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Need 1/4" Rod Stiffer than Steel

09/19/2012 4:05 PM

Carbon Fiber? Ceramic? I'm doing my own Young's Modulus research, but meanwhile you may have suggestions. I have a 1/4" diameter rod, that is 10' long, firmly supported at both ends. The only weight that it is supporting is a thin rubber sheath, and its own weight. The sag we are getting with any stainless steel is proving problematic.

I'm looking for a substitute that is available in 1/4" rod, and 10' lengths.

And also somewhat reasonably priced. I can't afford Beryllium or carbon nanotube. Up to 10X cost of steel would be reasonable.

Less brittle than glass.

Significantly stiffer than steel. There is some minor vibration involved.

Any specific ideas? Trying to locate something asap. I wonder if there might be a steel cored ceramic rod that would work.

Thank for your help.

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#1

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 4:13 PM

How are you using this? What is its function?

Obviously its mounted horizontally, since you say that sag is a problem.

But a horizontal rod by itself usually means nothing. You probably also have some stuff hanging from it, or otherwise interfacing with it. These all exert some additional force on the rod.

Please supply us with more information.

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#2

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 4:18 PM

How much does the rubber sheath weigh? Is its weight evenly distributed along the rod?Disregarding the weight of the sheath, a hollow rod would have less sag than a solid rod. If you explain completely what you are trying to do, someone here might have an idea. Are you absolutely limited to 1/4" round rod? The absolute highest Young's Modulus might not be the solution - the weight of the rod is a big factor. Could the end supports handle a tension rod load?

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#3

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 4:28 PM

What about carbon fiber? although 10' might be special order....I know they are available in 8' lengths at the carbon store....Reduce the weight and you reduce the sagging....

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#4
In reply to #3

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 4:34 PM

Carbon fiber was the first thought in my head. I'm looking at that. But wanted to get any additional input before I committed.

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#10
In reply to #4

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 6:39 PM

Does it have to be round? A square rod ♦ turned so, might add rigidity...

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#14
In reply to #10

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 7:31 PM

No it doesn't have to be round, but I'm not sure that at such a small scale, at such a long length that that would make any perceptible difference. Although the weight would be less, the cross section would be considerably decreased as well.

For that matter, if I were to do that, I could just use a rectangular cross section that is 1/4" x 1/8".

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#15
In reply to #14

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 8:08 PM

Yes, that looks better...

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#5

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 4:38 PM

What about a tube filled with foam?

We just went through this with someone wanting to build a cheap welding curtain.

Don't remember the outcome.

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#16
In reply to #5

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 8:19 PM

Seems like it would be difficult to get foam through to the center of 10'.

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#19
In reply to #16

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 8:28 PM

That's 5 feet from each end.

I used to mix the foam, pour it into a plastic syringe stick the syringe into some tubing and fill the tubing with foam. Then, pull the syringe off the tube, squeeze the tube end with one hand and withdraw it through my thumb and forefinger to unload the hose. You'll have to play aroung, but, it's not that tough.

Or run the foam through a static mixer and shoot it through a hose, as it is withdrawn.

It's doable, I'm just not sure it's a good answer to the problem.

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#6

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 4:46 PM

The buna sheath is .7mm thick. Haven't weighed one, but negligible.

No nothing hangs from it. I'll describe the assembly. The rubber sheathed rod is inside what is essentially a 1/2" inside diameter tube. The tube has a 9' slot at the 12:00 position and at the 6:00 position. A foamy solution flows through the top slot and exits through the bottom slot. My rubber sheathed rod is suspended, centered in this tube. When air is blown into the rubber sheath, it expands... sealing off top and bottom slots. When air is released, the flow resumes.

The purpose of the rod is to keep the rubber sheath centered along the entire length of pipe while it is deflated, and not touch the inner walls of the pipe. If it sags in the center it will droop down and partially cover the bottom slot. Unacceptable.

The flow does have some pressure, but it is small, as it is all a slow flow of bubbles.

I've used steel rod on shorter lengths, but now am R&D on a 10' length and steel sags too much.

