Friday, 7 March 2014

Geneva Stop Gear on a 19J Rotherham, 1902

Geneva Stop Gear.
Click on an image for a larger version.
In my last post on isochronism I mentioned the Geneva Stop Gear, the next watch I opened up for restoration was a very nice size 12, 19 jewel Rotherham from 1902 – the first I have had in for months – and it not only has Geneva gear but it is fully function rather than broken or removed to give a longer run time.

So, here is a picture of it (Top) and I will attempt to explain how it works.
The mainspring is in the barrel with the gear teeth on its top edge and that powers the watch via the centre wheel pinion, it rotates around the arbor (axel). One end of the mainspring it attached to the inside of the outer wall of the barrel and the other end is attached to the arbor. To allow the spring to be wound, the arbor has a longish extension on the other side that goes through the face plate and is pinned to one of the winding gears (the left most gear in the second picture).
The 12 Rotherham 19J Keyless, 1902.
On the other side of the barrel (shown) there is a collar fitted to a squared off section of the arbor which has a small “nib” protruding (at 5 o’clock to the centre in this picture). This nib engages the cut out in the Geneva gear. In this picture the spring has been pre tensioned just over one turn and is as it will be with the watch fully wound down, if you look at the piece of the gear clockwise from the nib you can see that it is locked against the collar having a convex profile so it cannot lose that pre-tension.

The segment anticlockwise however is concave as are the next 3 segments and when the watch is wound the “nib” moves anti-clockwise pushing the Genva gear a little clockwise and after one full circle it will engage in the next cut out, this cycle can be repeated three more times until the nib reaches the next convex section of the gear at which point it will be locked in a similar way to the position shown thus preventing the spring being wound further.
The arbor is prevented from turning backwards by the “click” or ratchet operating on the winding gear, bottom left of the pinned winding gear in the second picture.
As the watch runs the barrel rotates around the arbour until it comes back to the state shown in the picture and the watch will stop. But during those four revolutions the spring will not have the very weak power from a fully wound down spring, because of the pre-tension, and will not have the maximum power of a fully wound up spring because of the four turn limit.

One more method of making a watch more accurate but see also my post Does non-functional geneva gear matter?

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