Is this mainspring ok?

[omx5o]

Hi,

I’m in the process of servicing an Oreba movement. After the disassemble and inspection, this is how I found the mainspring. It’s not S-shaped in any way and is not flat as you can see in the image. Is this mainspring tired and in need of replacement or should it be fine after a clean?

thanks.

[AdamC]

Hi,
I’m in the process of servicing an Oreba movement. After the disassemble and inspection, this is how I found the mainspring. It’s not S-shaped in any way and is not flat as you can see in the image. Is this mainspring tired and in need of replacement or should it be fine after a clean?
thanks.

I’d replace it with a modern alloy spring. It’s old and the fact that it’s not lying flat would be enough for me to replace. It’s also likely that performance will be affected.


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[rogart63]

Looks pretty tired. If you could find one replace it. 

[GeorgeC]

That is a mainspring that is "set" and would now allow for a long duration of powering the movement nor the movement's amplitude specification.  Not being flat is most likely due to the movement being installed by hand - not a mainspring winder.

Definitely replace.

George

[omx5o]

Thanks all, that's a definitive replace then. I suppose the next question is how interchangeable are pocket watch mainsprings. Do I need to source the exact type for that caliber, or is it possible to use something more generic? The movement/watch is an Oreba and the main plate and bridges all have "84" stamped on them. The case has "3714" stamped on it. The barrel is 15mm diameter and the spring is about 1.5mm in height.

[rogart63]

Someone probably have an old pocket watch mainspring book.  I have checked but only had for watches. Could be tricky to find the exact spring. as new springs are much better then the old one where. show us a picture of the movement. If you have one. 

Found some Oreba movement on google. http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&b0&2ustu&1386777618

https://www.ebay.co.uk/itm/OREBA-HALF-HUNTER-SWISS-LEVER-15J-MOVEMENT-SPARES-OR-REPAIRS-N27-/382293150576

[transporter]

This is the problem I have every time I can’t find an exact match for a mainspring, you will need length height and thickness of the mainspring and then look for one that is nigh on the same, I cheekily ask here as some like old hippy to name but one will off the top of their head give you a size of a spring that will fit, it might be shorter but thicker etc etc but their experience is priceless, as of yet I still haven’t figured out how far off the exact measurements I can go to find a mainspring that would work. It’s wizardry you know!!!!!

[rodabod]

A white spring of the same dimensions won’t have the same strength as a blue one, so this should be taken into account. 

[omx5o]

Here is a picture of the movement. I’ll get accurate measurements once the vernier turns up. I see Cousins supply an exhaustive list of mainsprings if all else fails.

[transporter]

17 hours ago, rodabod said:

A white spring of the same dimensions won’t have the same strength as a blue one, so this should be taken into account. 

SO does that mean that you will need a longer spring to compensate for the lack of strength or a thicker spring? ( I’m thinking thicker an I correct? ) and if so how much thicker or longer??

[mikepilk]

2 hours ago, transporter said:

SO does that mean that you will need a longer spring to compensate for the lack of strength or a thicker spring? ( I’m thinking thicker an I correct? ) and if so how much thicker or longer??

I understand that a coiled spring can be modeled as a series of small cantilever beams. The strength of such a beam is dependent on the moment of inertia = width*thickness³/12                     (that's thickness cubed)

i.e the strength of the spring is FAR more dependent on the thickness than the width.

So try to get the thickness as close as possible

[transporter]

Cheers mate, I’ll keep that in mind when I get stuck, but be prepared for a PM with my question lol

[omx5o]

Thanks, that's interesting math behind it. I now have the spring dimensions, 1.9 x 0.25 x 520 x 15.8 mm. My M.Cutmore book states that the unwound spring should occupy radius/3 of the barrel, so that's something else to keep in mind.

Cousins have a 1.9 x 0.23 x 500 x 16.5 Non-Automatic ( GR56991 ). That looks pretty close?

[mikepilk]

1 hour ago, omx5o said:

Thanks, that's interesting math behind it. I now have the spring dimensions, 1.9 x 0.25 x 520 x 15.8 mm. My M.Cutmore book states that the unwound spring should occupy radius/3 of the barrel, so that's something else to keep in mind.

Cousins have a 1.9 x 0.23 x 500 x 16.5 Non-Automatic ( GR56991 ). That looks pretty close?

Looks good but the diameter is too big to push straight in to the barrel. You will have to unwind it and re-wind it in to the barrel.

[omx5o]

6 minutes ago, mikepilk said:

Looks good but the diameter is too big to push straight in to the barrel. You will have to unwind it and re-wind it in to the barrel.

I measured the internal distance of the barrel. Is that correct? If I measure the external diameter (excluding the teeth) then it's 16.9mm.

[mikepilk]

The wound spring is 16.5mm, if the internal barrel diameter is greater than that it will fit 

[omx5o]

Just an update. I received the new mainspring from Cousins today. Checked it against the barrel and as expected it was a fraction too big to just drop in. So I carefully unwound it by hand and used a winder to get it into the barrel. This is where it got a little interesting as I had problems the first time I used this winder because I used the wrong size so I selected what I thought should be correct size for this one. However, when winding I discovered that with about an inch of spring to go it suddenly jammed and I realized that the winder barrel was full and stopped the spring being completely housed. Fortunately I worked around the problem by continuing to insert the winder into the watch barrel as normal and then manually feeding in the inch of remaining spring into the gap around the edge of the barrel. I tried pressing the winder button but it was too stiff since the spring was jammed in, but fortunately with a little tap of a hammer the spring popped into place. So far so good.

Then I noticed that the spring's inside coil was just a little too big for the arbor which meant it just freely rotated without catching. So I carefully used pliers to shrink the size of the inner coil. The arbor then slotted firmly into place and after returning the barrel lid, it was winding successfully.

After that, I re-assembled and oiled the watch by closely following Mark's course videos. The movement is now happily ticking away with an amplitude of 274 and a nice steady beat. I think the balance might need realignment as there's a large gap between the dots on the timegrapher, but so far I'm pretty chuffed with it all.

Thanks for the help.

Peter

[mikepilk]

Great job we done