There you go.

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#7
In reply to #6

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 4:56 PM

How about using a steel rod and then including electro-magnetics at discrete points over this assembly as a means to pick up the rod without touching it?

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#8

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 5:11 PM

1.-Use steel tubing, butt weld one end and tap a brakes bleeding valve to it.

2.-Weld a hydraulic check valve to the other end.

3.-Fill the tube with a power or manual greaser allowing air to escape thru the bleeder at the other end.

4.-Close the bleeder when only grease comes out of it.

5.-Keep pumping untill you read a couple thousand Psig on the manometer, detach the greaser and make sure it doesn't leak.

6.-Here you go, a poor man's Young's modulus research, whatever that is !

Don't worry about an explosion, there won't be any gas expansion, in case of failure, you'll only hear a sharp crackling sound and your tube is gonna hang limp.

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#9
In reply to #8

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 5:22 PM

I am assuming the rod is not supposed to rotate around its axis, so a little pre-forming (bending the tube upwards) might help.

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#13
In reply to #9

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 7:21 PM

If I go with a metal, that may be possible. But there is very little room for error, as the top slot also can not be covered if I bend too far. Not sure if I could achieve the precision needed in calculating that bend. But it's something to look into.

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#38
In reply to #9

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 9:14 AM

I have had to do this (bending the rod) in aerospace applications. It works for a while but tends to reform due to memory.

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#42
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Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 9:34 AM

Yes, memory is the problem with prebending. this needs to be stable long term.

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#17
In reply to #8

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 8:24 PM

I'm confident that my tube doesn't hang limp unless I want it to, but interesting grease bomb. Unfortunately, if it leaked or cracked it would be somewhat disastrous. I think I need to keep grease out of the equation.

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#47
In reply to #17

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 10:35 AM

I'm confident that my tube doesn't hang limp unless I want it to,
Then you are younger than i am.

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#55
In reply to #47

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 1:21 PM

Viagara? LOL

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#25
In reply to #8

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 11:05 PM

This suggestion is a good idea. Prestressing a tube with internal pressure will stiffen it up. Significantly. Grease does not have to be used--you could consider pumping, say, epoxy resin into the tube and letting it set up under high pressure. As long as the resin doesn't reduce in volume when it hardens, you have your prestressing. You could also use cementious grout but the pumping might pose a challenge. I've used a grease gun to pump epoxy...it works good but you only get one use out of the gun!

The other suggestion, of tensioning the rod will also be useful. I recall a veneer clipper that used a rotary knife, about 8'-6" long, to clip veneer coming off a veneer lathe at high speed. The clipper knife was about 1/4" x 2". It was like a piece of wet spaghetti when not tensioned but when put under hydraulic tension it was amazingly stiff. You might consider using the external pipe as your structural member for the tensioning of the internal rod.

Jon.

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#11

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 6:55 PM

You need more than stiffness. You need very good stiffness-to-weight ratio.

Given the rubber is .028 thick, the maximum permissible sag would be .097". I doubt that a pultruded carbon fiber rod (the stiffest variety of carbon fiber rod) would be able to meet this. A carbon fiber tube might. Goodwinds makes .25" carbon fiber pultruded tubes, and can make them in ultra high modulus.

Perhaps you could tension the ends to reduce sag.

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#12
In reply to #11

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 7:12 PM

That's a good specific suggestion. I'll look into it. And yes, I will be tensioning the ends.

Let me backtrack on the inside diameter of the outer tube. I'll have to remeasure it tomorrow at work. I think it's 1/2", but I'll have to check again.

And I just found out that we may try to do the same thing with a 20' length in the near future. That will pose a greater challenge. Some out of box design adjustments may be required. Tension will certainly be a factor in that case.

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#20
In reply to #12

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 8:53 PM

Well if you can tension the ends then would a coated stainless steel wire work?

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#22
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Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 9:01 PM

No, I won't be able to attach the air inlet in order to inflate the rubber tubing.

