Best Practices for Not Crushing Thin-Wall Tubing

[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!