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Thread: Moment of Inertia- section of round tube

  1. #1
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    Moment of Inertia- section of round tube

    ipt>
    Hi again,

    working of some plastic cantilever snaps which are on a ring shaped part. I found a calculator for half round tube, but I'm looking for 1/18 round tube, base spans 20 degrees. p.s. I'm terrible at calculus.

    Thanks for your help

  2. #2
    Lead Engineer Cake of Doom's Avatar
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    I = 0.5 x M(b^2 + a^2)

    M = mass, b = radius to outside of tube, a = radius to inside of tube wall.

    Or do you mean the Second Moment of Area? The two terms get interchanged regularly over here.

  3. #3
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    Thanks for your response. I have deduced that I am looking for the other one... to which I = b h cubed over 12 for a rectangle.

  4. #4
    Lead Engineer Cake of Doom's Avatar
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    No worries.

    From your first post, I thought you were looking for the properties of the tube, not the cantilever parts. Sorry about that.

  5. #5
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    I may have found it...

    I = R^3 t ( a + sin (a) cos (a) - ((2 sin^2 (a)) / a))

    where R is external radius
    t is thickness
    a is angle from vertical to one side. ie 2a is full sweep of section.

  6. #6
    Lead Engineer Cake of Doom's Avatar
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    Ix = π (do^4 - di^4) / 64

    do = outside diameter, di = inside diameter.

    This is the simpler method for the second moment.

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