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Reincarnation - 2" PVC piston valve

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Reincarnation - 2" PVC piston valve

Unread postAuthor: Slauma » Sun Dec 20, 2009 1:13 pm

Here's my second pneumatic, it's a 2" piston valve. This is basically a scaled down version of my first cannon that I could never get to work very well (I think it had a lot to do with trying to pressurize it with just a bike pump with a ridiculously over-sized chamber), so I decided to start over and make some improvements. I figured I should name it too, so this is also where I came up with the name. Here it is

Image

Features:
- 2" PVC piston valve
- 0-210 psi pressure gauge
- 115 psi safety pop-valve
- Threaded barrel adapter so barrels can be interchanged
- "Bike pump pressure isolation chamber"
- 1/2" ball valve pilot
- Duct tape to hold it all together

The smaller chamber you see with a hose running from it was my solution for using a bike pump to pressurize it. I found that using the bike pump directly I was never able to get the piston to seal against the pipe face; it would always leak because I could never pump fast enough to get a fast enough pressure build up to keep the piston seated with a tight seal. With this setup the bike pump is first used to pressurize the smaller chamber (which is isolated from the main chamber by the smaller ball valve you see at the top), then once there's enough pressure you just flip the ball valve and the air flows quickly enough and builds enough pressure fast enough to seal the piston. Then you continue to pump to pressurize the main chamber. Before firing the small ball valve is closed again to isolate the smaller chamber so that it doesn't add to the pilot volume.

I have a golfball barrel (sleeved 2" PVC) on it at the moment that can chuck golf balls a couple hundred yards, even with the ball valve pilot. Hopefully I'll be replacing the ball valve with a QEV soon and maybe even rig it up with a pistol grip. The back end of the piston valve sits pretty nicely on your shoulder, kind of like a stock, so with a pistol grip I think it would have a very rifle-like feel to it.

I just finished building it and still want to make some improvements to it though. I'd like to find a nice clean way to mount the smaller chamber and a barrel support, other than duct tape. The duct tape looks pretty clean actually, but it's not very solid. Something like a huge screw/band hose clamp? I haven't really found anything yet that would suit, any ideas? Let me know what you think, I'm open to any suggestions for improvements/modifications. Thanks for looking.
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Unread postAuthor: Gippeto » Sun Dec 20, 2009 2:28 pm

You've obviously been paying attention, and have a couple brain cells to rub together. That's good. :)

A couple things you seem to have overlooked...

It looks as though the gauge is threaded into a single layer of pvc..Preference is to tap only double layers.

The reducer looks to be dwv...again not the way to go. If it doesn't say NSF-PW ... or have a rating printed on it...don't use it. Opinion seems to vary on this issue...I prefer to err on the side of caution.

Nice work otherwise. :)
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Unread postAuthor: Slauma » Sun Dec 20, 2009 3:10 pm

Thanks Gippeto.

Yes, you're correct about the reducer being DWV. I know it's no excuse, but it's the only thing I could find for what I needed. I was iffy as to whether I should use it or not, but I feel safe (whether I really am or not) using it as long as I don't take the pressure up too high. The highest I've taken it so far is 65psi and after seeing the results with a golf ball I don't know if I'd even want to go much higher with it. The small segment with the pressure gauge threaded in is double layer (sleeved at least) however.

After doing a little research it appears that the particular company that manufactured this piece uses the same material and processes for manufacturing SCH40, SDR, and DWV pvc; the difference in the pressure bearing capability lies in the wall thickness. I looked at the reducer, and in comparison to the 4" endcap (which is rated SCH40), the wall seems to be just as thick, if not thicker, so I decided to use it.

In your opinion though, if there were to be a failure in the gun, do you think it would be the reducer? And any ideas on how exactly it would fail?

I, luckily I suppose, have no experience with PVC failure. The most I know is that low temperatures and the possibility of high stress (like dropping it on an edge) is too close to asking for a failure for me.

One thing I do as a safety precaution is never store my cannon in temperatures below 60°F and never play with it outside too long on really cold days so the PVC doesn't cool down too much and possibly get brittle.
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Unread postAuthor: Gippeto » Sun Dec 20, 2009 3:29 pm

IMO the reducer is the most likely area for a failure to occur. PVC failure mode is shrapnel with sharp edges driven at fairly high speed.

I've not had a failure with pvc either... but then I've never pushed my luck with it. :roll:

I believe it was MrCrowley that had a fitting or fittings fail inexplicably. They were even pressure rated. With this in mind, it just makes sense to me to build as safe as possible.

Later
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Unread postAuthor: cannon monkey » Sun Dec 20, 2009 3:36 pm

what did use for a piston im finishing my 2 inch piston valve up today :)
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Unread postAuthor: Slauma » Sun Dec 20, 2009 4:22 pm

Ideally I'd have the valve housing made out of aluminum, and all metal parts and connectors. But that's not going to happen.

I used 1.25" pvc pipe couplings connected with 1.25" pipe with o-rings in between.
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Unread postAuthor: turner » Sun Dec 20, 2009 6:39 pm

That looks really nice. I love the bike pump chamber thing also.
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Unread postAuthor: cannon monkey » Sun Dec 20, 2009 9:53 pm

mm i be posting mine xmas morning once i know it done and works im waiting glue dry now very nice cannon u got there
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Unread postAuthor: michaelgohjs » Sat May 22, 2010 7:32 am

great idea.. but i was wondering if using a spring at the piston would solve this problem?
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Unread postAuthor: cannon monkey » Sat May 22, 2010 8:01 am

michaelgohjs wrote:great idea.. but i was wondering if using a spring at the piston would solve this problem?



not really sure what to say cause this is on an old topic but yes a spring would help fix that problem

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most big random like i ever seen

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