Best Practices for Not Crushing Thin-Wall Tubing

[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.

[JamesCH95]

Originally Posted by MrForbes;1415752[B

]I guess I have understood the problem too long, I never even consider using a thru bolt on a piece of thinwall tubing.[/b] 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.

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.

[TD78]

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.

[ghesla]

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.

[JamesCH95]

Originally Posted by ghesla

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.

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.

[Gdeaver]

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.

[Chris is me]

Originally Posted by JamesCH95

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.

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?

[Oblarg]

Originally Posted by Chris is me

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

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.

[JamesCH95]

Originally Posted by Chris is me

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?

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.

[Chris is me]

Originally Posted by JamesCH95

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.

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.

[JamesCH95]

Originally Posted by Chris is me

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.

Rodger that, makes more sense than creep!