Mainspring in Focus

The term ‘Creep’ refers to the slipping of the welded bridle around the outside of a lubricated barrel Creep.

To compensate, some brands have grooved the barrel wall to stop the bridle from slipping. The problem is, though, that the torque generated from the winding with a grooved wall sends too much power to the escapement.

The creep is something of a compromise.

Shortening the bridle, which is spot welded to the spring, and straightening it, reduces creep but leaving the watch with elevated amplitude.

The magnet test with determine whether a mainspring is good quality and made of nickel/cobalt/chromium as these materials are anti-magnetic.

The term Strength, when used in regards to the mainspring is the thickness and width measured in Newton Metres – torque.

The mainspring should be the height of the barrel wall less 0.5mm clearance.

It is important to have the correct spring for the barrel is to ensure the watch gains the maximum power reserve possible i.e. number of turns of barrel. To do this, you need to figure out how much space you have inside your barrel. First things first, you need to work out the area of the barrel. To find the area of the circle, you have to square the radius and multiply by pi. But don’t forget you have to compensate for the barrel arbour, which takes up roughly one third of the barrel’s diameter.

Here’s a working example for you:

If a barrel of 10mm has a radius of 5mm then the area would be 78.57 (25×3.1428), but that’s before you’ve subtracted the area occupied by the barrel arbour. Since the barrel arbour tends to occupy one third of the would-be functional space of the barrel and the mainspring in either a relaxed of wound state occupies another third (the final third remains for the tensioning of the spring), you need to perform the following calculation:

The equation for working out the length of the mainspring is thus:

Pi (R2 – r2) / 2e

R= barrel radius
r = arbour radius
e = spring thickness

You have to double the thickness of the spring to account for the necessary space.

In similar equations, N will stand for the number of turns between the wound and unwound extremes of the spring and L the length of the spring.

To find the number of turns to unwind a barrel (N’), do the following equation:

N’ = 1/e (R – the square root of R2 – (Le/Pi)

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