pocket watch mainspring dimensions and replacement

[luiazazrambo]

Hello Clock and Watchmakers,

I have an old key wound pocket watch where I think i should replace the main spring. I have a few questions here where i would love to read about you opinion.

I start with a statement or observation. The mainspring barrel of this watch had a "geneva stopwork", but it is broken now, possibly the owner used too much force with the winding key. Somebody has removed the maltese wheel when it was not functioning any longer. This video explains how the geneva stopwork helps with isochronism: geneva stopwork 

First question which is probably the easiest one, is the end of the mainspring ok or is this the result of some creativity?

Second question. When i measured the mainspring height the outer coil was 1.98 mm and as you went in it got shorter and shorter and the inner coil is only 1.85 mm. Is this a manufacturing issue? Is this how a mainspring would wear out by touching the lid or the bottom of the barrel, would it wear out this way? Or is it just because it was designed in this way?

And my last question, would a modern alloy metal mainspring provide the same power as a blue steel providing both have the same dimensions? I always wondered on this one and it is a general question, not about this particular watch.

Best regards,

lui

[clockboy]

I have not worked on a Geneva but replacing the spring with a modern alloy spring should be fine. Noticed that the stop lever is a tiny bit bent this may cause issues. I am not sure how the mainspring is held in the barrel and I have not seen a new mainspring with that style of end. Someone who has had experience with a Geneva might be able to give advice.

[JohnR725]

12 hours ago, luiazazrambo said:

The mainspring barrel of this watch had a "geneva stopwork", but it is broken now, possibly the owner used too much force with the winding key. Somebody has removed the maltese wheel when it was not functioning any longer.

there is one other reason why it's missing a piece? at some time in the past when it was serviced either because somebody was lazy they didn't put it back on or they just didn't want to figure it out. So just never put it back together. It unfortunately is all too common that one or both pieces are missing.

12 hours ago, luiazazrambo said:

First question which is probably the easiest one, is the end of the mainspring ok or is this the result of some creativity?

that is one of the ways that people use to do ends. Or as you put it is basic creativity.

12 hours ago, luiazazrambo said:

When i measured the mainspring height the outer coil was 1.98 mm and as you went in it got shorter and shorter and the inner coil is only 1.85 mm. Is this a manufacturing issue?

though it should be the same with typically unless? If somebody got creative in other words they didn't have the right size mainspring and they modify it that's what you might be seeing.

then the general prevailing theory for most people is that modern mainsprings for the same thickness will be stronger then the original blued steel spring. But there's some other things that come into play that make download more complicated like the particular curvature the spring. So basically you really don't want to go any thicker than whatever you have now a lot of it will depend upon the escapement you have.

[oldhippy]

I would say the the reason why one part of the geneva stopwork is missing is because the screw hole is worn, this is a common fault. That old mainspring needs to be replaced with a new one, it doesn't matter about the alloy that today's modern ones are made off. It isn't an expensive movement they are common, so it isn't going to affect its value.