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Problematic 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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Problematic piston valve.

Unread postAuthor: Biopyro » Mon Jul 09, 2007 8:54 am

As any members of the UKSGC will know I have been having problems with my valve since day one, first this doesn't work, then this, then something else. It's driving me nuts. It is a barrel sealing valve inside a 50mm tee. The barrel is 40mm.
The piston itself is made from moulded polymorph plastic, so is a very good fit. The seal is 4 layers of tyre inner tube, which I have tried both stuck onto, and not stuck onto the piston (I bolted it in the centre instead). The piston and barrel are perfectly flat on each other (this was a suspected problem).
When the rubber is bolted on, it slips out into the barrel and breaks the seal above a certain pressure. When it's glued on, it won't seal without some force behind it, so I added a spring. Now it seals, and doesn't slip but the piston doesn't move when the pilot is vented. I don't know if this would happen still without the spring, because of the other problems. When the piston didn't move I poked it backwards from the barrel port, it no loger sealed, but would move backwards when forced by air from the barrel port.
Additional information is available here.
I really appreciate any input for any of the previous and current problems, since it seems like I have tried everything and I'm probably overlooking something really simple any it's driving me nuts!
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Unread postAuthor: Infernal Maveric » Mon Jul 09, 2007 2:39 pm

Where is your fill valve? I can't be arsed to trawl through 4 pages of posts to find out.

if the fill valve is not behind the piston, that's your problem, happened with my new copper piston gun.

If it is behind the piston, may I suggest that you lube the piston up like HELL with cooking oil/WD40/Durex lubrication ECT to allow the piston to move back.
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Unread postAuthor: spanerman » Mon Jul 09, 2007 2:57 pm

as i said on uksgc try a less stiff spring and remeber 'anythings possible with enough lube' (quote of a gay mate lol)
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Unread postAuthor: Infernal Maveric » Mon Jul 09, 2007 3:04 pm

Eww, and to think he was probably talking about fisting :(
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Unread postAuthor: dongfang » Mon Jul 09, 2007 3:12 pm

Hi,

Your description of the problems make me think of the part of the tee that is in front of the piston, surrounding the barrel. It there pressure there, or is the piston such a good fit that there is not? How large is the volume of that space?

It is the pressure there that moves the piston back.

And, how far does the end of the barrel stick into the tee? Can you see it when looking into the tee from the chamber port?

Regards
Soren
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Unread postAuthor: Biopyro » Fri Jul 13, 2007 12:08 pm

Yes, the fill is behind the piston, and I think I might try lots of lube, I have some silicone lubricant grease or something in the garage.
I don't think a weaker spring will be enough to seal it, nor do I think that's the problem, but I'll try.
There is pressure in front of the piston, and the barrel extends halfway into the tee, clearly visible from the chamber port (before it was glued on at least). The volume of that area the the chamber are one and the same, so, around 2l or whatever. I know that the chamber is being pressurized.
I am beginning to wonder if it might be the small difference between the 2 diameters, 50mm and 40mm, but I really hope it's not.

Is it common for the seal to be glued onto the piston, or just secured by a bolt in the centre?
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Unread postAuthor: dongfang » Sun Jul 15, 2007 4:50 am

Hi,

OK I see; the barrel extends into the tee and is visible from the chamber. I believe your chamber is pressurized when you tell me so :lol:

I have seen both methods of attaching a seal to a piston.

In my own first tee valve (which is not finished yet - see the thread
I put the seal O-ring at the barrel end, not the piston.
Having lost any illusions about lathe access, I have given up grooves for the O-ring, and now plan to make the barrel seal this way:
In a 50 mm socket, I insert a 50-40 mm reducer so that it is sunk in by 2 mm or so. In that space, I put a hard 5.34 mm O-ring. The curved head of the piston will hopefully ensure that the ring does not get pushed into to barrel.
If you are having nothing but trouble with the seal on the piston, stay tuned and I will tell you if my seal is any good once I get it built.

Edit: I see you use metric - where do you live?

Regards
Soren
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