Wednesday, 31 December 2014

The dreaded bodged impulse jewel "fix".

Waltham 1908 model Balance
A frequently fatal (to the watch) previous repair that I find quite frequently, both recently done to be able to sell a watch and others done many years ago.

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.

Waltham 1883 movement. The lever
pallet is the dark bar like piece to the left,
the pallet forks are on its right side.
The escape wheel is below it (not quite in
position). This is laid out tangentially  as
an English lever but has a club tooth
escape so is a Swiss Escapement.
As the balance oscillates to-and-fro the impulse jewel contacts the pallet (lever) fork on each swing flipping it from side to side - the "tick" of a watch - this lets the escape wheel at the other end of the lever (on a Swiss escapement) or to its side (English escapement)move on one tooth for each swing and through gears this makes the second hand turn once a minute, the minute hand turn once an hour etc.

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.
A violent knock causing the balance to swing round violently and the impulse jewel hitting the pallet fork from the wrong side, as the fork can't move twice in one direction but only back and forth this stresses the impulse jewel and perhaps breaks it. The same can happen if the watch is fitted with a mainspring that is too strong for it and the balance turns by more than 360 degrees, although less violent if this goes on for a long time the jewel will eventually break.


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.

The Problem

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.
So when faced with this problem the impulse jewel is sometimes replaced with a metal pin, possibly shaped but often not. This may very well work quite satisfactorily for a while but the pin, particularly if brass will eventually wear and if steel may very well damage the pallet forks. Then you have a problem because the pin will probably now be stuck in the roller and if you can get it out the roller is likely to be damaged.

The repair

Waltham 1883 model from 1903/4
Although spare impulse jewels or rollers with a jewel fitted are available for a few types of American and Swiss watches  the alternative these days is usually to find a spare roller from a scrapped movement complete with impulse jewel, at least it means that those donor movements that were beyond economic repair will give life to another watch.

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.
This is may not be easy even when the movements are the same make and model and made at about the same time, the Waltham 1883 model shown had a brass pin replacing the impulse jewel and I had to try five rollers from other similar movements before I found one that would fit on the staff securely and that would work the action.

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.

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