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#18

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 8:27 PM

Use a square steel rod and bend it upward in the middle the amount of the sag. This can be achieved by fixing the ends at the appropriate angle, you can experiment. This is better than trial and error cambering ( putting a permanent bend into it) although that can be done.

A square section makes it easy to prevent rotation about it's polar axis.

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#21

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 8:56 PM

My first thought was "fishing rod" but then you drop in the idea of 20' in the future.

Can it be fixed to the inside of one half of your outer cylinder at around the 3 O'clock location, or even the 1 O'clock position? That way, the flow disturbance at the outlet will be minimised/eliminated and it would then be scalable to 20 feet and beyond.

Or have I got it totally wrong?

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#23
In reply to #21

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 9:07 PM

No the inside surface must be absolutely smooth with no internal wires or fastening devices. Must only be attached at the ends. And if anything interferes with the inflating of the tubing it will not inflate evenly.

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#27
In reply to #23

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 12:01 AM

Does it have to be a single tube?

If not, two tubes vertically spaced with a thin web welded between them (like an 'I' beam) would be much stiffer in the vertical plane.

or if you wanted it stiff in all planes, three tubes in an equilateral triangular cross section with webs between (as used for radio masts) would give good stiffness in all planes

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#24

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 9:45 PM

If you check the deflection formulas (depends on how the ends are attached/supported), I think you will find that the moment of inertia is too small for any normal construction material. Things will get 8x as bad at 20' vs 10'.

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#41
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Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 9:30 AM

Exactly. Hence my question, which is about hearing suggestions for non-normal construction materials. Carbon fiber tube or rectangular or round rod still seems to be in front. Also, some type of filled pressurized tubing might be a possibility.

Frankly, I don't see 20' being possible, no matter the material, with this current design.

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#26

Re: Need 1/4" Rod Stiffer than Steel

09/19/2012 11:32 PM

Cambering or tilting the end connections of the rod have been mentioned. As has tensioning of the rod. Maybe you need to tell your bosses that the materials available do not permit 10'. Would they accept a series of shorter tubes/rods? Say 2 at 5'. Flow thru the tube could be split, and then rejoined. With low flow rates, you could use only half of the apparatus. Segments could also be combined to make 20'. Since this tube/rod seems to be acting as a valve, would some other type of pneumatic valve work?

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#40
In reply to #26

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 9:19 AM

It needs to be one continuous smooth rod. That is integral to the design function.

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#28

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 12:44 AM

Titanium is considerably stiffer than steel, and much lighter. I don't think the price is more than 10X that of steel.

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#29
In reply to #28

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 1:53 AM

what grade steel are you using?

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#39
In reply to #29

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 9:17 AM

On the 5' model we are using 316. But there really is no SS that has a stiffness to weight ratio that comes close to being sufficient over a length of 10'.

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#30

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 2:02 AM

1/4" is so small for a prestressed concrete with inner wire or rod and the yalasit's idea deserves a better application like the Vanguard rocket,anyway it will get a bigger diameter at the middle of the rod..I don't accept the idea you need a higher young modulus material, steel is what you need.I would think again air inlet and rod attachments.-

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#31

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 2:13 AM

BAMBOO STRIP

You will need to deviate a little from 1/4" round to 20x2 mm sector of a circle .

We will choose the right Bamboo species ,treat the split

But carrying 10' long is going to be expensive.

mm

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#32

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 4:26 AM

If it doesn't have to be round, can it be suspended?

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#33

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 5:50 AM

Can you anchor both ends thereby transferring the sag problem to both ends. In other words put lateral tension on the rod? Tensile strength and anchor points on either end would be the driving factor as to how much the rod would be able to sag.

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#34

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 6:09 AM

Does the outlet have to be at the bottom, or could the mechanism be angled to allow the rubber to sit on the bottom while deflated, and simply add a pouring lip to the outlet?

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#62
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Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 5:29 PM

Thank you GM1964, but the slots do indeed need to be at top and bottom because of the architecture of the adjoining parts, and the fact that the surface that is being coated by emerging foam is directly below. Also the bladder needs to be central. Symmetry is crucial so that the flow is consistent. The consistency is easily disrupted by any deviation from complete symmetry along all axes.

