SpudFiles

Are you absolutely sure your pilot isn't leaking to atmosphere?

If someone gives me an inventor assembly, I can get you a drag force at at whatever velocities you like.

ramses wrote:Are you absolutely sure your pilot isn't leaking to atmosphere?

I'll be checking that today when I re-assemble it. Last I checked (couple of months ago), the pilot dropped from 270PSI to 240PSI over 5minutes, after that it was leaking so slowly it didn't matter.

I wonder if Guinness World Records would accept this as a record for "farthest distance for an object fired from a spudgun"
Knowing their criteria, it is probably too vague as they like to have at least one control. For example, there's a record for farthest distance to fire a pumpkin, with a pumpkin being the control. With my 'experiment', neither the cannon nor projectile could be controls and repeated by others.

(Reply to earlier posts...)

I did some brainstorming and I think I figured out your low opening pressure problem. Ok, I guarantee I figured it out*


First I'd like to direct your attention to an old high-speed video by clide:
http://gbcannon.com/projects/flipbooks/clearvalve.html

As you can see, when a sealing face is not secured to the rest of the piston completely, the face and piston will not move together in a uniform way. It will move long before the valve actually "opens", which in your case will cause it to dump the pilot volume.
Below is a quick approximation of your valve and what it will most likely do upon firing. There are a couple solutions to this, but the easiest is just to glue the face down.

*Guarantee covers both your entire purchase price and eternal salvation.

Unfortunately, my sealing face is epoxied to the washer behind it and I just went up to the garage to double check the epoxy hasn't come unstuck. The sealing face is quite secure and except for a chunk of epoxy 2mm x 2mm that has been removed, the epoxy isn't cracked.

Interesting solution though, not one I would've thought of.

Ok, that should mostly rule out the flexing issues, but it still could be the seal on the pilot instead. You may eventually want to replace your current pilot with a QDV-ish design. A couple washers, orings and a short length of copper pipe with porting holes drilled in the side would work fine.

Ah yes, It had been recommended to me before to go with a QDV pilot design so hopefully this will be the motivation I needed. I'll see what I can whip up this afternoon.

AAh, but you forgot the force acting on the back of the sealing face. That is 806 lbs pushing to the left.

(we are all neglecting the diameter of the rod, but whatever.)

Okay, I'm back with some numbers.

Pilot charged to 260PSI, dropped to 255PSI over 5 minutes, didn't drop any further.

Pilot charged to 260PSI, chamber charged to 40PSI, pilot dropped to 255PSI over 5 minutes once again.

Pilot charged to 110PSI, chamber pumped slowly up to 145PSI before the piston opened prematurely. There was a slight leak out the barrel port (roughly 1PSI/minute) and I monitored the pilot pressure while the chamber was being pumped up; there was no change or fluctuation in pilot pressure until the piston opened.

Back in November when I filmed the cannon up at my friends bach, on one occasion the piston opened prematurely at around 130PSI with 220PSI in the pilot.

Even if there was a problem with the sealing face, I can't think of why that would open the valve.

Edit: If I were to get a piston machined, what would be some recommended materials to machine it from? Any type of metal would probably be too expensive.

... But plastic definitely wouldn't last... And there's not much else you can use. If you're gonna get one machined I would think aluminum would be your best bet.

Polycarbonate, HDPE, Delrin Acetal, Nylon, and some other plastics are pretty strong, but I have no idea how they would hold up to the repeated impacts of being slammed around in a hybrid. I'd sure they would all be able to handle the gas pressures involved, but I think you might need some insane bumper to dampen the impacts repeatedly. I feel like HDPE might wear down, polycarbonate could crack and fall apart eventually, and I haven't had enough experience with other plastics to know what would happen to them.

Unfortunately 2" solid rod stock of any material isn't the cheapest thing in the world.

I feel like aluminum would be solid, but eventually it might get squished a tiny bit lengthwise, which could increase the diameter just enough to keep the piston from sliding.

You might be right about the aluminum, but I don't think plastic can take the heat generated in Crowley's hybrid.

I think heat should be okay; I mean, the rubber has to take it.

I would suggest aluminum. I might be able to get a bar end from work for cheap, which I could machine, and then ship over there. The bumper might need a revision to cope with the increased mass. You're probably looking at at least one pound. I can't even drill out the back, because the pilot plug has to be there.

I still want that CAD model! (this means you, Dewey!)

I had an interesting idea for a piston. You can substitute the ABS plastic based on necessity. It might be worth looking into though. The impact absorption would help extend the lifetime of the piston. Tell me what you guys think!

EDIT: This was with intent of being used in MrCrowley's Hybrid.

ramses wrote: I would suggest aluminum. I might be able to get a bar end from work for cheap, which I could machine, and then ship over there. The bumper might need a revision to cope with the increased mass. You're probably looking at at least one pound. I can't even drill out the back, because the pilot plug has to be there.

If that ends up being an affordable option I would be interested, could work out cheaper to buy from you rather than pay for some stock here and then get someone else to machine it for me.

I just realised today that the piston isn't exactly straight. Since there is a tiny bit of play between the bolt and the inside diameter of the washers, when everything was epoxied together it ended up with a slight lean. It only became noticeable once I put the piston inside its barrel nipple housing with the sealing face sticking out. The sealing face ends up being on a 3° (ish) angle. I'm surprised the piston ever worked in the first place

I don't think it is the cause of my leak out the barrel today though, I think the sealing face has had it. The seat has cut about 4mm in to the rubber.

MrCrowley wrote:
Edit: If I were to get a piston machined, what would be some recommended materials to machine it from? Any type of metal would probably be too expensive.

what dia is your piston, i just started a new job at a high pressure aluminium die casting place, we a few hundred kgs of 2.5'' od about 10cm of alu that we remelt. If you want i could ask if i can take one and send it ino who can machine it then he can send it to you.

i think Polycarbonate
would be better, good heat resistance and great impact resistance.