Quarter repeater mainspring thickness

[AlanB]

In repairing a generic Swiss cylinder quarter repeater pocket watch with no repeat mainspring I'm trying to work out how to determine the size of a suitable spring.  Barrel ID 7.6mm, Arbor 2.77mm height around 1.45mm.

Clearly only one turn is used plus setup, but the only actual figure I can find in Saunier who says in the previous half century 3.5 turns was considered good (and Saunier is about 1880).  Calculating off that would suggest about a 0.15mm thick spring 128mm long. That feels intuitively a little thick, but I don't have another repeater conveniently to hand to check so any pointers welcome.  Photo shows the repeat train assembled without the bridge since that hides everything.

 

Alan

 

[nevenbekriev]

Yes, the arbor usually makes about 3 to 3.5 turns. But usually spring takes 2/3 to 3/4 of the free space in barrel, not 1/2, so take it for the calcullations. This way the change in torque is smaller.

I have a picture for You, this one is little older, but no mater