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3" Piston Problems

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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3" Piston Problems

Unread postAuthor: fireman565 » Thu Feb 14, 2008 10:53 pm

OK.. working on my fourth spun gun. I've made two combustion (simple), and one pneumatic involving an Rainbird anti-siphon between a 4'x2" barrel and a 4"x2' chamber, (also simple in design).

This time around I'm working on 3" barrel-sealing valve, 2"x4' barrel with a 4"x2' chamber using am Orbit 1" valve as a pilot. During planning and experimenting I was able to finally start gluing up the 3" valve. I actually got it to work up to about 60 psi and the leaks took over. No problem, right? Stop the leaks. Wrong. My leaks were first around the screws holding in the threaded bushing. Once I got the leaks there stopped, (first problem) I noticed air moving through the Orbit valve while charging. Not a massive amount, but enough for it to drain down in a matter of seconds. Next problem, I had the dreaded 'honking'. But the honking was not from the Orbit valve, but the piston. Has far as I could tell the piston was sealing off, releasing, bouncing off the bumper, and traveling about half way back to the barrel. On the next firing, it wouldn't seal, instead 'honk'. Not much of a problem because to stop the honking, I would just run the ramrod down far enough to push the piston all the way back, pressurize the gun, and the piston would seal. But still annoying.

In an attempt to remedy these minor problems, I noticed I had the pilot valve backwards, duh. Changed the Orbit valve around. Now things have gone to crap...lol

Upon charging the gun, the piston seals, but air is blowing out the pilot (Orbit) valve at a pretty good rate, but still holding some pressure. I have modded it with a blowgun, plugged the solenoid hole, disassembled and cleaned, and pulled the small, white plastic piece from the diagphram to enlarge the equilization hole. It seems that when I charge the gun, the pressure overwhems the Orbit valve and forces the valve open, hence the reason I pulled the small white plastic piece. Now it does build some pressure, and when released, it all exits the valve now instead of the barrel. The piston is sealing, its now not releasing. Its actually staying sealed even after 'firing' the gun. Weird thing is the gun actually worked with the Orbit backwards with minor air bypassing the diagphram. With it turned correctly, its a major bypass!..

I understand the concepts involved in this type gun. The piston was at one point releasing, at one point the pilot valve was equalizing and working. All this working, with the pilot valve backwards...lol up to about 60 psi because of the leaks. Trying to correct the leaks, turning the valve around in the right direction, its now not working....lol Go figure. I even tried to put everything back like it was, but it won't work.

Just testing the gun at first, it shot a 2" section of 1-1/2" pipe duct taped solid all but through a 55 gal. drum. The potential for the gun is there, I've just misplaced it somewhere.

Sorry so long.

EDIT: I have read the many tutorials and how-to's, it just seems that this design is a little different than normal.

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Last edited by fireman565 on Tue Feb 19, 2008 12:18 am, edited 2 times in total.

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Unread postAuthor: LikimysCrotchus5 » Thu Feb 14, 2008 11:59 pm

Very nice for a fourth build.

I would say to replace the sprinkler valve with a 3/4 inch QEV, which i think will be more reliable.

Your piston seetup is also somewhat peculiar.

I have never heard of a piston valve honking so im not sure how to help on that.
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Unread postAuthor: fireman565 » Fri Feb 15, 2008 12:03 am

Yeah.. Out of everything I have read, I couldn't ever find anything on the piston honking. I know it was, or was the reason it was honking though. I could 'reset' the piston by pushing it back to the bumper, pressurize the gun, and it would work. But, after firing, some of the time, I had to reset the piston or it would sound like an Asian elephant in heat!
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Unread postAuthor: clide » Fri Feb 15, 2008 12:18 am

If your sprinkler valved worked backwards then the only thing I can think of is that you have the same problem as this guy: http://www.spudfiles.com/forums/please- ... 13119.html
Do you have a hole through the center of your diaphragm?

As for the piston valve, is it honking when you are trying to fill it or when it is fired?
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Unread postAuthor: fireman565 » Fri Feb 15, 2008 12:30 am

Do you have a hole through the center of your diaphragm?

I honestly don't remember a hole in the middle. Its the same valve. The very center of his valve looks 'modded' for some reason, or either I'm just seeing the 'epoxy' material he used to plug the center hole.

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As for the piston valve, is it honking when you are trying to fill it or when it is fired?

