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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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Piston valve trouble

Unread postAuthor: Gun Freak » Wed Mar 31, 2010 8:46 pm

I finished my first tee piston cannon last night. This morning I tested it. Here's the problem. The piston won't seal, and the air goes out my pilot valve. Pilot is a 1/4 BV. Piston is a bolt piston. Housing is a 3/4" tee. It is all PVC. I am thinking that it is just the piston seal. It probably isn't good enough. But what are some other problems that one might run into when making a piston cannon? Thanks.

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Unread postAuthor: saefroch » Wed Mar 31, 2010 9:08 pm

Be sure the piston head is made of softish rubber, and well-secured to the piston. Have you checked that the tee is fairly well-greased and the piston slides well? Can you hear it thumping as it hits the stop at least? Also be sure that the piston doesn't have the chance to slide it's back out of a housing at the back, and let the head drop down off the stop. Also check that the head is lubricated with something like vaseline, that's a BIG factor. Above all, make sure your piston is long enough, and the head is soft and greased.
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Unread postAuthor: thedeathofall » Wed Mar 31, 2010 9:11 pm

There are several possibilities that could cause this problem. First, is there any way you could take pictures? this would help tremendously in getting this problem solved.

From personal experience however, there are a few causes of not getting it to seal.

1. barrel porting is not cut straight. If it is not a 90 degree angle cut, or has burs/dirt on it, it may not seal.

2. not enough initial pressure to seat the piston. Sometimes, especially if the piston is heavy or tight, there needs to be a lot of force to push it forward the first few times. Did you lube the piston?

3. the sealing surface is not good enough. My first two pistons were made with tire tubing for the face. They both failed becuase it was not thick enough. See if you can get a hold of 1/8 inch thick neoprene sheet from Mcmaster or similar places. I have yet to find it in a store.

I hope this helps a little. Once again, pictures speak much much louder than words.
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Unread postAuthor: saefroch » Wed Mar 31, 2010 9:17 pm

Really anything that's soft and can make a seal will work for a piston face, if you apply a solid nonpolar lubricant and apply enough pressure. Pump quickly, don't expect a newbie hybrid to seal at 40 or even 60 gspi.

From my experience, if you've got a messy piston stop, you'll get a failing piston, not one that fails to seal right off the bat. It will seal, then when you let it seal, it will like burp out all the pressure that you put in past the piston, sliding it back only a bit.
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Unread postAuthor: jhalek90 » Wed Mar 31, 2010 9:25 pm

WELL, the best seal i have found is called gasket rubber.... its available is a 6''x6'' sheet at most any hardware store... i got mine at Ace, and i have seen it at gilroy's ect.

The problem could also be that your piston is not tight enough. if to much air gets past your piston.... it may not seal before the air pressure has a chance to go around it and push on the sealing. face.

In a 3/4'' T i assume you have a 1/2'' piston.... if thats the case get a wooden dowel for the piston.... it worked for me.... and just super glue some of the gasket rubber i told you about onto the face of it.

Hope that helps
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Unread postAuthor: Crna Legija » Wed Mar 31, 2010 9:45 pm

try to kink the hose you fill it with then dump the air into the cannon
get pics of you piston if you can, it will help lots. how tight is your piston you should be able to suck it back and blow it forward with your mouth

the part you piston seals against is it cut flat 90* and is your piston sealing face flat

if you can get some pics
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Unread postAuthor: Gun Freak » Wed Mar 31, 2010 9:54 pm

Sorry I wasn't very clear in my description, I forgot a few things. The piston is spring loaded. I figured this would help because less force would be needed to seal it. Second, I cut the sealing face almost perfectly 90 degrees so I'm sure that isn't the problem. I EVEN hooked it up to my compressor and that wouldn't even seal it. The piston is a bolt with a few nuts, washers, and rubber gaskets for a good seal. I just didn't lube it. And about the pictures, it's kinda hard since everything is glued together. The piston is somewhat serviceable, it slides in a piece of 3/4 pipe that has a female adapter on the other side, but I kept it in place with a washer held in by the lip in the F.A. so it will probably be tough to get out. I may be able to get a few pictures, but not until tomorrow, since I'm not at home right now. Thanks for the helpful replies.

~GF~
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Unread postAuthor: deathbyDWV » Wed Mar 31, 2010 10:32 pm

saefroch wrote:Also check that the head is lubricated with something like vaseline, that's a BIG factor.


Hahahahahaha... That's what she said...

On a more serious note...
Are you sure that the piston is square with the end of the barrel when it pushes foreward? You should check how much it can wiggle crooked.

Also when you fill it, does it only seal a little and just leak alot or does it not seal at all?

Btw gunfreak...

[sarcasm]It's SO helpful that you put ~GF~ at the bottom of you post. I'm much too lazy to look up and to the left a couple inches...[/sarcasm] :lol:
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Unread postAuthor: saefroch » Wed Mar 31, 2010 10:47 pm

The back of the piston needs to seal, yours seems to be waaaayyyyyy heavy, which will kill the effectiveness of the gun. I would removed the spring, as this plus the weight of the piston will kill the opening speed. And it seems to me like it's possible that the piston has wiggle room inside the tee. Is there air going out the barrel at all? Or just out the pilot? If it's only going out the pilot it's likely that the piston is refusing to even move forward in the first place, and you've got a spring issue.

