How to stiffen 1/2" aluminum tube?

[Myka]

Originally Posted by hillegom

If you are going to bend a new one, what about inserting a steel rod before you bend the aluminum. It will be harder to bend, but a lot stronger. And it will still look shiny

I thought about trying to get a steel rod in there, but I overthought it and didn't think about inserting the rod before bending. How brilliant! I'll give this a try.

[Reef Pilot]

From my aircraft building experience: Two ways to do it.

1. Cut two pieces of same tube about 3 inches. Use a suitable size round file to shape the ends to fit over the bent tube to form a triangle corner support. Leave the ends slightly longer to form tabs where you drill and rivet to the main tube. And/or you can rivet the sides for extra strength and stability. Shape and file the edges to make it fit perfectly and look pretty. This will be the most professional finish.

2. Cut two small triangle aluminum gussets to fit on each side of the bent tube, and rivet (4 rivets per gusset). File to round the edges and it will look pretty good as well.

Also note that you should not mix aluminum and steel. Dissimilar metals cause corrosion and will eventually lose strength and loosen up.

[Myka]

Originally Posted by WarDog

Is that your foot? What the hell did you do to your toe?

That's just a bit of nail lacquer that I haven't managed to pick off yet.

Originally Posted by Reef Pilot

Also note that you should not mix aluminum and steel. Dissimilar metals cause corrosion and will eventually lose strength and loosen up.

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.

[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.