Calculate Mainspring Thickness and Length

If you are replacing a mainspring with a different width, then you will want to determine the thickness so as to have the same strength characteristic. This calculation, performed iteratively, determines the correct mainspring thickness and length even if the old mainspring is incorrect.

Change the items in the yellow boxes and see the Results below.

Calculator

Change to Mainspring Width

Barrel Measurements:

Inside Diameter of the Barrel:

Diameter of the Barrel Arbor:

Old Mainspring Thickness:

Old Mainspring Length:

Old Mainspring Width:

New Mainspring Width:

Results:

Needed Mainspring Thickness:

Needed Mainspring Length:

Formulae:

T_{MS_n} =

(

L_{MS_n}

L_{MS_o}

W_{MS_n}

W_{MS_o}

)

T_{MS_o}³

)

^⅓

Formula:

L_MS =

x ( D_B² - D_A² )

2 x 4 x T_MS

where,

T_{MS_n} is the new mainspring thickness
L_{MS_n} is the new mainspring length
L_{MS_o} is the old mainspring length
W_{MS_n} is the new mainspring width
W_{MS_o} is the old mainspring width
T_{MS_o} is the old mainspring thickness
L_{MS_n} is the mainspring length
D_B is the inside diameter of the barrel
D_A is the outside diameter of the barrel arbor

This calculator, provided by Horology - The Index, is one in a series of repair calculations.