Valjoux 7758 mainspring replacement.

[bangy55]

My first time servicing a 7758. Started to open the mainspring arbor and was surprised to find what appears to be a small gear affixed (permanently?) to the bottom end of the arbor. Feels like some kind of Teflon material. I assume it drives the moon phase wheel? Any help here would be greatly appreciated. Not sure how to proceed since if it isn't supposed to be removed from the arbor I'm not sure how to deal with the spring.

Edited February 21, 2021 by bangy55
Problem solved.

[bangy55]

Sometimes the obvious answer to a problem is just too... well, obvious. The pinion isn't attached to the arbor, it's attached to the barrel, which of course, makes perfect sense.

[clockboy]

That is an interesting problem. It is obviously friction fitted to the arbor but not being made of metal makes its removal difficult. Wondering if when the barrel lid is removed it also lifts the wheel. If it was my watch I would have a play if it as a customers I might well just leave it alone.If the wheel becomes damaged it could be a big problem returning a broken watch is not recommended.

[bangy55]

1 hour ago, clockboy said:

That is an interesting problem. It is obviously friction fitted to the arbor but not being made of metal makes its removal difficult. Wondering if when the barrel lid is removed it also lifts the wheel. If it was my watch I would have a play if it as a customers I might well just leave it alone.If the wheel becomes damaged it could be a big problem returning a broken watch is not recommended.

I appreciate your observations and conclusion Clockboy, because they are exactly the same as mine as I turned off the lights and went to bed. Ten minutes later, it dawned on me that it was possible, but unlikely, the pinion could be attached to the barrel and not the arbor. Since the turning of the arbor is intermittent, it wouldn't make sense in a train designed to keep track of the passage of time. The barrel however, operates continuously. So, I jumped up and attached a pin vise to the ratchet side of the arbor and turned it while I watched the geared end. I was relieved to see the arbor end turning and not the leaves. I felt silly, but it does seem to be a backassward arrangement on first blush. Thank you for your interest and input. It is a customers watch and yeah, I sure didn't want to return it worse off than when I got it.

[clockboy]

Yes I agree a strange design indeed. But that’s what it is and it works, why it was decided to make a gear out of resin is also strange. The never ending learning curve.

[nickelsilver]

The pinion is in beryllium copper and it's friction fitted to the cap. It isn't to be removed in service, other than removing the lid itself. It drives the hour counter, and also acts as the bearing for the lower barrel arbor surface. If you take the barrel apart, note that this cap fits over the barrel, rather into it. You will want to get a screw driver thin enough to fit the gap, and give a little twist (carefully), working your way around. It will pop right off after a few twists.