Valve selection

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getgray
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Tue Sep 06, 2022 5:17 pm

I'm working on helping a high school robotics team update their T-shirt cannon. They have been directed to move to metal parts away from PVC. I've got a copy of the GGDT and I'm fuzzy on the valve selections;

chamber sealing pilot, barrel sealing pilot, burst disc

Are any of these available in a 1" size in brass? If I use generic, the performance drops substantially, so I'm assuming a "normal" valve (whatever that is) isn't enough.

Thanks,
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getgray
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Wed Sep 07, 2022 10:44 am

Anyone know what I should use for a "pulse valve" in GGDT's valve parameters? They seem to be an option for fast high flow valves?
getgray
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Wed Sep 07, 2022 9:48 pm

What about these? They look like nice, fast valves. "Dust Collector, Pulse Valve, Series JSXFA" What valve type would one use to model them in GGDT? Chamber seal I think, but grateful for some verification.

Company link:
https://www.smcusa.com/products/JSXFA-2 ... lve~161147

Their video:


Datasheet:
https://www.smcpneumatics.com/pdfs/JSXFA.pdf
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Thu Sep 08, 2022 12:42 am

I'm pretty clueless about what you need. Maybe some more details about the cannon might help. There are some pneumatically minded guys around here who will likely chime in. The only thing I really know how to do is make a piston cannon. Sounds like you want to buy a valve, but I can't think of any purchased valve that won't suck. Performance requirements may dictate what valve is needed.
getgray
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Thu Sep 08, 2022 1:42 pm

Building launcher for t-shirts. Rolled up. And yes, I would prefer to purchase a valve. A lot of times the larger companies will donate parts like that to kids robotics teams. I've reached out to SMC and Asco for a similar one they make. They both have high Kv values relatively speaking I think, but I'm not sure how to enter their parameters into GGDT. That is, what kind of valve type to use. I *think* these are chamber seal types, but not sure. They don't have pistons per se, or maybe the membrane IS the piston.

The goal is to launch a T-shirt in a gym from half court into the upper or lower bleachers. So not a high bar I don't think. Hopefully using relatively low pressure. But I need to understand the valve characteristics in order to size the barrel and chamber appropriately.

If building a valve is going to help me keep pressure low, then I'd be grateful for suggestions there, too.
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Fri Sep 16, 2022 2:34 am

Piston valves are the best option. When you say you would build one,, what tools are available to you??

I am not sure how to specify piston valves in GGDT.

What is your time line? Use at least a .75" valve.
getgray
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Fri Sep 16, 2022 9:29 am

Building one would be my last resort. I've got a small metal lathe and might be able to muddle through it, but I prefer a off-the shelf solution. If I made it, I need to make so I can use in metal, air pressure safe parts. Need is very soon. We can't proceed with the overall project until we know what the cannon part is going to be.

I contacted SMC about my project and they are donating a pair of their 1.5" Pulse valves and some solenoid activated pilot valves to activate them. Rep is bringing them by today. I'm hoping one of them will work. Similarly I am not sure how to model them in GGDT, that is, what valve parameters to use. I think they would be chamber types?

https://content2.smcetech.com/pdf/JSXFA.pdf
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jackssmirkingrevenge
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Fri Sep 16, 2022 11:15 am

getgray wrote:
Fri Sep 16, 2022 9:29 am
I contacted SMC about my project and they are donating a pair of their 1.5" Pulse valves and some solenoid activated pilot valves to activate them. Rep is bringing them by today. I'm hoping one of them will work.
Guaranteed to work, basically overkill for a t-shirt launcher which is good because you should be able to get away with using lower pressures.

Here's an approximate model using 150 psi and a 12 inch long 4 inch diameter barrel, dumping a 21 cubic inch chamber yields almost 150 feet per second for a 200 gram shirt:

Image
hectmarr wrote:You have to make many weapons, because this field is long and short life
getgray
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Tue Sep 20, 2022 12:29 am

Thank you. I have the valve in hand now. It's a beast visually. They don't publish a Cv for it because they said Cv is generally applied to valves with "continous flow" and this one is a pulse so didn't typically apply. Rep has some approximate Cv's which were pretty high, and he's inquired with Japan to see if they can confirm. Right now, what he has says Cv = 52. Seat diameter is larger than 1.5" I'd say. Maybe 2", I'll have to open it to measure. Open time or dwell time, I don't know what to use. It's not published and it's only going to stay open as long as there is enough pressure differential. It is a diaphragm only design, no literal wire spring to close it, but the diaphragm is pretty stiff to push open by hand. The diaphragm in this case is the spring. I think it would fit the "chamber seal" type of valve, except for this diaphragm's spring force. I don't expect that's accounted for in the model. He probably meant to account for that using piston mass and diameter.