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#35

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 8:10 AM

If there is an absolute need to keep the cross-section as small as possible, then I suggest that you use a POST-TENSIONED braided steel wire rope, either galvanized or SS. That would eliminate the sag due to self-weight IF you are not hanging anything from the member.

The supporting structures on either end must be capable of resisting the high tensile loading!

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#36
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Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 9:05 AM

No Moosie, I can't use rope. There is an air inlet on the end that inflates the rubber sleeve, that goes through the end of the rod. Can't do that with a braid.

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#37
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Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 9:12 AM

Could you post a cross section view of one end? A picture (sketch) will clear up any mis understanding.

So far this has been a good thread.

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#43
In reply to #36

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 9:37 AM

We need to see some pics, especially at the ends.

What is this used for?

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#45
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Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 10:07 AM

Unfortunately, this is a unique proprietary design and I'm not able to go into any more details as to its use or photos. I have to make due with being vague with any descriptions beyond what I've already given. It's basically a type of valve for a very specific purpose.

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#44

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 10:04 AM

Okay, so Alumina and Carbon Epoxy seem to have the highest Specific Modulus (stiffness:weight) of all commercially available materials. Alumina might be too brittle. I'll have to investigate. Carbon epoxy is promising.

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#46

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 10:34 AM

From what I understand from the OP and the subsequent member contributions and clarifications from Out of Box Experience is

  • the rod must be hollow section because air is transported down it
  • tensioning of the rod still results in excessive sag
  • midsection external support is totally out of the question
  • radical alternatives to the inflatable valve arrangement won't do ( I saw some alternatives proposed here)

Here's my two bob's worth...

The hollow section could be exploited to accommodate a support mechanism and still transport air.

I propose feeding a guy wire down the centre of the hollow section rod. The guy is anchored to rigid supports at each end with a tensioning mechanism at one end.

The steel rope can be tensioned far more than the hollow section rod it is supporting could ever be. This would be about as straight as you could get (no?). If you used spider web silk and unobtainium composite filaments you could pull it really, really straight I guess.

Back to the hollow rod. I propose a circular hollow section (CHS) of the required size with a gland screwed onto one end and a T on the other end to take a gland on the straight leg and an air coupling as needed on the 90 leg. The glands would permit the steel rope to pass through them into the CHS and be sealed off air tight at the points of entry.

To support the CHS mid section, to transfer the load (the weight of the CHS) to the steel rope, disc spacers could be introduced into the CHS during the steel rope threading exercise. I'll guess 1 disc per foot would be enough.

These spacers would be dimensioned to support the steel rope and fit inside the CHS in a coaxial arrangement where the centre (steel rope) was the load bearing element and the CHS outer (and its sheath) was the load.

The discs would need to be perforated to permit adequately unhindered air flow through the tube. To cut down on weight, discs shaped like rimless wagon wheels would be OK I guess. Plastic most likely. 3D printer could pop them out no problem if you couldn't find any ready made in the market.

Would that work?

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#48
In reply to #46

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 11:04 AM

Your specs are correct except that, no the rod does not need to be hollow. I just need to be able to drill a single port (air path) into one end and out the side within the last couple inches or so, so that I can inflate and deflate the rubber sleeve.

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#49
In reply to #48

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 11:23 AM

You reckon solid bar sags even when well tensioned. Can you tension some CHS enough to not sag? It would be lighter and sag less.

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#50
In reply to #49

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 11:42 AM

That seems to be the direction I'm moving. Actually, I contacted the carbon fiber tubing company that was suggested, and am waiting for information. If I can properly grip the ends of CHS carbon tubing, securely enough to put high tension on it, yet not crush it, that may be the way to go. Or I might try stringing wire or cable thru the center for tensioned support as you've suggested, and use a thick walled carbon fiber pultruded tube over that. Might work, if I can work out a way to get a sealed air feed port into one end of it, despite the wire.

Getting closer. I appreciate everyone's resourceful ideas.

