# Thread: Moment of Inertia- section of round tube

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

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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.

2. 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. 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. 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. I may have found it...

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

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

6. 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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