Best Practices for Not Crushing Thin-Wall Tubing
 

As anyone who has had students put bolts through aluminum tubing knows, it's really easy to crush the tube by overtightening. For especially thin-walled tube (1/16'' or thinner), it can be difficult to not do it.

I was wondering what teams consider a "best practice" for avoiding this. I've used both washers and wood inserts - the former doesn't work quite as well as the latter, but is easier.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

Unless you have to, don't through-bolt, place your nuts on the inside of the tube, so that you're only putting load on one side of the tube. For this reason, rivets are my preferred fastener for working with this type of material, since they do this automatically, and can do so blind, right in the center of a tube.

Wood or plastic inserts also work well for the ends of tubes. Plastic has the advantage of being tappable.

If this can't be done, use versa block style construction, clamping onto the tube, allowing the strongest part, the edges, to support your parts. This can be done even without advanced machining. Use a few small screws with through holes to locate your part on the tube, and then bigger screws on the outside to provide the real clamping force.

Short of this, make sure that the parts on both sides of the tube are supported by the whole tube, not just the middle. Again, the edges, where you gain the full support of the sides parallel to your fastener, are the strongest parts. Make sure your part takes up the whole height of the tube when possible. A cool trick to achieve this is to make an aluminum plate the height of the tube to slip under your nuts, distributing the load.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

We use a torque wrench and make sure to set it to a low torque.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

wood in the tube, I have been doing this for years. Typically during the field construction, I will rip down some .875x.875 square dowels from 2x4 for this purpose. The wood adds less weight than going to thicker wall tubing but provides plenty of crush support.

We actually did this last year with some fiberglass square profiles also, to provide a better surface for mounting a bushing all the way through the tube.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

Depending on the application, rivet nuts or rivets are an option too. There are many situations where you don't need to go all the way through the tubing.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

We've been experimenting with these in the preseason, and our impressions have been generally good. I've heard some complaints about them coming loose and spinning in their holes, though - we might try dabbing the outside of them with some red loctite before installation to prevent this.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

In addition to proper tightening. We find it easiest to just cut out a small piece of PVC to length and run the bolt right through it. Works like a charm.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

1. Important to tighten adequately
2. Ribbed ones are less likely to rotate
3. Bit size is important

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

I've got 2. and 3. covered. Tightening, I'm still unsure about. I've been following the "stroke length = min rivnut grip width + max rivnut grip width - material thickness" formula and hoping that works, since I can't really "feel" when the rivnut has tightened fully and I don't want to damage the threads.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

Using washers on both sides work well for us when we through bolt also stuffing the tubing with wood or plastic.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

sometimes a little crush isn't such a bad thing :p

we often tie off the ends of our "469 signature" 1/4" tubing by heating with a torch and then crushing flat in the jaws of a vise ... at the right angle, of course!

then we drill a hole through that and rivet it to whatever. works pretty nicely!

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

To answer the OP's question, there are a few ways to do this.

You can bypass this problem entirely with rivets or rivet nuts, as previously mentioned. If you are installing bolts the OTHER way (as in Bolt Head -> Tubing -> Thing You Are Screwing The Bolt Into), you can drill a tool clearance hole through one side (3/8 works well for a #10 screw).

The other method that Greg touched on is to insert some kind of material inside the tube. Wood is a great choice as it is rigid and light - I am honestly surprised more teams don't use wood inserts to increase tube strength for very little weight. If you are looking for something you can tap, a smaller plastic insert (delrin) can work pretty well depending on how much load those threads will see.

Finally, depending on how much tube access you have, if its easy, just slide your favorite kind of hollow tubing in the hole and run the bolt through its center.

If you can't avoid it, washers will help slow down the crushing process which helps I guess. Don't use .04 wall tube without support in this manner!

As long as they are installed properly, this should not be an issue, especially if you get the rivet nuts with the knurling on the outside. If this happens, then you haven't been installing rivet nuts fully and need to redo this.

A properly installed rivet nut can take a lot of load. We lifted our robot via four eye bolts screwed into four rivet nuts in the chassis. This is something I was very nervous about until we actually did it and tested it, but rivet nuts can do the job!

