Stiffening Aluminum Tubing

How about 12" diameter x 0.375 wall and ~240:1 gear reduction?

Hi Tornado, The space available for the tube to fit is just under 14" wide, and the fabric pile-up thickness won't allow for a 12" tube. Having read yours, and some of the other comments, I'm inclined to go back to see if we can modify the space to use a larger tube. I do like the thicker wall to help avoid buckling, and the single tube is the lighter, simpler, approach. Thanks!

This doesn't sound like the best way to do this....how about something like this?

https://www.youtube.com/watch?v=O0fnxQrwHFc

This is what we have for gym dividers at the local athletic facility--even looks like the same brand. One advantage is the intermediate supports along the span, allowing low MoI shafting and small cable reels.

The OP may have an artistic reason for wanting a flat expanse of curtain, free of interwoven cables passing through grommets. It would be interesting to hear more.

Tornado - exactly. The pile-up would be too tall for the storage space available (the fabric would be in sight lines). Rented fabrics and drops can't have holes punched for cables. However, for some of the other masking applications in the room, this is a great way to achieve the long spans needed.

SE - can't be done this way, the drums have to wind up rented drops/fabrics that can't have cable holes or grommets punched in. Yes, the big advantage is shorter spans and smaller tube. I should have included more information about the space available for the tube to fit in. While the Porter fold-up method is great for wide curtains, and allows for intermediate support, the result is an overall height of drive-tube and cable drop and folded curtain that is too tall for the space we have in this installation. Added to the no-holes-in-fabric limitation, we skipped this general methodology.

Something close to B or C would probably work, though it would be better just to skip the inner tube and just use thicker stiffeners, or run the stiffeners across the entire diameter.

An option for welding would be to drill regular holes through the outside pipe where the internal stiffeners meet the wall, cover all the holes with tape, then close off the ends, purge the entire pipe with argon, and remove the tape from one hole at a time to plug weld the internal webbing.

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Going with a slightly larger diameter would probably be a much better solution. Any chance you can get agreement for 12"? The stiffeners and even increased wall thickness aren't going to improve deflection, just resist buckling better. A larger diameter would allow you to achieve decreased deflection.

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The early Atlas missile models used sell skins about the thickness of a dime without internal stiffeners. The secret was pressuring to prevent buckling. Pressurizing your tube however would introduce a when new set of problems and expenses.

Agreed. From the MoI standpoint, the internal structures will be less effective than tube diameter and wall thickness, to say nothing of fabrication problems.

Truth - the welders were smiling about your drilled skin plug weld idea. We love it!!! The CNC guys were kinda rolling their eyes and scratching heads about how to fit the tube into our machine to drill the holes in proper alignment(s). But yes, this could be done and would be a great solution to tying the stiffeners to the outer tube. I do have to ask - with good pre and post flow settings on the TIG, it seems we could get enough shielding gas at each hole without pressurizing the tube. This might lead to a new discussion that's more about welding than the best example for the tube stiffness - but were you joking or serious about that... and what issue do you see at weld that would require pressurizing the tube behind/beside each plug weld?

Spades - this is why I posted to CR4!!!! YES!!!! The light bulb finally came on in the dark caverns of the old noggin. Yes. This can be a solution. A paddle wheel of sorts. We can put most of the weight into the vanes, not the tube, and with plenty of gussets as you describe. AND!!! Attachment/removal of the curtains is much easier because a stagehand can reach both sides of one "blade" to tie on the curtains via holes in the edges of the vanes. And we can rapidly CNC punch those holes rather than drilling the outer tube. With the tube design we were looking at all sorts of ways of bolting the fabrics to captive nuts, adhesives and leader strips, all kinds of surface-of-the-drum methods that all led to a thicker wind-up which in turn led to a smaller allowable bare tube diameter. With this no-skin idea we can add about an inch to the diameter, and we can have all kinds of thin-skin, thick-skin, no-skin jabs around the shop floor for a few weeks, at least. We like it a lot. Anyone else have good/bad comments on the no-skin methodology???

