Working with thin tubing

Our team is thinking more about switching to 2x1 tubing in 1/16" wall thickness, primarily to save weight. The obvious issue that sticks out is clamp crush which would weaken the tube. The other issue is bearings and for that, I’ve heard bearing blocks are a must have. How would we avoid clamp crush especially when using thin tubing on primarily structural applications where things need to be rigid? We don’t have access to a welder so our options are rivets or bolts.

Be careful with rivets. The 1/16" wall doesn’t hold onto them super well. We ended up having to bolt+washer a lot of our stuff. Our rivets got super loose by the end of the season. We used 3/16" aluminum ones. Maybe different rivets yield different results

Most of the time you don’t need to clamp things so tight that they end up crushing

We fill tube with wood blocks in the areas that will have a crush load. Since our shop is next to the school’s wood shop, we can make the blocks easily. We plane stock to the right thickness and table saw it to the right width, then cut it to length on a chop saw. This works best when thickness and width are tight fits inside the tube, so we usually hammer it into place. Doesn’t take much force, but it won’t just slide in, either.

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To prevent clamp crushing, there are a few answers. Both are easy:

  1. Don’t tighten your bolts so hard. Tighten them until the tubes start bending, then back off.
  2. Add plates to both sides. If you’re using gussets to bolt two tubes together, just adding a gusset on each side of the tube will prevent bending. If you are only bolting to one side, you can make a small 1/16" or 1/8" plate matching the hole pattern to spread the crush load around on the tube.

Rivets will not be an issue with regards to crushing tubes, as they only load on one face. You can achieve the same result with rivnuts (we buy them from McMaster) if you need something which can take higher loads.

Yes, bearing blocks are a must. In fact, I also strongly dislike adding bearings to tubes ever, as even bearing holes in 1/8" tube can result in oval-shaped bearing holes at the end of a season, especially on high-load parts.

The most likely reason for Chad’s issue is that the grip range was incorrect. All rivets have a grip range, which is the thickness that a rivet will stack up. For instance, if I were trying to rivet a 1/8" plate and a 1/16" tube together, I would need a grip range that included 3/16". When you have properly selected rivets, this usually isn’t an issue.

However, rivets generally have less clamping force, since you can’t crank down on them. They also have lower strength in pretty much every direction than a screw. As such, you should always be using many rivets where you could have gotten away with just 2 or 3 screws.

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We only have a few sizes of rivets in terms of length. We probably did have the wrong grip size. I will keep that in mind next time we order rivets

Another option I haven’t seen posted yet… clearance holes. When bolting two pieces together, drill all the way through, then grab a bigger drill bit and drill through just the outside edge of the tube. That way, your bolt head passes through the clearance hole and you’re essentially just sandwiching two plates. No risk of crushing the tube at all.

That’s largely what we do (every situation is different, though!), regardless of the wall thickness. We rarely bolt two tubes directly to each other, much more often bolting them together using gusset plates. Depending on the situation, that means the bolt goes through the tube and the nut outside of the gusset, or we use a rivet nut inside the tube and put the bolt into that through the gusset. We’re a big fan of Vex’s 0.1" tubing, it’s right between 1/8" and 1/16", and seems to work well with everything. Their more recent thinner wall tubing is great too, although the one place we used it last year we applied an extra plate on the outside before attaching a rivet nut to it. It held the 40-lb gas springs beautifully all season, though!

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So what I am getting at reading the responses is to use gussets to fasten pieces of tubing together. If using bolts, use a washer to prevent crushing. I’m assuming drilling a clearance hole like Jon said would be equivalent to using rivets and gussets since essentially you are fastening just one side.

3d printed inserts work wonders to keep thin wall from collapsing. Use it all over the place.

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Another thing you’ll learn to enjoy are rivnuts, or threaded inserts.

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What do you mean by 3d printed inserts? Do you have a picture I could see?

Basically the same thing as @Richard_Wallace’s wooden blocks, but 3-d printed from plastic rather than cut from lumber.

This. It’s literally what it sounds like. Measure the cross section and 3d print a block/spacer to fit inside. If you get the tolerances right, they’ll slide into place with minimal effort finagling the block.

Crush spacers work great. Here is an application of them:
image

The 3/8" spacer goes through the mating surface of the 2 tubes. This allows you to really crank down on the bolt without worrying about crushing the tube. To save weight (as we had a lot of these on our robot), we used 3/8" nylon stock instead of aluminum.

Honestly, we haven’t had any issues with rivets coming out of thin wall, just make sure you buy the correct grip range of rivet and they won’t come out.

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Would you slide these in or would you drill a clearance hole and then use a washer on the bolt?

Pay attention to the aluminum alloy that you use for your tubing. It’s fairly easy to find 2x1 tubing in 6063; not nearly as easy to find it in 6061. Both Tensile Strength and Yield Strength are substantially higher in 6061 aluminum.

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The inside faces (the faces of the tubes that meet) have .38" holes in them while the faces where the bolt head and nut rest have a regular bolt clearance hole. To assemble it, you put the spacer in one of the tubes first and slide the other tube onto the spacer, then put the bolt through and tighten.

Hopefully this exploded view helps better visualize it:

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What about using 2024 or 7075? I know those are more expensive but would using those alloys reduce crush even further?

McMaster PN 6546k2 is 2 x 1 x 1/16" in 6061.

Does anyone have any experience with the vex Polycarbonate 2*1 VersaFrame tube? How does it compare to both the .1" aluminum and the .05" aluminum variants strength wise? Do holes slot out at a concerning rate?

The use cases for polycarb vs aluminum are rather different. The main time to use polycarb is when you expect collisions and want flexibility to bend rather than break, or when you’re interested in using the tubing as a compliance spring. In the case of intake arms, both will often apply. (There are probably others I’m not thinking of right now.) Aluminum is more rigid but stronger and stiffer when it comes to supporting a load.

I’ve used the 1x1 several times (very pleased) and 2x1 Vex polycarb tube once (not so much, but I’d try them again in a different application). I haven’t attempted to use rivets with either, though if I did, I’d put washers on what is normally the blind side of the aluminum. Whenever I’ve used the 1x1, it’s been with #8 machine screws about 1-1/2" long and nylock nuts, snugged up but not really tightened; no issues noted with loosening, even though the screws did take shock shear loads holding on an intake wheel… For the 2x1, I was hoping to “glue” it to other pieces of polycarbonate for a ramp. It turns out that while the clear polycarbonate from the local home center works quite well with MEK, it makes the black polycarb used in the tube brittle.