Relation between mainspring thickness and height ?

[Endeavor]

Hello All;

I'm busy with a 1860'ish pocket-watch. The on the tail-end repaired mainspring (the original or a later replacement??) has a height of 3.44mm, a thickness of 0.2mm and a repaired length of about 430-440mm

The ID of the barrel is 16.8mm and the arbor diameter is 5.5mm.

If I use these measurements in the spring-calculator: https://www.vintagewatchstraps.com/mainsprings.php , I get

The problem arises when searching for a new spring (which has to be sourced from either CousinsUK or inside the EU).

A 3.44 mm height is not a common size and it's a very high spring for a pocket-watch. The closest 3.4mm high springs which I've seen had a thickness of 0.22mm, making the spring, with regards to its thickness, even stronger (too strong perhaps?)

If I were to go down in height, 3.2mm or even down to 3.0mm, more size options become available.

The spring calculator doesn't take the height of the spring into consideration, which perhaps seems a bit odd. I would expect that a 1.5mm high spring with a thickness of 0.2 is weaker than a 3.4mm high spring with the same 0.2mm thickness ?

So is there a relation in strength between the thickness of a spring and its height?

If I were to find 0.2mm thick spring with a height of say 3.2mm or even down to 3.0mm (which is about a 12% reduction in height compared to a 3.44mm spring), what are the consequences?

Would that spring be less strong and would the height reduction, considering its already "huge"-height, cause any problems?

Hope to hear your thoughts

 

 

[praezis]

Hi,

the height is of small influence to the momentum. It contributes directly,  while the thickness contributes with its cubic value! 

However a too low blade may wobble in  the barrel.

Many B-Uhren use H=3.5 mm. You will find them on Ebay(german) as Laco or Lange mainspring.

Frank

[Endeavor]

1 hour ago, praezis said:

Many B-Uhren use H=3.5 mm.

Thanks

Not sure if 3.5mm would fit, could be slightly too high. And the 3.5mm springs I've seen started with a thickness of 0.22mm, but I will keep looking and indeed, the German eBay has a nice selection

 

1 hour ago, praezis said:

However a too low blade may wobble in  the barrel.

That's were the rubber hits the road, how low is too low? Will 3.2mm still do or as a minimum a 3.0mm?

[praezis]

Depends on the inner net height of your barrel. Normal with that size you will use inner height minus 0.1 mm, but minus 0.2 will do no harm.

The mentioned B-U springs have thickness 0.20

Frank

Edited May 19, 2023 by praezis