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Second hand revolves in 50 seconds, not 60, on brand new movement

AvatarBob

Registered User
Jun 4, 2016
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I worked up the nerve to replace the upper gear-and-stem that holds the second hand on the new movement with the equivalent piece from the old movement, but it did not fix the problem, so I put the pieces back in place. So the next thing to try seems to be replacing both the upper gear-and-stem AND the lower gear that drives it.

What is the correct way to remove the lower gear in the photo below from the new movement? After careful examination of the old movement, the equivalent lower gear looked like it used a simple friction fit, and I carefully and successfully pulled it straight off the old movement with a pair of pliers without causing any noticeable damage. But I'd like to be a little better educated before trying anything like that brute-force approach on the new movement.

IMG_2506.JPG
 

Willie X

Registered User
Feb 9, 2008
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Two matching pry bars, equally spaced 180° apart. You may have to make them. They need to go in close to, or touching the shaft, and I wouldn't try to hard. The gear could have been pressed on with heavy pressure. If so you won't be able to remove it without a press and the proper fixtures.

I'm assuming you finally got the tooth count figured out? If not, no need to proceed ...

Willie X
 

RJSoftware

Registered User
Apr 15, 2005
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I'm counting 40 teeth on bottom gear and estimating 32 on top. Anyway, to remove that gear you could use hollow punch but be careful about bending/breaking pivot.

You can set shaft vertical in bench vice, loose, pinion gear set on ledge. Set hollow punch on top and hammer on top.

You can drill hole in wood to serve as ledge too.

The mesh between the two gears looks very shallow, this might work in your favor as a matching larger gear might fit. Even if you only had to fudge bushing hole a tiny amount. But I'm guessing you wont have to do that. Someone put wrong gear on top.

Yes, I'd only change top gear in your picture. Then test mesh and do something simple like open bushing holes a little, if it was to tight mesh.
 
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AvatarBob

Registered User
Jun 4, 2016
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Thanks, Willie and RJ. I am not mechanically inclined, and removing the gear sounds too risky for my low skill level, so I'm glad to have learned that before I got myself into trouble. And I finally learned for sure why I can't get this to work with the gears I have, I believe.

This is what I learned, which I suspect many nice people in this thread were trying to tell me, but which I didn't understand until now. I'm going to run through it here in case I'm still wrong, or if anyone else is interested in my gears-for-dummies education.

I was thinking that because both movements tick at 3600 bpm, and because moving the easily-removed top gear (the one that holds the second hand) from the old movement to the new movement still let the new movement function (albeit with the same 50-second rotation of the second hand that I'm trying to eliminate), that moving BOTH gears from the old movement to the new would produce the desired 60-second rotation of the second hand on the new movement. I thought that perhaps there were differences in the various gear sizes, tooth counts, and/or gear distances that were causing the different rotational periods of the second hand.

However, I got definite counts of the teeth on both old gears and both new gears from pictures I had taken when I was moving the one gear and could clearly see all four gears, and saw that the tooth counts were the same (30) on the old and new top gear that holds the second hand, and the same (36) on the old and new bottom gear on the escape wheel's axle. That prompted me to do more research, about how gear sizes affected things, and I finally found some articles that I understood better than before.

I learned that gear size by itself, tooth count by itself, and distance between gears by itself don't determine whether gears mesh. Rather, gears only mesh when the ratios of their "pitch diameter" to their tooth counts are the same. So if one gear has twice the "pitch diameter" of a smaller gear, the gears will only mesh if the larger gear has twice as many teeth as the smaller gear. And for every rotation of the larger gear, the smaller gear would rotate twice. That would be a "tooth for tooth" movement on both gears.

I had never heard of "pitch diameter", and I don't follow the explanations of how to determine it, just the general concept. But it appears to be less than the diameter between the tips of teeth and greater than the diameter between the bases of teeth. So not something that I could measure directly. But because my gear pairs DO mesh, I don't need to determine it: on both movements, a 36-tooth gear on the escape wheel's axle is meshing with a 30-tooth gear that holds the second hand. Even if the corresponding gears' diameter and teeth sizes are slightly different, as long as they mesh and turn together, both pairs of gears should produce the same second-hand rotational speed from the same escape-wheel-attached gear's speed.

Facts:

  • I could see on the new movement that it was ticking 60 times per minute, that the 36-tooth gear was directly on the escape wheel's axle, and that its second hand would rotate once in 50 seconds.

  • I could see on the old movement that it was also ticking 60 times per minute, that its 36-tooth gear appeared to be on the escape wheel's axle, and that its second hand would rotate once in 60 seconds.

  • In any meshed gear pair, there is a "tooth for tooth" movement when they rotate; and the gear diameters, tooth sizes, and tooth counts don't affect that.

Looking at the facts like that, it seemed that the only possible reason for the difference in the second-hand speeds would be a difference in the speeds of the rotation of the escape-wheel axle that held the 36-tooth gears.

I could see the escape wheel fairly well on the new 5-tube movement and had confirmed that it was rotating once every 60 seconds. But I hadn't realized that I had never been able to confirm that on the old movement because it was obscured by the big 9-tube pin roller. I had just been ASSuming, without giving it any thought, that the old movement's escape wheel was also turning once every 60 seconds because both movements are 3600 bpm and so both tick exactly once per second.

But I'm sure most of you know that's not always the case, and now I do, too. I'm scrapping the worn-out old movement, so I didn't have to worry about putting parts back together, so I removed the 9-tube pin roller to get a better look at the escape wheel. And found that it rotates once every 72 seconds, not every 60.

Aha.

So: On the old movement, in 60 seconds, the escape wheel turns 60/72 of a rotation, and so does the 36-tooth gear on its axle. So that bottom gear advances by 60/72 * 36 = 30 teeth in one minute. That causes the 30-tooth second-hand gear that meshes with it also to advance 30 teeth in one minute, which is exactly one rotation for that 30-tooth gear. So the second hand moves at exactly one rotation in a minute on the old movement.

But on the new movement, in 60 seconds, the escape wheel turns one full rotation, and so does the 36-tooth gear on its axle. So that bottom gear advances 36 teeth in one minute. That causes the 30-tooth second-hand gear that meshes with it also to advance 36 teeth in one minute. So the second hand rotates 36/30 of a circle per minute, which means that it takes 30/36 of minute to rotate exactly one time on the new movement. And 30/36 of a minute is 50 seconds, which is what I was seeing.

So the original issue I was seeing is due to an escape wheel with a different tooth count in the new movement. And that's the end of that story. :p
 

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