How to stiffen 1/2" aluminum tube?

[Reef Pilot]

Originally Posted by Myka

I was a bit concerned about that, but I thought it took dampness to cause an issue. What if I wrap the steel in something first, like Tuck Tape or vapor barrier plastic?
Otherwise, option #1 sounds nice.

That's true, but humidity is enough to start corrosion. With aircraft we use zinc chromate epoxy for all joints which seals against moisture, and is also a glue for extra strength. But you can't buy that stuff in a regular hardware store.

Not sure how heavy the fixture is, but bear in mind this only strengthens the bent tube area, and if your tube size isn't large enough, you could still get bending and flex with the weight.

If you need more strength overall, you can make the cut tubes (or gusset) longer. A larger gusset can also be rounded on the outboard side towards the middle to provide more clearance if necessary (and it will look prettier). For the gussets, I would use .040" or .060 sheet pieces. Your local metal shop usually have scrap bins, and should let you pick out small pieces at no cost. Oh, and use T6 6061 aluminum for superior corrosion resistance.

[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.

[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.

[Myka]

Originally Posted by Reef Pilot

That's true, but humidity is enough to start corrosion. With aircraft we use zinc chromate epoxy for all joints which seals against moisture, and is also a glue for extra strength. But you can't buy that stuff in a regular hardware store.

So is it a reasonable solution to seal the steel away from the aluminum then?

Not sure how heavy the fixture is, but bear in mind this only strengthens the bent tube area, and if your tube size isn't large enough, you could still get bending and flex with the weight.

About 18 lbs.

If you need more strength overall, you can make the cut tubes (or gusset) longer. A larger gusset can also be rounded on the outboard side towards the middle to provide more clearance if necessary (and it will look prettier). For the gussets, I would use .040" or .060 sheet pieces. Your local metal shop usually have scrap bins, and should let you pick out small pieces at no cost. Oh, and use T6 6061 aluminum for superior corrosion resistance.

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

[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.