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Piston valve not actuating....

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 not actuating....

Unread postAuthor: Metroid fan » Tue Mar 17, 2009 4:51 pm

I'm new here but I just am finishing my first gun with a piston valve. I made the valve from a lotion bottle filled with hot glue and with bike tire on one side. It fits into a 1 and 1/4 inch tee and then the back of my valve is a medication bottle that fits into a 1 inch coupling which is going into the tee. I've tried a whole bunch of stuff to get it working, I've wrapped tape around it and greased it I have sanded off some of the lotion bottle to get it to work smoother. Nothing works, every time I fire it I get a beautiful hissing sound which I am growing to hate. I just want to know, How easy should it be to move the piston? Do you guys have any suggestions for me? I can post pictures if you guys want me to... I'm just getting really angry that it isn't working yet.
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Unread postAuthor: jrrdw » Tue Mar 17, 2009 5:03 pm

Yes post pics of everything on it's own and semi assembled and fully assembled.

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Unread postAuthor: MrCrowley » Tue Mar 17, 2009 9:56 pm

Yes pictures will help a lot.

Try and take a picture of the piston inside the piston housing so we can see the gap. Either your piston is jamming, not a close enough fit inside the piston housing or your pilot valve is too small. Or even all three of these.

Your piston should be able to be moved by you pushing on it slightly, the gap shouldn't be noticeable between the piston and the piston housing but the piston shouldn't be too tight. For a 1 1/4" tee, i'd recommend a 1/2" ball valve as the pilot valve although a 3/8" ball valve would work. The bigger the pilot valve the better.
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Unread postAuthor: Metroid fan » Tue Mar 17, 2009 9:58 pm

Okay here are some pics, and again if someone could describe how easy it is to move their piston by hand it would be greatly appreciated.

Image

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Unread postAuthor: Lentamentalisk » Tue Mar 17, 2009 10:04 pm

Am I seeing that correctly? The way it looks to me, you have as big of a pilot volume as you do chamber... The pilot volume is supposed to be less then 1/100 the size of your chamber.
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Unread postAuthor: mark.f » Tue Mar 17, 2009 10:04 pm

Oh dear...

What does your piston seal against, for starters?

Secondly, that is one gigantic pilot space you've got there...

I think you should do a little more research, for starters.
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Unread postAuthor: Pilgrimman » Tue Mar 17, 2009 10:16 pm

The main problem I see is the pilot volume. Try to reduce the amount of space behind the piston.

BTW, I love the username :D Metroid is a sick game :D
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Unread postAuthor: Gippeto » Tue Mar 17, 2009 10:22 pm

You don't think surface area may have something to do with our young friends problems?

Or have I had one too many? :? (occasionally a possibility :D )

But I will agree that there is some..."extra" pilot volume.
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Unread postAuthor: Metroid fan » Tue Mar 17, 2009 10:25 pm

I know the pilot volume is rather large :D so you guys think that is the only thing? I assumed that pilot volume didn't have a whole lot to do with the piston not sucking back... But I can make the pilot volume a tad smaller... Lol.
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Unread postAuthor: Gippeto » Tue Mar 17, 2009 10:29 pm

I think the problem is that you have too large of a sealing face (diameter) for the size of pilot (diameter again).

You need differential force (pressure x area) to get the piston to actuate.

But, without some dimensions, it's hard to say.

What are the diameters?
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Unread postAuthor: Metroid fan » Tue Mar 17, 2009 10:39 pm

The large side fits almost perfectly into a 1.25 tee and the small side fits with a seal into a 1" coupling. I also forgot to mention that the sealing face fits onto a one inch coupling and will seal at almost 1 psi. If that helps.

By the way the chamber has much more area than the pilot. The pilot is only about half as long.
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Unread postAuthor: jook13 » Tue Mar 17, 2009 11:04 pm

One thing that bothers me about this is your quote
I assumed that pilot volume didn't have a whole lot to do with the piston not SUCKING back


It doesnt so much create a vacume effect as it drops pressure behind the piston. Make sure the sealing face on the piston is larger then what it is seated against. Basically, make sure there is some surface area on the front of the piston so the air in front of the piston can push it back when you release the pilot volume.
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Unread postAuthor: Carlman » Tue Mar 17, 2009 11:26 pm

O my, that gun looks.. um well it doesnt look..

aaannnnyyyway, id sugest you check out some other piston guns in the showcase and keep your eye on the pilot setup...
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Unread postAuthor: Metroid fan » Tue Mar 17, 2009 11:37 pm

Well I've seen how small the pilots were but i can't figure out why that would have so much of an effect... It's just some spare parts from my first pneumatic so don't worry I can replace it, but I wanted to see if it would work or not. Really though, why would pilot volume change anything? I want to understand the physics behind all of these statements. I've run some stuff in GGDT and when I change the pilot volume it has no effect on the fps... I put it pretty high too, like 100 inches cubed.
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Unread postAuthor: jook13 » Wed Mar 18, 2009 12:05 am

To be honest that is a good question. I can see how a bigger pilot would lower performance. The larger the pilot, the slower the pressure behind the piston drops and therefore the slower the piston gets pushed back. A properly made piston should theoretically work no matter what the piolt size. The key there is properly made piston. A good piston with o-ring seals would fire at the slighted drop in pilot pressure. A good example of that is a QEV. My qev has fired on a few occasions when I disconnected from the filling schrader and a little air leaked out. If the piston is not tight enough and the pilot is emptied too slow, air from the chamber has time to get around the piston and exit with the pilot. Sorry for the ramble, hope any of this helps.

I still dont see how there is any surface area on the front of the piston to push it back when you pilot the valve. That is my main guess to your problem. Read this thread, it is immensly helpful:
http://www.spudfiles.com/forums/piston- ... t8157.html

Also, are you using duct tape to hold the cannon together to retain access to the piston? Besides the obvious safety issues, I dont see that remaining air tight. And could your piston be so airtight that air doesnt get around the piston and into the chamber?
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