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Mainspring strength questions

cspinner

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May 9, 2022
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How much strength does an additional 0.02mm of height add to a spring? I have a Felsa 690 bidynator that is overbanking like crazy (see attached). I pulled the mainspring and it appears to be white alloy. Width is 0.11 mm (per spec posted on Ranfft), but the height was 1.42mm (compared to spec of 1.40mm). However, I’m not certain that slight of difference would cause this extreme behavior.

prior to this, both the 4th wheel and escape wheel were sluggish as they had zero end shake. I had to adjust the jewels as necessary to correct that. I am wondering if a prior watchmaker replaced the mainspring with a stronger one to compensate for that. Before servicing this movement, the amplitude was really low (<180). Would like to get opinions on whether I’m barking up the wrong tree.
 

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gmorse

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Hi cspinner,

The strength is directly proportional to the height and inversely proportional to the length, but proportional to the cube of the thickness, so the latter has a far greater influence. I doubt if the slight difference in height would have any great influence on the effect you're seeing. The video is really too short to see what's happening, if you post it on YouTube or similar and then link to it in your post, it will be more efficient.

Regards,

Graham
 
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cspinner

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Hi cspinner,

The strength is directly proportional to the height and inversely proportional to the length, but proportional to the cube of the thickness, so the latter has a far greater influence. I doubt if the slight difference in height would have any great influence on the effect you're seeing. The video is really too short to see what's happening, if you post it on YouTube or similar and then link to it in your post, it will be more efficient.

Regards,

Graham
There was a slow motion aspect to it that got removed on the forum uploader. Here it is on YouTube. If you look carefully, the balance wheel is swinging all the way 360 degrees in both directions and slamming against the fork with tremendous force each way.

I’m at a loss what it could be if not the mainspring. With the balance removed, I could tell by manually nudging the pallet fork back and forth that it seems far stronger than I would expect. These movements also have this double wheel that drives the sweep hand pinion/arbor (attached). Do you think that has some form of adjustment to control power and it somehow loosened up?

9999B913-0854-46DE-8F2D-6B32BBB71175.jpeg F2484EF4-AD4B-4A86-8A2F-052134F588F1.jpeg
 

Skutt50

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Mar 14, 2008
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It is of course impossible to tell from your photo but the mainspring to me seems to be thicker than 0,11mm.......

Did you measure it to 0,11? Is it a proper mainspring for an automatic movement?

Your thought about the mainspring being replaced with a stronger to compensate for the binding jewels is quite possible. It could be worth a try to replace the mainspring with a weaker.
 

cspinner

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May 9, 2022
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It is of course impossible to tell from your photo but the mainspring to me seems to be thicker than 0,11mm.......

Did you measure it to 0,11? Is it a proper mainspring for an automatic movement?

Your thought about the mainspring being replaced with a stronger to compensate for the binding jewels is quite possible. It could be worth a try to replace the mainspring with a weaker.
Yes, I mic’d it in a few places and it’s definitely 0.11 mm. What’s confusing is my width measurement of 1.42mm is larger than what’s posted on Ranfft, but according to this photo of a NOS spring, it’s still 0.03mm smaller than it possibly should be.

C75A4E5A-121E-48CD-B54B-5D9D394C2F99.png
 

gmorse

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Hi cspinner,

You can check the inside height of the barrel to see whether the spring will fit in there properly.

As this is an automatic movement, Skutt's question is a valid one because the outer bridle on auto springs is different, it allows the spring to slip when fully wound, but you do need to use the correct braking grease on the barrel wall, (nowhere else!).

Regards,

Graham
 
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cspinner

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May 9, 2022
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Hi cspinner,

You can check the inside height of the barrel to see whether the spring will fit in there properly.

As this is an automatic movement, Skutt's question is a valid one because the outer bridle on auto springs is different, it allows the spring to slip when fully wound, but you do need to use the correct braking grease on the barrel wall, (nowhere else!).

Regards,

Graham
Yes, it is a two piece automatic mainspring. The bridle is wound into the barrel and the tail of the spring catches the notch in the bridle. I should note that the watch was working consistently (though losing 4-5 minutes per day, and low amplitude) before I started the service. The automatic winding functioned fine as I did a test wear for a few days without needing to manually wind it up.

Do you know if these Felsa's came with a blued steel spring from the factory? I am not sure how to check that and am unsure how common these alloy springs were in the 40's. Since the white alloy variety tends to be stronger than the blued steel, replacement with the same dimensions will result in a stronger performance. If the watch originally came with a blued steel spring, then that could contribute to what I'm seeing.

