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Choosing Replacement Mainspring - Help


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I am looking for a “holed” replacement mainspring (without bridle) for an old fusee pocket watch and a fellow forum member pointed me towards one on the cousins website. 

https://www.cousinsuk.com/category/filter/mainsprings-by-size-watch-pocket

Two issues:

First, the spring is simply listed as “non-automatic” and doesn’t say how it is terminated. However......

Second, the spring is substantially longer than the original, the original spring was roughly 410mm and the cousins one is 580mm. Are mainsprings easy to alter, maybe snip the spring down to size and drill or punch a hole in the end?

 

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27 minutes ago, Robbie010 said:

I am looking for a “holed” replacement mainspring (without bridle) for an old fusee pocket watch and a fellow forum member pointed me towards one on the cousins website. 

https://www.cousinsuk.com/category/filter/mainsprings-by-size-watch-pocket

Two issues:

First, the spring is simply listed as “non-automatic” and doesn’t say how it is terminated. However......

Second, the spring is substantially longer than the original, the original spring was roughly 410mm and the cousins one is 580mm. Are mainsprings easy to alter, maybe snip the spring down to size and drill or punch a hole in the end?

 

Search for the barrel diameter vs the length and make sure you have the height correct and strength close (thickness.)

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59 minutes ago, jdrichard said:

Search for the barrel diameter vs the length and make sure you have the height correct and strength close (thickness.)

Thanks. 

Only 2 options come up:

2.50 x 0.20 x 17 x 580

or

2.50 x 0.20 x 15.5 x  480

The original spring was:

2.50 x 0.20 x 17 x 410

Not sure where to go from here, my instinct is to go for the 15.5 x 480 as the spring is still longer than the original, even though it is for a smaller barrel??

 

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 The Cousins spring will come with a "tongue end", which, if you have a hook in the barrel that worked with a hole end before will work A-OK with the tongue end. Length wise, there's a lot of leeway with it until you get to really small stuff (try fitting a spring with 20mm extra length in a JLC 852 caliber with a 5mm barrel inner diameter haha). The Cousins spring will work fine. Additionally, the way the fusee works you don't actually use the whole length or "number of turns of wind" that the barrel can produce. It will make about  6 turns of wind, and the fusee will use about 4 to 5 of that.

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5 minutes ago, Robbie010 said:

Not sure where to go from here, my instinct is to go for the 15.5 x 480 as the spring is still longer than the original, even though it is for a smaller barrel??

The barrel diameter size is only Important if you're shoving the spring indirectly. If you're going to use a mainspring winder it's not an issue. Then if the spring is just a little bit longer a little bit shorter it's not an issue. If it's dramatically longer yes you can break it and shorten it. Then you can even make a hole in the end of the spring but it can be a pain to do. I would just go with the spring is really close and see if it works with the end that it has.

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33 minutes ago, JohnR725 said:

The barrel diameter size is only Important if you're shoving the spring indirectly. If you're going to use a mainspring winder it's not an issue. Then if the spring is just a little bit longer a little bit shorter it's not an issue. If it's dramatically longer yes you can break it and shorten it. Then you can even make a hole in the end of the spring but it can be a pain to do. I would just go with the spring is really close and see if it works with the end that it has.

Oh dear. 🤬

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410 was surely too short.

My calculator says for barrel 17 mm:
optimal length = 503
recommended = 554.
You may use any of both Cousins' springs.

Frank

Mainspring_app.jpg.42fae93a25dc8dd67c5dec8cd685f3e0.jpg

 

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1 hour ago, praezis said:

410 was surely too short.

My calculator says for barrel 17 mm:
optimal length = 503
recommended = 554.
You may use any of both Cousins' springs.

Frank

Mainspring_app.jpg.42fae93a25dc8dd67c5dec8cd685f3e0.jpg

 

Interesting calculator! Thanks

I can only assume it has something to do with the age of the watch, being an old fusee pocket watch?

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1 hour ago, Robbie010 said:

Interesting calculator! Thanks

I can only assume it has something to do with the age of the watch, being an old fusee pocket watch?

