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Piston valve trouble

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: saefroch » Thu Apr 01, 2010 9:05 am

Ah do you know how a piston valve works? It has absolutely nothing at all to do with a vacuum. Difference in area is all that it is.


I'm looking at one right now, and I know how they work, or at least the one that I made.

From your leak description it sounds to me like your piston is refusing to seal, then refusing to slide back. That's an odd combination for me, since in my amateur experience with tacky piston valves I've never encountered such a combination.
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Unread postAuthor: Crna Legija » Thu Apr 01, 2010 9:42 am

so the one you made do you suck the piston back?
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Unread postAuthor: saefroch » Thu Apr 01, 2010 10:04 am

It's not an inline, made from galvanized schedule 40 steel plumbing. The way it's designed, if you don't open the pilot valve quickly enough, it just empties the chamber until it's almost empty, then the piston shoots back and you get a really whimpy discharge, that happens at about 20 psi. If you open the ball valve sluggishly, you get a pretty weak shot. So yeah, the pressure change from the piston valve has to be sudden enough to generate enough pressure to draw back the piston. At least I think that's how it works.
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Unread postAuthor: saefroch » Thu Apr 01, 2010 12:51 pm

Okay, this is going to sound dumb and complicate the problem.

I did the exact same things as gunfreak, built a piston made out of fasteners, with a neoprene washer as the sealing head, and I KNOW that my gun works with my hot glue piston. It DOESN'T work now. I have EXACTLY the same issues he has. Creepy is the only word I can find. I'm gong to troubleshoot this some now, but I can say for certain that it is not his gun or firing mechanism that is the issue, purely the piston. I pump it up, bit of leakage down the barrel, but I can get it up to like 60 psi, then I discharge and it all just goes out the barrel. Gonna play around with it, open up the gun a few times, try to troubleshoot, and post any results.

This is a piston issue, and I'm working on fixing it. I will post again when I find the issue or fix it.
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Unread postAuthor: Gun Freak » Thu Apr 01, 2010 12:55 pm

Yeah my issue is definitely the piston, not anything else. My gun is PERFECTLY constructed. Only the piston... I guess not so much.
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Unread postAuthor: saefroch » Thu Apr 01, 2010 2:10 pm

EDIT: Don't Double Post /facepalm And I even read the forum rules recently.

I think i've got it fixed. That didn't take all that long at all, I built one from one trip to sears. The biggest issue is getting the neoprene washer in the front from getting pulled off past the nut. You'd need to find a washer that has a much smaller hole in it, what I suggest is buying a washer of the next side down, shoving that forcefully on in front of that, then putting the nut on, in front of small neoprene, big neoprene which would actually make the seal, then metal washer, then nut. That would be the front of the piston. I'll post pics once I get this sorted.

I'm pretty well done working on the problem that is now ours, I might come back to this later, but it's pretty well got me stumped. I've worked on this about half an hour, made some serious progress, but not really gotten anywhere. I made a piston that I think is very much similar to yours, Gun Freak. My piston seals to the sealing face at about 70 psi, and from there up to 170 psi it functions perfectly well. But when I try to fire, it just discharges all the air out the pilot. The piston looks to be slid back, to the open position just fine, except when I pull it out, it becomes obvious that... okay, typing this out, I think I just thought of a solution, I'm just going to edit this post from now on until I give up or fix it.

Image
Image
Image

Good as my work, I'm editing this instead of posting another:

It works. Amazing. Incredible. I now have a real piston that I can use on a piston hybrid. It works very well, but I'm not sure if it's as good as my hot glue-based one, and i'm not about to try to find out.

The fix: Put another nut on the back end, and tighten that as best you possibly can against the nut next to it. This will prevent it from coming apart. If you have enough nuts (hahaha) double up on all of them. The wacky black stuff that's my piston head is the inner tube I got my schrader from. It works very well, and this piston seals at 70 psi and works now. I'm going to have to do some shots with this.

One more thing: The inner tube is replaceable if you can force a smaller neoprene washer, put that in front of a larger neoprene washer (front refers to the barrel side), and tighten those in place with nuts, those two washers can serve as a piston head. You might be able to put a small metal washer in place of the small neoprene washer, but I know that doesn't work for me, the small metal washer hits the sealing end of the barrel gizmo.

Gun Freak's a genius, (sarcasm)i'm godly at troubleshooting(/sarcasm). Any questions?

Edit2: The piston's a bit sluggish coming back, so i'll still be working on this. If nobody objects to it*cough* moderators *cough*, i'm going to hijack this thread to post the changes that I make to the piston from Gun Freak and the community's benefit. Which means i'll be making another run to the hardware store to try to remove my ghetto sealing end on the piston, and maybe trying to reduce piston weight, and seeing if I can do anything at all to make it pull back faster.
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Last edited by saefroch on Thu Apr 01, 2010 10:32 pm, edited 2 times in total.
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Unread postAuthor: MrCrowley » Thu Apr 01, 2010 2:41 pm

I'm looking at one right now, and I know how they work, or at least the one that I made.

