Force of latex tubing

[Ben]

Yesterday, I extracted a force equation from a spreadsheet that was meant to show how much force a piece of latex rubber tubing applied when stretched. The spreadsheet was meant for tubing being folded in the center and pulled like a slingshot, but after asking the guy who runs the company that makes the tubes I learned that it still applies when a fluid is inside of the tube. Finding which elongation percentage applies during that situation is what matters. Based upon what I know so far, I'd estimate that percentage to be 500%.

The formulas in the spreadsheet were overcomplicated. It appears that whoever made the spreadsheet didn't know much about simplification. I've reduced the 500% elongation formula to this:

Force = 460*pi*(I*W+W^2)

In the formula, I = internal diameter and W = wall thickness in inches.

This formula is fairly accurate and shows that I didn't buy the right tubing for my CPS water guns at all. Back in 2004, I thought that thickness was what mattered completely, but as it turns out internal diameter matters some as well. McMaster-Carr alone offers several tubes that offer more force than the one I bought. The good thing is that my homemade CPS water gun's design allows for changing most anything and this isn't a problem.

At this time I'd also speculate that stacking tubes over another does not increase the thickness as it would in this formula. It seems to have noticeably less of an effect than would increasing the wall thickness.

Anyway, I've emailed Primeline Industries about some tubes I'd be interested in buying. All of these tubes have at least 250 pounds of force. With luck, a few of these tubes will be affordable in small enough quantities for me to buy and resell. This will be a great deal to builders. Homemade CPS water guns are much better than air pressure ones, especially when they can be noticeably more powerful. I can't wait for a response.

[Silence]

Nice. I look forward to the performance of your upgraded CPH!

So, does a larger internal diameter provide more power? Interesting...although I'm also interested in hearing about their reply. Congratulations on the find.

[Hunt_and_Annoy]

Nice. I might want to try something like this someday.

[WaterWolf]

So, this suggests that a 1"ID-1-1/2"OD-.25"W tube would be far more powerful than something like a 1/8"ID-3/4"OD-.3125"W piece, despite the fact that the later has a much thicker wall.

I have most of the parts needed to test this on my CPH.
I'll buy a few more things from my local hardware store, compare and post the results.

[Ben]

Actually, it's not more powerful, and it's less useful. I tested it (see the backpack mod article for the modified gun). Read from this page too: http://www.sscentral.org/homemade/supercps.html

Do not expect larger diameter latex tubing to work substantially better than smaller diameter tubing. While larger diameter tubing has the potential for more water flow, it also has a lot of dead space to fill, is larger, and the dead space makes it heavier as well. After I made this water gun, I tried an "upgrade" to larger diameter tubing, only to be disappointed. I would suggest using smaller tubing with larger tubing barbs. I plan on developing a way to stretch latex tubing over barbs 3/4 inch in diameter or larger. Smaller tubing is lighter due to the lack of dead space, smaller by nature, and allows for layering of tubes.

Also, to reiterate what I've said before, force matters little. It's pressure that counts, not force. I didn't know what I was talking about. And this equation was for the force of rubber tubing used like rubber bands, so it's not even right. To get a good performing water gun, you want high pressure combined with large tubes, ideally with as little and as smooth of turns as possible.

In LRT, you want thick tubes with small diameters to get more pressure. But, small diameters limit flow. I emailed Primeline Industries several weeks ago about a 1/2 inch ID or 5/8 inch ID tube that had a wall of 3/8 inches which should be extremely powerful, but I never got a price quote back. I think I should email them again.

I've been considering making an equation (a surface integral simplified) to calculate the approximate pressure generated by a tube, but I never got around to it. I'd need to do some research and compare against reported pressures. It's possible though.

[WaterWolf]

So on McMaster.com, what does the "Maximum Pressure" number for LRT really mean.
Is that how much pressure it exerts on the water when expanded?

I hope you can get some good stuff from Primeline. I might buy a bit from you if you do.
One question though, since you would be dealing with much higher pressures, mightn't a CPH guns using this material need a narrower pump?

[Silence]

Yes, maximum pressure is the pressure at which the bladder is fully filled. The pressure does drop off as you shoot, like with an air pressure water gun, but not noticeably. Unless you look for it.