Yes, it’s very possible that it’s broken previously and had been shortened. 
 

In short, for a given thickness (0.2mm in your case), there is an optimal length which will give you the longest running time. Too short and you don’t get enough turns. Too long and you occupy too much space in the barrel and limit the number of turns. The length has negligible effect on torque developed, unless taken to extremes. So, aim for the optimal length (in my opinion). 

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The other thing to bear in mind is that the full length (and number of potential turns you can therefore get) matters less for a fusee watch as the number of turns will be limited by the fusee stop-iron.
 

Whereas on a modern watch where we use every single turn of mainspring, it does matter as we want it to run for as long as possible. 

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As a rough guide the mainspring should take up 1 third of the barrel, barrel arbor 1 third and space between the arbor and spring 1 third. 

Not have to use part of the spring is very bad practice and it can cause strain on the part of the spring that is being used all the time. The strength of the spring should be as near to the old as possible, to strong and you not only risk of doing damage to the movement but the chain could break. No point have a chain and the spring is to short because part of the chain will not be used. There is a reason why the chain is a particular length.    

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7 hours ago, praezis said:

My calculator says for barrel 17 mm:

My reading skills are lacking what exactly is the diameter of the original barrel? The only thing I see is the 17 mm which is what the replacement springs have.

 

6 hours ago, Robbie010 said:

Interesting calculator! Thanks

I can only assume it has something to do with the age of the watch, being an old fusee pocket watch?

The calculations here become interesting. The calculations do not know that this is a fusee watch. Do the calculations take into account that the reason why we have a fusee is the mainsprings back then were very poorly made? Or the characteristics of a blued steel spring versus a modern spring is at going to change anything?

Or even American watch manufactures they had different mainsprings corresponding to the number of jewels. Specifically they changed the thickness of the spring. So a seven jewel watch would require a stronger spring than a 21 jewel watch does the calculation take that into account or does it even know what type of the escapement we have all things that will change the outcome it does not know. It gives us an approximation based on a modern watch of what will probably work hopefully. Or basically this is a good starting point to give you a clue.

 

24 minutes ago, oldhippy said:

Not have to use part of the spring is very bad practice and it can cause strain on the part of the spring that is being used all the time. The strength of the spring should be as near to the old as possible, to strong and you not only risk of doing damage to the movement but the chain could break. No point have a chain and the spring is to short because part of the chain will not be used. There is a reason why the chain is a particular length. 

In the very early days of timekeeping they discovered that the poorly made mainsprings did not produce equal force. Especially when you're dealing with earlier escapement's it would really screw up timekeeping. To compensate for the un-equal power of the mainspring the fusee was invented. In order to work properly you're supposed to adjust it to get the best part of your mainspring and then hopefully you will have a more equal force and better timekeeping.

Modern watches don't need fusee's. If you look at the curvature and the length of modern springs notice they have a interesting back curve. That's designed to equalize the force of the spring. They'll also have much longer Springs C don't reach the end extreme force and you get a more equal force. Which is why typically modern watches will run for two days just to get better timekeeping over the first 24 hours.

 

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On 3/5/2021 at 9:41 PM, JohnR725 said:

… a seven jewel watch would require a stronger spring than a 21 jewel watch does the calculation take that into account or does it even know what type of the escapement we have all things that will change the outcome it does not know. It gives us an approximation based on a modern watch of what will probably work hopefully. Or basically this is a good starting point to give you a clue

All true, but I am not sure what you try to say.
Keep away from calculation - look up docs or surrender? Try and error until hopefully successful?

Mainspring calculation helps when no lists or docs are available. It proposes dimensions that will work, even for unexperienced beginners.

Needed starting points are 1) barrel diameter and 2) spring thickness. 
Sole task of calculation is: find the optimal spring length that gives the most revolutions of the barrel.
Fine tuning (of thickness) is where knowledge and experience come into play! 

Rules and equations are found in many watchmaking textbooks. You can calculate afoot or cast these rules into an app like above.

Frank

 

 

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