I don't think the 'no structural outer shell' option will perform anywhere near as well as a tube. Remember, the critical place for material is at the top and bottom. The only reason to have internal stiffening members is to prevent the wall from buckling.

Imagine each of these radial sheets. Wouldn't it be improved if material were taken from the middle and placed at the outer edge. Take that to the natural conclusions and you have a tube.

If the tube doesn't need to rotate, an elliptical x-section might be good. (For different reasons involving air flow, there is a condenser/cooling tower manufacturer--Evapco--that uses elliptical tube in its designs.)

Agreed. Anyone remember PK Ripper Floval (flat oval) tubing bmx frames coming out in 1979?

I like the way you are thinking for this! A thin shell could be slid over your structural vanes to give a smooth surface to support the fabric. A clamping strip could be added for holding the fabric.

Drew K

I forgot to mention a couple things....

If the internal stiffeners are connected in the center and to the wall of the outer tube, you can get away with far fewer than 8. A simple 'x' in the cross section (one sheet that spans the diameter and two additional sheets that form a plane perpendicular crossing at the center) will go a long way toward keeping the walls from buckling, especially if Gussets (pie shaped) are welded in perpendicular to the axis of rotation at the significant loads and supports.

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Also it is critical to remember that your material is no longer T6 in the weld and the HAZ... the heat treatment is no longer. For that reason and to minimize distortion and internal stress, carefully design the welds. Intermittent welds spaced for non overlapping HAZ is a start.

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It is worth noting that all of the above can be avoided if the better solution of a larger diameter can be used.

Truth - thanks for the follow-up. In this project, it's not the theatrical people who goofed, rather the theater design people (architects and zoning height restrictions). The space allowed for the theatrical fly system is woefully inadequate. The stage viewing area is at least 20 feet high, and depending on seat location, as much as 22 feet high. However the clear and available fly space above is only 14 feet. So - any rag that hangs and touches the floor can only fly out 14 feet, leaving at least 6 feet still showing to the audience. Hmmm. A big fat "whoops" or "who planned this like this" ensues. So we were called, after the fact and as usual, to come up with a solution. There are many solutions, but the customer/users and administrators are most interested in roller-drops. We just can't fit a bigger tube diameter and that is what led to the stiffener discussion. Thanks for the reminder about HAZ and changes to the T6. I fully expect the engineer to specify some manner of stitch welding. We always skip around to avoid heat build up as much as possible. It will be an interesting time when we can "caulk gun" an aluminum connection that performs as good as a weld. We love the 3M adhesive tapes for applying skin to framing. We have not yet seen something that would work here. Thanks again for the suggestions and follow-up.

Perhaps consider one roller tube parallel and behind a larger diameter fixed tube over which the slides...or the support tube could roll as well is sliding isn't an option.

As nothing is being rolled around the larger tube, the diameter will not increase. Also if you have the roll up tube turning the correct direction, the tension of the curtain will be mostly up,

Just use a thicker tube.

Yes. The simple method should win the day. When I did the numbers on a 10" x 0.5" wall tube, and on a 0.75" wall tube, and then a 1.0" tube, my math was showing more deflection as the weight of the tube increased. My limited understanding of this leads me to think the self-weight of the tube is increasing faster than the MoI. Perhaps because there is still only one outer edge, no matter how thick the material? Which is why I proposed Example A as an alternative to get two outer surfaces via two tubes. I'll admit I don't have the education and experience needed to fully understand everything here. It sounds like the old intro-class discussions I've heard on why a hollow tube is stiffer than a solid rod???

"what happens if we want to wind up a cable with a 240 pound weight at the center of the tube?"

What if the Fat Lady gets tangled up in the curtain when it's raised. You can't put all the what if's into the design. Are you designing a spool to lift a curtain which has a known weight. Not Crane Hoist which again would have a weight limit on it. That can be over loaded and fail.

Why 6061? Why not 2024.

Your surface to spool up the curtain does not need to be perfectly round. So open it up to be able to weld the vanes in place.

