Dick Weber wrote:
> Hi Mike,
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> I hope this is crystal clear.
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Well, Dick, I'm an EE not a PE, so I need a little more hand-holding.
I may want to use a mixture of different yield strengths...
I realize that I have to make a snug fit or the "rattle room" makes the
displacement different for each tube.
Maybe I missed some brain trick to apply your advice to my question.
Be gentle. Please?
Mike - W8MM - EM79sd
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K5IU Reply .....
Ok, I'll be gentle...
Assuming a snug fit, here's what you do. Find the following quantities.
First, find the net area moment of inertia of the composite section. Use the
following equation:
net area moment of inertia of a composite tube = I = PI
*(Do**4-Di**4)/64
where Do = outer diameter of outer tube and Di = inner diameter of inner tube.
Now find "sec mod outer" by dividing "I" by the outer radius of the outer tube.
SMO = I / Roo where Roo = outer rad of outer tube
Then mutilply SMO by the yield stress of the outer tube. This gives the value
of bending moment the composite tube can take where both tubes have the same
yield stress as the outer tube. (No, this is not the answer.)
Now, find "sec mod inner" by dividing "I" (same I) by the outer radius of the
inner tube. ( Not Di/2)
SMI = I / Roi where Roi = outer rad of inner tube
Then mutilply SMI by the yield stress of the inner tube. This gives the value
of bending moment the composite tube can take where both tubes have the same
yield stress as the inner tube. (No, this is not the answer either.)
Now, THE ANSWER is the lessor of these two bending moment values. This works
because Young's modulus for both tubes are the same or are real close.
Be sure to work in the same units when doing this. I suggest you use lb and
inch.
How'd I do?
73,
Dick / K5IU / BSME / MSME / PE (sorry, no EE, but was 18 hrs short of BSEE also
a LONG time ago)
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