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#51

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 11:59 AM

Updated info:

The ID of the outer tube is actually .75", not .50"

That gives me more leeway. Not too much more, but perhaps just enough. Before getting specific numbers from Goodwinds re: the Specific Modulus of their various carbon fiber tubes and rods, I'm suspecting that might work, either alone or with a tensioned wire or cable running thru center. Especially with the available room inside now being 3/4" instead of 1/2". In fact, given the 3/4" ID, it sounds like perhaps a 20' length may no longer be considered impossible.

I'll let you all know when I get some material specs on the carbon.

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#53
In reply to #51

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 12:51 PM

3/4" would make things far easier. A 1/2" hollow carbon fiber pultruded tube of about .06" wall thickness would be light and stiff enough, even without tensioning. Intermediate modulus (42 msi) or even standard modulus (33 msi) would probably work, if the tubes are all unidirectional fibers evenly tensioned, with minimum resin content. Avia composites was quite good at this, and I assume that Goodwinds (who bought them) is too.

There are huge reductions in self-supporting deflections with thin walls, large diameters, and light materials. The modulus of standard carbon fiber is not all that much different that carbon steel (33 msi vs 29 msi) but the densities are much different (by a 1:4 ratio). The modulus of the fibers is best used only when all the fibers are in line with the stress and perfectly straight. Woven fabrics have a far lower effective modulus.

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#56
In reply to #53

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 1:27 PM

Yep, I believe you're right. I'm waiting to hear if I can get 10' lengths. Their standard stock only goes up to 5'. Now presuming this works, and looking ahead to a 20' assembly, I wonder if it's possible and practical to string a wire or cable 20' and be able to get it straight enough and strong enough to string a pultruded tube over it for that length. Or a non-metallic cord of some sort that won't stretch or relax.

This assembly is called a Bladder Support Rod... the bladder being the rubber sleeve. Its sole purpose is to suspend the bladder down the center of the outer tube without touching the inside surfaces of the outer tube.

Oh, hold on.. just noticed you said 1/2" diameter. The outer tube ID is 3/4" diameter, but the inner still needs to be 1/4" OD.

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#65
In reply to #56

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 7:23 PM

I begin to understand your problem, does the bladder have to be continuous over the full length? If you could use shorter lengths of bladder joined end to end, then perhaps at the join, you cold have a support frame between the inner and outer tubes - do you understand my suggestion?

The construction would be: central tube with bladder over it, the bladder being sealed to the tube at both ends, then a support between the inner and outer tubes, followed by the next section of bladder sealed to the inner tube - repeated as required.

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#64
In reply to #51

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 7:11 PM

Can the rod or tube not have anything attached to it externally along its length?

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#66
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Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 7:59 PM

That's correct. Nothing can be attached to the internal 1/4" x 10' rod, its entire length, except at the ends. The rod & bladder must all be one continuous length.

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#67
In reply to #66

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 8:36 PM

Ok, does the centre have to be a rod or tube, since all it is doing is supporting the bladder - could it be a triangular cross section lattice structure made from carbon fibre, or a geodesic structure , again made from carbon fibre - think of the fuselage structure of the Wellington bomber.

I think the real difficulty will be that this centre structure has to also support the weight of the bladder and resist the flow of the material - for 10' at this size that is a tall ask!

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#52

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 12:47 PM

Can you use Alumina or cordierite or Mullite

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#54
In reply to #52

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 1:16 PM

I already mentioned Alumina. Something that long, thin and brittle would snap during maintenance or handling. I haven't looked at the others.

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#57

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 2:02 PM

I seem to recall that carbon fiber is hydrophillic.

Better check on that, cause I'm not sure.

Don't know what makes up your "foam" but it may be an issue that you haven't faced with metal.

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#58

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 3:57 PM

Is the use temperature is room temperature or higher. Have you considered glass ceramic coated alloy 42. Glass you will need is high alumina containing which will give relative high modulus and protect metal form oxidizing and improves modulus of Alloy 42. This can also be done for cheaper cast iron and steel

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#59

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 4:07 PM

Maybe this would work. Make the outer tube in two halves, and, weld ball bearings once every 2 foot in the positions shown. They should be offset in each half by a foot as shown in the lower diagram which is clearly not to scale.

