pic: Arm joint idea

This is an arm joint idea that I came up with earlier this week instead of studying for my finals. I was trying to figure out a nice solid way of making an arm joint to connect round tubes (because they’re so awesome) and this is what I came up with.

There are two main parts that actually need work to make; the clamp on the mast, and the stub for the arm. Both would be welded from steel tubes and strips. The idea is that the mast-side of the joint is made to slide off (or up and down for tensioning chain) and is held on with hose clamps. The arm stub would be 2" od (perhaps) and about a foot long (perhaps), with an actual arm member with an id of 2" clamping onto the outside. Modular and easy to assemble, minimal bolts, and strong. (perhaps)

Interesting idea, I like the concept. I have two major concerns:

  1. Have you done the analysis on the hose clamp? Remember that it has to hold against chain tension, which can be extremely high, especially for #35 chain, which it nominally looks like the sprocket is for.

  2. The bolt attachment that indexes the arm to the sprocket is going to be under a lot of torsional stress. I would recommend putting a plate or something on the other side that is keyed to the sprocket shaft somehow, or an extension from the tube that will mate more solidly with the sprocket. Otherwise I think you will see a lot of bent metal on the arm tubing and possibly a damaged sprocket as well.

A slight typo I made in the first post… it would use t-bolt clamps, not hose clamps (still same concept though). We used a similar method of attaching an arm on our 2007 robot and had no slipping problems due to chain tension. It is worth noting that that was an aluminum/aluminum interface, not a steel/aluminum interface, so the coefficient of friction will be less for this design. I don’t think it will be slipping due to chain tension, though. (maybe from torsional loads from a robot pushing sideways on our arm, but that makes the slipping a design feature to prevent broken parts. :slight_smile: )

As for the bolt attachment, all of twisting moment is being accounted for by the 2" steel tube. I expect the only force you will see on the indexing bolt will be shear stress, and the sprocket won’t see any bending moments either.

Looks great Scott! Hey are you guys signing up for NURC?

I can’t wait to see if we can even use an arm this year!

Round tubes are easy to work with if the OD of the tube matches the ID of a square/rectangle tube. This gives you an easy flat surface to work with, face mount things to, press bearings or bushings into, etc.

Heat guns can be good persuaders to make the OD match the ID. :slight_smile:



There’s a very good chance! The high school team will probably have a go at it again this year… and me and my dad are thinking about having a grown-up team, just for fun.

Thats pretty cool. But I don’t see why you didn’t just use square tubing the whole way.

Yes, square tubes aren’t as torsionally stiff, but I’d think the lack of a joint would make up for it.

I have bad memories of team 852’s fiberglass tube arm getting snaped off at the base at CalGames '07.

The round tube will be a bit lighter, and more torsionally stiff (as you said) than a square tube of the same size.

The other reason I see teams go with round is that they are using PVC pipe. I know you’re aware, but you can’t just go and by that at Home Depot (or anywhere… I’m not sure, I’ve never tried to find PVC square tubing). The teams often like to use the PVC as it’s so elastic and rather hard to break.

Also, about your last statement, i think we should separate the material vs. shape argument. Without knowing anything about their robot that year, I’d imagine that the failure wasn’t from the shape of the tubing, but rather how they were holding onto it. You really need to clamp it, fastening to it is a bad idea.

This design would probably work just as well with a square tube acting as the base for the second segment of the arm, it’s just another way of getting the job done. Even so, I like the round tubes because you can clamp arm components together with t-bolt clamps. It seems like a stronger and easier to maintain system to me.

I really prefer using a round tube for the mast, though. In 2007 our mast was made of a 3" diameter, 1/16" thick 6061 T6 aluminum tube. This thing was plenty strong for a FIRST robot, and it only weighed 0.70 lbs per foot. Since most square profile tubes of similar size don’t come thinner than .125", they will weigh at least twice as much. (you could cut speed holes in a square tube, but that takes too much time and effort for us :wink: )