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How do you calculate the moment of inertia for a hollow shaft with a truss-like struc

  1. Oct 16, 2012 #1
    To give you a better idea, I have it drawn out here: http://tinypic.com/r/eq6ln5/6

    I am calling the thickness of a rod t and the thickness of the shaft t2. I am using the basic equation Ixx = (integrate over area)(y^2)dA on different sections and then adding them all together, following the guidelines of this website:


    I split the shaft into sections, as follows: http://tinypic.com/r/2j5mc8l/6
    I calculated the moment of inertia of each section and added them together.

    Referring to half of one rod (aka half of the inner diameter) as L, I have come up with this equation:

    =2*L^3*(SQRT(2)/2)^3 (three rods)
    +2*(t*L*(1-(SQRT(2)/2))) (top of center rod)
    +L*t^3/12 (middle horizontal rod)
    +(PI()/4)*((L+t2)^4-(L)^4) (hollow cylinder)

    Am I going about this the correct way? Any suggestions would be appreciated.

    Thank you!
  2. jcsd
  3. Oct 19, 2012 #2
    Re: How do you calculate the moment of inertia for a hollow shaft with a truss-like s

    Calculate the Ixx of the ring
    then calculate the Ixx of an entire length of internal bar (spanning the diameter) and multiply by 4

    If you want I could show you the process of manually calculating the second moment of inertia of the structure.
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