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A stay open piston valve concept.

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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A stay open piston valve concept.

Unread postAuthor: boyntonstu » Mon Oct 26, 2009 10:33 am

Does it pay off to consider a piston valve that flies away from the seal, hits the bumper, and is retained until the next pressurization?

A tapered plug or a mechanical latch comes to mind.

IOW If locked, the piston could not bounce back towards the seal and slow or stop the flow.


Also, the pilot exhaust could be sealed and it may save power.

BoyntonStu
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Unread postAuthor: qwerty » Mon Oct 26, 2009 10:47 am

Ive never had a problem with pistons bouncing back and i think you will find that not many people have had this. Also about the piston sealing back on the housing: Theres no need. It would only save a miniscule amount of power for the effort put in. And also QEVs dont bounce back and also seal on the back of the housing when fired.
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 26, 2009 10:50 am

Some thoughts on how piston bounce may affect power with lightweight projectiles here.
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Unread postAuthor: Ragnarok » Mon Oct 26, 2009 11:23 am

Actually, HEAL already does this, sealing the piston back against the rear of the valve - for both of the reasons you mention, avoiding piston bounce and to try and minimise any air flow back through the pilot.

The latter is unlikely, as the valve is already built with a check valve, but as what I did (basically, a stack of rubber washers) acts as a bumper, the small effort applied so that these were airtight and it could seal back against them would have been stupid not to spend.
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Re: A stay open piston valve concept.

Unread postAuthor: Technician1002 » Mon Oct 26, 2009 11:36 am

boyntonstu wrote:Does it pay off to consider a piston valve that flies away from the seal, hits the bumper, and is retained until the next pressurization?

A tapered plug or a mechanical latch comes to mind.

IOW If locked, the piston could not bounce back towards the seal and slow or stop the flow.


Also, the pilot exhaust could be sealed and it may save power.

BoyntonStu


When barrel sealer pistons open the area on the piston face becomes exposed to the chamber pressure. Chamber pressure rapidly vents out the barrel. Piston recoil is a problem only if these two items are present;

1, a really bouncy bumper. Key learning, use an impact energy absorbing bumper. Test your bumper choices by hitting them with a hammer and checking for bounce. Some materials work much better at absorbing impact. Materials used to deaden road noise in cars and in homes are excellent choices. Carpet padding, felt, etc are excellent choices.

2 A large diameter ratio between the valve seat and piston OD. A piston that opens while the pilot contains a fairly high pressure will suffer if the chamber pressure drops faster than the pilot area pressure drops. This pressure can re close the piston early. A close diameter ratio ensures the piston remains closed until most of the pilot pressure is already bled off. Too small of a pilot volume and flow restrictions between the chamber and piston make this problem worse.

With little piston bounce and little pilot area pressure when the piston pops open, the piston should be slammed hard into the bumper to remain there until most of the chamber and barrel pressure is gone, long after the projectile has left. There should be no need to trap the piston in the open position.

Below is a sound wave of a piston cannon discharge with no projectile. The waveform shows no sign of the air flow stopping early from a piston bounce.
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Unread postAuthor: psycix » Mon Oct 26, 2009 5:05 pm

Most pistons that bounce back actually bounce after the projectile leaves the barrel.
When all air rushes past the piston a vacuum may be created sucking (actually not-pushing resulting into pushing from the other side not to be neutralized anymore) the piston forward.
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Unread postAuthor: AngryChauncey » Tue Oct 27, 2009 12:41 am

Yeah I don't know if you would really notice the difference in performance, but it would look sick if you took it apart to show someone. By the way Technician that sound wave (audacity? great program...) is a really interesting way of seeing how the air flow behaves. I might just have to try that. What kind of mic did you use to record that? [/quote]
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Unread postAuthor: Ragnarok » Tue Oct 27, 2009 7:26 pm

I doubt that it results in a particularly dramatic performance increase. What does happen will depend on the projectile, and unfortunately, I don't have any performance data on how much HEAL's set-up improves things.

Still, at worst, it's not going to harm the performance, and it also means the parts around the rear of the piston only have to take one impact from the piston each shot, which should improve their lifetime.

Potential for performance improvement, no particular downsides and a longer lifetime. If you can do it simply and cheaply, why not? I can tell you as much as HEAL 2 will still be built like that.

EDIT: Error in one sentence.
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Last edited by Ragnarok on Wed Oct 28, 2009 7:11 am, edited 1 time in total.
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Unread postAuthor: Technician1002 » Tue Oct 27, 2009 7:37 pm

AngryChauncey wrote:Yeah I don't know if you would really notice the difference in performance, but it would look sick if you took it apart to show someone. By the way Technician that sound wave (audacity? great program...) is a really interesting way of seeing how the air flow behaves. I might just have to try that. What kind of mic did you use to record that?


The Mic is a studio dynamic mic that is no longer manufactured. It is a Yamaha Be100. Any good dynamic mic should work well. Avoid the condenser microphones. Many of them clip at high sound pressure levels. To prevent over recording and clipping, I used a mixing board and dialed in a lot of attenuation. I recorded near the breech so the pressure wave did not overwhelm the recording while providing good sensitivity to the hiss.

In short, it was recorded with a recording studio setup, not just a mic plugged into a soundcard.

I second the Audacity plug. Great program.
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