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Piston Hybrid

A place to ask general spud cannon related questions.
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Unread postAuthor: ramses » Wed Mar 31, 2010 4:21 pm

If you get the ratio of piston OD to seat OD right, they can be at the same pressure. you need to make sure the piston is more or less airtight with whatever pipe it is in, otherwise the mix behind the piston could POSSIBLY ignite, forcing the valve closed and possibly causing your chamber to explode. at the very least, your rubber sealing face will be trashed, as well as maybe the seat of the valve
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Unread postAuthor: SpudFarm » Wed Mar 31, 2010 4:31 pm

ramses wrote:If you get the ratio of piston OD to seat OD right, they can be at the same pressure. you need to make sure the piston is more or less airtight with whatever pipe it is in, otherwise the mix behind the piston could POSSIBLY ignite, forcing the valve closed and possibly causing your chamber to explode. at the very least, your rubber sealing face will be trashed, as well as maybe the seat of the valve


Only one way to find out..

And, would you build a gun that ruptures if it jams something in the barrel? :P
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Unread postAuthor: saefroch » Wed Mar 31, 2010 5:41 pm

Hmm, i'll draw it out and think about it, definitely something to consider.
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Unread postAuthor: ramses » Wed Mar 31, 2010 6:42 pm

SpudFarm wrote:
ramses wrote:If you get the ratio of piston OD to seat OD right, they can be at the same pressure. you need to make sure the piston is more or less airtight with whatever pipe it is in, otherwise the mix behind the piston could POSSIBLY ignite, forcing the valve closed and possibly causing your chamber to explode. at the very least, your rubber sealing face will be trashed, as well as maybe the seat of the valve


Only one way to find out..

And, would you build a gun that ruptures if it jams something in the barrel? :P


well yes, because I know mine will fail elsewhere first. I use EMT conduit as my 2" barrel, and I know the compression fitting will break before I will rupture the chamber. 1000 psi on 2" diameter is 3141.5 lbs of force. The barrel will definitely rip off first.
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Unread postAuthor: saefroch » Wed Mar 31, 2010 8:13 pm

The self-removing barrel was actually a problem I ran into with a very ghetto design, never thought of that as a safety feature...
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Unread postAuthor: ramses » Wed Mar 31, 2010 10:49 pm

well, it's still not very safe, but better than splitting open a steel chamber filled with >2000*F, >1000PSI gas next to your face.
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Unread postAuthor: saefroch » Wed Mar 31, 2010 10:55 pm

Agreed. Safety is paramount.

Any ideas on the construction of a lightweight hybrid piston?
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Unread postAuthor: ramses » Thu Apr 01, 2010 9:28 pm

depending on your bore, a solid aluminum cylinder can work. I have never tried UHMPWE, but it is cheap enough to be worth a shot. For my valve upgrade (1.5" porting) I decided on two aluminum disks held together on a .5" Al threaded rod, supported against an Al tube. I have a bolt circle calculator, and I set it up for one stud. It gave me a preload of, IIRC, 3000 lbs on the bolt. If you have a lathe and feel like making a crapton of swarf, you could machine an aluminum spool.
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Unread postAuthor: saefroch » Thu Apr 01, 2010 10:22 pm

No lathe. Solid cylinder for a piston hybrid? I would need to see a diagram for this... How do you get metal to seal to metal?
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Unread postAuthor: Moonbogg » Fri Apr 02, 2010 12:17 am

saefroch wrote:No lathe. Solid cylinder for a piston hybrid? I would need to see a diagram for this... How do you get metal to seal to metal?


You can drill and tap a hole and then bolt a rubber disc to one side of the piston and that will seal the barrel. Its similar to a regular pneumatic piston and how they seal.
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Unread postAuthor: saefroch » Fri Apr 02, 2010 8:38 am

Gotcha, thanks. And aluminium taps so easy, so that shouldn't be too hard. Being that i'm working on pistons a lot lately, I've gotta try that. Just gotta go get more rubber.
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Unread postAuthor: ramses » Fri Apr 02, 2010 9:00 am

remember, weight increases as the square of diameter, and as the diameter increases, the length must increase also. You should calculate the volume, then mass, of the piston before you make it.

And yes, I did tap the end and attach a piece of rubber. If you can find the rubber that is inter-weaved with fibers, that is much less likely to rip itself off and out the barrel. I had some trouble until I started using it.
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Unread postAuthor: psycix » Fri Apr 02, 2010 9:04 am

Solid aluminum?
I'd drill/lathe a volume out of the back and fill that with foam.

Or use two plates and a pipe (or hollow cylinder)
And run a threaded rod through the center that threads into the back plate, and sticks out of the front plate to bolt the sealing face on.
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Unread postAuthor: saefroch » Fri Apr 02, 2010 9:08 pm

I was actually thinking of using an aluminium nipple, sanding it down a wee bit by chucking it into a drill press, since it's .02" too large in diameter to fit into the corresponding pipe size, then cutting down a dowel to fit into that, then dampening the dowel and heat-sealing it into the nipple (with threads chopped off) with a blowtorch. Viable?- What I mean by that is could that type of design hold up and work in a piston hybrid- Aluminium schedule 40 if that's a structural issue.
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Unread postAuthor: Pete Zaria » Mon Oct 04, 2010 9:27 pm

Edit: Sorry, didn't realize I was kicking up a dead thread until after I posted. My bad.

ramses wrote:...otherwise the mix behind the piston could POSSIBLY ignite, forcing the valve closed and possibly causing your chamber to explode. at the very least, your rubber sealing face will be trashed, as well as maybe the seat of the valve


A pop-off valve or "safety" burst disk in the pilot volume behind the piston should safeguard against this thoroughly enough.

Getting an absolutely perfect seal on a piston is very hard to do. One thing in your favor is that, the combustion will likely start on the other side of the chamber from the piston, so hopefully, pressure will build up and start to move the piston back (opening the chamber to the barrel) before the flame front reaches the piston and could potentially ignite anything behind it.

I'd still put a pop-off or something behind the piston just to make sure.

Just my two cents, and take them with a grain of salt - I've never built a full-scale hybrid.

Peace,
Pete Zaria.
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