Even in slow motion, the balance swings with such force it's hard to visually track. It almost looks like it can make a whole second revolution if it weren't for the jewel slamming against the fork.
 

praezis

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Feb 11, 2008
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Since the white alloy variety tends to be stronger than the blued steel, replacement with the same dimensions will result in a stronger performance.
Don't worry about that! There is no "the white alloy spring". Some kinds are a tiny bit weaker, some a tiny bit stronger than steel springs. Generally they replace blue steel ones just fine.

Frank
 
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WoodyR

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Jan 27, 2021
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I recently serviced an old manual-wind ETA movement and replaced the very set mainspring with the recommended size. It knocked at anything over about 80% wind. I replaced the 9010 on the balance pivots with HP1300. That had little effect. I bought the next weaker mainspring and returned to 9010 on the settings. All good now. My takeaway is this: sometimes I'll just have to buy more springs to find the one that works.

To echo what Graham wrote, you're using the correct amount of the correct braking grease in the correct spot, right?
 
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cspinner

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May 9, 2022
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I recently serviced an old manual-wind ETA movement and replaced the very set mainspring with the recommended size. It knocked at anything over about 80% wind. I replaced the 9010 on the balance pivots with HP1300. That had little effect. I bought the next weaker mainspring and returned to 9010 on the settings. All good now. My takeaway is this: sometimes I'll just have to buy more springs to find the one that works.

To echo what Graham wrote, you're using the correct amount of the correct braking grease in the correct spot, right?
Sounds like that's what I'll need to do. By "next weaker", do you mean dropping from a 10 dennison (0.11mm) to an 11 (0.10mm)?

Of course my luck ran out. As I continued to troubleshoot, the impulse jewel eventually fell off. Fortunately, I didn't lose or break it - so I'm only out the time to shellac it back into the roller.

As for the braking grease, yes. But would that have any contribution to this issue? Just curious the thought there.

Thanks for all the advice so far.
 

WoodyR

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Jan 27, 2021
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Sounds like that's what I'll need to do. By "next weaker", do you mean dropping from a 10 dennison (0.11mm) to an 11 (0.10mm)?

As for the braking grease, yes. But would that have any contribution to this issue? Just curious the thought there.
You'll have to contact your part supplier to see what's available regarding the weaker springs. Since the impulse jewel was colliding so violently, you might have to go down 2 steps. Height is important so it operates (centers) correctly between the barrel floor and lid.

From what I gather, the quantity, application location and type are all important regarding braking grease in automatics. Not enough and the balance might knock (it won't slip or slip past the target torque). Too much and the balance might not have enough amplitude (slip too easily).

Hopefully you can remove the barrel without taking off the train bridge and train.
 

cspinner

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May 9, 2022
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You'll have to contact your part supplier to see what's available regarding the weaker springs. Since the impulse jewel was colliding so violently, you might have to go down 2 steps. Height is important so it operates (centers) correctly between the barrel floor and lid.

From what I gather, the quantity, application location and type are all important regarding braking grease in automatics. Not enough and the balance might knock (it won't slip or slip past the target torque). Too much and the balance might not have enough amplitude (slip too easily).

Hopefully you can remove the barrel without taking off the train bridge and train.
Thanks for that clarification. I always thought the grease was to provide a way for the spring to "slide" around the barrel at full wind while the automatic rotor continues to do its thing, but grippy enough NOT to slip during normal operation or when winding up. Based on what you're saying, it sounds like the grease needs to allow the end of the spring to slide even during normal power delivery (in order to meet the target torque as you say)? Or did I interpret that wrong?
 

cspinner

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May 9, 2022
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Yes you are starting to get it. Automatics do need to slip and they do need the braking grease in the correct place.
Delivering power is not all about having the mainspring fully wound.
I have additional data after trying more troubleshooting steps tonight. Well, first it took me a good hour to reinstall the impulse jewel. Once that was done, I stripped, cleaned and regreased the mainspring barrel wall. Then with the barrel reinstalled I placed a mark on the barrel lid and wound it such that the lid turned a full 20 turns (more than confident the mainspring should have been rotating along the wall). I did not feel any “snap” or “snags” during the wind up. Next, I held back the click as I let it back down. The barrel lid unwound 5-6 turns in that process. So, it takes about 5-6 good turns before the bridle begins to slide. Is that acceptable?

next, I double checked my escapement, specifically the lock. It seemed normal to me and not too small of a lock. I attached a video of the fork being manipulated with power applied. Not sure how well the sound translates in the video but you can hear how overly strong it is.

finally, I reinstalled the balance after letting down the mainspring. In the second video, I begin slowly winding the mainspring. At only THREE CLICKS of wind it starts to rebank again (you can hear the galloping sound). Then you can see it return to normal as power is used up. I have not messed around with thicker oils yet as I feel that will yield inconsistent results. All pivots are lubricated with 9010. Nothing on the pallet arbor pivots. I removed the 941 from the pallet stones. I also didn’t lubricate the end stones of the balance yet.

