Calculating the Size of a Mainspring •

Discussion in 'Hints & How-to's' started by bangster, Apr 4, 2015.

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  1. bangster

    bangster Moderator
    NAWCC Member

    Jan 1, 2005
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    #1 bangster, Apr 4, 2015
    Last edited: Apr 4, 2015


    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

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