|Waltham 1908 model Balance|
Horologists please excuse some simplification and omissions here! Likewise I hope the non-specialist will not be too phased by a little bit of jargon.
What is the impulse jewel?
The impulse jewel is part of the balance wheel assembly, it is a small, often "D" shaped pin like jewel and is mounted on the "roller table".
The picture above right shows the balance assembly from an unrestored Waltham-1908 movement, the roller is the small disk in the centre of the picture with the balance staff (axel) going through its centre and the impulse jewel on the right - it is easier to see if you click on the image for a larger view.
By some clever engineering the impulse jewel also gives the balance a "kick" to keep it moving, it does this once every swing on a Swiss Lever escapement with a club tooth escape wheel and on every other swing with a traditional English Lever escapement with a ratchet type escape wheel.
On an average size 16 or so pocket watch the roller is about 3.5mm / 0.14" in diameter so you can see that the impulse jewel it pretty small. And sometimes they break.
Why do they break?
Often through human intervention or error! Of course age and long use can eventually break them and the shellac that traditionally held them in place can fail but more often I think the problem is normally one of two things:
|A shaped impulse jewel in the "roller"|
the entire piece is 3.5mm across.
Secondly a less violent knock can cause the impulse jewel to over-ride the pallet fork and end up on the wrong side ("overbanking"), it cannot then get back to the correct side and the watch stops. There is a system to try and prevent this occurring but it is not 100% effective, especially with a single roller.
At this point everything is recoverable if you know what you are doing. But there is what appears to be an irresistible temptation to try and force the balance back into action, and trying to force the balance to move against this little jewel will break it.
A broken impulse jewel can be a very big problem now, and in the case of some watches long out of production, it has been for at least 50 years. You see impulse jewels for most old watches and particularly those with single rollers are no longer made or available as New Old Stock (NOS). They also come in a huge range of shapes and sizes so trying to find a replacement from those that may be found can be very difficult.
This is particularly so for old English watches and as some of these have very long impulse jewels these are also the most likely the break. Larger sized watches in general are also more susceptible.
The Bodged repair
|My ancient roller removal tool.|
|Waltham 1883 model from 1903/4|
The roller is first removed, most easily with a tool like the one shown above.
A replacement roller then has to be found that is the right diameter, thickness, with the right diameter centre hole and of course the correct impulse jewel in the correct place on the roller.
|A replacement roller being pressed |
on to the balance staff.
The roller then has to be pressed into position with the pin precisely aligned to where the pallet fork will be so that the movement will in beat.