Plan is for a 3" barrel, figure same size for chamber for consistency for now at least, Measured our shirt at 330 gm, measured the force it takes to move the shirt with a medium tight roll at only .04 PSI, which seems low, but that's what it too. I put a 4.6 oz weight on the roll to get it to slide, spread that across area converted to lb, to determine PSI.

Used your suggestion for Open and dwell. Used 75ft/s to lob shirt to target distance (different calculator).

I find I only need 18psi, which seems low. The old PVC cannon, using a 1" sprinkler valve needs about 100psi to do much of anything (similar size chamber but a 36" barrel which might be part of the issue there).

Image
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jackssmirkingrevenge
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Wed Sep 21, 2022 12:11 pm

getgray wrote:
Tue Sep 20, 2022 12:29 am
Used your suggestion for Open and dwell. Used 75ft/s to lob shirt to target distance (different calculator).

I find I only need 18psi, which seems low. The old PVC cannon, using a 1" sprinkler valve needs about 100psi to do much of anything (similar size chamber but a 36" barrel which might be part of the issue there).
Rehosted your image as it wasn't showing up:

Image

You used a significantly larger chamber volume for a smaller barrel volume and lighter projectile, hence the need for lower pressure for the desired velocity.

For the latter, as a T-shirt is not exactly an streamlined and stable projectile, you're likely going to need to launch it faster than that to get the required range.

Is the plan to dump a small firing chamber for each shot, or connect the valve to a larger tank and momentarily open the valve for firing?

The latter will give you better efficiency, that is assuming you're in a situation where that is important and the intention is to fire repeatedly.
hectmarr wrote:You have to make many weapons, because this field is long and short life
getgray
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Wed Sep 21, 2022 4:39 pm

First, thanks for the help on this. (and for that rehost of the image).

re chamber volume: I just set it the same as the barrel for a start point. I'd like to keep the pressure low but the a smaller barrel will save on costs so I'll work to balance it once I get a feel for how the valve will do. I planned to do some testing with PVC pipe and fittings, appropriately shielded for safe testing. Before I go buying stainless pipe.

Design wise, it would be good to determine some physical parameter they can change to change the launch velocity the best. Pressure seems very sensitive from "never leaves barrel" to "80ft/s". Harder to control when only a few PSI difference.

For barrel length my intuition tells me longer would give more directional "projectile steering", but in GGDT, shorter seems to work better in the models I've tried. The existing/old robot's PVC cannon barrel that I inherited is 36" long (might be part of their issues). But 12" does seem short being as a rolled T-shirt almost fills it. That said, I've seen the "Bleacher Reacher" high pressure gun and it's performance, it has a short barrel and seems OK at directional targeting.

re T-shirt projectile, I used a couple of projectile calculators that account for air friction, and I used the worst case for projected area when determining the drag coefficient of the T-shirt. I used the dimensions of the existing court and bleachers for a lobbing target. The shirts are currently wrapped with rubber bands to preserve cylindrical shape, but it would be great if they had enough launch velocity to unfurl mid-air and have a soft landing. I don't want any Mrs Flanders Episodes of course :) . But having the ability to adjust velocity and go further is going to help, in case they ever want to go outside from basketball to where they may need longer travel. But for now, happy with Basketball arena.

re plan: The plan is to use (2) pressure tanks. 1) A "storage tank" (or tanks) to store air at a higher pressure, say 100psi. And 2) the "Chamber", which would be at a lower pressure I hope. These would be connected with a pressure regulator. The Cannon will be robot mounted and for now, be a single shot device. It will drive out onto the court, give a warning and fire the T-shirt. Return to base where it gets a new t-shirt. I'm not sure exactly how this SMC pulse valve will behave, but the idea is to activate it and hold open (dwell) for only as long as necessary to get the desired velocity. The Pulse valve will be activated by a secondary electrical solenoid controlled pilot valve, also provided to us by SMC. The pulse valve has a minimum pilot valve spec, I think it requires a pilot of 5mm, but the SMC guy sent one that was supposed to work with it correctly. My understanding form the parameters I've tried is the chamber pressure will be mostly exhausted with each firing. I don't have the response time specs on the Pulse valve, yet.

Today I did take some measurements of the 1.5" NPT pulse valve. The exit opening from it's diaphragm is ~1.86" and I measured that it takes ~6.25 lbs force to hold it open. Doing the math I came up with 2.71 in2 for the exit opening, and spreading that force over the area, 2.3 psi to hold it open. If I did the calcs right. So not as much a psi requirement to open as I expected based on feel (and lack of experience)

Hopefully that answered your question. If I have the valve modeled right in GGDT, and it's results are close, I'm feeling better about being able to get it working. I did notice that the dead volume has a pretty big effect on this design, so I need to work toward keeping that small. Limited with PVC fittings, bu twith stainless distillery spools, I can get a flat back plate with a 1.5" nipple that will minimize it.
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