Need Torsional Strength

[MadMango]

I have this arm that is rigidly mounted at one end, with a rod at the other that supports a load of 330lbs out at a distance of about 15inches.

Currently the arm is made up of 2 C-channels that are fitted together to create a closed box section. The arm measures about 35" long x 4" wide, with the box section 2" high. The arm does not deflect under load, but it does twist along it's axis.

I have heard of torque tubes, and am wondering if I will get more torsional strength out of the arm if I were to replace the 2nd C-channel with several 2" dia torque tubes?

Material of arm is all mild steel, HRPO, with the everything welded in place. "Happy the Hare at morning for she is ignorant to the Hunter's waking thoughts."

 

[butelja]

If your channels are welded toe to toe forming a box section, then you already have a torque tube. It is crucial that they be welded along their length, rather than at the ends only. If only the ends are welded, it is not acting as a closed section. The cardinal rule for dealing with torsion is to use closed sections. Whatever you do, don't get rid of 1 channel and keep the other. The remaining channels torsional strength will only be a fraction of what it was when acting as a closed section.

Is it possible to go to a single larger tube? Possibly a 4"x4" or 4" pipe? A single large thin wall tube will carry the torsion most efficiently.

Good references for analyzing this are Roark & Young, Formulas for Stress and Strain, and Blodgett, Design of Weldments.

 

[prex]

If the two channels are continously welded together to form a rectangular pipe, it will be difficult, if not impossible, to get a substantially higher torsional stiffness in the same space. However I don't see how two channels might form a section 4"x2": what kind of channels are they?
As you find no sufficient stiffness I suppose that either the channels are not fully connected or that the fixed end does not sufficiently hold the arm in torsion.
prex
motori@xcalcsREMOVE.com

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[MadMango]

Thanks for all the info. The one C-channel fits inside the other, both open ends facing each other, slide together to form this box section. The inner C-channel is chain welded down the 35" length of the arm, plus plug-welded on the sides every 8" or so.

Upon further thought, I am going to abandon the idea of torque tubes, and will insteal lay a "Z-shaped" member in the C-channel, and cap that with a sheet of 14ga steel. My thoughts for this is that the 35" span was too long, so if I can divide my span into maybe 12" closed sections, the torsion won't be as bad. "Happy the Hare at morning for she is ignorant to the Hunter's waking thoughts."

 

[boo1]

I agree the two "c" channels (assuming closed section) are very effecient in torsional loading. The shear stress is found from:
Shear stresss=T*c/J
So the fuction is a linear relationship to the distence to the outer fiber and polar moment of inertia. The built up torque tubes would be of little improvement.