Help selecting a mainspring

[Sjk4x4]

id like help with selecting a mainspring for an unmarked pocket watch.  

I have taken the measurements of the existing mainspring and have looked online for a replacement. The issue is the strength.  The original spring is .21 mm thick. The replacements I've found are .19mm or .23mm thick.  Which would be a good choice? I read going with the stronger may break something in the old watch.

Are there recommendations?

[HSL]

I think if you write some more meassurements of the spring someone here can help you with this. Especially if it is to an unknown movement. I know I seen some theories of this before in the forum but here is a small attemt to explain why and what to think off. This might not be correct, my memory isn't what it used to,  but then someone will correct me.

When choosing a mainspring one should be aware of the parameters involved. There are three measurements to keep in mind. Width, thickness and length. When speaking of the strength of a mainspring one could say it is proportional to its width, as an easy example one could say a mainspring which is twice as wide as another would develop a force four times as great.
The thickness of the mainspring will have a different relationship to the force generated, here the relationship is the cube of its thickness. An easy example once again would be to take a twice as thick mainspring which would be eight times as strong.
The length of the spring is then deciding the going time of the barrel.

If when choosing a new mainspring there wouldn’t be a correct one there then is the classical relationship between the barrel inner diameter, arbor, and the mainspring.
Beginning with estimating the thickness one could measure the inner diameter of the barrel and divide the result with 100.
When the spring takes up around 2/5 of the space inside the barrel then the length is good.
If you can´t find the correct thickness one could choose the next thickness below the one you measured up. Just keep in mind the length of the spring should then be slightly longer.

The width should match the previous mainspring or be slightly smaller, since the mainspring barrel has a barrel lid which you should be able to pop into place without getting the mainspring stuck between the lid and barrel.

When you decided the size of the mainspring you have to look which bridle you have in the end of the spring, these looks different depending on manufacturer of the movement and the type of winding system. On a manual wind the bridle is locked in a fixed position while on an automatic winding watch it has the ability to slip a bit when the spring is wound up.

[JohnR725]

Always nice to give us pictures of the watches just because we like to see pictures of watches and who knows we might recognize the watch. If the mainspring is blued steel versus the modern steels the modern steels are supposed to be stronger. A lot of it depends on the number of jewels in the watch if you go to strong you could have over amplitude issues. If it's a seven jewel watch they go to week it might not run. But I don't think it's going to be that critical ago with the weaker one.

[Sjk4x4]

JohnR725, theres a post I made asking for help identifying a movement and there’s lots of photos of the movement. It seems to be an old example of a recased  pocket watch. It has two jewels, on the balance. Now that you mention it, the spring IS blued. I put it back into the watch and it has ran overnight but the amplitude seems low. I’m guessing it’s from around 1875

[Sjk4x4]

I will try the math route for a backup check, but it never really was my strong suit 

[HSL]

6 minutes ago, Sjk4x4 said:

JohnR725, theres a post I made asking for help identifying a movement and there’s lots of photos of the movement.

There are lots of movements around , but in your post you just ask for some advice of a movement with a 0.21mm thick mainspring. Well there are thousands of them out there and not only John but probaby many more people are interested of which of the thousands of choises it is......   Which one did you ask about?

[JohnR725]

12 minutes ago, Sjk4x4 said:

amplitude seems low.

 

13 minutes ago, Sjk4x4 said:

theres a post I made asking for help identifying a movement

As a reminder when asking questions you need to give enough information. It's good that you referenced this other reference because this is a cylinder escapement. A cylinder escapement for watch made a long time ago. Do you know how to properly set up a cylinder escapement? You know what sort of amplitude you should expect out of a cylinder escapement? Or what about your timing machine to do what the lift angle for cylinder escapement is? You'll note that I keep using the word cylinder simplistically the amplitude sucks if it runs in the keeps anything that resembles time within 5 to 10 minutes a day you're doing great. There's a reason we don't use cylinder escapements anymore.

[Sjk4x4]

Thank you for the reminder about the info. Posting on a forum is new to me so thanks for the patience. I’m not sure if this is even the correct way to attach a link to the referenced photos.

