Cause for regular variance of around 30 degrees in amplitude

[patchwerk]

I am working on a 18/0s Elgin 484 movement from the 30s. After cleaning, and doing a lot of work on the balance it runs but the amplitude varies in a regular pattern by about 30 degrees. Because of the regular pattern I figured it was a problem with the train, and replaced the train wheels from another movement just like it. The exact same pattern emerged. The x scale in the images below is in hours and the period of the pattern is around 6 minutes. Any ideas?

 

Thanks!

[Nucejoe]

Are there endstones on any train arbor and have you checked them?

[Nutiborskoku]

It could also be the mainspring, irregular wear of the barrel arbor hole or top barrel hole in the base plate or barrel bridge (side shake too high) or the endshake of the barrel itself.

[praezis]

Period of those waves is 6 seconds.

Escape wheel needs 6 seconds per revolution...

Frank

[AndyHull]

3 hours ago, praezis said:

Period of those waves is 6 seconds.

Escape wheel needs 6 seconds per revolution...

Frank

I presume from that, the culprit is likely to be a dirty, damaged, or worn tooth on the escape wheel.

My money is on a tiny spec of dirt, so small as to possibly require a microscope to see it, causing just sufficient drag to create that periodic "limp" in the motion of the escapement. 

I had a similar issue, I posted about here.

 

Edited September 15, 2019 by AndyHull

[patchwerk]

Thanks for all the responses!

@praezisThe period is 6 minutes, not 6 seconds (the x axis is hours:minutes on that chart), so I do not believe it is the escape wheel. The six minutes makes me think it is the third wheel, but I have swapped it out for others twice with the same effect.

@Nutiborskoku,I have used different barrels and arbors with it but will check the barrel bridge and mainplate holes.

@Nucejoe, no endstones.

[AndyHull]

Does it do this with the hands removed?

Might it be a canon pinion issue?

https://elgintime.blogspot.com/2012/01/mechanics-of-canon-pinion-friction.html

Do the hands turn correctly and uniformly?

 

Edited September 15, 2019 by AndyHull

[praezis]

3 hours ago, patchwerk said:

Thanks for all the responses!

@praezisThe period is 6 minutes, not 6 seconds (the x axis is hours:minutes on that chart), so I do not believe it is the escape wheel. The six minutes makes me think it is the third wheel, but I have swapped it out for others twice with the same effect.

@Nutiborskoku,I have used different barrels and arbors with it but will check the barrel bridge and mainplate holes.

@Nucejoe, no endstones.

Oh, you are right, thank you!

But the principle stays the same, you can find the source this way. 6 minutes can be

- the revolution of a wheel, or

- the passing time of two meshing teeth

Frank

[patchwerk]

5 hours ago, praezis said:

Oh, you are right, thank you!

But the principle stays the same, you can find the source this way. 6 minutes can be

- the revolution of a wheel, or

- the passing time of two meshing teeth

Frank

The six minutes interval roughly correlates with the 3rd wheel in a standard 18000 movement (right?). How would I calculate which other wheel pinion and teeth interactions happen that regularly? I have more scrap movements of these types and can try replacing more of the train wheels but would love to learn how to approach this more surgically. 

[praezis]

To calculate, you need teeth numbers of wheels and pinions and known revolution times (e.g. 2nd wheel, 4th wheel).

Once I calculated these times for a calibre that I often work on (18000, small second):
1 tooth of the barrel (=2nd wheel pinion tooth) = 5 minutes
1 revolution 3rd wheel = 7.5 minutes

I found that most often the tooth is to consider, less often the wheel.

Frank
 

Edited September 16, 2019 by praezis