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

Last year we predicted some crushing issues. We decided to use some delrin blocks milled to fit tightly inside of a length of 2x1. They had a hole in the center to allow some wire rope to fit through, but it could be used for bolts as well. Took about 30 minutes to make a set of them.
This method worked great for us. Our 2x1 did not crush where we had the delrin.

EDIT: We used washers at one point as well, but they actually ended up warping.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

We love this since there is only one step. Part the tube to length.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

I've had my teams put wood in the frames for years, as well.
Just ensure you go against the grain or the block can split.
I've even threaded the wood, when needed.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

The solution that I would recommend would be to only bolt through a single wall. That can be done by either the use of a rivet nut, an access hole on one side for the fastener, or to keep the bolt head accessible a sleeve around the bolt. The access hole is a pain since it makes it difficult to remove the fastener.

Filling the tube with something isn't bad at the end of a tube but is a pain in the middle of the tube. It would be the strongest method since it allows the fastener to work on both sides of the tube.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

I guess I have understood the problem too long, I never even consider using a thru bolt on a piece of thinwall tubing. We use a lot of thin wall tubing, but we always figure out another way to attach it. Usually it's done with a gusset of similar thickness to the tubing wall, attached with several small rivets.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

I'm in this boat. Rivnuts, weld nuts, access ports, pop rivets, brackets etc all work well to solve this problem without the need to create additional parts to brace tubing. Most of these methods save weight, time, cost, and complexity vs some sort of insert.

Rivnuts, when installed properly in the right-sized and de-burred hole, work like a champ. No need for loctite or glue. We used dozens of them on our 2014 robot with great success.

In a few situations where I have 'needed' to put a bolt through a tube (thin-walled or not) I will weld an insert into the tube to support the fastener. But I haven't 'needed' to do this in FRC though.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

I have, in the past, used aluminum spacers...especially at the ends of a tube. My team has used spacers from this source: http://www.aluminumspacers.com/. Teams with a lathe can make their own...but for teams that don't have lathe access, I find it to be a good source.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

Last season we used some 3d printer parts inside the tube.
It worked well and has allowed us to have precise holes in the insert.

We used some to support dead shafts, avoiding support them only in thin walls.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

Why did you feel that this was necessary?

Some quick bearing load calculations assuming 1/32in wall, 3/8dia dead axle, and passing it through two tubes (thus 4 walls), and a 35ksi yield strength: 1/32in x 3/8in x 4 x 35,000psi YS = 1640lbf of bearing strength through just the walls of the tubes.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

This post focuses on Al tubing. We have over the years used fiberglass protrusion tube many times with great success. Thru bolts, bearings and shaft present a problem for pultrusions. We fill the area for the bolt with a mixture of chopped carbon fiber and laminating epoxy. Fibers should be chopped to 1/4". We trace out the inner profile of the tube on a piece of 1/4" plywood to make a dam. It is inserted into the tube and held in position with a few drops of super glue.The epoxy mixture is then packed in. We warm the area with the heat gun and it is ready in about an hour latter. Also works for setting sleeve bearings and shaft into the protrusion. Use just enough epoxy to wet out the carbon. I cut the carbon in a home made glove box for safety. Use care when measuring out the un-wetted fibers. Pultrusions are great but require different fabrication methods.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

I would definitely be concerned about creep more than pure yielding. We had all kinds of creep failures when we mounted screws directly to VersaTubing last year without gussets or other material support.

That said, might be easier to just make a gusset plate and slap it on the outside?

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

We did this with our catapult last year - we used slices of C-channel to support the steel bar that our pneumatic pistons attached to, since we were afraid the thin-wall tubing would not hold up so well after repeated shots.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

Why do you think it was creep? My understanding is that creep typically takes many hours, or days, to manifest.

I'd think a plate on the outside of the tube would be a lot easier and probably lighter than an insert, too.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

Creep is the wrong word, I think. What I meant is that wear on the hole from vibration or cyclic loading of the bolted part happens much more quickly on .04 wall tube than any other size. This kind of wear is something that I've been able to ignore elsewhere in FRC due to the short life span, but in this tubing you can't. We added some plates to increase the effective material thickness at the hole site and riveted the plate to the nearest neighboring holes.

Re: Best Practices for Not Crushing Thin-Wall Tubing
 

Rodger that, makes more sense than creep!