Best Practices for Not Crushing Thin-Wall Tubing

[Oblarg]

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.

[Joe G.]

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.

[Arpan]

Originally Posted by Oblarg

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.

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

[Greg Needel]

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.

[MrRoboSteve]

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.

[Oblarg]

Originally Posted by MrRoboSteve

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.

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.

[John Retkowski]

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.

[Mike Marandola]

Originally Posted by John Retkowski

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.

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

[MrRoboSteve]

Originally Posted by Oblarg

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.

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

[Oblarg]

Originally Posted by MrRoboSteve

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

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.

[mlantry]

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

[Chris is me]

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!

Originally Posted by Oblarg

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.

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!

[Daniel_LaFleur]

Originally Posted by Greg Needel

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.

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.

[Mr V]

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.

[MrForbes]

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.