Quote:
Originally Posted by philso
The "strength" of a square, rectangular or round tube is also different from an open profile such as an I-beam or channel. My empirical experience has been that the tubes, in general, resist torque much better than the open profiles. This characteristic may be more important in FRC robots than the ability of a particular profile to support a static load. Perhaps someone with the appropriate background can offer their comments (I am just an EE but I have had to deal with mechanical issues a number of times over the last 30+ years).
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For thin walled Sections In torsion:
Theta = (TL)/(GJeff)
T = applied torque
L = Length of the section
G = Shear Modulus (material property for 5052 Al this is ~26GPa)
Jeff = Effective Area Moment of Inertia or Torsion Constant for the section (I'm fuzzy on the terminology here)
For closed sections:
Jeff = (4 t (Aenc)^2)/S
t = material thickness
Aenc = Area Enclosed by the section
S = circumference of the section
For open sections:
Jeff = (s t^3)/3
t = material thickness
s = arc length of open section (similar to circumference, but ends don't meet)
Applying a 10 Nm load to a 50 mm diameter circular section x 100 mm long x 2mm thick yields the following:
Closed Section:
Theta = .01 degrees
Open Section:
Theta = 5.26 degrees
There's also stress calculations I could go into, and this gets more complicated with different thickness walls on parts of the section and warping of open sections, but I think you get the idea. If anyone is interested in more detail PM me.
This is why you rarely see open sections in automotive sheet metal. You'll always see a bunch spot welds down the length of a section.
I'll try to post the bending equations for thin walled beams later when I get time.