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A Question for the Piston Hybrid Owners

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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A Question for the Piston Hybrid Owners

Unread postAuthor: mark.f » Sun Nov 27, 2011 10:06 am

This is about a pneumatic cannon.

I'll get right to it...

For those of you who have a piston hybrid with a larger piston and a smaller "spool" that seals against the back of the tee and helps to exhaust the pilot... like this one from SB-15, do you ever have to "adjust" the compression on on both sealing faces (the actual piston's sealing face against the barrel port and the face on the rear pilot port) periodically to get both to seal correctly? I notice Mr Crowley traded in his old rear gasket for an o-ring seal...

I can't decide on whether or not to use at least one o-ring seal on a DFTV that's in construction or to use two gasket seals like on the above mentioned piston hybrid valves. I would definitely use two gasket seals if I heard that sealing was a minor issue and didn't require frequent upkeep. Otherwise, the logical thing to do would be to use at least one o-ring seal (on the back "spool" or to seal in the barrel port) with a hard stop. Either spot will create excessive o-ring wear, though, and sizing parts and o-rings is a little tricky in 1" and 1.5" SCH-40 (at least for somebody without a lathe trying to use standard cylindrical parts).

Anyways, thanks for reading.

Edited by jrrdw.
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Unread postAuthor: Gun Freak » Sun Nov 27, 2011 11:11 am

Instead of the two gasket seals I'd use a gasket on the seat, obviously, and an o-ring in a nipple in the back to seal the spool so you wouldn't need to fine tune the distance. I've thought about that before also.

Strange that I'd post on this topic... 8)

Adding to my previous post: the o-ring should be as close as possible to the end of the nipple so it exhausts right away. I guess that's the benefit of using a flat seal in the back— it starts exhausting the pilot as soon as the piston moves.

Edited by jrrdw.
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Unread postAuthor: mobile chernobyl » Sun Nov 27, 2011 2:25 pm

GF - Diagrams go a long way! :wink: even if they're done in MS Paint lol...
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Re: A Question for the Piston Hybrid Gallery

Unread postAuthor: SpudBlaster15 » Sun Nov 27, 2011 3:25 pm

mark.f wrote:do you ever have to "adjust" the compression on on both sealing faces (the actual piston's sealing face against the barrel port and the face on the rear pilot port) periodically to get both to seal correctly?


Once the spacing it initially set up, it only requires adjustment if the sealing face becomes damaged, which isn't a problem with the hard, thick rubber material used on my valve.

O-ring seals are interesting, but in my experience the smaller ones don't last very long when exposed to numerous cycles of seating and unseating in a port. I think you're better off going with a gasket setup.
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Re: A Question for the Piston Hybrid Gallery

Unread postAuthor: MrCrowley » Sun Nov 27, 2011 3:55 pm

mark.f wrote:I'll get right to it...For those of you who have a piston hybrid with a larger piston and a smaller "spool" that seals against the back of the tee and helps to exhaust the pilot... like this one from SB-15, do you ever have to "adjust" the compression on on both sealing faces (the actual piston's sealing face against the barrel port and the face on the rear pilot port) periodically to get both to seal correctly? I notice Mr Crowley traded in his old rear gasket for an o-ring seal...

I found that on a few occasions when I pushed the piston forward to seal against the barrel and adjust the gasket to seal against the pilot port, I would try pumping the pilot volume but it would leak at the gasket and require more adjusting.

I also had numerous problems with getting gaskets to seal, mostly I tried 1/8" - 1/4" gaskets and most worked for a fair amount of time but a few would deform under pressure and leak, others would get cut up after awhile.

I'm really happy some members convinced me to go with an o-ring as I don't have to make extra adjustments, I just push the piston forward then turn the spool rod to push the o-ring inside a copper adapter, doesn't matter how far in it goes as, obviously, it can seal where ever inside the copper adapter.

It also worked first time and every time since, no leaking at all. The only issue was the o-ring being pulled out of its groove after every shot but I've since fixed that by removing the teflon tape I used for the old o-ring to make it fit and buying an thicker o-ring that actually fits and seals without the teflon tape.

I'm sure a gasket seal can be just as reliable once the right gasket is found.

What's this for anyway? I assume it isn't for a hybrid since this is in the pneumatic section.
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Unread postAuthor: SpudBlaster15 » Sun Nov 27, 2011 5:58 pm

I should clarify that the setup on this piston is a dual diameter gasket seal, configured such that the lateral area of the smaller gasket seals against the inside of the port (Much like an o-ring would), and the cross section of the larger gasket seats against the end of the port. In my experience, this has proven to be more reliable than either a single flat gasket joint or an o-ring.
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Unread postAuthor: mark.f » Sun Nov 27, 2011 9:08 pm

Everybody wrote:ongoing forum joke concerning Gun Freak


Guys guys, I think it's important to note this is for a pneubmatic and not a hybrid. :D Although I will say it's shaping up to be something completely different than the normal barrel sealing tee valve (even with the DFTV setup).

SB15, thanks, that clarified a lot. I'm a little constricted on space (as I'm trying to make it look polished and pretty), but I believe I can implement that in the 1" SCH-40 I'm using... by splitting a piece of 1" and gluing inside the port. Then, I can probably use tap washers to use such a setup.

Thanks for all the input, guys.
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Unread postAuthor: jrrdw » Sun Nov 27, 2011 11:44 pm

Thread cleaned...
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