Cheater-bar moment capacity

[davidosterberg]

I am trying to determine the moment capacity of connection of two tubes where one tube fits tightly in the other. I attach a drawing to show how I mean.

Clearly the moment is transfered through friction and through surface pressure. In my particular case the the tubes are made of composite materials with low shear strenght and low transverse compression strength. I would therefore be especially interested in these quantities as a function of the bending moment.

The question is similar to the one discussed in thread507-274127 but I thought I'd have another go at it.

Thanks / David

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[BAretired]

I think I would consider the tubes a loose fit, disregard friction and consider the compressive force F acting at each of the four contact points between the two tubes.

F = M/2(L-b) where b is the length of bearing required to resist the force.

If the outer radius at each corner of the tubes is 2t and the inner radius is t, the tubes are not in contact for a length of 2t from the inside of the outer tube wall. If a is the required width of bearing, the bending moment in the wall of the outer tube would be F(2.5t + a/2) and the tension would be F.

It might be necessary to experiment with different ratios of a:b to find the best combination.

BA