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Originally Posted by Ben Piecuch
Has anyone found data on the torsional stiffness (or polar moment) of Extruded Aluminums? (8020, Bosch, etc...) I'm trying to compare a 2"x1" x1/8" box section to a similarly sized extruded piece. Surprisingly, the bending strengths and weights are very similar, which is swaying me toward the extruded piece. However, this piece will probably have some torque applied to it's end, and my feelings are that the extruded piece will have a fair amount of twist.
If I don't get any data from here, I'll try to rig something up myself and post some numbers here. Thanks.
Bengineer
Team 228
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The polar moment of inertia of a shape is the sum of the intertia in two normal directions.
For instance, with a cylinder, Ix and Iy = pi*D^4/64
the polar moment is Ij = pi*D^4/32
I'm reasonably sure the give you the moment of inertia, so you can just double it for a symetrical piece and it will be the polar moment. In your case, you just add the Ix and Iy values.
Matt