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Piston valve specs and optimization?

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: Technician1002 » Fri Jul 31, 2009 1:19 pm

pvcmaster wrote:
Floating designs offer a number of advantages, including greatly reduced breakout friction and longer seal life. Floating pneumatic piston seals are suited for applications in which the air pressure does not exceed 200 psi (or in hydraulic designs where a small amount of leakage is permissible). Floating O-rings are NOT suitable as rod seals. http://www.rlhudson.com/O-Ring%20Book/designing-dynamic.html


I found this while researching Technician, i hope it helps.

I can move the bumper to the valve itself instead of the piston, but i am concerned about how to keep it in place, or if a rubber washer or two is even sufficient to cushion the impact.


Very nice.. Thank you. When the piston moves in the QDV and when first filling it does have some leakage. It was accepted as a trade off for low friction.

Their drawing is soo much better than my crude drawing.. Very nice find.

Edit.. Update.. I received permission to use the picture.. Thanks guys..

Click on the photo to see it full size.

Please just credit us as shown below:

Source: RL Hudson O-Ring Design & Materials Guide. Used with permission.
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Floating O rings..
Source: RL Hudson O-Ring Design & Materials Guide. Used with permission.
Last edited by Technician1002 on Fri Jul 31, 2009 5:41 pm, edited 2 times in total.
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Unread postAuthor: randompkguy » Fri Jul 31, 2009 5:35 pm

@pvcmaster: good catch

http://www.rlhudson.com/O-Ring Book/Gland Dimensions/table51.html
that's the table I have used for all my piston designs

Thanks for trying to answer my questions btw. I'm not sure how powerful the design will be seeing that the force on a 1/2 inch projectile at 1000 psi is only 196 pounds, while the force on a 2 inch projectile fired at only 100 psi is 314 or so psi.

I know that if you take a chamber and shrink it to 1/10th its original size you will have ten times the pressure with the same amount of air as the original chamber, therefore you should get a 1000% power increase (ten times) but as I said, you still only get 196 pounds on a 1/2 inch projectile, so it seems high pressures wont give that much of an fps advantage at such small barrel diameters...
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Last edited by randompkguy on Fri Jul 31, 2009 7:27 pm, edited 1 time in total.

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Unread postAuthor: randompkguy » Fri Jul 31, 2009 6:36 pm

I can move the bumper to the valve itself instead of the piston, but i am concerned about how to keep it in place, or if a rubber washer or two is even sufficient to cushion the impact.


On my 2" piston cannon, I epoxied a 3/8" rubber disk to the endcap and have used the cannon at up to 120 psi so far with no problems. I use the same rubber for my 3" porting cannon which is in a 4" tee as well and it's still in great condition, so you should be fine with a few washers.
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Unread postAuthor: Technician1002 » Fri Jul 31, 2009 7:33 pm

randompkguy wrote:@pvcmaster: good catch

http://www.rlhudson.com/O-Ring Book/Gland Dimensions/table51.html
that's the table I have used for all my piston designs

Thanks for trying to answer my questions btw. I'm not sure how powerful the design will be seeing that the force on a 1/2 inch projectile at 1000 psi is only 196 pounds, while the force on a 2 inch projectile fired at only 100 psi is 314 or so psi.

I know that if you take a sealed chamber and shrink it to 1/10th its original size you will have ten times the pressure with the same amount of air as the original chamber, therefore you should get a 1000% power increase (ten times) but as I said, you still only get 196 pounds on a 1/2 inch projectile, so it seems high pressures wont give that much of an fps advantage at such small barrel diameters...


The problem is very well stated.. Thanks for bringing this up as it is often a poorly understood subject.

Force, mass, and area and acceleration and how they are interrelated.

It is time for long drawn out discussion.

First the relationship to the projectile force from pressure. That is simple to understand. Bigger is more area is more force. Looking at smaller is less area is less force. That part is well understood.

Let's cut a few projectiles out of a sheet of 1 inch steel for example and look at the force/pressure/mass/acceleration relationships.

Let's do a 1/2 inch, a 1 inch and a 2 inch steel slug 1 inch long.. The area of each varies over a long range. However if you apply 100 PSI to all three of them, they will all accelerate at the exact same rate.

As the area increased in each case, the mass increased by the same amount.

Now do the above with only 1/2 inch steel plate.. Now each slug will have half the mass. Each slug will again accelerate at the same rate regardless of the diameter, but the 1/2 inch long slugs will out accelerate the 1 inch long ones as they are half the mass for the same respective areas.

The correct way to find the acceleration is not by being disappointed that you only get 196 lbs of push. That is only part of the story. 1 96 lbs of push on an object 8 times lighter will be very impressive because it will have more force per square inch than the 2 inch object at lower pressure..

I hope this makes sense.

The above example ignores the real world flow, friction, and other factors. So sue me.. :D This is a concept of physics only.
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Unread postAuthor: randompkguy » Fri Jul 31, 2009 9:56 pm

thx thx thx :D its been a while since i took physics....
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Unread postAuthor: pvcmaster » Mon Jan 25, 2010 8:48 pm

Okay, so it is finally finished! (Sorry to bring back this thread) I took everything i've read here and technicians advice, some math, etc.. to finally design what i think is my best barrel sealer. It is ridiculously loud with that QEV piloting it, and totally lived up to my performance expectations. I'll probably start branching out into some of my own pneumatic designs now that i've got the basics. Thanks for all your help everyone, here are some pictures:
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Unread postAuthor: deathbyDWV » Mon Jan 25, 2010 9:22 pm

Looks good. Got any damage pics or video?
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Unread postAuthor: pvcmaster » Mon Jan 25, 2010 9:34 pm

not yet, i'll add them soon along with pics of the internals :)

btw your user name is hilarious :lol:
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Unread postAuthor: Gun Freak » Mon Jan 25, 2010 9:47 pm

that gun is beast
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 26, 2010 2:44 am

That QEV alone would make a fearsome valve, let alone as a pilot! Looking forward to the damage :D
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Unread postAuthor: jrrdw » Tue Jan 26, 2010 8:47 am

Looking good, I hope a video is in our near future. Didn't you have a gulf ball barrel planned for this?
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