wood they used was not veneered. You get solid plank with varnish and polished. The simplicity of the external design and the high specification of hard wearing woods is fit for purpose, but of course we know the longer terms benefits now. Most have knock mark which shows its not from Marks and Spencer and there is even a key on some (“I wasnt late – look at the clock”). The star is of course the movement. The plate, pallets, arbours, bushes, the whole lot could have come out of a  Messerschmidt 109e (plane, ww2, sorry. It was awesome) for the build quality. They cost a sunken galleons treasure chest to restore mechanically but your cant repair good quality with bad quality. Well you can actually. Punch bush, flatten and reposition the escapment, polish the arbours. To do it properly you need a spring change and that bit is not fun. Its 55m of .5mm carbod steel held with alloy bendy wire that has to go into an aperture with 2mm tollerances, usually with some sweaty mods to the coil size with a spring winder. If it uncoils half way through Im Ok. Thats because I wear a full really expensive full leather apron, gloves fit for Apollo and one down from a welding mask.