Here's the problem. The piston won't seal, and the air goes out my pilot valve.


Okay, we're all going to feel like idiots if this is the cause of your issue, but is your pilot valve closed? There should be no way for air to go out the pilot, since it should be closed leaving the only exit for air down the barrel or pressurized in the chamber and pilot volume.
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Unread postAuthor: deathbyDWV » Wed Mar 31, 2010 10:52 pm

I think he means that no air goes out the barrel and it only goes out the pilot. It this is true then you need a tighter piston.
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Unread postAuthor: Gun Freak » Wed Mar 31, 2010 10:54 pm

It is filled through the pilot, then closed, the when I open the pilot, all the air goes out it.

deathbyDWV wrote:Btw gunfreak...

[sarcasm]It's SO helpful that you put ~GF~ at the bottom of you post. I'm much too lazy to look up and to the left a couple inches...[/sarcasm] :lol:


Haaha :D Fine then, I'll stop.

I will try replacing the piston with maybe wood or something light.

~GF~
~GF~
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~GF~
~GF~
~GF~
~GF~
~GF~
~GF~
~GF~
~GF~
~GF~
~GF~
~GF~
~GF~
~GF~
~GF~

:D :D :D :D :D
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Unread postAuthor: saefroch » Wed Mar 31, 2010 11:00 pm

Wood might work, but dowels can be fragile. Take a part that is attached to your tee out, lube that up, and pump it full of hot glue and you've got yourself a body-sealing piston, just screw your head into one end of it and point that forward.

*pwhew* that was close. So you fill through the pilot, close the pilot, removed connection to pressure, and open pilot and all the air runs out. I think you've got a very basic issue here, especially considering nothing's going out the barrel. It's most likely that you're just not generating enough vacuum pressure with the pilot to suck back such a heavy piston. Be sure to really whip open the ball valve quickly, fill it to at least 100 psi, and if that doesn't work the only thing I can think of is that your piston is just too heavy to budge.

On afterthought, it could be that it's getting stuck to your piston stop, because of the shape you close for the piston that's possible, it might be getting caught on something and not kicking back. If you test it again, set the piston in the back position when you being to pressurize.
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Unread postAuthor: MrCrowley » Thu Apr 01, 2010 12:32 am

It's most likely that you're just not generating enough vacuum pressure with the pilot to suck back such a heavy piston

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.

Ok so you try to pump up the gun and it leaks out the sealing face? I think the problem is that the bolt your sealing face is attached to is leaking. Even if you use a washer on the sealing face, air can leak out through the bolt and washer and out the barrel. Solution? Epoxy up the sealing face to cover the bolt and washer (but still so that it has rubber to seal against the barrel).

If that works and you try to pilot the valve and the air just goes out the pilot your problem is that your piston isn't a good enough fit in the housing. This can be tricky business but be more worried if your piston moves too easily rather than not easily enough. If you are also using rubber washers/gaskets to help make it a tight fit, use lube. It will help greatly.

edit:
BTW, I wouldn't ask your parents where the vaseline or baby oil is. Could get a little awkward :D
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Unread postAuthor: thedeathofall » Thu Apr 01, 2010 12:47 am

BTW, I wouldn't ask your parents where the vaseline or baby oil is. Could get a little awkward


Ask your girlfriend though... she might know :lol:

On a side note (which side I wonder), I think you mistated the problem at first. when you fill the gun, does air leak out the barrel? yes or no?

When you open the pilot, does the all the air go out the pilot? yes or no?

If the second is true, then you need to remove the spring, make sure you have a tight seal between the piston and the housing, and if possible, get a light piston.

If the first is true, you need to get a better sealing face. Like I said earlier, 1/8 inch neoprene sheet or a THICK neoprene washer. The thicker the better. (yes that is what she said...)

Also, what size is your barrel porting? I will assume because it is PVC, that it is 1/2 inch. This may be to big to make the pressure difference that McCrowley was talking about. You may have to change housing sizes or move into metal parts so you can make the barrel porting and sealing face smaller.

Once again... hope it helps and works out for ya.

*on a side note (the other side of above), it feels soo good to be back....
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Unread postAuthor: reedfe » Thu Apr 01, 2010 7:23 am

MrCrowley wrote:
BTW, I wouldn't ask your parents where the vaseline or baby oil is. Could get a little awkward :D


Tehe no problem, I already know :D

I'll try lubing it and maybe removing the spring. All I have is expoxy putty... will that work? I don't think so... but....

Anyway the air is leaking out the barrel when pressurized and then it all comes out the pilot when triggered. It must be what MrC was talking about. The washer and bolt seal. I'll see.

EDIT: SORRY POSTED IT ON MY FRIENDS ACCOUNT. ITS GUN FREAK :D
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