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Idea for QDV Valve

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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Idea for QDV Valve

Unread postAuthor: EricT » Wed Jun 06, 2012 11:06 am

I'm making a PVC QDV in a 2" T, 2 x 2"-1.25" adapters, 1.25" ported pipe, and a 1" coupler for a piston. I found that the 1" coupler with two plugs on the ends makes a perfect piston when the plugs are not completely inserted. This leaves a gap on the ends just the right size for loose O-rings.

Here's the issue I'm trying to figure out. I don't like the idea of putting my hand behind the bumper and pulling a rod to actuate the valve. The rod is simple but I have another idea. A vacuum could be created behind the piston to suck the piston rearwards and thus actuate it. This might have a few advantages. No friction from rod on piston, even lower back pressure in pilot area, and remote actuation. This is simple enough except I'd like to use an air cushion bumper instead of beating the crap out of foam or other material. To accomplish this I've thought of creating a sort of limited-check valve. The valve would be situated at the rear of the pilot/bumper area. The valve would normally be open and allow a small amount of air to to escape as a vacuum actuates the valve, but when the piston starts to accelerate back the pressure differential would cross a threshold and the valve would close, creating a sealed air cushion.

Sound like a good idea? Any ideas for constructing it?
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Unread postAuthor: samiam0295 » Wed Jun 06, 2012 12:00 pm

Your idea is more or less identical to the operation of barrel sealing piston valves normally operate. There is a pilot volume behind the piston that is exhausted, creating low pressure behind the piston that allows the pressure in the chamber to rapidly push the piston back. Tons of designs out there for that principal.
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Unread postAuthor: EricT » Wed Jun 06, 2012 12:13 pm

Yes, except it's a 1:1 ratio, atmospheric instead of chamber pressure pushes the piston back, pilot vacuum instead of back pressure, and the air cushion bumper. My main concern is with the air cushion and the possible limited-check valve. If it stays open too long the piston will slam into the rear cap. If it closes too easily the piston won't be actuated by the vacuum. I've looked around for others using air cushions and have only seen one design.
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Unread postAuthor: ramses » Wed Jun 06, 2012 12:31 pm

The issue you will have is that by drawing air out of pilot area, you reduce the capacity of the air spring to function. Also, any check valve you use will add to the "dead volume" of the pilot (in the sense that it is a pump)

Honestly, I would use a mechanical arrangement to pull a rod, and simply make sure that the rod is a good fit in its exit hole and in its hole in the piston. Friction with the rod is negligible. Even if you add the rod's mass to the piston mass, it is likely to be about as much as your projectile. Your projectile will move only (eg) 2" for the piston to open up 2".

In either case, the force on the piston changes exponentially with displacement. At 1/2 travel, you will have about 15 lb/in^2. At 3/4 travel, you will only have about 60 PSI. But at 1/32 travel, you will have 480 PSI.
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Re: Idea for QDV Valve

Unread postAuthor: mattyzip77 » Wed Jun 06, 2012 12:41 pm

EricT wrote:I'm making a PVC QDV in a 2" T, 2 x 2"-1.25" adapters, 1.25" ported pipe, and a 1" coupler for a piston. I found that the 1" coupler with two plugs on the ends makes a perfect piston when the plugs are not completely inserted. This leaves a gap on the ends just the right size for loose O-rings.


I actually tried this before with a 1 1/4 coupler with plugs and it did not work. Hope you have better luck than me. As soon as you pressurize it the o rings fall off!! For a 2 inch tee, a 2 inch rubber sanding drum is the best piston you can get without having to use a lathe. I say this all the time, and no one listens to me it seems!!!
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Unread postAuthor: EricT » Wed Jun 06, 2012 1:36 pm

Thanks for the feedback guys. Good food for thought. If I were to abandon the air bumper idea and keep the vacuum actuation I wonder what type of bumper would stand up to the forces. I've considered standard closed cell foam, memory foam, pipe foam, and cloth.

How thick were your O-rings? I'm using 1/8". Seems pretty tight so far. Guess we'll see.
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Unread postAuthor: POLAND_SPUD » Wed Jun 06, 2012 2:57 pm

duh just use an air cylinder to open the qdv... ->> problem solved
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Unread postAuthor: shardbearer » Wed Jun 06, 2012 11:41 pm

I think that the simplicity of mechanical activation and only one valve is one of the big benefits of a QDV, it's what convinced me to make one. If you are going to add a pilot valve, it just adds complexity without many benefits, especially with the vacuum system, check valves, and air cushion. More complexity is not only harder to make, but leads to more points of failure. What happens if your air cushion doesn't work? Do you have a vacuum pump? What will you use as a pilot? If you try to use a higher vacuum to make your valve open faster, your bumper will be less effective or even nonexistent. This will require a decent amount of math and adjustment of the bumper for different vacuum pressures, and you have to get it right the first time or you'll blow out the back of your tee.

My very similar QDV cannon is a 1 1/2" tee (you don't need a 2" one) with a 1 1/4" pipe running through it, piston is a piece of 3/4" pipe with rings cut from endcaps to hold O rings. I found that 1 3/8" OD 1/8" thick work well. By the way, 1" pipe and 3/4" couplers/endcaps will fit perfectly inside 1 1/4" pipe, but 1" couplers are way too big. All I have left to do is go buy another O ring (I lost one of mine and only bought 2), and drill and thread my tank to attach the quick disconnect.

I decided to start simple and work my way up, something I learned from programming. Make one system, perfect it, and then add the other in, making them separately first if at all possible. If you make your whole cannon without testing any part of it first, troubleshooting will be much harder. I am starting with just a string trigger, and working my way up to the rod with shaft collars.

Also, try to keep access to the chamber side of your valve even when it's done. The bevel on the inside of the ports is surprisingly important to the O-rings.
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