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First Design, T-shirt Cannon, feedback on my initial design.

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: garybdavis » Wed Nov 07, 2012 9:11 am

Lockednloaded wrote:...in order to change pressure on each shot, attach a knob to you palmers reg (to make pressure adjustment easier) and dial in your desired pressure before sliding the valve forward, filling the gun.


Do you know if the Palmer regulator will bleed off output pressure when you dial it back down? In other words, you have been shooting at 300psi and you want your next shot to be 200psi. So, you have to back off the regulator to 200psi before you charge the pressure chamber. Most regulators will bleed off the air from it's output side when you do this, but I don't know if this one will. I guess I can test it when it comes in. A knob would be simple enough - just epoxy a cut off allen head wrench and a knob.

I may still not fully understand the detailed function of a slide valve. Are they spring loaded so they return to one position or manual movement? There are two positions - release air from port 2 to atmosphere (pilot trigger) and flow from port 1 to port 2 for fill, correct?

Also, what is the most common source for one and is size important (ie the QEV is 3/4 but the slide valve can be 1/4)?

Thanks for everyone's help.
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Unread postAuthor: Lockednloaded » Wed Nov 07, 2012 9:43 am

My Palmers reg doesn't bleed off pressure, but it doesn't need to in this case.

The slide valve has a sleeve that has to be manually pushed forward to pressurize your gun, then manually pulled back to fire. They can be spring loaded with some sort of modification, but you won't want that for your project.

From my experience, you'll probably want at least a 1/4" slide valve to pilot a commercial 3/4" QEV. You can usually find good deals on eBay, or you can just order on from McMaster, they're called tank valves on there IIRC
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Unread postAuthor: garybdavis » Wed Nov 07, 2012 10:55 am

Yes, I've found the slide valve in McMaster-Carr and on ebay as well. I think ebay has them in 3/4" which would match the pilot on the QEV. Makes for nice simple plumbing.

Please verify one thing for me. When the slide valve is pulled back to fire the pilot, where does the air dump from the pilot go? Is it to a small hole that vents to the atmosphere? If so, that will work perfectly. In fact, I can just set my reg for 300psi and plug the nipple (with a orifice hole drilled in it) between the slide valve and the QEV. That way the pressure chamber will fill slowly and the user can watch the pressure gauge as it fills up. They can release the slide valve when it gets to the desired pressure. But, this is all dependent on if the slide valve venting to the atmosphere...

Edit: I ordered a 3/4" QEV from ebay for $30/shipping. The description says it vents to the atmosphere. And I ordered a couple of push button 1/8 NPT valves from McMaster-Carr, so I'll have plenty to play around with. :)
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 07, 2012 1:35 pm

garybdavis wrote:Please verify one thing for me. When the slide valve is pulled back to fire the pilot, where does the air dump from the pilot go? Is it to a small hole that vents to the atmosphere? If so, that will work perfectly. In fact, I can just set my reg for 300psi and plug the nipple (with a orifice hole drilled in it) between the slide valve and the QEV. That way the pressure chamber will fill slowly and the user can watch the pressure gauge as it fills up. They can release the slide valve when it gets to the desired pressure. But, this is all dependent on if the slide valve venting to the atmosphere...


Image

Image

The male thread connects to the QEV pilot area.

The air vents to the atmosphere through the set of ports closest to the male thread.
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Unread postAuthor: garybdavis » Wed Nov 07, 2012 2:49 pm

Sweet. Now I understand it. You did those drawings just for me didn't you?

I think I need to modify the design to have the slider directly behind the QEV so there is as little volume of air there as possible. That will make it fire quicker/better? So, I'll mount the pressure gauge on the R side of the QEV instead.

Edit: Make that the A side of the QEV for mounting the pressure guage, unless I wanted to know the barell pressure when firing...
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Last edited by garybdavis on Wed Nov 07, 2012 4:47 pm, edited 1 time in total.

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Unread postAuthor: jackssmirkingrevenge » Wed Nov 07, 2012 3:33 pm

garybdavis wrote:Sweet. Now I understand it. You did those drawings just for me didn't you?


this forum is usually quite helpful to those who ask politely, but we're not *that* friendly ;P

originally posted here: http://www.spudfiles.com/forums/semi-au ... 23626.html

I think I need to modify the design to have the slider directly behind the QEV so there is as little volume of air there as possible. That will make it fire quicker/better? So, I'll mount the pressure gauge on the R side of the QEV instead.


Yup, doing it right.
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Unread postAuthor: garybdavis » Wed Nov 07, 2012 4:44 pm

Enough research. Time to grab the safety glasses and start testing a few ideas... :glasses7:
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Unread postAuthor: garybdavis » Fri Nov 09, 2012 9:28 am

Cobbled up a few parts and did a couple of tests. Can shoot a 220gm t-shirt 80 ft at ~160psi. The pressure chamber is a 3'x3/4" piece of pipe (what I had lying around the shop). The barrel is 2 1/2" EMT x 14" long.

The only thing I've manufactured so far is the barrel. I turned a plug from a 1/8" thick piece of plate on the lathe and welded in a 3/4 metal pipe threaded on one end - screws right into the QEV.

My son and I played with it for quite some time. Now I see the addiction of spud guns. :lol:

Next items to be tested:
Palmer Reg set to 300psi with 48ci HPA
3/4 QEV
Smaller pressure chamber
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Unread postAuthor: garybdavis » Wed Nov 14, 2012 4:11 pm

Got my slide valve and Palmer regulator in. The silder really made a difference in the piloting of the QEV. At 200psi, I'm shooting 120ft or so with a nice arc. I'm not completely happy with the layout, so I may remote mount the tank with a hose.