I’m still very puzzled at how a mainspring that is technically weaker than the replacements for this movement (unless I’m really screwing up the measurements) can be this overpowering.

I’m chatting with NAWCC member Mel in the meantime and his eBay store has a mainspring that will fit, which is 2 steps down in strength. Do you predict that will be enough?
 

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WoodyR

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Jan 27, 2021
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I have additional data after trying more troubleshooting steps tonight. Well, first it took me a good hour to reinstall the impulse jewel. Once that was done, I stripped, cleaned and regreased the mainspring barrel wall. Then with the barrel reinstalled I placed a mark on the barrel lid and wound it such that the lid turned a full 20 turns (more than confident the mainspring should have been rotating along the wall). I did not feel any “snap” or “snags” during the wind up. Next, I held back the click as I let it back down. The barrel lid unwound 5-6 turns in that process. So, it takes about 5-6 good turns before the bridle begins to slide. Is that acceptable?

next, I double checked my escapement, specifically the lock. It seemed normal to me and not too small of a lock. I attached a video of the fork being manipulated with power applied. Not sure how well the sound translates in the video but you can hear how overly strong it is.

finally, I reinstalled the balance after letting down the mainspring. In the second video, I begin slowly winding the mainspring. At only THREE CLICKS of wind it starts to rebank again (you can hear the galloping sound). Then you can see it return to normal as power is used up. I have not messed around with thicker oils yet as I feel that will yield inconsistent results. All pivots are lubricated with 9010. Nothing on the pallet arbor pivots. I removed the 941 from the pallet stones. I also didn’t lubricate the end stones of the balance yet.

I’m still very puzzled at how a mainspring that is technically weaker than the replacements for this movement (unless I’m really screwing up the measurements) can be this overpowering.

I’m chatting with NAWCC member Mel in the meantime and his eBay store has a mainspring that will fit, which is 2 steps down in strength. Do you predict that will be enough?
Are you sure you're installing the balance correctly? In other words, are you sure you have the impulse jewel on the correct side of the fork? Do you know how to do that?
 

cspinner

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May 9, 2022
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Are you sure you're installing the balance correctly? In other words, are you sure you have the impulse jewel on the correct side of the fork? Do you know how to do that?
Yes I do. Same technique I've used for many other watches. If the impulse was on the wrong side of the fork it would be overbanked and not run at all. Or are you referring to something else? The watch is running in the videos I've posted.
 

WoodyR

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Jan 27, 2021
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Is the beat error close-ish to zero? Do you know how to visually check this with the balance installed?

I can't DL videos. Can you post the last 2 to YT? I can't tell anything from the first video you posted there. The balance is moving too fast and the camera is too far away. I use a loupe with my phone when I take super slo-mo.
 

cspinner

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May 9, 2022
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Is the beat error close-ish to zero? Do you know how to visually check this with the balance installed?

I can't DL videos. Can you post the last 2 to YT? I can't tell anything from the first video you posted there. The balance is moving too fast and the camera is too far away. I use a loupe with my phone when I take super slo-mo.
Beat looks fine- impulse jewel is pretty much centered in line with the fork arbor. here are the videos above on YouTube:

 

WoodyR

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Jan 27, 2021
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The first video looks fine to me.

The second vid isn't a super slo-mo so I can't see the knocking. The only thing I get from it is the abrupt and odd way the balance wheel stops.
 

cspinner

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May 9, 2022
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SOLVED!!!! I got a donor movement and compared Hairspring, mainsprings, pallet fork, etc. As it turns out this little double wheel that drives the sweep second pinion and the rest of the train is designed to contain a bit of friction between the wheels. In my movement, the double wheels spin freely. In the donor, they turn somewhat in lock step and are difficult to force turn separately from each other. This is the only component i replaced and now my movement runs with a healthy 225 degrees of amplitude even at full wind.

I would like to understand the theory behind this though and how the original wheel lost its tightness. Perhaps even a way to tighten it…

7418CC68-2A50-4792-A386-5371FE04A928.jpeg
 

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