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Valve type...Piston, Shuttle or QEV?

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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Valve type...Piston, Shuttle or QEV?

Unread postAuthor: velocity3x » Wed Jul 01, 2009 8:56 pm

It seems to me that people have different ideas on the nomenclature valves should carry. As a result, I'm confused as to what people look for in a valve in order to categorize it as Shuttle, Piston or QEV. This is a barrel sealing valve I make and to me....it fits in all categories.

My question is: In you opinion, what category does this valve fall in and why?

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Unread postAuthor: Hotwired » Wed Jul 01, 2009 9:38 pm

QEV = commercial dump valve for pneumatic cylinders

Also, what is rather unique to QEV valves is that they have flexible diaphragm/rubber shuttle type seals, NOT massive (as in high mass) pistons.

Shuttle valve... if you really wanted to call it that but very much worse flow if the inlet was from left side than right.

Many QEVs can operate as shuttle valves.


What I would call it is a questionably functional barrel sealer. I'm wondering what surface area you're thinking will push back the piston.


Good drawings mind you ^^
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Unread postAuthor: mobile chernobyl » Wed Jul 01, 2009 9:47 pm

Barrel sealing, very similar to a regular QEV actually. In the spudgun world it would be "barrel sealing" because it seals off the barrel, or outlet, and not the chamber, or pressure vessel. Either way the pilot is the same.

Do you make your o-ring grooves with a radius like pictured, or do you just have them with a squarish profile? Either way cool drawing, looks like SolidWorks? I love that program lol.

Oh yea how do you get the middle (where the inlet comes in, and the conical section of the piston rests) section of the large body with a larger ID? I'd assume you just use a boring bar? That's always a tricky machining maneuver... But I guess since it's not used as a sealing face the surface doesn't need to be perfect.
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Unread postAuthor: velocity3x » Wed Jul 01, 2009 9:59 pm

mobile chernobyl wrote:Barrel sealing, very similar to a regular QEV actually. In the spudgun world it would be "barrel sealing" because it seals off the barrel, or outlet, and not the chamber, or pressure vessel. Either way the pilot is the same.

Do you make your o-ring grooves with a radius like pictured, or do you just have them with a squarish profile? Either way cool drawing, looks like SolidWorks? I love that program lol.

Oh yea how do you get the middle (where the inlet comes in, and the conical section of the piston rests) section of the large body with a larger ID? I'd assume you just use a boring bar? That's always a tricky machining maneuver... But I guess since it's not used as a sealing face the surface doesn't need to be perfect.


I cut ALL O-ring seats with a .125 ball end mill.

Hotwired..."What I would call it is a questionably functional barrel sealer. I'm wondering what surface area you're thinking will push back the piston. "

Although I appreciate your opinion, here are the facts on the valve:
Charged at 300 psi, it will stand for over 1 hour without loosing a pound of pressure.
Using 300psi co2, a GB is still climbing out when it crosses the ridge line 1200 yard down range.
I hope it'll work better when I get it beyond that "Questionably Functional" stage.
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Last edited by velocity3x on Tue Nov 24, 2009 9:35 am, edited 1 time in total.
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Unread postAuthor: Hotwired » Wed Jul 01, 2009 11:21 pm

Fair enough, the surface area available to crack open the valve - the edge of the o-ring didn't seem sufficient.

Consider the rest of that sentence void but still sticking with the barrel sealer name though :P
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Unread postAuthor: Technician1002 » Wed Jul 01, 2009 11:30 pm

It is a barrel sealer. The face of the piston faces and seals against the barrel, not the chamber. That is the difference.

The chamber sealer has high pressure in the pilot to hold the piston against the chamber in front of the piston and has low pressure (barrel) off the side of the piston.

A barrel sealer has high pressure on the side of the piston and pilot area to hold the piston against the low pressure area of the barrel.

This is the fundamental differences in configuration.