The foam should rejoin after it passes the balls and anyway it has the other side of the rod to flow down. The bladder may not seal the bottom opening but it should still seal the top OK.

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#60
In reply to #59

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 4:52 PM

But, you forgot this, "This assembly is called a Bladder Support Rod... the bladder being the rubber sleeve. Its sole purpose is to suspend the bladder down the center of the outer tube without touching the inside surfaces of the outer tube".

So, no contact allowed. The bladder seals the slots in the outer pipe.

I thought of washers first,, but...................................

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#61
In reply to #59

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 5:20 PM

Randall, I appreciate the effort you went thru illustrating your idea, but as I said, there can be nothing between the inner and outer pipe. The foam, in fact, will not rejoin on the other side, resulting in non-uniform dispensing which will be apparent. The other thing is, the bladder when inflated must seal both top and bottom slots instantaneously, otherwise the shut-of aspect will not be instant and foam trapped below the upped seal will continue to ooze out.

The design works perfectly in models up to our current largest, which is almost 6' long. But the jump to 10' gives us the issue of the center of the rod dipping down and causing a reduced foam flow towards the center.

Any type of object or support within the chamber that is touching either the bladder or inside of the pipe disrupts the consistency of the foam emerging, across the length... even tiny wires or pins. Absolute consistency is essential.

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#63

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 5:44 PM

if money is not a problem, then try inconol or titanium

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#68

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 8:55 PM

I thought that titanium was stronger than steel but less stiff by a half.

A quarter inch square rod, over a ten foot span, should deflect about five inches under its own weight. The deflected shape is parabolic. If the ends are rotated to camber the rod, the upward shape will be circular so that the rod would not be quite straight. Imagine a circular curve upward with a parabola re-arranged to hang from the top of it. It would be at the correct elevation in the center but slightly high towards the ends. With some adjustment, the best fit could be discovered.

Since the deflection is proportional to the span to the fourth, you would have a sag of 5"x24=80" with a 20' span, an impossibly large amount, perhaps it could be made to work if it was tensioned but I doubt it. Cambering it would make it too far off straight to be useful.

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#69

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 9:07 PM

I recommend a design change. My thought is a type of rotary valve, using a tube-in-tube design*. The inside tube has slots like the outside tube. Rotate the inside tube to open and close the passage.

* Doesn't have to be a tube. I would try a solid bar with a slot machined through it. Something like HDPE so it could fit snugly to minimize leakage, yet rotate easily without binding.

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#70
In reply to #69

Re: Need 1/4" Rod Stiffer than Steel

09/20/2012 11:29 PM

I'd agree with that.

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#71

Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 1:41 AM

Another option would be to scale up the inner component diameters, so that the strength/deflection of the centre components are achieved, then spec the outer tube to maintain the same clearance as in the smaller unit. This may take some fine tuning, as there will be a longer distance for the foam to flow.

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#72

Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 4:42 AM

As the outer tube has a slot top and bottom over it's entire length i would expect that a longer tube will 'blow out' in the center when you activate the bladder/valve making your efforts a waste of time.

If it works at 6' an upscaling of ALL dimensions should work at 10'. I realise that the flow is controlled by the gap/slot so that dimension would be the only one to remain as is, unless you increase the speed of the sheet being coated to use up the extra material.

Jim

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#73

Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 5:12 AM

Another stupid suggestion: make the outer tube from a material which sags by the same amount as the inner rod/tube.

Incidentally as the whole thing gets longer, there is an additional advantage to having the bladder support rod a tube instead of a rod. You can add extra holes along its length so that the bladder inflates at the same time along the whole length. With the current design the inflation starts at one end and then works its way in a "wave" to the other end: not a problem over short lengths but but may leave a slightly diagonal cut off on longer units. Just thinking out loud, so to speak.