It's honking when pressurizing or filling the gun. From what I can tell, its the seal on the piston that somehow rebounds off the bumper from the previous firing and stops at just the right spot that when trying to refill the chamber, it doesn't seal, and the 'seal' actually acts as a reed on a horn...lol As soon as I move the piston back a tad, it will seal on the 'barrel' end and pressurize.
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Unread postAuthor: clide » Fri Feb 15, 2008 12:52 am

Ah, I think I see your problem with the sprinkler. Is that brass fitting the only thing plugging the solenoid hole? The inside of the solenoid has two holes, one leading to the top of the diaphragm and one leading to the valve outlet, so if you just capped off where the solenoid was then air would still be able to flow between those two holes. You need to either fill that area up with an epoxy or you could just plug the small hole leading to the valve outlet and continue to use that cap.

Have you tried firing the valve with a projectile, or even just a barrel on it? Having some pressure able to build up downstream of the valve will help it open better and it may not honk under normal firing circumstances.
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Unread postAuthor: fireman565 » Fri Feb 15, 2008 1:16 am

Ah, I think I see your problem with the sprinkler. Is that brass fitting the only thing plugging the solenoid hole?

Flare brass fittng, pretty pink tape, that's it. Thanks Clide, that may be the problem, though, there's alot of air moving through the valve while filling. I am going to epoxy it up though.

Have you tried firing the valve with a projectile, or even just a barrel on it?

LOL.. Yeah.. During my test-and-tune time, when I had the valve backwards, at about 50 psi, it all but punched a hole through a 55 gal. drum with a piece of 1.5" pvc duct taped into a projectile. With the valve in the correct position, the piston seals. The thing is, once the air is released, the piston is still sealed, as if pressure is still remaining. I have to push the piston off the 'barrel' side and you can literally hear the remaining pressure release. But this is with the pilot (Orbit) valve pointing in the correct position, not backwards...lol
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Unread postAuthor: dongfang » Sat Feb 16, 2008 5:56 pm

Hi,

That's the most beautiful diagram I have ever seen from a spudder... wow!
Very nice and clean construction, too.

If it were my gun, I'd be pretty proud of it. And I'd further:

- Try to troubleshoot it by eliminating the sprinkler valve for a moment. Instead, put in a ball valve, a threaded or rubber plug or just about anything else. Does the valve fire when you remove it?

- I see no pressurized frontal area on your piston. You probably know that it's the high pressure present in some small part of the front of the piston that makes it slide back and open the valve - when the high pressure behind it it eliminated, by opening the pilot valve. Where is that frontal area in your design? Usually one will make the piston a little larger than the diameter of the sealing surface, just for that purpose.

--

I am testing a valve these days (the large one of the 2 in my profile pic, in fact), with just a few mm of difference between piston diameter and sealing face diameter. That means, the frontal area that I'm talking about is quite small. It still opens pretty fast. The explanation is: Once it has opened a little, the back pressure from the projectile also helps...

Regards
Soren
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Unread postAuthor: DYI » Sat Feb 16, 2008 8:25 pm

I may just be tired and stupid, but from your diagram I can't see the pressurised area on the front of the piston. But you said you know how barrel sealing valves work, so there must be some area exposed to pressure on the front of the piston...
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Unread postAuthor: fireman565 » Sun Feb 17, 2008 12:04 am

Clide had part of the problem figured out. I pulled the brass plug, an epoxied the solenoid hole. That helped the pilot valve seal.

I also got rid of the 1" tee valve that the gauge and male quick connect were mounted. I tapped the piston valve and threaded the gauge and male quick connect into it just behind the piston. Trying to eliminate some pilot volume.

The main problem why it wasn't working....
I basically back-tracked my problem to the pilot valve wasn't releasing air quick enough. The only thing different I had done to the pilot valve was I re-threaded the blowgun into the top. I finally figured out that the threads were not letting the diaphragm open up all the way. I took off a couple of threads and I also sanded down the white plastic center piece of the actual diaphragm. How much? Down to where it was nearly level with the black rubber of the diaphragm.

Now its sealing faster and working great.

As far as the pressurized frontal area on the piston...
I must of missed that somewhere. I didn't take that into consideration for lack of knowing. But... it has some somewhere because the gun is scary now. The noise is something I didn't think would be so great. Now to pull it apart one more time and make sure everything is sealed up.

Thanks for the help guys.

EDIT: I went back and added basically a 2" section inside of the TEE from the barrel side to add a little more frontal pressure. I got to noticing that when fired, the piston wasn't moving back hardly at all, even though at 40 psi it launched a potato 150 yds. It seemed the seal was actually trying to blow out the barrel after fired. I'm hoping this is a result of not enough frontal pressure.
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Unread postAuthor: dongfang » Tue Mar 04, 2008 11:07 am

Hi,

Nice that u got it working.

If it opens at all, just a little, then the back pressure of the projectile on the front of the piston will actually help it open even wider... so these valves don't need a very large pressurized frontal area. But they do need a little.

I wonder if a weak burst disk - a plastic bag or someting between the piston and the projectile - could be useful for diagnosing valves that just won't work.

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