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Hi,

I need to order a new main spring.  According to Cousins website the correct spring is 1.60 x .10 x 300 x 9 non-auto but it is not available. The two closest are 1.60 x .10 x 280 x 8.5  @ £17.50 and 1.60 x .10 x 340 x 9.5  @ £7.50.  I’m tempted to go for the 9.5 for a couple of reasons.  Any reason that’s likely to be a bad decision?

Thanks.

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8 minutes ago, KenD said:

Hi,

I need to order a new main spring.  According to Cousins website the correct spring is 1.60 x .10 x 300 x 9 non-auto but it is not available. The two closest are 1.60 x .10 x 280 x 8.5  @ £17.50 and 1.60 x .10 x 340 x 9.5  @ £7.50.  I’m tempted to go for the 9.5 for a couple of reasons.  Any reason that’s likely to be a bad decision?

Thanks.

The two measurements of particular importance wound be the height and the strength,  the length would obviously correspond with the increase/reduction of half mm of barrel diameter. There is a big difference in price considering that the more expensive one is the shortest. The longer one might be ok ? But then it is taking up more room in the barrel, might it effect the unwinding ? I wouldn't have thought so for just that small amount.  Ideally a pro might reduce the longer one to suit. But there is some information that might help, do you have the one that was fitted to measure up, though not necessarily the right one.

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I would go for the dearer spring. You won't need to remove the spring from the carrier ring and then use a mainspring winder to get it into the barrel, for a start. Also that spring is closer to the needed dimensions, especially the length. The length plays a part in the mainsprings strength. If you double the length you will half the force (strength) of the spring and vice-versa. A spring with 20 mm less length would be about 7% shorter, so technically would be 7% more strength, but I find halving this number is closer to real-world findings, so the spring would be about 3 to 4% more strength/force.

On a mainspring that ideally kicks out 300 degrees of amplitude, a 3% increase in amplitude would be 309 degrees.

Increasing or decreasing the length of the mainspring will affect the power reserve to a greater or lesser degree. It depends how much shorter or longer it is.

I've attached a lesson regarding mainsprings, focussing on the size and strength of the spring within the barrel, you might find helpful. Unfortunately it is a PDF converted from a PowerPoint presentation, but only a slide was lost that was originally a video of fitting a mainspring

 

Lesson 5 The mainspring.pdf

Edited by Jon
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2 hours ago, Jon said:

A spring with 20 mm less length would be about 7% shorter, so technically would be 7% more strength, but I find halving this number is closer to real-world findings, so the spring would be about 3 to 4% more strength/force.

On a mainspring that ideally kicks out 300 degrees of amplitude, a 3% increase in amplitude would be 309 degrees.

Hi Jon, do You think that relation spring torque - amplitude is linear? I would rather guess that the amplitude should be proportional to the square of the torque. I had once idea to check it, but still haven't.

Edited by nevenbekriev
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Thanks all.

 

Movement is a Helvetia 800C. Don't know any history e.g. if the broken mainspring is original. Watch was bought recently as non-working.

13 hours ago, Jon said:

I've attached a lesson regarding mainsprings

Really interesting piece that Jon, thanks. I'd only been thinking of length relating to power reserve. I don't have a mainspring winder (yet) so I'd like to avoid winding the spring into the barrel by hand.

16 hours ago, Neverenoughwatches said:

The two measurements of particular importance wound be the height and the strength,  

The old spring measures at 1.60 x .11 x ~290. Sorry, can't be exact on the length. The piece I still have is 275, but the second piece escaped during measurement. Best measurement I had for it was ~13. The micrometer does zero accurately so I'm pretty confident of the .11

image0a.thumb.jpeg.cbd4f24d7da90fe8b068461894cc9e06.jpeg

Also, I'm getting the barrel ID at 9.5 on a vernier.

I'm now wondering about going with a .11 spring? Given all of the above, maybe 1.60 x .11 x 300 x 9?

Any thoughts?

Cheers.

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On 4/19/2024 at 3:55 PM, Neverenoughwatches said:

The two measurements of particular importance wound be the height and the strength,  the length would obviously correspond with the increase/reduction of half mm of barrel diameter. There is a big difference in price considering that the more expensive one is the shortest. The longer one might be ok ? But then it is taking up more room in the barrel, might it effect the unwinding ? I wouldn't have thought so for just that small amount.  Ideally a pro might reduce the longer one to suit. But there is some information that might help, do you have the one that was fitted to measure up, though not necessarily the right one.

One thing I would point out here is that, Cousins give the barrel diameter and this is important because when you come to fit the new spring, if the barrel diameter is smaller than the internal diameter of the washer the new spring sits in, you will have to manually unwind it from the washer to fit it in the barrel. If the spring barrel size is correct (or smaller) you will be able to drop the new mainspring straight into the barrel by just pushing it out of the washer. The other thing with barrel sizes is that, if you are really having to make measurement compromises (i.e on both thickness and length) you need to make sure the spring is going to wind into the barrel. 

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On 4/19/2024 at 7:05 PM, Jon said:

On a mainspring that ideally kicks out 300 degrees of amplitude, a 3% increase in amplitude would be 309 degrees.

Increasing or decreasing the length of the mainspring will affect the power reserve to a greater or lesser degree. It depends how much shorter or longer it is.

Thanks for the PDF lesson. I've never really considered the effect of changing length on spring force - I've always focussed on primarily matching thickness (remembering bd³/12 from old structures lectures), and accepting 20-30mm difference in length only effects power reserve. I hadn't considered it changed the strength.

But, as you point out, the effect on strength is not great - a quick search online revealed little, but I found this from David at vintagewatchstraps 

https://www.vintagewatchstraps.com/mainsprings.php

Effects of Length

If you can't find a spring that is close in length to the one that you want, can a longer or shorter spring be used? The theoretical torque generated by a spring is inversely proportional to its length, but this supposes that the spring is free. In a watch barrel, many of the coils of the spring rest on top of one another and friction between them must come into play. But if it didn't, then a spring that was say 380mm instead of the 360mm you really wanted would be in theory 5% weaker. By the time friction has also had its say this is negligible. Even a 400mm spring would be only 10% weaker in theory and less in practice. This is less than the difference between fully wound and 24 hours later. So there is quite a scope for using springs of lengths that are different from the ideal. It is the thickness of the spring that is the most important factor, which is why it is often called the "strength" of the spring.

 

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4 hours ago, ScrewDropper said:

One thing I would point out here is that, Cousins give the barrel diameter and this is important because when you come to fit the new spring, if the barrel diameter is smaller than the internal diameter of the washer the new spring sits in, you will have to manually unwind it from the washer to fit it in the barrel. If the spring barrel size is correct (or smaller) you will be able to drop the new mainspring straight into the barrel by just pushing it out of the washer. The other thing with barrel sizes is that, if you are really having to make measurement compromises (i.e on both thickness and length) you need to make sure the spring is going to wind into the barrel. 

You could potentially install the spring into a larger barrel and remove it from there and then wind it into an appropriate winder barrel to fit the barrel you are working on

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