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Calculating Torsional Force - Compression Springs

  1. Sep 18, 2014 #1
    Let's say I've got a helical compression spring of a known length and strength, and I want to know how much force it would take to twist the spring around the axis of its coils, effectively making the diameter of each coil smaller, so it acts a bit like a torsion spring.

    I know Hooke's law is probably relevant, just not sure how to go from data on a linear force to that of angular force.

    I'm working on a doohickey that compresses a spring fixed around a rod with a 90 degree handle on one end - The rod is inside a tube with a channel cut down its length, with a notch at the end of the channel so the rod can be locked into place and hold the spring compressed. I'd like to calculate (roughly) the amount of force I'll have to apply to the handle in order to move it out of the notch and subsequently let the spring/rod assembly fly forward (ignoring drag from the air, friction of the parts against each other, etc).

    - Apt
  2. jcsd
  3. Sep 18, 2014 #2
    angular deflection = (M*pi*D*N)/(E*I)

    where M = applied moment, or torque
    D = mean coil diameter
    N = number of active coils
    E = young's modulua
    I = second moment of area of wire

    Use consistent units.

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