New CPH no LTR

Build a homemade water gun or water balloon launcher and tell us about it.
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adronl
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New CPH no LTR

Post by adronl » Thu Jul 09, 2009 5:05 am

Well here we go I am about to make a new very compact CPH and where latex fails I believe that gum rubber will not. LTR has a durometer of 35A GRT has a durometer of 45A so they have very similar properties. Gum rubber however can stretch to twice its length and has a tensile strengh of 2000-3000psi at least what mcmaster sells. Another advantage is with LTR you can only get a maximum wall thickness or 3/16" for both 3/4"&1/2" ID for 1" you can get 1/4". With Gum rubber 1/2" ID starts with 1/4-1/2" walls 3/4" starts with 1/8"-5/8" walls

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Last edited by adronl on Thu Jul 09, 2009 5:32 am, edited 1 time in total.

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SSCBen
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Re: New CPH no LTR

Post by SSCBen » Thu Jul 09, 2009 4:46 pm

I'm very interested in this. Connect a pressure gauge and let us know what sort of pressures you're getting. ;)

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captianfear
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Re: New CPH no LTR

Post by captianfear » Thu Jul 09, 2009 5:30 pm

Would gum rubber have to be replaced as often as LRT?

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adronl
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Re: New CPH no LTR

Post by adronl » Thu Jul 09, 2009 5:38 pm

Well I worked out my design layout and it may not be as compact as I first speculated but it will resemble a supersoaker gun or a riffle more. Also there are plans for me to add a trigger system using small metal cable like on your bicycle's brake system.

As far as a pressure gauge I suppose I need to get one any way for testing Just to make sure I got this right put the psi guage on the firing end to get a level correct. I also read something about determining psi based on output, time, and nozzle size

I believe that the life of gum ruber will be substantially longer. From my tests so far it show no signs of fatigue after several tests at filled at 100psi to it's max capacity. As advertised it returns to its origional shape with no warping or stressed areas. I am pretty sure LTR would bust at that pressure in one test much less returning to its origional shape without warping and stress.
Last edited by adronl on Thu Jul 09, 2009 5:43 pm, edited 1 time in total.

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captianfear
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Re: New CPH no LTR

Post by captianfear » Thu Jul 09, 2009 5:57 pm

That could be a great improvement in homemade water guns the. I like the bike cable idea. Would there be enough force to return the firing valve to a closed position? Could you post a picture of you design if you have it?

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SSCBen
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Re: New CPH no LTR

Post by SSCBen » Thu Jul 09, 2009 6:58 pm

adronl, how can you know what pressure the tubes operate at if you don't have a pressure gauge? Are you saying that you filled them from a 100 psi supply? If you did, then they won't necessarily operate at 100 psi, in fact, to fill them they have to operate at less than 100 psi (flow requires a difference in pressure).

If you're unaware, the rubber tubing itself generates pressure. You can't fill it at any set pressure--you have increase the wall thickness usually to increase the operating pressure. The tube itself generates pressure by squeezing the water.

(As for the mentioned procedure based on output, it'd be complicated and approximate at best. It's easiest to use a gauge.)

LRT doesn't appear to degrade from the outside, especially from only a few cycles. You must measure the pressure the tube generates by itself and wall thickness after each cycle. Anyway, the selling point of LRT is that it is highly resilient and will return to it's original shape, so I doubt gum rubber is better in this respect, though I don't think it has to be if it operates at a higher pressure.

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adronl
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Re: New CPH no LTR

Post by adronl » Thu Jul 09, 2009 7:40 pm

The point of that statement is that GRT took the load of having 100psi forced into it and sustained without bursting I also did stress tests at said psi. I do have a pressure gauge but not on the water side which I soon intend to get my gauges are for the air side and I can only verify that there is a constant air pressure of that level being forced upon the water.

Also what about the question I asked can you not determine the psi of the water by the volume, rate and diameter with a math formula

Also from reading the forums I have heard of and seen the evidence of LTR bursting on first attempts and from your own statements that is does stretch out, deform, and deteriorate after several uses.
Last edited by adronl on Thu Jul 09, 2009 7:46 pm, edited 1 time in total.

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SSCBen
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Re: New CPH no LTR

Post by SSCBen » Thu Jul 09, 2009 7:46 pm

I see. The pressure itself won't break the tubing--only the expansion will. Injecting water at 100 psi into the tube won't necessarily do anything unless the amount of water is enough to pop the tube. ;)

As for a simple formula to determine pressure based on other factors, yes, it is possible, however, it'd be only approximate and would require data collected when the pressure is known (or computer simulation that I assume no one here will do). So it's not really feasible.

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Silence
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Re: New CPH no LTR

Post by Silence » Thu Jul 09, 2009 11:04 pm

If you limit the diameter that latex rubber tubing can stretch to, as Larami, Hasbro, and Buzz Bee Toys do, then you may be able to preserve the thickness of the rubber.

Regarding your question about approximating the pressure, well, you can combine the formulas for volumetric flow and the volumetric flow coefficient to get a relationship between pressure and stream velocity or output. (Note that, for some reason, the second page refers to flow as F, not Q.) However, finding the flow coefficient, Cv, is a nontrivial task and generally requires either simulation or experimentation, as Ben said. Generally you would initially have to measure the pressure and other properties of a system, then calculate the Cv, and finally be able to approximate things like the pressure as you change the other properties.

I'm curious about the appearance of your gum rubber tubing...it seems to comprise a strip of rubber fused into a helix.
Last edited by Silence on Thu Jul 09, 2009 11:10 pm, edited 1 time in total.

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adronl
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Re: New CPH no LTR

Post by adronl » Fri Jul 10, 2009 2:54 am

Well here it is I am trying to decide if I want to put the valve half way down the gun or at the firing end. having it at the end will make it harder to rig up a trigger but I won't have to hold the barrel up to prevent loosing water between firing last it will be less laminar though. If I put it about half way I will have a much more laminar flow, easier to rig up the handle and the last part you already know. Soooo

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adronl
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Re: New CPH no LTR

Post by adronl » Fri Jul 10, 2009 2:56 am

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Parts for 3 more guns

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Silence
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Re: New CPH no LTR

Post by Silence » Fri Jul 10, 2009 3:05 am

My recommendation for the trigger valve: put it near the nozzle. A long stretch of tubing won't make the stream noticeably more laminar because the bore is too wide and the nozzle at the end may disrupt the flow anyway. And you already mentioned that the water could spill out of the nozzle.

Also, a structural support connecting the pump to the pressure chamber case could help.

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adronl
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Re: New CPH no LTR

Post by adronl » Fri Jul 10, 2009 3:15 am

I literally just finishing this and have not yet added the structural stability yet. There will be an assembly at the rear for a handle and trigger I hope depends on how complicated it gets regardless it will look nice. when I get done but for now the glue hasn't even cured some of it was put on less than 15 minutes ago

Few hours later now I think the glue has cured paint has dried did a little reinforcing but no where near done ill have to be carefull but about to perform a first test I'll post a picture tomorrow of the progress so far with out final structure added
Last edited by adronl on Fri Jul 10, 2009 6:04 am, edited 1 time in total.

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cantab
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Re: New CPH no LTR

Post by cantab » Fri Jul 10, 2009 2:44 pm

Interesting geometry. If it works for you that's good. Most would have put the CPS chamber opening towards the front.
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captianfear
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Re: New CPH no LTR

Post by captianfear » Fri Jul 10, 2009 5:47 pm

Is the firing valve going to be on the end of that long white pipe at the top of the design?

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