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efficient piston valve?

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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efficient piston valve?

Unread postAuthor: no-limit » Sat May 24, 2008 12:01 pm

I just made a piston valve with a piston that does not fit very good, it's really loose. There is also a lot of pilot volume. I use C02 and and can get it to seal the barrel pretty quick. It does take a lot to get it to come back, anything below about 40 psi and it just won't. When I take it up to 60 psi and above it seems to work well. I am using a 1" sprinkler valve for the pilot.

The thing I would like to know is, if it works at all is it as efficient as it can be? Is there something I'm missing? Is the speed in which it comes back changeable or is mine coming back, because it comes back, as good as any. Is it requiring more air, looseing it thru the pilot. The sprinkler was honking and then I shortened the distance to the sprinkler valve and it is gone.

Let's see did I miss anything? I'm already in the process of making another one. I thought the chamber might be to large and use up to much C02 so I poured 3 lbs. of marbles down the barrel. The hose is 6' so the kids don't have to stand to close. I was thinking of making it 4'.
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Unread postAuthor: STHORNE » Sat May 24, 2008 12:08 pm

Well first of all, you have a TON of unneeded hose that would reduce performance. So you will want to trim those up and make them as short as possible. But since you are using a sprinkler valve for your pilot, I'm not sure how much that would effect it. But I do know it would effect speed.

If I were you, I would ditch the pilot hose and just thread in a 1/4" ball valve right into the sprinkler valve and use that as your trigger. It will have a much faster exhaust performance and won't be as cluttered as the hose. And you wont have as much of a chance of catching that hose on anything and damaging the gun.
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Unread postAuthor: elitesniper » Sat May 24, 2008 12:12 pm

To much pilot, is your piston serviceable? it looks like you just glued on a bushing..oh well...but its your loose thats the problem im pretty sure.
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Unread postAuthor: Carlman » Sat May 24, 2008 12:14 pm

STHORNE wrote:If I were you, I would ditch the pilot hose and just thread in a 1/4" ball valve right into the sprinkler valve and use that as your trigger.


no-limit wrote:The hose is 6' so the kids don't have to stand to close


read.

anyway nice gun mate, how about just make a different piston for it or is it not serviceable?
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Unread postAuthor: Gippeto » Sat May 24, 2008 12:23 pm

What I TRY to do with a piston valve;

Minimize piloted volume, the accepted rule of thumb around here is D/4. Which means that your piston should travel 1/4 of your barrel diameter when fired. ie. a 1" piston will travel 1/4"

Using GGDT, size your pilot valve to eliminate piston bounce.

Minimize piston weight WITHOUT increasing piloted volume.

Use a bumper which will not compress under air pressure. (Closed cell foam, will compress under pressure.)

Minimize the amount of air that can bleed back from the chamber to the pilot volume. A tight fit, o-rings, or u-cup seals all work well. Incorporate a check valve if possible (but not needed)

Ensure a frontal area on your piston. (Barrel seat should be smaller than your piston diameter)
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Unread postAuthor: no-limit » Sat May 24, 2008 12:56 pm

I can't seem to find any parts that will let me service the piston, so once it's put together it's done, including the chamber with it.

The question was if it works at all is it working at it's best. It does work well but is there somewhere I can get it to work better? Make it come back faster.
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Unread postAuthor: STHORNE » Sat May 24, 2008 1:05 pm

Carlman wrote:
STHORNE wrote:If I were you, I would ditch the pilot hose and just thread in a 1/4" ball valve right into the sprinkler valve and use that as your trigger.


no-limit wrote:The hose is 6' so the kids don't have to stand to close


read.

anyway nice gun mate, how about just make a different piston for it or is it not serviceable?


I know. I was just pointing out what could be effecting performance and also, if all parts are pressure-rated (which they are), there really is no need to be cautious as of firing far away (just wear ear protection).
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Unread postAuthor: elitesniper » Sat May 24, 2008 1:05 pm

All you need is a female and male adapter and a bushing for your sprinkler valve.
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Unread postAuthor: MrCrowley » Sat May 24, 2008 4:12 pm

no-limit wrote:The question was if it works at all is it working at it's best. It does work well but is there somewhere I can get it to work better? Make it come back faster.

Definitely not, it sounds like the piston is pretty damn loose, and having to fire it at over 40psi is quite bad. I can get a 2" piston to fire at less then 7psi.

Sorry for the waste of money, you really should've checked with us if you had doubts. Because we put a huge emphasis on the whole 'make sure your piston is a good fit', because that's usually 90% of the problem.

I understand your need for the 6' hose, but at least use a smaller diameter hose. The one you have is quite big. You can get ones with only about a 1/8" hole, which will reduce pilot volume of the sprinkler valve.

The only thing you can do is try and saw of the bushing and see if you can get the pipe out of the tee using a flat head screwdriver and hammer. I've done it before but you have to be careful you don't damage the tee.

Also where eye protection* and remove the glue and primer marks from the tee socket once you've got the pipe out.


*Got hit 6 times in the face yesterday taking apart fittings. Twice in the eye. I did have protection on, but it was a mighty thump, went to fast to even see it hit me.
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Unread postAuthor: no-limit » Sat May 24, 2008 4:14 pm

I don't want my question to get buired.

The question was if it works at all is it working at it's best. Am I getting maximun thrust?

You answered while I was writing this.
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Last edited by no-limit on Sat May 24, 2008 6:41 pm, edited 1 time in total.
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Unread postAuthor: MrCrowley » Sat May 24, 2008 4:16 pm

no-limit wrote:The question was if it works at all is it working at it's best. Am I getting maximun thrust?

No. I said that above. :)
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Unread postAuthor: keep_it_real » Sat May 24, 2008 6:19 pm

I know you didn't ask about safety but I'm going to tell you anyway...

You should check the elbow on top for cracks while shooting it because the kick could but a pretty large amount of force on it. That's if your shooting it on the ground like in the picture.

It looks pretty sweet though.

On thing for performance. It looks like turning the ball valve not only pilots the sprinkler valve but also empties some pressure behind the piston (ignore this if that's not right). I would attach a different ball valve on the hose only for piloting the sprinkler valve.
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Unread postAuthor: no-limit » Sat May 24, 2008 6:48 pm

I'm using a blow gun for piloting the sprinkler valve. Waste of money,HA, this is the 4th one I've messed up but a least it works. I will just keep it and build another one.
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