Yes, you could cut the tube in half, weld in the vanes then weld the tube back together.

Good Answer.

Drew K

The stress and potential for distortion as well as giant HAZ make this impractical. Leave the majority of the pipe/tube intact , just bore regular holes and plug weld the stiffeners to the wall.

Ozzb- Why not 2024? I don't know. 6061 is what we have used for almost every other thing we have ever built. What advantages do we get with 2024? We did consider constructing a dodecagonal, then just an octagonal, tube all from flat stock. That was a lot more cutting and welding, which led to the tube-in-a-tube approach. Now it looks like we are headed back toward an octagonal design, minus the outer skin. True, we can't put in all the what if's. We were asked a what if (drum used as cable hoist) and I shot that one right down. Plus, we already build a hoist for that type of theatrical trickery and it works far better - potentially adding a sale, while keeping the designers and performers safe. Which is (almost) always a good thing.

Stiffer 2024 http://asm.matweb.com/searc/SpecificMaterial.asp?bassnum=MA2024T4

6061 http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6

My thoughts are to change the direction of some of the stresses in a more favorable direction. If you use 6 or so steel cables arrayed in a circle inside to pre-stress the tube it will deflect less because as it tries to flex it will increase the tension on the cables on the bottom and put the tube in compression.

Drew K

What about a square truss design inside a square tube or skinned surface...

We use square truss every day on our events. We've been taught by all the manufacturers - Tomcat, Xtreme, Thomas, Global, etc. that the truss is designed to be used in one orientation - and tilting the truss weakens the load capacity significantly. Cranes with lattice booms are really strong, because the booms don't rotate along the long axis. Is that correct? If for some reason a crane boom did have to rotate in that axis, would that lead to round booms?

From training, and experimentation, it seems that a square tube, in the single-point-up orientation is not as strong as a round tube of the same cross section.

I can see how a square tube with internal stiffeners running from corner to corner in a tee would make for a much stronger structure than the hollow square tube. And construction of this would be far easier, just some flat pieces welded to the square shape. All this has me considering using steel - if we can get the weight down by using thinner materials. Which mostly goes against the convention of using aluminum when weight is the driving factor over cost.

I knew this post would lead to some interesting alternate ideas! Many thanks to everyone.

Fill the tube up with concrete?

Why do you have to use aluminium? Why don't you use something else, like one of the steels?

Concrete and steel are too heavy for the roof weight capacity.

High strength steel, 4130 chro-moly is a good example, sometimes require less weight than aluminum to meet similar requirements.

If the design is limited by deflection rather than stress it might not help. I wouldn't swear about chro-moly, but for ordinary steels the elastic modulus is same for the various strength grades, ~ 200 GPa.

Correct, but even though steels don't show a lot of variation in Young's modulus, those values are about 3 times the value for aluminums. Steels are typically less than 3 times the density of aluminum alloys.

The higher value has a larger effect than simply allowing a thinner side wall. With the diameter constrained, steel will have effectively a larger diameter because the material will be further on average from the center than the thicker wall aluminum.

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The benefit of using 4130 or similar will be ability to use thinner material. The stiffness will be roughly the same for thicker and thinner sidewalls until failure of the thinner wall by buckling. In this case that would be a very thin sidewall indeed.

Seems to me that your best approach may be to use extruded square tubing. Why does it have to be round?

If it has to be round for some reason, why not look at filling the tube with structural grade expanding foam. Strong and lightweight.

Here is one more idea that might provide the least deflection of the options thus far....

Okay, there is a limit on the maximum allowable size of the curtain rolled up on the pipe...which is keeping the diameter at 10" max. So how big is the diameter of the curtain roll at its maximum?

Consider constructing a non rotating, approximately 'u' shaped channel with thin wall round tubes at the tops of the 'u' for rigidity and so that the curtain can easily slide over. t

That channel supports a shaft maybe 3 or 4 inches in diameter for winding the curtain. The winding shaft is held parallel to the channel with swing arms such that the shaft can move way from the bottom of the u channel as more curtain is wound up.

