Periodic drop in amplitude every 5 minutes

[steve1811uk]

Hi, I have an Orient 'Explorient' 48748 movement that is experiencing a periodic drop in amplitude every 5 minutes. The drop in amplitude is from 260 to 235 degrees and lasts for around 90 seconds. I suspect an issue with the 3rd wheel that I think has a rotation period of five minutes. I am going to strip down to investigate. Any idea what could be causing this? Has anybody seen this before and fixed it? Unfortunately I do not have a spare 3rd wheel or know how to source the part as orient parts seem to be very hard to find. I am hoping to find some piece of dirt on a pinion leaf. Thanks in advance.

[clockboy]

Look at the suspect wheel with good magnification it could be a bent tooth.

[watchweasol]

Hi Have you recently dismantled it/serviced it , If not as clockboy said look for a damaged wheel or some dirt jammed in a pinion causing a bind .

[steve1811uk]

1 hour ago, watchweasol said:

Hi Have you recently dismantled it/serviced it , If not as clockboy said look for a damaged wheel or some dirt jammed in a pinion causing a bind .

It has been serviced. I inspected the third wheel just now and found no issues. Also working out the timing of that wheel indicates a rotation every 9 minutes 22.5 seconds (not 5 minutes). I think I just worked it out out in my head at least, cannon pinion (12 teeth I think, 1 tooth every 5 minutes) pushes against the teeth of the minute wheel. The friction will be at a high point every five minutes if the minute wheel is binding. Need to get the dial off and give those wheels another clean and check for binding.

[watchweasol]

It doent take much to interfere  with the workings of a watch a slightly bent tooth , wheel not flat or even out of round all will cause intermittent problems. inspection under magnification  required I think.

[steve1811uk]

Wasn't the cannon pinion, it had 15 teeth. Also removing the minute wheel had no effect on the issue. I think that puts the issue is on the motion side. One thing I have not managed to check is the centre wheel pinion. If that has 12 teeth then that will be it. Will investigate further how the barrel meshes with the centre wheel pinion.

[steve1811uk]

Centre wheel has been found to have 12 pinion leaves each one transitioning every 5 minutes. Barrel has been found to be tilting due to worn bottom arbor hole. The tilting causes the meshing of the teeth to become tight (dropping amplitude) as the barrel tooth transits across the pinion leaf. I can see wear points on the brass barrel teeth indicating that. I am going to close the barrel arbor hole with a rounded punch to try and get the barrel to run flatter. Will post progress soon.

[steve1811uk]

Well the watch is back on my wrist and the periodic amplitude variation is better but not gone completely. In the process of working on the issue I actually made the situation much worse with amplitude dropping to 180 degrees from 220 degrees. What I had done to cause that was to close the oval upper barrel arbor hole and make round with very little play. What I had done in effect was to move that barrel arbor hole back and away from the centre wheel and fixed the barrel arbor in a slightly tilted position. This caused the barrel teeth to run at the top of the centre wheel pinion and actually ride up the pinion and bind every 5 minutes. I fixed the issue by using cutting and smoothing broaches and staking set (6 or 7 small indents on the side of the arbor hole that was away from the centre wheel) to effectively move the barrel arbor hole back towards the centre wheel. Now getting 270 degrees at full wind and dropping to 255 degrees periodically (probably not solvable without replacing parts, centre wheel pinion and barrel teeth have some wear). All's well that ends well.