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determining expansion/shrinkage of tubing with air?

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determining expansion/shrinkage of tubing with air?

Unread postAuthor: JDP12 » Sat Sep 19, 2009 8:28 pm

Hey all- I have a question that may be fairly difficult without experimenting, but here goes-

I would like to be able to determine the expansion of tubing when air is forced into it and how quickly it would "snap" back to normal length.

In other words- think of surgical tubing. when you pump air into it, it expands and contracts. Then when you bleed the air out, it reduces and elongates again.

What I'm looking for is a way to tell this about different kinds of tubing just by their materials and specs on like mcmaster- is that really possible? I"m looking for something that expands/contracts and then snaps back in a very short time- so a pretty tough tubing.

Is that a possibility?

Thanks
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Unread postAuthor: covey12 » Sat Sep 19, 2009 8:49 pm

well, from all of the surgical tubing paintball grenades i made, the harder the tubing the quicker it goes back, for example the surgical tubing "snaps back" really slow, but something like an airhose would snap back a lot quicker, but need more force to make it expand

you probably knew that already, but idk how else to explain it


may be something like this http://www.mcmaster.com/#tubing/=3pfimz
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Unread postAuthor: JDP12 » Sat Sep 19, 2009 8:56 pm

i know thats where i'm looking- i'm more looking for material recommendations and wall thickness recommendations please.

Thank you though

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Unread postAuthor: btrettel » Sat Sep 19, 2009 8:58 pm

Water gun people approximate the expansion of latex tubing with simple formulas like "it gets 3 times longer and expands about 750% in diameter". You're right that experimentation can find this.

Solid mechanics isn't my field, but I'm reasonably certain deriving an equation for the expansion of an infinitely long thin tube (which is reasonably close to this) with an internal pressure isn't too difficult. I do not know enough about the subject to do this, however.*

Most practicing engineers probably would use FEA (i.e. computer simulation) and/or a book with derived formulas to calculate something like this. There are many books with nothing but derived formulas for stress and strain (i.e. stretching/compressing) in materials.

As for the time to snap back to normal dimensions... that'd be more difficult to calculate. I can't say a simple analytical formula is not possible as I don't know much about the field but I don't imagine it would be particularly simple.

To reiterate, experimentation is likely all you can do.

-----

* I have looked at this problem before due to an interest in water guns but didn't make much progress as I knew nothing about solid mechanics then. It's also worth noting that rubber has a non-linear stress-strain curve, so latex tubing would probably be difficult to model without linearization.
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Unread postAuthor: JDP12 » Sat Sep 19, 2009 9:00 pm

Ok- thanks for the opinion- I just want to be able to get the right stuff right away if possible- even though some experimentation would be necessary for its application.

But perhaps some materials recommendations?? As in hardest/toughest to expand?

also-would reinforced tubing like this- 5394K22 work? or would the reinforcements not expand- would ineed un-reniforced? I have a feeling i'll have to order a bunch of this to test but i'd like to be able to cut it down as much as possible
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Unread postAuthor: btrettel » Sat Sep 19, 2009 9:13 pm

Material "stiffness" is described by numbers like the modulus of elasticity and the shear modulus. This isn't necessarily what you want as the material dimensions also make a big difference. Which would expand more: a tube of aluminum foil or an aluminum tube that is 1/8" thick?

How much expansion do you want? I haven't noticed expansion (that I can see with the naked eye) in non-rubber tubing aside from some cheap vinyl tubing. And the expansion of the vinyl tubing was small.

Reinforcements are far stronger than the tubing and would severely restrict how much expansion you can get.
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Unread postAuthor: JDP12 » Sat Sep 19, 2009 9:17 pm

true. OK thank you.

I guess expansion isn't as important as the quickness of expansion and the quickness of "de-expansion" - its more the snap that matterse. However how much it expands will play a role as well- Because the lengths are gonna have to be able to be kept reasonably short.

I guess i would hope for an expansion rate of maybe 40-50% or so? That is, when is expanded it is 40-50% shorter than when not.. I'm not sure so I'm gonna have to do some experimenting for sure but i'd like to narrow down the material type and thickness as much as possible first

Like on the mcmaster site they use the Durometer scale- I'm assuming the harder it is, the tougher it is to expand and the harder it snaps.

I'd like to be able to easily keep it under 200 PSI, so i don't want to have to go to crazy pressures to get this...

At least I don't want to YET
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Unread postAuthor: btrettel » Sat Sep 19, 2009 10:35 pm

Latex is probably the most common material for what you want. It's the only one I know that will expand that much and have the resiliency to return time after time.

The Durometer scale is like the modulus of elasticity I mentioned earlier. It says something about the material, but nothing about the geometry (in particular, the thickness of said tube). So keep that in mind.

It's worth noting that latex tubing will equilibrate at a certain pressure. If you put gas in at 100 psig and the tube's equilibrium pressure is 25 psig... the gas will expand to 25 psig. You can easily put too much gas volume in and pop the tube if you're not careful.
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Unread postAuthor: JDP12 » Sat Sep 19, 2009 10:37 pm

Ok- what about possibly neoprene tubing?
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Unread postAuthor: btrettel » Sat Sep 19, 2009 10:53 pm

I don't know much about it. McMaster-Carr says it's harder than latex but less resilient. Resilience is extremely important here given that you do want the tube to contract. Neoprene likely will be permanently deformed from only a few expansions given my experience with latex.

It's possible that nothing like what you want exists. Layered latex tubes should be the closest thing.
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Unread postAuthor: jimmy101 » Sun Sep 20, 2009 10:32 am

I take it that you are basically trying to store more compressed air (more energy) in a volume than you can with a rigid container? Part of the energy is in the compressed air and part in the extended hose?

Off hand I can't think of a single example of that out in the real world. I would say that is what you want to find. If it is possible, even if not particularly practical, then there is some device somewhere that uses this approach.

I suspect though that the high mass of the hose means that the hose won't spring back all that fast. Of course, you haven't said how fast you need. Figure the density of the hose material is roughly ~1,000x the density of air at 1 ATM. Pressurize the air to 10 ATM and the hose density is still ~100x that of the air. The air will move much more quickly than will the hose. If you are thinking of some type of gun, then I would suspect that the hose would be moving too slowly to contibute much to a projectile.
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Unread postAuthor: POLAND_SPUD » Sun Sep 20, 2009 10:49 am

given my experience with latex
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Unread postAuthor: btrettel » Sun Sep 20, 2009 11:53 am

Incidentally, there was some discussion about using condoms in water guns. They can be bought in bulk for reasonably cheap and in theory could make a really thick latex tube. You'd run into some problems with inadequate longitudinal expansion though (probably NSFW link). No one does this, probably because balloons would be much less awkward more than the longitudinal expansion issues.

The only advantage of a expanding gas chamber as far as I'm concerned is its ability to maintain a relatively constant pressure without a regulator. This can be advantageous where cost, space, weight, or number of parts is a concern. If very high flow rates or pressures are desired, don't bother as the latex does not snap back instantly and getting high pressure is rather difficult.
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Last edited by btrettel on Sat Jan 16, 2010 1:00 pm, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 20, 2009 1:17 pm

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See, you don't need me any more *sniffle*
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Unread postAuthor: MrCrowley » Mon Sep 21, 2009 4:57 am

jackssmirkingrevenge wrote:Image

See, you don't need me any more *sniffle*

Wow I didn't know that thing still existed :D
Seems pretty crap, wonder why I didn't make his head come off/explode/smash in to pieces... :?
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