pic: REV Robotics building system

[taichichuan]

Hey Gang,

What is the load bearing capability of the 1" extrusions? That is, how does it compare with 1" square 1/8" square 6061 aluminum tube? If I've done the math correctly, the Rev extrusion seems to be lighter than the 1" square aluminum tubing. So, I'm trying to determine if it's a better option than welding the 1" tube for a frame.

TIA,

Mike

[GeeTwo]

Loading in what direction?

For most deflection and torsional loading (including buckling), the stiffness depends on the moment of inertia of the cross section, which, for a given amount of material, increases as the square of the distance from the center. For a given amount of material and diameter, a hollow round tube will have the greatest moment of inertia (be strongest), with a hollow square tube being a little bit behind. Shapes like I-beams and rectangular tubes are designed to have a larger moment of inertia in one dimension (the longer dimension) than another to accommodate a larger load in that direction.

For tension loading, I understand that shape makes less difference unless your material forms fibers.

Shapes like the REV and 80/20 extrusion require more weight for the same strength; their purpose is in easy fabrication using tee nuts and similar fasteners.

[Lireal]

Originally Posted by GeeTwo

For most deflection and torsional loading (including buckling), the stiffness depends on the moment of inertia of the cross section, which, for a given amount of material, increases as the square of the distance from the center. For a given amount of material and diameter, a hollow round tube will have the greatest moment of inertia (be strongest), with a hollow square tube being a little bit behind. Shapes like I-beams and rectangular tubes are designed to have a larger moment of inertia in one dimension (the longer dimension) than another to accommodate a larger load in that direction.

Can you be a little more clear? Which has less deflection, the tubing or the extrusion?

[EricH]

Originally Posted by Lireal

Can you be a little more clear? Which has less deflection, the tubing or the extrusion?

Just to be a total annoyance (and to give you a taste of real engineering):

What's the load case, the size, and the wall thickness? Oh, and how is it supported? You see, the answer depends on knowing those... OK, OK, I'll go a little easier than that.

What he's saying is, essentially, more material at the outside of a piece of material will resist deflection better, given the same mass and size. BUT, extrusion may be tailored to resist deflection better in one direction. (I-beam being one example--it'll bend better if you're trying to bend it in the direction of the open sides than if you try to bend it towards the flanges.)



Moment of inertia (and in this case, that's MASS moment of inertia) is something you'll tend to hit somewhere around 2nd-year engineering coursework, possibly a hair sooner. It's not that hard in concept; the simple definition works out to how much torque it takes to rotate an object about a given axis. Now, the actual application of that definition, and the formulas to help determine it, are where that gets fun, as the shape of the object in question plays a role (which is why that was brought up in the explanation). If you're interested in running through some of the math, let someone know; that can be arranged...

[GeeTwo]

Originally Posted by Lireal

Can you be a little more clear? Which has less deflection, the tubing or the extrusion?

Originally Posted by GeeTwo

For a given amount of material and diameter, a hollow round tube will have the greatest moment of inertia (be strongest), with a hollow square tube being a little bit behind.

That is, tubing is the stiffest shape there is for loads which may be required in every direction sideways to the length.

[MechEng83]

Originally Posted by EricH

Moment of inertia (and in this case, that's MASS moment of inertia) is something you'll tend to hit somewhere around 2nd-year engineering coursework, possibly a hair sooner. It's not that hard in concept; the simple definition works out to how much torque it takes to rotate an object about a given axis. Now, the actual application of that definition, and the formulas to help determine it, are where that gets fun, as the shape of the object in question plays a role (which is why that was brought up in the explanation). If you're interested in running through some of the math, let someone know; that can be arranged...

I'm pretty sure the discussion is about AREA moment of inertia (units of [length]^4). Though your description of mass moment of inertia (units of [mass]x[length]^2) is related to rotational inertia, it's not germane to this discussion.

Regarding OP's question:
We found the extrusion to be heavier than the thin-walled tubing we've previously used for frame elements. Are you possibly referencing 1/8" wall square tubing? If so, I'm fairly confident the tubing is stiffer in bending and torsion than the Rev Extrusion. As far as axial stiffness (along the length) it's going to be proportional to the cross sectional area, which is proportional to the weight/unit length for the same material.

[Greg Needel]

Originally Posted by MechEng83

I'm pretty sure the discussion is about AREA moment of inertia (units of [length]^4). Though your description of mass moment of inertia (units of [mass]x[length]^2) is related to rotational inertia, it's not germane to this discussion.

Regarding OP's question:
We found the extrusion to be heavier than the thin-walled tubing we've previously used for frame elements. Are you possibly referencing 1/8" wall square tubing? If so, I'm fairly confident the tubing is stiffer in bending and torsion than the Rev Extrusion. As far as axial stiffness (along the length) it's going to be proportional to the cross sectional area, which is proportional to the weight/unit length for the same material.

FYI the for REV extrusion are as follows.

Cross Sectional Area: 0.38133 in^2
Moment of Inertia X: 0.03394 in^4
Moment of Inertia Y: 0.03394 in^4
Yield Strength: 21000 Lbs./ sq. in (psi)
Modulus of Elasticity: 10007000 Lbs./ sq. in (psi)


The debate as to which is stronger really does depend on your loading conditions and use case. If you are talking in the context of FRC style robots, I highly doubt you will see a notable difference in strength per piece (as small deflections) don't really effect much in this case.

Where you will see the difference is in how you join them. A welded 1x1 tube is going to be better than a piece of extrusion with nuts and gussets, but there again it depends on your use case and for FRC scale applications you can't go wrong with either.

[MechEng83]

Originally Posted by Greg Needel

FYI the for REV extrusion are as follows.

Cross Sectional Area: 0.38133 in^2 (FTFY)
Moment of Inertia X: 0.03394 in^4
Moment of Inertia Y: 0.03394 in^4
Yield Strength: 21000 Lbs./ sq. in (psi)
Modulus of Elasticity: 10007000 Lbs./ sq. in (psi)

Thanks Greg. Always good to hear specs from the figurative horse's mouth.
I'll file this info away for comparison discussions with my team.