The barrel is 14"x2 1/2 EMT with a custom welded mount. The pressure chamber is one I welded up just for testing. It's 1.375 ID by 5" long. I'll buy a new piece of pipe for the final version once I'm satisfied with the results. I'm thinking about making two different pressure chambers for it. one that is about this size and good for shooting around 200psi. And one that's about double the volume for shooting longer distances at the same pressure.

A few tweaking questions for you guys:
The 3/4" slider is big so it has a great feel to it (easy to get your and around it). Would a 1/4" slider be more or less efficient?

The slider will leak sometimes if you are using it well above 250psi mark if you don't slide it hard to the fill position. This slider is rated for 250psi and seems well made. Is that normal?

I think I can get around 20 to 30 shots per 48cu in tank. This at 200psi with around 8 to 9 cu in pressure chamber. At 300psi, it seems to be more like 15 shots, but obviously the shirts fly farther. I looking for the sweet spot between efficiency and component reliability. I know the higher pressure I run at the more stress it puts on the QEV. Is it really more efficient to run at a higher pressure and smaller pressure chamber or is 8 to 9 cu in at 200 to 250psi about right? I'm getting enough distance to make me happy.

Last question, does anyone see anything terribly unsafe about the configuration?

Thanks
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 15, 2012 12:41 am

Looking good!

Currently looks like something you would use to lob t-shirts at zombies :D

Video of it in action?

garybdavis wrote:I'm thinking about making two different pressure chambers for it. one that is about this size and good for shooting around 200psi. And one that's about double the volume for shooting longer distances at the same pressure.


Why not keep the same chamber and tweak the pressure?

The 3/4" slider is big so it has a great feel to it (easy to get your and around it). Would a 1/4" slider be more or less efficient?


It depends on the porting, generally moar porting = better piloting = moar power.

However if you have a smaller pilot volume using the smaller slider, this would offset smaller port area.

The slider will leak sometimes if you are using it well above 250psi mark if you don't slide it hard to the fill position. This slider is rated for 250psi and seems well made. Is that normal?


Inspect the o-rings for wear, especially look at the porting to make sure the edges have been properly bevelled. A bit of lube (something that the o-rings won't absorb) goes a long way to keep wear down.

Is it really more efficient to run at a higher pressure and smaller pressure chamber or is 8 to 9 cu in at 200 to 250psi about right? I'm getting enough distance to make me happy.


Instead of looking for higher pressures, it might be worth looking into your projectile - packing makes a big difference, you might want to consider for example packing the t-shirt into a large paper cup in the barrel.

Last question, does anyone see anything terribly unsafe about the configuration?


I don't like the elbow which attaches the tank to the reg. It's unsupported and potentially can have a lot of stress put on it, I would consider an external structure that doesn't make it bear all the load. Otherwise, looks great!
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Unread postAuthor: garybdavis » Thu Nov 15, 2012 9:04 am

jackssmirkingrevenge wrote:I don't like the elbow which attaches the tank to the reg. It's unsupported and potentially can have a lot of stress put on it, I would consider an external structure that doesn't make it bear all the load. Otherwise, looks great!


Yes, the 1/8NPT connections to/from the regulator make for a weak physical connection point. It's good enough for some testing, but not for continuous use. I'm thinking about ordering one of their regulator mounts and a braided hose. When I do that, I'll remove the elbow between the tank and regulator and use a close nipple instead. Only problem is what to mount the regulator to. I suppose I need to create some type of a handle or stock for the whole thing.
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Unread postAuthor: Technician1002 » Thu Nov 15, 2012 10:48 am

Any home welded pressure tanks (the elbow and chamber) should be hydrostatic tested. If you later want greater range, look for a slightly larger valve and chamber. I launch t shirts 200 feet at 100 PSI with a home built 1 inch valve. This put shirts into the stands from cross the field.
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Unread postAuthor: garybdavis » Thu Nov 15, 2012 11:07 am

Technician1002 wrote:Any home welded pressure tanks (the elbow and chamber) should be hydrostatic tested.


I assume any of the places that test scuba tanks will test my home made pressure chamber? What pressure do you have them test it to (I plan on running 300psi max)? The other two I can see are to use threaded pipe and fittings, a small scuba tank like a spare air. or maybe a small CO2 tank that are rated/stamped from the manufacture.

I'm trying to keep the pressure chamber small and run at 200 to 300 psi to keep the overall air consumption down. I guess I need to do the math to verify that's true. In other words, which would give me more shots per 3000psi 48cu in tank; large pressure chamber at 100psi or smaller pressure chamber at 300psi?
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 15, 2012 3:35 pm

garybdavis wrote:I assume any of the places that test scuba tanks will test my home made pressure chamber? What pressure do you have them test it to (I plan on running 300psi max)?


I doubt they would be willing to do that... but surely you could do it yourself, fill it with water or oil and get pumping to 1.5 times your planned operating pressure.
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Unread postAuthor: jrrdw » Thu Nov 15, 2012 4:39 pm

"Spud Cannon" or any version of the term tends to be bad words in a lot of stores, hardware and such. It's why we post WIP threads and take our chances. It's also part of the fun.

Kinda like popping your 1st wheelie on your bike... :)
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