Identifying Mainspring With Nothing But Barrel?

[kendavies]

Hi,

 

As a 'newbie' a little introduction seems polite: flushed with the success of fitting a new ETA 955.412 to a Rotary and curious about the workings I took apart the old movement and re-assembled it. Well, most of it, I had the salutary lesson of watching a small calendar part ping off never to be seen again but no matter - it was for experience. From that straight into mechanical watches and I have bought a couple of cheap 'trench' watches off ebay to get some experience on. By 'cheap' I mean around £10 so I can afford to make mistakes though I'd rather hear them tick again.

 

I have stripped the first - a very basic no name movement with no jewels and a cylinder escapement, all seems OK other than there is no sign of the mainspring so my question is; given just the barrel can a reasonable guess be made about the required mainspring?

 

The internal diameter of the barrel is 12.1 mm, its depth is 2.22 mm and the external diameter of the arbor is 4.0 mm

 

Any answers, suggestions or questions gratefully received  :)

[Geo]

A very warm welcome to the forum Ken, and thank you for a good introduction.

Regarding your question, you should be able to get a spring that will suit your watch. Some sizes you have which is fine, you will also need to know the thickness of the spring and the end fitting where it drives the barrel. The width of the spring will have to be just slightly smaller than the inside depth of the barrel. I would guess at 2mm. I would have a look at Cousins website and see how many permutations are close to what you require.

Let us know how you get on, I'm sure someone on here will give you more advice to consider.

[anilv]

Hi Ken,

 

Generally, the first thing you should look at is identifying the movement, this will help in ordering spare parts. There will usually be a makers mark (cartouche) and a movement reference.

 

In most watches from the 50s and newer the marks will be under the balance. On older watches the markings will be under the dial.

 

Having established the movement ID, it is then simple to look for a suitable mainspring from websites like 'cousinsuk'.

 

There is a useful reference http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&&2uswk  where you can locate your model of movement and it will give you the dimensions on the mainspring and other useful stuff.

 

Coming back to your watch, trench watches from the world war 1 period are quite tricky to identify but a picture will enable all of us on this site to pitch in and the combined knowledge may help.

 

Also trench watches, if advertised as non-running, will usually have a broken balance staff as they did not have shock protection. Not a really big problem but a bit challenging for someone new to the hobby.

 

Looking forward to hearing more from you.

 

 

Anil

[kendavies]

Hi Anil and Geo,

 

Thank you for your suggestions so far, I am familiar with Cousins having bought the necessary tools from them (I love tool shopping :)) There are no markings anywhere to give a hint, foolishly I did not photograph the movement before dismantling it and my pencil drawings won't be much help! The movement is held in place by one large cock and another that holds the balance, it does not look very elegant. This is what it looks like whole:

 

 

Clearly the crown has been replaced at some time.

 

I did think at first that the balance staff was broken but looking at it closely when disassembling I am pretty certain it was simply dislodged from its cock.

 

I have another coming in the post that looks very similar so that might give some clues and I'll see how many springs on Cousin's website might fit the bill.

[Geo]

Looks like a converted pocket watch Ken. Could you post pictures of the movement?

[WillFly]

Looks like a typical, pin-set "transition" watch from the 1910s/1920s - often paraded as "trench" watches on eBay - and not uncommon. It should be possible to get a spring for this if you can identify the movement.

[anilv]

I have one which I wear once in a while.....it's a great conversation piece as most people cannot believe it is around 90 years old..

 

will your swatch be around in 90 years?

 

Anil

[kendavies]

Hi Everyone,

 

When I get the chance I'll prop the movement back together and photo it - now I've found that holding a loupe in front of the lens turns my phone's camera into a good macro lens! I went over it very closely last night and there are no markings anywhere that I can find.

 

It seems to me that the answer to my original question is "No", I have to identify the movement before being able to select a mainspring. I assume that the length/width ratio of the spring reflects the reserve/power associated with it - is that correct?. With a simple movement like this, which I imagine contains more friction than a jeweled movement, is it fair to say that it is likely to have a thicker spring than a better movement would?

 

Ken

[TxWatchbreaker]

If you cant id the movement, you are not lost. I remember from watchmaking school that there is a formula for calculating mainspring length from the barrel inside diameter, the arbor diameter and the power demands of the watch based on the gear train. The formula used to estimate the mainspring size when you know nothing about the mainspring is Discussed here : http://www.nawcc-index.net/CalcMainspringEstSize.php. There are other easier formulae when you know the thickness to calculate the length or when you know the length to calculate the thickness. The end styles would be apparent from the barrel and the arbor. Hope this helps. Who says highschool algebra was useless...

[TxWatchbreaker]

Iink above appears broken. Just google "calculate estimated mainspring" and you will find this formula

[kendavies]

Hi,

 

Thanks TxWatchbreaker - the link you give seems to be correct but doesn't appear to work as a hyperlink, non hypered it is:

 

www.nawcc-index.net/CalcMainspringEstSize.php

 

that will help greatly - thank you.

 

Bit busy this time of year - I'll report back once I've sorted it all out :)

 

Ken

[bobm12]

Welcome to the forum Ken!

[kendavies]

Hi Chaps,

 

I promised to prop the movement together so you could see it - here it is without the balance both with and without the back plate. I am already wondering how on earth I'm going to get it back together with everything in place:

 

 

I have also had a go at the mainspring calculations, I could not get the calculator on the website to give anything meaningful but working through the calculations by hand gives a suggested mainspring with depth 1.9 - 2.0 mm, thickness 0.12 mm and length of 426 mm which seems plausible though there is nothing matching that on Cousins website. Those calculations are based on having 30 hours per full wind which would equate to 27.25 full turns of the mainspring.

 

I feel I am getting closer, especially as I've just discovered my wife has an ultrasonic cleaner hidden in a cupboard...

 

Ken