Mainspring Strength

[rodabod]

Hi,

I've recently fitted a new white mainspring in an Omega bumper, cal 28.10 as the original blue spring was set. The mainspring was selected according to Cousin's look-up table and I believe others have used the same. 

The watch is in great condition and has been meticulously cleaned and lubed. However, the watch occasionally knocks the bankings when left perfectly dial up and fully wound. 

I want to rectify this, so am wondering if anyone has an opinion on whether going from the current 0.1 strength spring to a 0.095 spring might help since it's only just beginning to bank occasionally. 

As I understand it, the force (and therefore torque) provided by the spring is related to the cube of the thickness. Therefore 0.1 cubed = 0.001 and 0.095 cubed = 0.00086.

So..... would anyone like to hazard a guess that if I dropped the maximum torque by 15%, would I be ok?

[oldhippy]

This is an auto there for it cannot be fully wound. If you can fully wind it then you have the wrong mainspring. The m/s should have a slip spring attached, which allows the spring to slip around the inside of the barrel

[vinn3]

that is the "main spring clutch"  as patented buy Movato.  Vinn

[Marc]

 

This is an auto there for it cannot be fully wound. If you can fully wind it then you have the wrong mainspring. The m/s should have a slip spring attached, which allows the spring to slip around the inside of the barrel

 

None-the-less the torque delivered by the m/s is still a function of its thickness, albeit slightly complicated by the added influence of the slipping bridle.

Whereas a manual m/s is said to be fully wound when it is physically impossible to wind it any further due to the coils binding, an auto m/s could be said to be fully wound at the instant before the bridle starts to slip since it is not possible to wind it any further even though the arbor can still be rotated, either manually via the crown or by the auto works, as doing so simply causes the bridle to slip.

The torque at which the bridle slips is governed primarily by three factors; the coefficient of friction between the bridle and the barrel wall, the contact area between the bridle and the barrel wall, and the force with which the bridle is pushed against the barrel wall.

The coefficient of friction is a function of things like the material from which each of the sliding surfaces are made, their surface finish, and how the interface is lubricated, hence the importance of using the correct breaking grease.

The contact area is down to the shape of the bridle.

The contact force is down to the thickness of both the bridle and the m/s.

All else being equal, a thinner m/s will have a lower contact force and will subsequently slip at a lower torque. Conversley, too thick a m/s will hold on tight until a greater torque than is required by the movement is generated before slipping, causing the balance to over bank or "gallop".

Using a thinner m/s will inevitably result in a loss of amplitude. Different people have different bench marks for what constitutes acceptable amplitude and for me it's around 275-280 at full wind, and over 225 after 24 hours (slightly lower values for Seikos). If your balance is over banking then its amplitude is in the order 350 degrees. I don't know what the relationship is between m/s torque and balance amplitude but lets assume for the sake of argument that it is linear so a 15% reduction in torque equates to a 15% redudtion in amplitude. That still leaves you with around 300 degrees at full throttle.

I would certainly give it a go and see what happens.

[steve1811uk]

Just interested if you tried the thinner mainspring? What was the resulting reduction in amplitude?

[GleaveandCoLuke]

We supply this as Omega 330 (= 28.10) and our size is 1.10 x 0.095 x 300 x 9 (auto  as it is a bumper movement), with no issues.  So can't say it will solve the problem but using that size spring should be fine.

Edited May 27, 2021 by GleaveandCoLuke

[rodabod]

On 5/26/2021 at 9:55 PM, steve1811uk said:

 

On 5/26/2021 at 9:55 PM, steve1811uk said:

Just interested if you tried the thinner mainspring? What was the resulting reduction in amplitude?

Hi Steve. I did indeed purchase the 0.095 spring and it still banked, albeit less often, so only when it was fully wound and either dial-up or dial-down. 

Edited May 29, 2021 by rodabod

[steve1811uk]

On 5/30/2021 at 12:20 AM, rodabod said:

 

Hi Steve. I did indeed purchase the 0.095 spring and it still banked, albeit less often, so only when it was fully wound and either dial-up or dial-down. 

Thanks, I have had a similar issue with an Orient Star 'Explorient'. I think I have solved that with a thinner mainspring, early days but fingers crossed.