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  1. #1

    Default Calculating the Size of a Mainspring


    The formula I use is:

    L = .3927 [(D*D) - (d*d)]/T

    L = length in inches
    D = inside diameter of the barrel or space available for the mainspring to unwind, in inches
    d = diameter of the arbor in inches
    T = thickness of the mainspring in inches (not the width!)

    (D*D) = D squared

    With this formula you can vary the thickness of the mainspring a little (plus or minus 0.002" but favor the thinner side) and maybe find a suitable replacement.

    —David LaBounty

    For open springs, let D =the diameter of the mainspring wheel. The width will be determined by the depth of the barrel or the available amount of winding arbor. —bangster
    Last edited by bangster; 04-04-2015 at 11:35 AM.
    1. Check out the Repair Hints & How-To's forum. You may find your answer there.

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