CRP Bracket Thickness

Currently Competition Robot Parts (CRP) sells structural brackets with a thickness of 0.080 inches and functional brackets (motor mounts, bearing plates etc.) with a thickness of 0.125 (1/8) inches.

My reasoning behind the 0.080in thickness is to provide a slightly lower-weight solution than VEX’s or REV’s brackets while still giving my customers the strength they’re used to.

The .125in thickness is so that the bearing plates can be doubled up if needed to provide support for the full width of most FRC-used bearings.

On my team (5413) we only use .063in (1/16) and .125in (1/8) aluminum for all of our sheet metal and have never had issues using structural brackets (similar to the ones I sell) in .063 thickness.

I’m thinking about making my structural brackets available in .063in thickness and possibly heavy duty versions in .125in. I would like some opinions on this, do you think your team would be comfortable using structural brackets in the .063 thickness? Would there ever be a case where you would want .125in thick brackets for high-load structures? If you had a choice between the two options, which would you prefer?

This goes for anything structural: Wall thickness alone is not a determinant of strength.* What matters is the profile, I.E. the cross section of the part in the direction of the load. More specifically, the second moments of area of the part determines its ability to resist bending loads. Look at I-beams: more material further away from the neutral axis gives greater stiffness than equivalent solid sections.

Right angle brackets are tricky because of the bending loads. If you can reduce or eliminate the bending with gussets, etc, the problem goes away.

With the theoretical stuff out of the way, I’ll remind people that airplanes were once made almost entirely of bent sheet metal, with thicknesses much less than 1/16… Just design with a little thought to the forces acting on your system and how to better resist them.

As far as “comfort”- generally, if you are able to prove that the bracket can withstand x amount of force, either through static analysis/testing, people’s “comfort” will become irrelevant. Proper design and analysis, then validation, is greater than a gut feeling on strength.

(*obviously there are many exceptions… pressure vessels come to mind)

Material and (in case of many materials) tempering matters as well.

However, assuming similar materials and form as Vex gussets, I am confident that we would have used .063 gussets for the 0.04 tube of the superstructure of our 2017 robot. All that this structure supported was a ramp to direct a gear towards our holder/pusher, a gear pusher, and the control boards which filled the side walls. We would probably not have used it for the high-reaching arm of our 2018 Bayou Robot.

On a related subject, anyone working on polycarb structural gussets and some sort of polycarb rivet, or a polycarb adhesive-and-cleco system to build polycarb tube structures?

Right here bud

97447A050 and 7592A31 on McMaster.com

The first is an aluminum 3/16" x 0.126 to 0.25" rivet. My idea here was to create entirely plastic parts, with any metal having been used as a temporary clamp only.

The second is a bonds-everything epoxy. While I did not try this one, I did test about a dozen different plastic/polycarb adhesives and didn’t find any which would acceptably bond Vex polycarb tubing to itself or the polycarb sheet we use (from Home Depot). Most of them came off by hand with only an inch of leverage. MEK worked great sheet-to-sheet, but did not stick to the tube. When I tried using more, it actually made the tube brittle. By “a system”, I meant tubes and gussets and an adhesive which are all known to work well together.

You don’t really want fasteners that elastically deform for parts that are going to see any reasonable load. Why the aversion to aluminum rivets? They work just fine for both polycarb tubing and polycarb gussets - 2791’s claw in 2014 was built with a lot of this stuff.

The aluminum rivet is far more appropriate than a plastic one.

Did you try an epoxy that specifically listed polycarbonate? What surface prep did you do?

Yes. Everything I used listed polycarbonate either on the package or its website. I tried at least one epoxy type (Loctite brand IIRC), MEK, and a few viscous adhesives. I did both smooth and scuffed (lightly sanded and wiped) surfaces, and tried them both “end to face” and “face to face”. As expected, the thinner adhesives/solvents largely worked better on smooth material and the thicker ones (e.g. epoxy) preferred the scuffed surfaces. The VF tubing was so resistant to adhesives that I considered trying polystyrene glue. If the tube in my junk drawer at home hadn’t solidified, I would have.