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ETA 2824 Mainspring

chadders1966

By chadders1966
in Watch Repairs Help & Advice

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chadders1966 WRT Addict

chadders1966

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Hi

I need a replacement mainspring for an ETA 2824 - the bridle came off the original when I was re-inserting. Cousins UK have the size as 1.23 x 0.132, Ranfft 1.20 x 0.115, Borel (and General Resorts I think) 1.25 x 0.125. However the original ETA parts listing says 1.23 x 0.125, which agrees with my measurement of the existing spring.

Does anyone know where I might get a spring exactly 1.23 x 0.125? Cousins don't carry one, and none of the listings on Ebay actually state the size. Alternitavely is there any way the bridle can be re-attached? Thanks

Stephen

 

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mikepilk Distinguished Member

mikepilk

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2 hours ago, chadders1966 said:

Hi

I need a replacement mainspring for an ETA 2824 - the bridle came off the original when I was re-inserting. Cousins UK have the size as 1.23 x 0.132, Ranfft 1.20 x 0.115, Borel (and General Resorts I think) 1.25 x 0.125. However the original ETA parts listing says 1.23 x 0.125, which agrees with my measurement of the existing spring.

Does anyone know where I might get a spring exactly 1.23 x 0.125? Cousins don't carry one, and none of the listings on Ebay actually state the size. Alternitavely is there any way the bridle can be re-attached? Thanks

Stephen

 

I'm in the process of re-assembling a 2824 and had exactly the same problem. I went with the Cousins suggestion (GR 3149X) and it fits fine.

I wouldn't worry about getting an exact match.  Cousins must have sold a lot of these for such a common movement so expect they're selling a suitable spring.

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chadders1966 WRT Addict

chadders1966

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47 minutes ago, mikepilk said:

I'm in the process of re-assembling a 2824 and had exactly the same problem. I went with the Cousins suggestion (GR 3149X) and it fits fine.

I wouldn't worry about getting an exact match.  Cousins must have sold a lot of these for such a common movement so expect they're selling a suitable spring.

Hi,

I can see that it probably would work as it is the right height, but a thickness of .132 is quite a bit above .125 - perhaps the description is wrong and it is really .125, any way you could measure it? I'm not entirely sure what the problems of too thick a spring are. Excessive wear? Overbanking? Shorter reserve?

Stephen

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mikepilk Distinguished Member

mikepilk

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51 minutes ago, chadders1966 said:

Hi,

I can see that it probably would work as it is the right height, but a thickness of .132 is quite a bit above .125 - perhaps the description is wrong and it is really .125, any way you could measure it? I'm not entirely sure what the problems of too thick a spring are. Excessive wear? Overbanking? Shorter reserve?

Stephen

Ranfft has :

2824         1.20 x 11.0 x 0.115mm
2824-1/2  1.25 x 11.0 x 0.125mm

Cousins has the same spring for all 3 variants :   1.23 x .132 x 420 x 10.8 (GR 3149X) 

I can't measure the new mainspring as it's already fitted. The old one measures 1.23 x 0.13 x 420 - same as the Cousins one.

Like you, I was a wary due to the increase in thickness. If the spring is assumed to be a series of beams in bending - from the (very little I remember of structural mechanics) the moment of inertia is bd³/12, where 'd' is the thickness of the spring i.e it varies as thickness cubed. But as the new spring seems to match the old, I'm happy.

(A too powerful spring can result in too high an amplitude and 'knocking' where the balance jewel hits the back of the pallet fork)

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chadders1966 WRT Addict

chadders1966

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1 hour ago, mikepilk said:

Ranfft has :

2824         1.20 x 11.0 x 0.115mm
2824-1/2  1.25 x 11.0 x 0.125mm

Cousins has the same spring for all 3 variants :   1.23 x .132 x 420 x 10.8 (GR 3149X) 

I can't measure the new mainspring as it's already fitted. The old one measures 1.23 x 0.13 x 420 - same as the Cousins one.

Like you, I was a wary due to the increase in thickness. If the spring is assumed to be a series of beams in bending - from the (very little I remember of structural mechanics) the moment of inertia is bd³/12, where 'd' is the thickness of the spring i.e it varies as thickness cubed. But as the new spring seems to match the old, I'm happy.

(A too powerful spring can result in too high an amplitude and 'knocking' where the balance jewel hits the back of the pallet fork)

The spring I took out matched ETAs specifications exactly, 1.23 x 0.125 x 40. This makes me think it may be an original as I can only seem to find springs that match either the height or thickness, not both.

I was happy to reuse the spring as it was in good condition, but somehow the bridle/attachment that pushes the spring against the barrel came loose and I now have it in two pieces. I have no idea how that happened, or if it can be fixed.

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mikepilk Distinguished Member

mikepilk

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20 hours ago, chadders1966 said:

The spring I took out matched ETAs specifications exactly, 1.23 x 0.125 x 40. This makes me think it may be an original as I can only seem to find springs that match either the height or thickness, not both.

I was happy to reuse the spring as it was in good condition, but somehow the bridle/attachment that pushes the spring against the barrel came loose and I now have it in two pieces. I have no idea how that happened, or if it can be fixed.

I almost always fit a new spring. I've cleaned and refitted springs that looked fine but I ended up with low amplitude. It was only after re-stripping and examining all the parts again that I discovered fitting a new mainspring solved the problem.

Most of the mainsprings (I usually buy the Cousins generic ones) are rarely a perfect match with all the original specs. I never worry about getting all the measurements identical. As long as width fits the barrel, close enough is usually good enough.

I just fired-up the 2824 I'm assembling and it's showing 295 deg amplitude. Probably increase a bit after 24h but I don't expect the amplitude to get too high. 

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