Torsional Stiffness of Extruded Aluminum

[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

[Ianworld]

I don't have the numbers but what i can tell you is that it should resist torsion.

A 1 sided piece of aluminum(flat bar) resists bending in one direction
A 2 sided piece of aluminum(L bar) resists bending in two directions.
A 3 sided piece of aluminum(U channel) resists bending in two directions and torsion.
A 4 sided piece of aluminum just does that better.

If you have a piece of each of those shapes you can try it. The difference is pretty obvious.

[Matt Adams]

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

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

[Gary Bonner]

Item has torsion specs for their extrusions. You have to register on their website to get to the e-catalog. www.itemamerica.com