In essence this provides for larger effective diameter by switching the position of the curtain and the structural material. The structural material benefits from being on the outside, the curtain doesn't.

Most of this is a bit over my head, I'm an Electronics Engineer, not a Mechanical Engineer. However, after reading the question carefully, I believe I've come up with an educated response:

Have you asked anyone involved in Theatre building design and maintenance about this? Rolling up a 'theatrical curtain' seems like just the sort of thing they think about every day. Who knows, there may even be a company out there that has the solution waiting in a file cabinet, ready for the day the machinery finally gets to be fabricated full-scale.

Hey Tex, howz it going?

I'm still thinking about this one.....please give me some time to figure it out. As I informed you beforehand in our email exchange, I've never designed anything involving aluminum as a structural member.

But, initially I have thought of something like this: using your Case A scenario, but modifying it significantly, in an order to beef-up the cross-sectional area, Section Modulus and Moment of Inertia, in an effort to prevent the crippling problems.

You could use a tube-within-a-tube concept, by employing a fully-intact inner smaller diameter tube that is fully outer-wrapped with a ring (or cover if you will) of a larger diameter tube tight-fitting which has been cut into segments. The number of segments would be up to you. All you would have to do is TIG full penetration weld longitudinally where the segments abut one another. You may have to roll flat sheets of the aluminum to produce the required outer layer ID so as to provide a tight-fit at the inner and outer tube interface.

Have you investigated using a Composite tube instead, possibly utilizing Kevlar or similar material, which is commercial available from manufacturers? It could provide very strong material properties, weight reduction, and significant reduction in fabrication efforts and subsequent costs.

Just thinking outside the box, ya know....

Perhaps a used sailboat mast?

Drew K

This is just an off the wall thought, how about a layer of Kevlar composite material over the alum as a stiffener, extremely light, and won't add a lot of weight. Driveshafts are made this way for drag racers that hit these driveshafts with multiple hundreds of horsepower. Just out of the box thinking from an old dragracer.

A very good idea Lockduke.

Hopefully the composite will adequately bind to the aluminum tube!

I have seen some of the destroyed drive shafts that were damaged from other parts breakage and the composite material is still attached to the outer tubing even where it hit parts of the chassis coming out of the trans and get beaten about in the drive shaft loops that the material is still attached to the point of needing a die grinder to get them out of the chassis. All in all a pretty durable material not always cheep but a very good and safe material to work with and bullet proof as well.

On my opinion the tube has to be dimensioned for deflection and NOT for stress. this the reason you can use 6061 since both 6061 and 2024 have almost same Young modulus and as long as the stress level is low enough no reason to go to a stronger alloy.

I would suggest you make following computations before you take the decision:

- deflection in the middle under own weight without the 240lbs drapes

- deflection under the concentrated load of 240 lbs

- deflection under own load and distributed drapes load

Make the sum of the first 2 and compare with the third value. This will give you an indication with how much you should increase the inertia moment of the section.

There are also possibilities to add on the out side by welding and I have the feeling that if you need an increase it has not to be as important/complex as you intended.

Of course due the application the section should present a minimal inertia moment which ever the reference axis angle is, a circular section is optimal but not compulsory.

I suggest you make those computations because I do not obtain same results as yours.

This is an example where the computation can direct to the optimal solution.

Nick

Don't forget to include the self-weight of the tube itself in the deflection and stress computations.

Hi Moosie!!!!

SO glad you are busy, and had time to drop by CR4.

I will re-do the numbers for round, and square tube as a test to see if my inclination is leaning the correct direction, or if my desktop is just tilted...

I don't know if the Kevlar wrap is practical in the budget. BUT!!! I am interested so I am going to do some testing in the shop on a smaller (6") tube we already have, rough the surface for improved adhesion, and wrap in spiral woven fiberglass. I've done this for some other projects for appearance, but not for strength. Then we will have some real numbers to crunch. Should be a fun experiment. I'm thinking if the 6" tube deflects 1" without the wrap, with a 500 pound center point load (hypothetical numbers), but then only deflects 1/4" with the wrap and the same load... that's a huge difference. How we could extrapolate that to a longer tube, or more or less wraps, I dunno.

