How to stiffen 1/2" aluminum tube?

[Myka]

Originally Posted by Reef Pilot

With the steel tube insert method, you could also use linseed oil (should be able to buy it a hardware store) inside and seal the ends (with epoxy). If there is no air (O2) then it will not corrode.

Ok. I'm thinking I'll try this option out first, and see what I can come up with. Thank you.

[Reef Pilot]

Originally Posted by Myka

I really don't want a gusset. You mean something like this, right?

Yes, but this one is not done very professionally. The corner edges should be rounded. I agree though, it is not the prettiest solution.

[sphelps]

This process doesn't need to be done with trial and error. Simple beam formulas exist that anyone could follow.

Max deflection of a cantilevered beam is dmax=P*L^3/(3*E*I)

P = Load or weight of fixture (/2 for 2 supports) (Newtons, kg*10)
L = beam length (meters)
E = Modulus (70e9 Pa for aluminium)
I = Moment of Inertia = pi*(d0^4-di^4)/64 for tube/pipe (m^4)

Simple stuff, easy to back calculate for I for a desired amount of displacement (ie 3mm) then determine the OD tube needed. Alternatively calculate the displacement a steal rod will create to get an idea if inserting a piece inside the tube will be worth while. I for a solid rod is = (pi*d^4)/64.

For example, aluminium tube 12.7mm x 8mm will displace 13.8mm with 2.5kg applied at the end of a 0.5m length of tube. A solid tube of steel with an OD of 7mm will displace 44mm with the same weight and length. You could conclude from this that the steel insert will not be very beneficial.

[sphelps]

Another option if you're rebuilding is to use stainless 304 tube. Switching to 304 from aluminum with the same dimensions will give you close to 3x the stiffness and and can be brushed to look just like aluminium. It's also available in solid rod for greater strength, although you don't gain that much. 1/2" Ornamental stainless tube looks to be around $3.50 a foot, solid rod is a bit more at closer to $5 a foot. That's metal supermarket prices which I believe will ship to Stoon, but you might find a local source cheaper.

Also I see you mentioned longer radius bends, keep in mind encase you didn't know, bigger radius means more flexibility, so shorter bends offer greater stiffness.

[Myka]

I didn't realize you could brush stainless or I would have used that right off the bat!

I'm going to try to work with what I already have (10 more feet of aluminum rod) and if that doesn't work then I'll just bite the bullet and hang it from the ceiling. Thanks for all your input Steve.

[mike31154]

That's definitely a wimpy looking piece of tube to be hanging 18 lbs or so near the end. Aluminum really isn't the best choice for something like this. Since you've already bent some up, have you tried hanging something close to 18 lbs to it to see what happens?

Not sure bending steel inside the alu will work all that well either. My guess (guess only) is that the aluminum outer will get crushed or deformed by the extra force required to bend the steel inside? That will ruin the look for sure.

My fixture hangs from the ceiling on spring wound retractable pulleys. Industrial, yes, but functional since it makes raising & lowering the fixture for maintenance a snap.

[Reef Pilot]

Or if you want something really strong (but still small diameter) you could use 4130 chrome moly steel tube (also a common item at metal shops). We use that to hang 400 lb engines on the front of a firewall. You won't bend it though, but a good tig welder can do a nice job for you.

[Myka]

Thanks for all your input guys. I think I done screwing around with it though, and I'm just going to hang it from the ceiling. I'll have to go play in the attic...

[sphelps]

You give up too easily. Take some some of that extra aluminum tube and cope in a knee brace on each support, rivet it in for a clean look. A knee brace deceases L which is cubed in the formula I posted previously, or in other words it's the most important variable.

[Myka]

Originally Posted by sphelps

You give up too easily. Take some some of that extra aluminum tube and cope in a knee brace on each support, rivet it in for a clean look. A knee brace deceases L which is cubed in the formula I posted previously, or in other words it's the most important variable.

I don't have a rivet thingy. I Googled "aluminum tube knee brace" and I got medical equipment. Do you mean like the pic I posted above?