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View Full Version : How to stiffen 1/2" aluminum tube?


Myka
09-13-2015, 02:44 PM
I made some aluminum arms to hang my ATI fixture. I used aluminum and brushed it to match the fixture. They look really good. However, my (cheap) tube bender only goes up to 5/8", but I couldn't find 5/8" aluminum tube, so I bought 1/2". As expected it's fairly flimsy/bendy. Any ideas how to stiffen it up? I could fill the tube with something - not sure what.

Madreefer
09-13-2015, 03:39 PM
Use that insulation in can. Used to fill cracks and seals. You can get it at any hardware store. It hardens pretty good and canbe cut easy. At least it's not heavy but it'll keep the shape

monocus
09-13-2015, 05:34 PM
2 part resin

Myka
09-13-2015, 07:13 PM
Use that insulation in can.

I actually did buy a can for this purpose, but wasn't sure if it would help much.
2 part resin
Any particular one in mind? I know there are a lot of different kinds.

mihaivapler
09-13-2015, 09:21 PM
any liquid epoxy resin would do the trick,mix it poor it in the tub and leave it to dry and will be very solid.also to make sure the tub will fill up use a little bit less hardner,about 10% so it will take a bit longer to set so it will flow

sphelps
09-13-2015, 09:39 PM
You'll probably need a knee brace or something to add the strength your looking for. Filling the tube with foam or composites wont do anything.

spit.fire
09-13-2015, 10:10 PM
fill it with concrete or epoxy

mihaivapler
09-13-2015, 10:34 PM
You'll probably need a knee brace or something to add the strength your looking for. Filling the tube with foam or composites wont do anything.

if the distance is short it does work,but for anything longer then 24" i don't think would work as the bar will bend

Myka
09-13-2015, 10:53 PM
Ok well, the bar is 5' long with a 90* bend at 16". The long end is inserted into a slightly bigger steel sleeve. The distance from the bend to the steel sleeve is 13". The steel sleeve runs to the top edge of the tank.

http://images.tapatalk-cdn.com/15/09/13/b159511b18b2ec35ff2c24f3bacd12f3.jpg

sphelps
09-13-2015, 11:25 PM
if the distance is short it does work,but for anything longer then 24" i don't think would work as the bar will bend

Epoxy has a modulus of 3 GPa or less, while aluminium is around 70. Doesn't matter how long the tubes are, it won't do jack.

WarDog
09-14-2015, 12:07 AM
Is that your foot? What the hell did you do to your toe?

intarsiabox
09-14-2015, 01:31 AM
I would just brace the bend with two pieces of aluminum flat bar. Just put a piece on each side of the tube at a 45 angle so the bend now looks like a triangle, drill a say 1/4" size hole through the bars and tube at both ends and put a 1/4" bolt through the holes and tighten up with a nylock nut. This method does take up some real estate on both the horizontal and vertical part of your tube and I'm not sure how much you have to spare. Hope this made any type of sense at all.

Myka
09-14-2015, 02:18 AM
Yes it makes sense. I was thinking along those lines too, but want to come up with a cleaner look than that.

I have some spared lengths so I'm going to try making a bigger radius bend. That might help enough.

If all else fails I'll hang it fron the ceiling, but I'm trying to avoid the industrial style look of that.

hillegom
09-14-2015, 02:42 AM
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

Myka
09-14-2015, 03:23 PM
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! :lol: :o I'll give this a try.

Reef Pilot
09-14-2015, 04:12 PM
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
09-14-2015, 04:18 PM
Is that your foot? What the hell did you do to your toe?

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

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
09-14-2015, 04:34 PM
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
09-14-2015, 04:39 PM
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
09-14-2015, 04:41 PM
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?
http://www.challengers101.com/KitImages/WingCov-0-1.jpg

Myka
09-14-2015, 04:43 PM
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
09-14-2015, 04:53 PM
I really don't want a gusset. You mean something like this, right?
http://www.challengers101.com/KitImages/WingCov-0-1.jpg

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
09-14-2015, 06:34 PM
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
09-14-2015, 11:50 PM
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
09-15-2015, 04:33 AM
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
09-15-2015, 04:04 PM
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
09-15-2015, 04:21 PM
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
09-16-2015, 01:54 AM
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
09-16-2015, 02:34 AM
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
09-16-2015, 02:39 AM
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. :lol: Do you mean like the pic I posted above?

intarsiabox
09-16-2015, 02:51 AM
Google "structural knee brace" and you will have your answer.

Myka
09-16-2015, 03:51 AM
Google "structural knee brace" and you will have your answer.

:thumbs: Ah ok, yeah we were talking about this sort of thing already. I'm not sure I could make it look pretty. I'm a woodworker, not a metalworker.

shiftline
09-16-2015, 06:49 AM
I'm thinking About building a similar thing but prob go with square steel or aluminum tubing

WarDog
09-16-2015, 06:58 AM
Mindy, you still haven't answered my most important question, lol.

Myka
09-16-2015, 02:41 PM
Mindy, you still haven't answered my most important question, lol.

I did, you're blind. :mrgreen:

WarDog
09-16-2015, 03:32 PM
Yes, apparently. Lol.

mike31154
09-16-2015, 04:25 PM
Rivet 'thingies' are quite affordable & easy to use, carpenter girl. And after you're done with the knee brace, it can be useful around the house & garage for other nifty work. If you've made up your mind about the ceiling attachment, a trip to the attic may not be needed, provided you can find a couple of studs in a favourable location with a stud finder.

Myka
09-16-2015, 10:34 PM
Well Mike, if I can make nifty things with the rivet thingy then I'm all in! I'll go find a rivet thingy...