pic: 2791's Brand New Arm

We used our 65 pounds to fabricate this new hanging arm. Inspired by 1625 (though it does not fold down like theirs), the arm hooks up to the reversible winch posted earlier to fold and unfold in order to grab the bar. We will be adding grippy material to the arm’s metal pole grabbers to get a secure hold, if not we have a backup plan to hold above the platform.

So this arm didn’t work more than once. The metal support failed under load combined with falling a few feet. The largest triangle’s outer edge (on the left) bowed out on both sides, I believe.

Does anyone have any suggestions for what to change in a redesign? I’m wondering if thicker material (the metal might have been 1/4" AL, so 3/8" would help), smaller triangles, and more standoffs throughout the arm (but not in the way of the piston) would help enough. I’m also looking into flanges.

Disclaimer: I didn’t CAD this, I’m not that smart yet.

Flanges will likely prevent the buckling problem you ran into (provided you have a way to bend it… 1/4" is pretty thick). Also, does it need such large speed holes in the supports? How close are you guys to the weight limit?

A picture of it attached to the robot would be helpful, too!

As I suggested before (might as well write it down this time):

Smaller truss pattern
More space along the edges of the material and in between triangles
Flanges are friends, not food
1/2" radius (or w/e you prefer) around standoffs
More standoffs (a pair of standoffs after each set of two triangles)

Flanges and more material, not more thickness.

Flanges are where you’ll get the most bang for your buck. Once it starts bending with no flange, there’s not much keeping it from continuing to bend. With a flange there is more material that needs to fail before that bending can continue.

Mathematically, bending depends on the moment of inertia, all other things being held equal. The moment of inertia of a rectangular section varies linearly with the length of the edge of rectangle parallel to the direction of the moment, and with the cube of the length of the edge of the rectangle perpendicular to the moment. That probably doesn’t make a lot of sense*, but it gives you a huge increase in strength for a small flange that runs along the edge of you aluminum arm.

*Because I can’t explain it well, not because it isn’t true.

Just a personal off topic request, could someone who CAN explain this well (or at least point me in the direction of the relevant equations) elaborate on this. I am an ignorant CS major but I am REALLY curious.

Sorry for threadjacking Chris.

Wikipedia does a good job with the math under this heading: “Second moment of area for various cross sections”

I tried writing more in depth descriptions, but I can’t get something down that uses the math.

In physical terms, think of it like holding 2791’s arm with one end in a vice. If you were to press down on in, it would take a lot of effort to bend it. However, to bend it away/towards you would take significantly less, right? It seems from Chris’s post that it was in this away/towards bend that the air failed. It turns out that adding just a little material in the plane that bending is occurring gives you much more strength.

TL;DR 2791 built an arm well prepared for tension/compression, but not strong enough for bending.

P.S. This all assumes I read Chris’s original post correctly. By bowing, you meant that head on the arm used to look like || and now looks like ()… right?

We are using a billet bar of 3/4" x 1" solid aluminum for the bottom “leg” of ours, about 15 inches long. The upper leg is 1/8 wall 1x2" tube aluminum. Our “wings” are .5" diameter solid aluminum rod bent slightly to get a better lock on the vertical tube.