Calculate an Estimated Mainspring | Horology

If the mainspring is missing or if the existing mainspring is a suspect replacement, use this calculator to determine the correct sizing.

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

Calculator

Estimated Mainspring

Teeth in Movement:

# of Wheels in Drive Train:

Wheel Teeth

Pinion Leaves

5 Wheels

Center Wheel or Center Pinion:

4 Wheels

2nd Wheel:

Main/Barrel Wheel:

Number of Needed Turns:

Expected Duration on a Full Wind

30 Hours

Number of Turns of Center Wheel:

8 Days

Number of Desired Turns of Barrel:

30 Days

(Override if desired)

Barrel Measurements:

Inside Diameter of the Barrel:

Diameter of the Barrel Arbor:

Results:

Needed Mainspring Thickness:

Needed Mainspring Length:

Formulae:

N_B =

N_C x

P₂ x W_C

W_B x W₂

N_B =

N_C x

W_C

W_B

T_MS =

( 2 x ( D_B² + D_A² ))^½ - ( D_B + D_A )

N_B x 2

Corrected Oct. 2020 with a very able assist
from Paul Bauerle.

L_MS =

x ( D_B² - D_A² )

2 x 4 x T_MS

where,

N_B is the number of turns of the barrel
N_C is the needed number of turns of the center wheel, i.e., number of hours: 30 or (8 days x 24) or (31 days x 24)
W_B is the number of teeth on the barrel wheel
W₂ is the number of teeth on the second wheel
W_C is the number of teeth on the center wheel, or center pinion for a 4 wheel train
P₂ is the number of leaves on the second wheel pinion
D_B is the inside diameter of the barrel
D_A is the outside diameter of the barrel arbor
T_MS is the mainspring thickness
L_MS is the mainspring length

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