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question about piston hybrid?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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question about piston hybrid?

Unread postAuthor: JDP12 » Tue Dec 21, 2010 8:11 pm

Unfortunately i can't post pictures cuz i forgot my cord for my camera..

But in the process of buildin my piston hybrid, I might have a problem.

I drilled a hole into the piston housing for a pressure input to pressurize the rear of the piston. But now the piston hits against the nipple that's threaded in there. It gives the piston about an inch of travel, which should be enough, but i'm worried about the stress of the piston hitting the 1/4" nipple threaded into the pipe. Do you think this could lead to anything cracking? I only plan on taking this to 7x or so, and the piston can't travel very far, so I don't think it can get alot of energy going..
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 21, 2010 8:18 pm

I take it the piston is airtight, say you pressurise the area behind it to 150 psi, that pressure is going to grow proportionally as the piston moves back so in all likelyhood the piston will never touch the nipple.
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Unread postAuthor: JDP12 » Tue Dec 21, 2010 8:32 pm

yea, but its similar to SB15's and MrCrowleys in that it unseats in the back after being moved.. I could easily modify it so its a normal piston though.

Thoughts on one or the other? Obviously a venting one will be better performance wise..
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Unread postAuthor: saefroch » Tue Dec 21, 2010 8:38 pm

A venting one would be my choice. I'd see if it's possible to extend the housing back farther and give the piston more travel, so you can install a bumper that the piston only encounters once it is no longer limiting flow.

If not... just let it crash into the nipple and pray it doesn't shear off the threads.
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Unread postAuthor: JDP12 » Tue Dec 21, 2010 8:41 pm

Yea. We shall see.. don't really feel like trying to redo my piston and everything so we'll see.
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Unread postAuthor: saefroch » Tue Dec 21, 2010 9:05 pm

Just a random question: Do you know if your piston is bouncing back shut?
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Unread postAuthor: JDP12 » Tue Dec 21, 2010 9:14 pm

No idea. Guns not in the firing stage et, still in construction stage. I doubt it will seal simply by bouncing back, i think it will need more pressure than that to properly seal.
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Unread postAuthor: ramses » Tue Dec 21, 2010 9:35 pm

jackssmirkingrevenge wrote:I take it the piston is airtight, say you pressurise the area behind it to 150 psi, that pressure is going to grow proportionally as the piston moves back so in all likelyhood the piston will never touch the nipple.


probably not , depending on the dead volume ( treating the pilot area as the head of a pump. I'm not talking about the volume from the sealing face to the projectile.)

I filled a similar area on my piston hybrid with caulk to minimize this. I also filled most of the nipple with caulk to further decrease dead volume. I did this to use the air as a bumper, but you could use it to spread the force of impact out. I also have a conventional bumper.
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Last edited by ramses on Tue Dec 21, 2010 9:36 pm, edited 1 time in total.
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Unread postAuthor: JDP12 » Tue Dec 21, 2010 9:35 pm

Hmm.. not a bad idea. May have to try it.
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Unread postAuthor: MrCrowley » Tue Dec 21, 2010 11:39 pm

you think this could lead to anything cracking?

If the nipple itself doesn't break off (if it is brass, it probably will be crushed to bits) and the piston isn't damaged, I imagine it will cause a leak around the nipple unless the nipple is threaded into a thick-walled pipe.

What size is your piston anyway? From experience with my piston, it would destroy the nipple on the first shot.
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Unread postAuthor: JDP12 » Tue Dec 21, 2010 11:43 pm

ah... bollocks. its a steel nipple.


It's a 1.25" piston.

Damn.. I'll have to get a new pipe, and extend my piston length so i can seal against something else, similar to what you did.
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Unread postAuthor: MrCrowley » Tue Dec 21, 2010 11:50 pm

Hmm maybe with a steel nipple you could find a solid rod of steel or similar which will sleeve inside it, and then drill a hole inside that. Could help reinforcing it to stop it from being crushed but I wouldn't be surprised if it damaged the threads and caused a leak.
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Unread postAuthor: JDP12 » Tue Dec 21, 2010 11:52 pm

Ah well, I'll prolly just have to redo the piston housing. THen see if i can get one of those fancy extender nuts for the piston.
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Unread postAuthor: MrCrowley » Tue Dec 21, 2010 11:55 pm

Ah one of these? Cost me $16NZD haha, ridiculous. Though it was a lot more useful than I first thought. Originally I had a fixed length between the piston and the 'spool' pilot piston thingy on the end which seals the exhaust, but you'll find out that this distance will change almost everytime you tinker with the cannon, so it was good that I used a nut coupler as it allows you to adjust the distance.
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Unread postAuthor: Moonbogg » Wed Dec 22, 2010 1:25 am

Could you take the nipple out and grind the back side down so it doesn't stick through the wall?

Also, maybe re-tap the pipe thread to something bigger and use a coupling, hopefully with a tight enough fit to not let either stick through the wall.
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