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Help on a 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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Help on a Piston Valve

Unread postAuthor: MajTom » Sun Mar 08, 2009 12:44 pm

I have been trying my hand at a piston valve for a few weeks now and when I finally went to try it out, all I got was the unsatisfying hiss of all the air leaving the pilot. When I opened up the chamber, the piston had not moved AT ALL. I tried it a few more times and still, nothing. I have been perusing the forum for a while, but I have not posted because of my somewhat limited knowledge (I have built a simple combustion and pneumatic). I based my design on this cannon by Mr Crowley http://www.spudfiles.com/forums/viewtopic,p,100616.html#100616.

I have formulated a theory about why the valve will not move, and I do not like it at all. I think that the small amount of surface area exposed to pressure on the sealing face is not great enough to force the piston back. The piston is a 1 1/4 end cap and a piece of 1 1/4 tee, which are reinforced by hot glue. I tried shrinking the pilot, so I do not think that is what it is. The piston is not super tight, but from what I have read, I still think it should work. Can you guys take a look at the pictures below and tell me what you think. Also, if the problem is what I think it is, can you suggest fixes so I don't end up wasting all that pipe and those fittings. Finally, I realize my chamber is quite oversized, but hey... bigger is better.

Image

The barrel goes about half way through the tee, so there is space in front of the piston.
Image

The full size Pilot Valve
Image

Take a look at this picture. The surface area does not seem to be enough. What can I do to fix this?
Image
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Unread postAuthor: mark.f » Sun Mar 08, 2009 1:05 pm

If that's 1-1/2" pipe the piston is sealing against I see no reason why it shouldn't work.

I suspect that your piston doesn't seal well enough for that small pilot valve. I have a 1/4 completed piston valve that uses 1-1/4" fittings for a piston, and I have not installed the O-rings yet, so there is considerable space around the piston for air to equalize faster than your pilot valve can vent the pilot volume.

Here's my simple suggestion. Wrap the piston in duct tape or electrical tape until it fits in the 2" pipe with a bit of drag, and then lube it up with lithium grease and try again. I see no reason why this piston valve shouldn't work other than the piston doesn't seal well enough.

If you're still having troubles, I would try a better pilot valve.
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Unread postAuthor: MajTom » Sun Mar 08, 2009 2:32 pm

Thanks so much Mark. You were right and your electrical tape solution worked perfectly for about 10 shots until the tape started to come off (that's all I could ask it to work I guess). And what a 10 shots they were!! That thing is powerful! Somehow I cracked a golf ball. :shock:

One question though: Where do you get lithium grease? I just used WD-40 right now and it seemed to work fine, but I have seen other people talk about it and I don't even have a clue what it is used for.
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Unread postAuthor: Biopyro » Sun Mar 08, 2009 3:27 pm

Well damn! I had a similar problem ( I think this was the problem) a while back. I ended up having to increase the size of the tee, and at the moment I am still looking for a better piston, so it's still out of action. :cry:
Hopefully this summer I'll get it going , but it seems very few things have a diameter of exactly 60mm.
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Unread postAuthor: Pilgrimman » Sun Mar 08, 2009 3:42 pm

Well, Mark beat me to it... I would've suggested the same thing... It worked for me.

Nice cannon! Those piston valves really pack a wallop don't they? :D
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Unread postAuthor: psycix » Sun Mar 08, 2009 4:30 pm

I recommend to reduce pilot volume. The long pipe nipple and the elbow are not needed.

Why not go like this:
Code: Select all
          Ballvalve
             || 
GUN==Tpiece
             ||
           Tpiece==Othervalve==quickconnect
             ||
          Gage

You can swap the components around if you want, and if you really want to use that elbow for the filling part, put it between the valve and the quick connect.
For even an even compacter design, use a cross piece instead of two T's. (if you can find one)
Using a sprinkler to pilot would also improve things.


When wrapping tape to enlarge the piston, make sure there is NOT ONE wrinkle in the tape for best performance.
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Unread postAuthor: mark.f » Mon Mar 09, 2009 1:14 am

MajTom wrote:Thanks so much Mark. You were right and your electrical tape solution worked perfectly for about 10 shots until the tape started to come off (that's all I could ask it to work I guess). And what a 10 shots they were!! That thing is powerful! Somehow I cracked a golf ball. :shock:

One question though: Where do you get lithium grease? I just used WD-40 right now and it seemed to work fine, but I have seen other people talk about it and I don't even have a clue what it is used for.


Glad I could help. As for the tape, maybe try duct tape next time. I've used duct tape on my pistons before and it lasted quite well, but hopefully my next one will actually use O-rings...

As for the lithium grease, you can get it at Wal-Mart. You can even get it in a spray can, if you don't want a whole tube of it.
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Unread postAuthor: MajTom » Mon Mar 09, 2009 7:04 am

Ok, so last night I both retaped the piston and shrunk the pilot volume as psycix suggested. I got the gun working again and think it is now more powerful than before, but that is just my observation, since I have no true way to compare it. This weekend I am going to work on a more permanent piston, maybe one made out of wood. I have access to a lathe, so I should be able to make one.
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Unread postAuthor: jmeyer1022 » Mon Mar 09, 2009 10:03 am

Good luck with your piston on wood, I made mine using a 2 1/4 hole saw then a large amount of time sanding until it fight nice, loaded it up with lithium grease and it seemed to actuate rather slowly. I'm not sure if it was catching or if it was a delay, but I'm piloting with a 1" sprinkler valve. I'm currently using the 1 1/4 coupler as you are, it works rather good but like you I'd prefer not having to put duct tape on. I'm thinking about taking thin sheet metal and molding it to fit nice, seeing as it would slide with minimal drag. Your options are endless just keep trying you'll find suttin
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Unread postAuthor: psycix » Mon Mar 09, 2009 2:04 pm

I have access to a lathe, so I should be able to make one.

And NOW you tell us that?
With a lathe you can make a perfect piston fit and even o-rings are easy to do.
Good luck.
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Unread postAuthor: MajTom » Tue Mar 10, 2009 11:58 am

And NOW you tell us that?


Ok, so I didn't realize that my grandfather had one in his shop until I was talking to my dad about it and he mentioned it. It will make things a lot easier when I can get down to use it. Anyway I will find some suitable material and make a properly sized piston that I will not have to tape.

Thanks everyone for your feedback.
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Unread postAuthor: psycix » Tue Mar 10, 2009 12:08 pm

You're welcome :)
Dont forget to post your shiny new lathed piston when its done!
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