Unfortunately, no I don’t know the lift angle and I see what you’re getting at. I believe I have the cylinder set in properly because it ran all night and was still going this morning, but is there more I should know? I have read up some on them but have not dig deep enough to find a good tutorial on tuning one. Seems like something I should do now.

i can at least put the hands on and see what time it keeps. My timer is one of the $170 imports and I’ve watched all of the videos I can find on setting it up also, because the instructions basically taught me to plug it in and turn it on:) 

Should I wait and see what time it keeps? Most people have said to replace an old spring on an old movement while I have it apart.

[AshF]

On 5/14/2021 at 7:32 AM, HSL said:

When the spring takes up around 2/5 of the space inside the barrel

Just wanted to point out that the diagram in this post says that d2 should be 2/5 of the inner diameter where actually it should be 1/5.

[Klassiker]

On 5/14/2021 at 6:00 AM, Sjk4x4 said:

I have taken the measurements of the existing mainspring

What did you measure with, and what results did you get?

[Sjk4x4]

I measured with Mitutoyo calipers set on mm.  the width varied between .19-.21 and the height was 2.40. I calculated the length to be 560mm or around 22 inches. Its amusing that on the supply house sites, the majority of the dimensions are metric and the lengths are in inches. Go figure.

going off an earlier suggestion that modern springs are stronger than ancient ones, I ordered a 2.1 height by .18 thick and 19.5 inches long. It was from Otto Frei. Do you think this will work?

I have questions on sources now. 

What sites are good for springs by dimensions?

and is there a place to pick up an assortment of jewels around 1.5 mm dia? The assortments ive found are for wrist watches.

[rodabod]

Your new spring will hopefully be strong enough. I've written enough times about how important the thickness is here before, but just to repeat that the force developed is relative to the cube of the thickness.

0.18 will hopefully be ok. You have the fact that the spring is new and not "set" on your side.

[Klassiker]

In a way, the most important dimension to know is the barrel i.d. Then you can select a spring with the same size o.d. and push it into the barrel straight out of the holder it comes in. The spring you have ordered should work fine, but don't throw away the old one just yet.

Cousins UK sell Generale Ressorts and they are listed in mm. I don't know why some mix mm and inches. You will also come across Dennison sooner or later. If you are in the US you might need to do a bit more decoding/conversion work.

I have the jewel assortment J47732 from Cousins, and there are several plain round jewels in there of 1.2, 1.4 and 1.6mm dia. I would need to measure the thicknesses. They are not domed.

 

[HSL]

On 5/18/2021 at 2:28 PM, AshF said:

Just wanted to point out that the diagram in this post says that d2 should be 2/5 of the inner diameter where actually it should be 1/5.

Correctly, sometimes my mind is somewere else..

[clockboy]

I have checked this rule with known good springs, wound in a barrel with no issues It’s the thirds rule which is a very good guide indeed.

 

[Sjk4x4]

i love the knowledge and the sharing here. Amazed!

 

The parts arrived today. Hopefully ill get to play with them this weekend and when the piece is together, ill post a photo.

 

thank you all for helping

[StuartBaker104]

There is a wealth of information on mainspring selection here, and some calculators for those who don’t like doing the maths by hand!

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

[Sjk4x4]

After a little bending to squeeze on the barrel arbor, the mainspring fit and locked.

Now is where everything went bananas   I bought two jewels.  One didnt have much of a dome, so I sprued it up and ran a diamond file around it. I grabbed it after with tweezers PING. I felt dumb even trying to look for it.

Second jewel-got it in place and secured. I started to hurry because company was coming and now I know never rush like that again.. the balance staff snapped into place cracking a jewel and a cap both. 

Now I need two more jewels so Ill put the tweezers down for a bit.

[jdm]

9 hours ago, Sjk4x4 said:

I bought two jewels.  One didnt have much of a dome, so I sprued it up and ran a diamond file around it. I grabbed it after with tweezers PING. I felt dumb even trying to look for it.

Actually jewels are easy to find. All you need is an €1 UV light and darken the room a little.

[Sjk4x4]

1 hour ago, jdm said:

Actually jewels are easy to find. All you need is an €1 UV light and darken the room a little.

WWWHHHAAATTT???? Thats amazing!  Thanks for that little 'gem' of advice!