Also it may be worth reviewing this thread so see if anything helps:-

http://cr4.globalspec.com/thread/66957

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#74

Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 8:04 AM

Interesting comments and suggestions overnight, some of which I can address. But meanwhile, as an update, here is the response I got from Goodwinds regarding their carbon fiber. FYI:

"I only have 8 ft lengths in the carbon rod available, and up to 12ft lengths in the tubes.

The tube will have less sag under his own weight for a given diameter (of course, the rod will be a bit stiffer when you actually apply a load to it)

I took a quick look at our .240 rod and tube and you're looking at about .75" in sag for the tube and about 1.5" for the rod over 10 feet (I measured only 8ft and extrapolated).

Pricing for a 10 ft long .265" carbon tube is $20.58 but you're going to really pay in freight, you're looking at hundreds of dollars for everything shipped over 9ft.

Please let me know if you have any other questions or concerns."

Hundreds of dollars shipping for $20 part. Yikes! I hate when that happens.

So let me ask y'all's opinion on this. What would be the effect if I were to use a small cable, wire or non-hydrophilic, non-elastic cord or string, ran it thru the center of this tube and tensioned the wire? Would that perceptibly decrease my sag across 10'?

I responded to the above email, by asking about the tube's hydrophilic properties and how it would be effected by moisture, both in it's Specific Modulus and dimensionally... as lyn had suggested.

As far as the other suggestions above... I need to keep the confidentiality of the design intact, but so as to eliminate ideas that are off track, I can add this to my previous description. The outer tube is not actually a tube. Essentially it is, but it is actually formed by two halves, something like two plates, coming together with a nearly half circle groove machined into each side, thereby forming what is essentially the inner surface of a 3/4" ID pipe with a groove at the top and bottom, because there is a small gap between the plates. The 3/4" dimension can not be changed, as further machining would break out into adjoining cavities.

So some of the above suggestions do not apply. But all other specs remain as stated. There can be no inner supports or anything to interrupt the continuous smoothness of the inside of the assy.

Regarding the redesign suggestion and using a rotating slotted inner core as the valve gate, although that is a good idea, it was actually one of the early design ideas of this project a few years ago, and evidently was rejected for some reason having to do with the breakdown of the foam that would be trapped within the slot when valve is closed, and the deposit of liquid that is released when it is opened again.

And as far as using the Inconel and the other metal mentioned... the deflection of both of those is greater than the pultruded carbon fiber tube.

Thank you all for your thoughts. Good thread.

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#77
In reply to #74

Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 8:36 AM

Thanks for keeping us all in the loop. Interesting challenge.

If it wasn't for the freight cost I'd say give the carbon rod a go with a tensioned wire down the middle of it.

Can't you go and pick it up yourself? How about packing it onto a Greyhound bus? Do they still do bus freight in the US?

While you are agonising over the freight cost, it might be prudent to tension up a length of wire that you might use and see how much it sags alone and then loaded with say a bit of PVC tube. If it looks like it will work then go for it. Order more than one carbon tube for spare stock to reduce the freight cost per unit.

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#78
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Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 8:49 AM

Yes, all of that is exactly what I will be doing. But still open to alternatives, of course.

Unfortunately, I live on opposite coast of where the rod is shipped from. Am going to see if there are any vendors on east coast, USA. But even so, a couple hundred $ isn't out of budget. Just seems wrong, principle-wise.

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#79
In reply to #78

Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 3:02 PM

I can't leave well alone so...

I'm not into the exotic materials, but following up on my previous suggestion using a square steel rod, If you extend the rod 4'-4" on either side, it will reduce the deflection in the middle to zero! It will cause a small upward deflection near the quarter points. I offer this to give an idea of what moment is needed at the ends to achieve this. If, instead, you hang weights closer in, you could probably adjust them to find the sweet spot where the upward and downward deflections are equal.

The 4'-8" is the same for round or square rod, but the square has a better stiffness to weight ratio for carrying the inner tube. The 4'-8" is on the assumption that the inner tube is continued over these cantilevers.

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#81
In reply to #79

Re: Need 1/4" Rod Stiffer than Steel

09/22/2012 1:00 AM

Not too shabby there mate!!

Good idea.

It won't completely straighten it though it will definitely make it straighter.