TX (DON'T MISTAKE FIBERGLASS FOR KEVLAR) there is a remarkable difference fiberglass to Kevlar and you can get Kevlar already impregnated with epoxy then put in plastic bag and pull a vacuum to set it off with an activator. There is a tremendous difference in the two products as far as strength. Or you can do it the old way and apply the resins then put in bag to pull vacuum to get total adhesion to the tube surface.

Thanks, but I took them into consideration

Thank You Nick, good advice. Combining round, with multiple laminar layers to get the max material on the outside edge, as others have suggested, seems to be the right direction. I'm going to re-work my numbers and re-post. Thanks Again, Tx

May I make a remark since rigidity(deflexion) is the parameter the value of E is important. I do not know if the added layers would bring much if their module is low.

Hey Tex,

You can also look into Aramid woven and non-woven cloth too that is impregnated with the resin.

Be careful about your "bench test", as it may not be accurate enough, depending of how you end support the tube. I also don't feel it is long enough to adequately simulate the long span tube you're envisioning. A 10-foot long tube test subject may result in more realistic data results. It is all a mater of the tube diameter-to-span ratio. The smaller the ration the more realistic the data interpretation will be.

Maximum Deflection (Delta) = (P*L^3)/(48*E*I) for a concentrated load at exact midspan.

If you go with a fully composite Aramid or Kevlar protruded tube, the manufacturer may be able to provide you with the necessary structural calculations, using your loading and deflection criteria. Who better to know the engineering properties than the manufacturer/fabricator, right?

Naturally, you'll have to compare the cost vs. structural benefit for each material.

I would not go with a fiberglass encased/wrapped aluminum tube, strictly based on lower rupture strength of that material.

Okay, lotsa good 'out-of-box' noodling around here! My favorite so far is the pressurized skin. My father used that one on a 24" diameter 45' long boring bar at Yuba Mfg. in the 60's. Went from droopy to stiff and stayed that way for a year. As long as you make it airtight and fill it with dry nitrogen it should be fine.
I've been OOB for quite a while, stand-by for off the wall! 2 suggestions:

1) Tensegrity.

2) Again something from antenna masts. 7 smaller identical diameter tubes in a welded bundle (six in a hex around a central tube.) I have made these out of 1/2" conduit, up 1 1/2". You need to make a prototype to really get how strong these structures can be for the cost. Haven't tried it with aluminum, but with conduit I use my little wire welder and a jig to hold the tubes in place, put a 1" long bead every 12" . To assemble stacks I use 3 6" pieces of 5/8" all-thread inserting one in every other tube around the outside ring. I take the bundles to site, stick them together and weld the ends in. I have tried many variations including pins, screws, and epoxy for final assembly and I haven't seen any advantage one way or the other. I think the really slick way to build these would be to make a spot welder that you could pull through a pair of tubes.

I like the tensigrity solution. I mentioned earlier about pre-tensioning the tube to combat bending, but thought of using a set of 6 or so cables arrayed around the inside of the tube but I can see how just using one would help and simplify the design. I wish I had a garage to test that out in a scale model just for my own pleasure.

Seems a way cheaper way of stiffening the op's tube than exotic laminates...if it would work.

Could some of the other posters on this thread comment on this design?

Drew K

I would like to know how you determined the stress for bending collapse of the section since using several documents on this subject I obtain about half of the value you give.

Stability loss under bending is a true problem and it is unfortunately not always taken into consideration, it differs from the stability loss of a column under compressive loads.

The doc I have compute depending on geometry and material properties a bending torque limit for the section. You give a stress value thus my question I shall be very thankful for indications.

According to the dosc I have the tube should be at least 6061 T4 to offer a good safety for the total loads: curtain + concentrated load.