Primeline, like most manufacturers, should be able to produce custom LRT of nearly every size. The problem is that to turn a profit, they must produce in bulk. Ben will need to sell all the extra LRT through an SSC store. We're probably talking about at least a thousand feet, or maybe just hundreds if Primeline already makes the right size.

The pump size depends both on the pressure and on your strength. I would probably have to use 3/8" to be comfortable. You can check out my spreadsheet in the second post of the PVC reference thread. It provides the relationship between pressure, pump force, and various pipe sizes. And for the record, 10 to 25 pounds is probably the range of pumping ability for most people. If you have a weighing scale, you could find something of appropriate weight to see how it feels.

[WaterWolf]

A 250 PSI CPS would be just amazing .
At 100 PSI, Ben has shown that 70 foot ranges are possible, so a 250 PSI would most likely have ranges well over 100'.

This is also getting towards some awfully high pressures, so if you're planning on using these in actual combat, face protection might start to become more necessary.

4" PVC is only rated at 220 PSI, so we should avoid hooking it up to that. Although CPH guns, tend to only use 4" pipes as slip-on covers for the LRT, while putting force on smaller diameter pipes, which can stand much greater pressures.

However, according to my calculations pumping up a 250 PSI pressure chamber with a standard .622" pump would require 75 pounds of pumping force, which is way beyond most people.

Even a CPVC pump (.5") would still need 50 pounds of force.

What would your solution to this be?

[Ben]

I'm not sure where you're getting this 250 PSI figure from. I doubt we could find a tube that generates 250 PSI. Such as tube might be over an inch thick. I don't think you'd have that problem.

We might be able to get 80 PSI maximum from the tube's I'm looking at with some layering. That's just a guess though. I really don't know.

As for the relationship between pressure and range, I wouldn't suggest thinking the problem is as simple as you've described. Supercannon II might be able to beat 70 feet at pressures as low as 40 PSI depending on the nozzle diameter used. I didn't vary nozzle diameter at all, so I don't know how the gun could perform at different diameters. 250 PSI might be so high that the optimal nozzle diameter would be something like 3/4 inches, which would increase the water usage to something extremely high. There's a lot of factors to it.

[WaterWolf]

I see where my confusion was.
In your first post you said:

All of these tubes have at least 250 pounds of force.

I understand what you had been saying now.


At McMaster, the 1/8"ID-3/4"OD-.3125"W tube has a maximum pressure of 85PSI.
If we could somehow get that onto a 1/4" or 1/2" hose-barb, you would have quite a powerful CPH right there.

Lack of flow would not be a problem, as-long as you were able to get a sizable hose-barb into it.

I'm going to make another McMaster order in the next week or so, so perhaps I'll buy a foot of that and experiment a little bit.

[Ben]

That 85 PSI tube is limited a lot by the diameter. If you find a way to stretch it, that might be a revolution, but in my experience, getting substantially larger diameter barbs into a smaller tube is impossible. We need a better method. Something that could expand the end but has a hole in the center is ideal.

[sbell25]

I might be missing something here, but what's wrong with the method that Killer 7 used to stretch his LRT onto a larger hose barb?

WaterWolf: the biggest problem you're going to have with that tubing is capacity. LRT expands in diameter by 8.5 times I believe. That means that when it's fully stretched, your tubing will only have an ID of around 1", giving you terrible PC capacity.

[Silence]

sbell25, Killer 7's method is designed for stretching LRT over a barb. If we want to slide an entire tube underneath, things become more complicated. But the underlying concept is the same.

You could probably get a brass tube slightly larger than the small rubber tube. If you taper one and connect the smaller tube there, you could coat the entire thing in silicone lubricant or Teflon paste.

[WaterWolf]

Still, there is the issue of limited capacity as sbell pointed out.
You'd need quite a long piece of LRT to get much water, so perhaps I'll just wait for Ben's shipment of larger, thicker material (If we can get it).

[Ben]

I was mainly talking about the difficulty of sticking a significantly larger barb onto a tube. Killer 7's method doesn't let you stick say a 5/8 inch barb into 1/8 inch diameter tubing, especially very thick 1/8 inch diameter tubing. There also is the capacity problem as mentioned.

I'm still waiting for an email. And even if I can get it, it might be as much as $500 to $1000 of tubing. I'd have to wait a bit to save money to get that. Of course, I'd take orders initially, but I don't think we'll get enough initial orders to cover all that.