The stress and deflection profile for what is effectively a beam would be like that of a cantilevered beam on bearings (not fixed at each end just supported). It will still sag a bit between the high points.

Maybe your idea in combination with an internal guy wire....

Mechanical tensioners from the cantilevered ends to the floor (base of the hopper) could replace weights and be infinitely adjustable.

The guy wire could be diverted downwards at the exit on each end to provide both tension for the internal support and contra-deflection force transmission for the cantilevered ends.

Rectangular you reckon....got me thinking now. Next installment.

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#80
In reply to #74

Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 5:13 PM

Hundreds of dollars shipping for $20 part. Yikes! I hate when that happens.


Fed X Express ships up to 119 inches length and 130 girth. Make a box 119" x 4" x 1". Then block the tube to one side and then the other (snake like) at 2.5 foot intervals. This will shorten it just enough to fit. Bend a couple of sheet metal end protectors and tape them into place.


A tension element down the middle of the tube will not make it straight, but could make it close if you held it to one side in the middle and to the opposite side at two nodes part way out from the middle, and back to the first side at the ends, truss style. (Look at the mast on a racing sailboat.*) Maintaining tension would be a royal pain.


If the truss was a simple triangle straight from end to middle, then the tension on the line would need to be about 1000 times the weight of the tube. If the tube weighs 3 oz (?) the tension in the line would need to be 3000 oz -- on the order of 200 lbs. Kevlar line starts to get thick enough for tension that the effective truss thickness is reduced. Carbon yarn would be a possibility too. A 1/16" pultusion would be a possibility that would be easier to handle than yarn, and stronger and stiffer.


Tensioning pultrusions requires something like a socketed aluminum end piece that has enough epoxy area to take the tension load in shear.



However, the foam sounds rather stiff if it cannot quickly come back together after going around a tiny tube support. This suggests that the force on the tube from the flow could be more than a couple ounces, causing the rod to bend away from the flow enough to block the outlet (even with fancy internal tensioned support).
When you consider that a carbon fiber tube already deflects too much (under just a few ounces of force) and that internal supports are less effective than the tubes outer skin, you can see that a couple ounces of force would easily overcome even a very stiff and strong tension member.


This would be much easier if you could make the inner tube larger in diameter. Deflection for a 1/2 inch tube would be far less. Is the exit nozzle so wide that the gap between a 1/2" tube and the 3/4" tube would impede flow too much?


*(Speaking of sailboat rigs: the tension in rigging wire can be several thousand pounds even on a smallish [24'] boat. Press on one of the rigging wires and you can easily deflect it with one finger.)

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#75

Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 8:08 AM

As I see it, in the OP's design, the bladder forces all of the product from the tube as it stops the flow but none of the suggested substitutes do.

I haven't been able to visualize the ends where the air is inserted, but the end of the bladder and the tube are sealed and the rod and the tube are supported.

In Randall's link, I posted that just fixing the ends reduced the deflection by 80%, that is not enough to solve this problem.

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#76
In reply to #75

Re: Need 1/4" Rod Stiffer than Steel

09/21/2012 8:16 AM

The current model is using a solid steel rod passing thru the rubber tubing bladder. The bladder is sealed against the rod by simply tying it off against the rod at both ends. The end of the rod has a hole in it a couple inches long that then veers off to the side and exits within the bladder. When air is applied, the area between the bladder and the rod is filled with air, thereby inflating the bladder and, as you said, pushing all the foam out of the chamber.

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#82

Re: Need 1/4" Rod Stiffer than Steel

09/22/2012 2:52 PM

I saw a TV program on how they make poles for pole vaulting. They were 17' long and one piece construction, each one custom made. You could contact a company that makes them and see what they can do for you.

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#83

Re: Need 1/4" Rod Stiffer than Steel

09/26/2012 12:08 AM

If you could anchor the end supports, thread the rod and put some nuts and torque on it you can probably straighten it out. If it still sags (not sure exactly what kind of steel it is) lean the end supports in such a way that you create a crown in the stressed rod equal to the remaining sag.

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