I used a formula from Roark, 5th Edition (I think, I only have photocopies of some of the pages) Table XVI Case M - Thin-walled circular tube under uniform longitudinal compression. It looks at local, as opposed to Euler buckling. I realise that's not exactly the situation discussed here, but I reasoned that if it can take that uniform compressive stress it can take similar stress due to bending.

Formula is -

Buckling stress s' = 40%*E*t/√(3*(1 - ν²))/r. Included 40% as it says tests show buckling occurs at 40 - 60% of theoretical. I assumed Poisson's ratio ν = 0.26 same as steel. I've also seen a formula s' = E*t/(4*r) which gives similar result, 658MPa.

I agree that in practice I'd use a safety factor at least 2 on those figures for an actual stress, perhaps your calculation includes this, hence about half my figure.

But as the buckling stress, with extra safety, is well above actual bending stress I concluded it's OK.

I presumed that you did what you did. There several studies concerning the bending which give totally different values. The loss of stability in presence of a bending moment is due to the section ovalisation which leads to a reduction of the resistance in bending. It is considered that the limit is reached when the ellipse (in the first phase it is an ellipse but at the end it gets a plane zone) has the small radius about .45 D and the big one at 0.55 D.

If you are interested to have more precise informations let me know.

Yes, I'd be interested in more detail, thanks.

I can see that if goes oval it could be in a runaway situation. I'm a bit surprised deformation as big as you quote is acceptable. Presumably you would recalculate the stress and deflection for the changed section properties.

Good style Codemaster. I give your answer a GA.

I agree with your calculations and suggestions wholeheartedly. However, I would include in the Live Loading an Impact Factor for a "Just-in-Case" scenario, where someone or a group of people tug or pull on the curtains for God knows what reason....say the curtain spools up incorrectly and bunches up or becomes misaligned, or the motor/gearbox quit unexpectedly. Sometimes, weird stuff happens.

I would suggest a I.F. of 1.33:1.0. Better safe than sorry.

OK thank. Good idea!

Thanks Codie!

I'm late to the party, and not sure how this might impact bending moments, but haven't seen it mentioned yet:

http://www.mdpi.com/1996-1944/7/9/6796/htm

The link speaks to compressive properties but I think a look into 'filling' the tube cavities with a stiffening foam-like substance may prove beneficial.

You might want to peruse this place, they have some cool stuff.....maybe something off the shelf...

...carbon fiber wrapped in kevlar...

http://www.rockwestcomposites.com/browse/round-carbon-fiber-tubing

I looked at their site a few days ago myself. Yes, some very cool products!

For a different approach, consider a pair of 2.5" trunnion rollers midspan, mounted on a counterweighted hook-like frame that reaches around the curtain from above, supporting it from below. Then maybe a 6" pipe would be enough.

I kicked this idea around, but thought the roller might cause a buildup of fabric prior to contact, or stress the fabric at contact point eventually leading to system or fabric failure....the roller could be powered at speed appropriate to adjoining surface, but then the flexibility of the roller would be a problem, to support the weight it would have to have firm contact....I dunno it might work but seems less than ideal and possibly problematic...

Solar your drawing could work if it had a double roller under the center line of the main tube with a pivot point at the corners of your brace and springs to lift the rollers against the main tube. It might elevate a percentage of the load from the center point of the middle of the main tube to help keep the load under failure specs. And with the springs as the roll gets larger the spring pressure increases as it accommodates the larger diameter of the fabric on the main tube. I wish I knew how to draw on my computer to make my point clearer. I wouldn't even guess what software I would need to do it.

Exactly! would that not be of any help for his needs and take the stress off the main tube. It seem too simple, my only worry would be the fabric might bunch up while it was being rolled up or am I overthinking it.

That was one of my concerns as well, but with a ~26' piece of fabric it seems to me it would have a great deal of resistance to this type of deformation....this would be specially true if the fabric was slippery like satin or plastic....it would more or less just be there for safety anyway so the weight would be minimal, therefore the pressure of the rollers against the fabric roll would be rather light....

Yes but I think there would be a little more spring pressure then you think trying to offset center of the tubing deflection, but then I'm no engineer and can not fathom the math involved to prove it.