The balance wheel swings to the left and the right. Halfway, the impulse pin enters and leaves the horn of the pallets.
The number of degrees of rotation of one beat is called the Amplitude. The amplitude of a modern watch should be between 275 and 315 degrees. For a vintage watch, an amplitude of 250 to 300 degrees is acceptable.
So, a strong amplitude is important, but a balance wheel can only rotate so much. At one point, the impulse pin will travel so far that it’ll knock against the outside of the horn of the pallets.
This is very bad because it affects timekeeping, and it can also damage the pallets or, more likely, the impulse pin.
What causes it?
This is a good example that progress can sometimes have adverse consequences as well.
There’s been a lot of progress in watchmaking. Modern mainsprings are made of white alloy, and they’re much better than the older ones that were made of blued steel. The blue steel ones would quickly lose power because they would set and lose their springiness. White alloy mainsprings are ‘unbreakable’, and they don’t get ‘set’.
Modern cleaning and modern lubrication are wonderful compared to the old methods of benzine, a hand brush, a box of wood dust, and animal fats.
Modern cleaning solutions get the jewels and pinions squeaky clean. As a result, friction is greatly reduced. This combination of much less friction and a stronger mainspring can cause the amplitude to increase so much that the movement starts to knock.
How can you spot knocking or rebanking?
Don’t worry. Rebanking is easy to spot because the sound is very characteristic. The repeating sound of the impulse pin hitting the horn of the pallets is very much like a galloping horse.
The following video shows the impulse pin hitting the horn of the pallets, and you’ll also hear the distinctive sound.
It’s also visible on a timing machine. The image below shows a typical pattern that you might see on the timing machine.
You can see sections that look like the usual two parallel lines of dots that are interrupted by sections of noise.
The balance wheel picks up speed and the amplitude climbs until the amplitude is high enough to knock. The knocking is represented by the dots that are all over the place, just like noise.
The amplitude drops because of the impact, and that’s why the healthy lines return. Until the amplitude gets high enough to start knocking again. This pattern repeats itself.
It’s also possible that the amplitude is so high that the impact doesn’t cause it to drop low enough to make the knocking stop. In this case, the timing machine will show the dreaded snowstorm of doom on the screen.
There’s an easy test if you want to check that it’s really knocking and not something else (besides the characteristic sound).
Remove all power from the mainspring and wind the crown just a few times. This way, the amplitude of the watch is much lower. If the issue is gone, you know that it was knocking.
How do you fix it?
If you can, fit a weaker mainspring. Try one that has a strength or thickness that is 0.005 or 0.01 lower. A new mainspring that’s slightly weaker will probably be more like the original mainspring the movement was designed for.
If this isn’t possible, there are some tricks but they go against the nature of a watchmaker.
Normally, you want as little friction as possible with a high amplitude. In this case, you want to lower the amplitude and one way to do that is to create more friction.
You could, for example, use a heavier oil in the escapement and wheel train. You can also lubricate the pallets bearing jewel or you can underoil the pallet stones. It’s not ideal but a knocking watch is useless.
The final methods aren’t very elegant but at least you’ll be able to use the watch.
Do you have any experience with a knocking watch? Let me know in the comments below.