Source air on side. piston seals against the barrel.. It's a barrel sealer.
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Unread postAuthor: POLAND_SPUD » Thu Jul 02, 2009 1:04 am

its quite cool...
but the ballvalve ruins it... jesus... a gun like this and you have a simple BV as a pilot valve


why there is so much pilot volume ? was it intentional ? if yes then what purpose it serves ?
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Unread postAuthor: spot » Thu Jul 02, 2009 4:19 am

I think there's not much pilot volume at all, and he uses the spring loaded ballvalve to dump it as fast as possible. If I recall correctly, his valve didn't actuate very fast when he was using a smaller pilot valve. He needs that much pilot volume for it to not constrict flow.
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Unread postAuthor: Hotwired » Thu Jul 02, 2009 7:57 am

What's wrong with a ball valve pilot?

I mean yes you can argue that rotating a ball valve is a PITA on a handheld cannon but this one is triggered.
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Unread postAuthor: POLAND_SPUD » Thu Jul 02, 2009 9:04 am

oh well... I am jealous.. that's it :D
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Unread postAuthor: velocity3x » Thu Jul 02, 2009 11:16 am

POLAND_SPUD Posted:
why there is so much pilot volume ? was it intentional ? if yes then what purpose it serves ?

Yes...it was intentional.
That's the volume of pilot air displaced by the piston in order to fully retract. I don't believe in the 2/d or 4/d or whatever rule. I think a piston should get the he*l out of the way.....completely.

spot wrote:I think there's not much pilot volume at all, and he uses the spring loaded ballvalve to dump it as fast as possible. If I recall correctly, his valve didn't actuate very fast when he was using a smaller pilot valve. He needs that much pilot volume for it to not constrict flow.

Spot,
You are correct. There's only 6.0 in3 of pilot air volume (holding back 14,000 in3 of compressed volume). Original, I did need the fast pilot air release to break a vacuum forming at the piston face seal. After doing some corrective machining (larger bore around the forward piston body), fast pilot air release is no longer necessary as the piston can now be released with not much more than a thought. Now I keep the trigger and linkage because I like the mechanical appearance.
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HissBoom of proper piston valve

Unread postAuthor: Technician1002 » Thu Jul 02, 2009 11:38 am

spot wrote:I think there's not much pilot volume at all, and he uses the spring loaded ballvalve to dump it as fast as possible. If I recall correctly, his valve didn't actuate very fast when he was using a smaller pilot valve. He needs that much pilot volume for it to not constrict flow.


This is a common misconception. It ignores the fact that;
1 air is compressible.
2 when a narrow ratio valve (diameter to seat) is used, the valve remains closed until the pilot pressure is very low.

As a result, a slow pressure drop due to a huge pilot space only creates a delayed opening, but when it does open, the air pressure rise between the projectile and piston face does indeed get it the he*l out of the way. Even the mass of the air in the barrel and the flow resistance is enough to make them pop open.

I understood the theory and had to test it myself. It is the primary reason the Mouse Musket was built. The Mouse Musket does use a small pilot valve. It does have a delay from the trigger to the main valve opening, but that main pop is impressive. This shot is a dry fire with no projectile. Opening times are faster with a projectile present.

Picture added of the Mouse Musket sound waveform at 30 PSI with small pilot ball valve. Audacity, a free audio editor was used to view the waveform. http://audacity.sourceforge.net/

In conclusion, there is nothing wrong with using a ball valve on a proper narrow ratio piston valve.

EDIT I almost forgot the most important part.
:wav:
Well Done velocity3x
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Attachments
HissBoom.jpg
Sound waveform of the Mouse Musket. The hiss portion is with the piston closed while the pilot pressure bleeds off. The slow rise in hiss it the manual ball valve being opened. I was in no hurry for the test.
Pow.jpg
Crack to full open time is very short.
Crack.jpg
Valve cracking open expanded with the time line above. Flat bottom is due to the sound system clipping. Note the duration of the main wavefront is only about 3 mS long. In contrast the Hiss is 200 mS long.
Crack.jpg (27.67 KiB) Viewed 550 times
hissboom.mp3.zip
Zipped hiss boom of the 30 PSI Mouse Musket for your analysis.
(15.65 KiB) Downloaded 33 times
Last edited by Technician1002 on Thu Jul 02, 2009 1:03 pm, edited 1 time in total.
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Unread postAuthor: velocity3x » Thu Jul 02, 2009 1:02 pm

Tech.....Even the mass of the air in the barrel and the flow resistance is enough to make them pop open.


Exactly....and pop open with explosive force!
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