Replacing the missing pendulum

Clocks often fall into a state of not working many years before they are presented to a clock repairer. The worst case scenario is when the owner takes it apart, fails to fix it and then stores the parts in a box for years. Being presented with a box of bits and the prospect of missing parts is always an interesting challenge!

A more common situation is when you are presented with a clock but the pendulum has been lost and you are asked to simply find a new one! They are all the same surely, just a standard item you’d have in stock!

Of course its not as simple as that as there are so many different types of clock out there made by different companies all of which made clocks with different numbers of teeth on the wheels and pinions and of course different lengths and styles of pendulum.

I was presented with a French striking clock recently with a missing pendulum so I’ll use this blog to explain the process of working out how long to make a new pendulum.

Assessment, repairs and cleaning

First let’s look at the movement of the clock. It is made by A.D Mougin and has a count wheel strike movement which was full of fluff and dust!

This is a picture of the back of the movement showing the count wheel. The spacing of the count wheel allows the hour to be struck, and one bell sounding for the half hour.

And this is the front with the motion work already removed. The click springs are not doing their job and holding the clicks in place at all.

And a look what what we have between the plates!

It was pretty obvious that this was going to need a lot of cleaning work so there is only one thing to do, take it apart.

Faults found when looking closely included:

• Click springs cracked and not acting on clicks
• General damage to most screw heads
• Escapement pallets worn
• Escapement arbor front pivot bent
• Strike spring eye torn out
• Some bent teeth on the escapement wheel

It certainly could have been a lot worse. I was expecting scored pivots and oval pivot holes in the plates, but it wasn’t that bad!

Here are some pictures of the issues, starting with the pallets:

The solution here is simply to move the pallets along the arbor so an unworn section of the pallets engage with the escapement wheel. You can see the square arbor section which is slightly tapered so a tiny amount of filing of the square (having removed the pallets from the arbor) will allow the pallets to be moved along slightly.

The springs whilst inside the barrels didn’t look too bad in that they were not set.

However after extracting the spring for cleaning, I wasn’t impressed with this which is going to need the end remaking before disaster strikes!

Nothing too challenging to be done in terms of repairs, although getting the tips of the escapement wheel straight and evenly spaced did takes some time with a pair of tweezers all under a microscope.

The movement was cleaned with white spirit and a stiff brush mostly. Serious grime on the plates was gently removed by using a grade 0000 of wire wool. Note that I do not use ammonia based cleaning solutions; they are just bad in my opinion.

A replacement pendulum

Before you even think about replacing the pendulum you need to do a train count (counting the teeth on the wheels and leaves on the pinions). From this you can work out how long one swing of the pendulum takes and from that with some basic calculations work out the theoretical length.

Lets start with the train count. For our purposes we only need to count the teeth on the centre, 3rd and escapement wheels and pinions. These are as shown in the table below:

	Wheel	Pinion
Escapement	40	7
3rd	70	7
Centre	84	8

Next we need to work out how long it takes for each arbor to turn. The centre wheel wheel we already know as this is connected to the minute hand. So an hour has 60 minutes each with 60 seconds, so it takes 3600 seconds to turn.

Moving to the 3rd, if 84 teeth of the centre wheel engage with 7 leaves of the 3rd, we can see this rotates in 7/84 * 3600 = 300 seconds.

Then to the escapement wheel, 70 teeth on the 3rd engage with 7 leaves of the escapement pinion, so the this rotates in 7/70 * 300 = 30 seconds.

So the escapement wheel has 40 teeth, so in one rotation of the wheel there will be 80 beats (a complete swing of the pendulum from one side to the other). In 30 seconds we will have 80 beats, so one beat (T) takes 30 / 80 = 0.375 second.

This means there are 9600 beats per hour.

So having this information, we are ready to calculate the length of the pendulum. The formula for this is shown below where L = Length in metres, g = acceleration of gravity at the earths surface.

So substituting our values we can see:

So lets call it 140mm, but how do we measure this 140mm as this is the theoretical length? I find the best approach is to estimate it from roughly the centre of the free length of suspension spring to about the centre of the pendulum bob.

Don’t expect a perfect timekeeper from these crude measurements, but it will get it close enough for you to be able to use the rating nut to correct it. My first attempt on the clock above got me to within 2 minutes a day; I was pleased with that.

A timing machine will help you to get the rating corrected faster. The clock here should be running at 9600 beats per hour. I recommend the MicroSet timer although it is only for sale in the US. I arranged to purchase my timer on a trip to California a few years ago and it has proven to be an invaluable piece of kit.

If you are based in the UK, then pendulum parts are available from Meadows and Passmore or if you are in the trade then Cousins is also a good alternative.