Unknown gear – Seth Thomas movement

[jpewslgl]

I have a Seth Thomas movement from an antique clock that I am going to be tearing down and cleaning soon. Before I do, I always take lots of time to mess around with the gears and take photos to see where everything should be when I go to re-assemble the movement.

I have come across a couple of gears that I have no idea what they are for. They are shaped like a clover and are located just below the winding arbors on both springs. It appears that one of the winding arbors has a small tab that connects and turns this gear, but then nothing else. Can anyone help me with this mystery?

Thank you in advance for all your help. I really appreciate everyone’s time in answering my questions!

Jimmy

[willofiam]

Hey Jimmy, I think what you have there is a gear that will only allow the mainspring to wind up and let down only so far. I had something similar on a different style movement, Seth Thomas 89A You would have to set it up right so you can get as much of the mainspring wound as possible. I could be wrong though, from the pics it is hard to tell what your little lever actually does, let us know. thanks William

[Bob Tascione]

Hi Jimmy,
William is correct. That is a “stop works” arrangement. It serves two purposes. One is to prevent forcing the spring too far when winding. The other and most important is to help the spring deliver an even, steady amount of power to the movement. When you look at a torque curve of a normal spiral mainspring you’ll see that the drop in force when unwinding is non-linear and is much more consistent after it’s been unwound a turn or two from full wind. The final wind delivers far more torque than the previous turns as the drop in power is non-linear. Also the “drop” in force is very rapid when nearing it’s last winds. This stop work will allow the spring to work between these two extreme points. When setting this stop work up I wind the spring fully with the locking finger lifted away from the locking wheel and then back the spring off one full turn. I then set the stop works up so the finger is in contact with the locking surface (the wider section of the locking wheel which is at about 4 o’clock in your picture). The finger can now enter into each slot, indexing the locking wheel forward by one slot with each rotation of the winding arbor for 6 or 7 slots until it’s stopped once again by the locking surface.

Hope this helps Jimmy,
Bob

[jpewslgl]

Thanks for the replies, that makes sense. But, of course, it brings up another issue The other winding arbor (for the chime) has the same stop wheel, but it appears that the locking finger has been broken off. Presumably by someone overwinding the mainspring. Any idea how to fix this, or where to locate a replacement? With the modern mainsprings, is it as much of an issue as it once was?

Jimmy

[Bob Tascione]

Hi Jimmy,
Not sure where or if you can buy that part. It would be pretty simple to make, especially having the one on the time side to copy. I’m guessing here but being on the strike side it’s probably not nearly as important as it would be on the time side. I’ve come across many movements that have had the stop works removed or intentionally made inoperable. This is especially true when it comes to the Geneve and Maltese Cross stop works in older watches. I personally like to bring them back to original keeping with the spirit of true restoration.

Hope this helps.
Have fun!
Bob

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