Piston valves do NOT use any kind of vacuum to actuate! Please show me a digram of your vacuum piston valve. I would love to see it.

The way it's designed, if you don't open the pilot valve quickly enough, it just empties the chamber until it's almost empty, then the piston shoots back and you get a really whimpy discharge, that happens at about 20 psi. If you open the ball valve sluggishly, you get a pretty weak shot. So yeah, the pressure change from the piston valve has to be sudden enough to generate enough pressure to draw back the piston. At least I think that's how it works.

Pilot volume is too big and pilot valve is too small. This is a trick Technician uses to get better performance or something. Also, your piston might not be such a good fit.

This has nothing to do with vacuum. Most likely, your piston valve isn't a good fit, so if you open the ball valve slower it has less flow, can't drain the pilot volume fast enough before the gases go around the piston and back in to the pilot volume so you're also draining your chamber pressure and when the ball valve reaches full flow the flow around the piston can't keep up and it actuates.

This is all to do with area exposed to pressure, the forces acting on that are and unbalanced forces. Nothing to do with suction or vacuums.
Don't see how your piston is any different from all the others.
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Unread postAuthor: saefroch » Thu Apr 01, 2010 3:08 pm

Okay, my understanding of a piston valve is hopelessly flawed. Feel free to direct me to a thread or explanation of this (maybe by our resident pilot volume reduction champion). My piston probably isn't any different from others, but I can say that it is different than the one I just finished working on, in that it is a solid cylinder, which probably doesn't make a difference.

Nevertheless, I managed to fix Gun Freak's piston issue.
EDIT: That sounds conceited. I THINK I made a version of his piston that works.
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Last edited by saefroch on Thu Apr 01, 2010 5:28 pm, edited 1 time in total.
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Unread postAuthor: Gun Freak » Thu Apr 01, 2010 3:26 pm

Here's some pics of my valve-

Whole gun-
Image

Piston valve-
Image

Pilot-
Image

Barrel view-
Image

Back view (through the female adapter with washer and spring)-
Image

Sorry I can't get any pics of the piston itself. It's all glued together, but I will soon be taking it apart.
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Unread postAuthor: ramses » Thu Apr 01, 2010 9:35 pm

saefroch wrote:Okay, my understanding of a piston valve is hopelessly flawed. Feel free to direct me to a thread or explanation of this (maybe by our resident pilot volume reduction champion).


here.
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Last edited by ramses on Fri Apr 02, 2010 9:00 am, edited 1 time in total.
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Unread postAuthor: saefroch » Thu Apr 01, 2010 10:31 pm

I already knew that. But what is even more amazing, is that I think the piston I built from Gun Freak's design is missing an equalizer hole. That would be the issue. I'll fix that in the morning when I'm up to moving far enough to pick it up.

I understood visually, but could not put my understanding into words. When the pilot volume is emptied, the net force on the piston changes, creating a virtual vacuum. But that's not really what produces the piston movement, it's the fact that after the valve is opened, pressure behind drops to atmospheric, and pressure in front is still high (This is my virtual vacuum, not a pressure below atmospheric behind, just a pressure difference), and piston flies back- so pretty much I knew what was going on but had a slight misunderstanding of exactly why, which led to all these problems. Of course, you intelligent pneumatic spudders already knew that. I'm just slow and sometimes ineloquent. Often wordy too.

Gun Freak- I would avoid gluing your piston into the mechanism of any gun, as the are often the first thing to go wrong (if anything). I hope I'm not being patronizing, as you should have already learned that, but I feel a need to be certain.

As always, if there is a misunderstanding (which seem to happen to me all the time with text (I love parentheses)) please say so. No, really, just do it, my ego can take it.
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Unread postAuthor: MrCrowley » Thu Apr 01, 2010 11:00 pm

I already knew that. But what is even more amazing, is that I think the piston I built from Gun Freak's design is missing an equalizer hole. That would be the issue. I'll fix that in the morning when I'm up to moving far enough to pick it up.

Unless your piston is o-ringed (or 100% air tight) it wont need an equalization hole and would be better off without one.

Gun Freak, if that is the face of your piston in the "barrel view" picture, i'd put my money on that it is almost certainly leaking out of the washer and bolt. Epoxy it up.

If you have other problems later on, we'll sort them out once your piston starts sealing 100%.
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Unread postAuthor: saefroch » Fri Apr 02, 2010 8:39 am

Wow, and I thought that an equalizer was my issue. Good thing I didn't add too big of one to mine. That could have ruined it. Oh well.
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Unread postAuthor: Gun Freak » Fri Apr 02, 2010 11:11 am

Yeah I don't need an equilization hole because the piston doesn't seal 100%. MrC, what should I do if it is all glued in already?
Did you get my PM?
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Unread postAuthor: MrCrowley » Fri Apr 02, 2010 3:41 pm

Take it apart haha. Not much else you can do.

Yup, I did.
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