Attempt at a pneumatic cannon

[aelous]

I used to play with a combustion cannon that my father and I had made, now recently I have come across a need for a pneumatic cannon. I found a design I like (Aqualab cannon). I tried to rebuild it, only using a 2" barrel. After many interesting dilemmas I have changed it like this: I am using a screw on end cap on the back end of the 3" chamber. I have installed a brass 3/8" "T" joint that has a schrader valve on one side and a 3/8" ball joint on the other. I found that the cap to our laundry detergent fits perfectly into the chamber. I have taken that cap and used 1/8" rubber (I believe it's a plumbing gasket material) to try and create a piston. The piston can move around 1" right now. I have a 1/16" equalization hole in the piston. The piston (which leads to my current problems) is the cap that has been cut down a little has 1/4" rubber on the inside (one layer), and then on the side that should cover the barrel it has a rubber grommet followed by the 1/4" rubber described earlier that should be sealing against the barrel, all bolted together with washers. Here's the problem: The piston will not seal against the barrel, and the foam ball I'm using as test ammo will simply pop out after two pumps from a bike pump. I have also used an air compressor to no avail. The piston will simply not seal. In my searches to figure out a solution I came across this website.

Some questions:
Should I be using a smaller barrel? The whole gun together is 3' long, which when you have a 2" barrel inside of a 3" chamber is a relatively small chamber. Could this be my problem?

Do I need to restrict the movement of my piston?

What can I do to make the piston seal better against the barrel?

Thanks for the help in advance, I realize that my description is relatively vague, and I will try to get pictures up as soon as possible, but for a diagram you can picture the cannon on the aqualab page, as that is my basis.

Thanks,
-Aelous

[BigGrib]

Umm I dont know a whole lot about pistons but i have seen a few diagrams and have a general understanding of them, I think your problem might be solved by decreasing the piston travel and possibly the addition of a small spring just to help the piston to seal against the barrel while your pumping the chamber. another thing is your equalization hole may be the wrong size (ie too big or too small) I'm thinking maybe too small, but I'm inebriated so I might just be talking to talk. who knows. peace

[dongfang]

Hi,

I have had a similar problem with a piston valve I made (search for Schweizer-Valve). It turned out that my pistion - which is very hard - did not seal perfectly flat against the barrel end. Well, in my valve, the rubber seal is an O-ring sitting in a groove in a reducer around the barrel end, but all the rest is standard.

The solution was to get some carbon paper, and carefully marking where the piston touched the barrel end reducer, then filing the reducer a little down there, etc, until they sat rather flat against each other. I marked up everything so I would put it back together in exactly the same way in each test and correct cycle.

I ended up filing away over 1 mm on one side.. tees are not 90/90 degrees here. Finally, I epoxied the reducer in the tee, using the piston to support it at the back while putting a big weight on it at the front. That worked.

I also had another valve that wouldn´t seal by itself. I fixed it temporarily by putting on the pilot (sprinkler) valve backwards, and putting a small chamber on it at the other end. Then, I could "shoot" the main valve shut and leave it with pressure on overnight. It helped to press the O-ring into shape.

Regards
Soren

[Sparkie]

I am using the rubber out of tap washers to seal my barrel face and if it doesn't line up perfectly it just will not seal. The rubber from tap washers is very hard to compress. Did not matter how much air I pumped into the chamber it just kept leaking out of the barrel.
Also using a bike pump you may not be getting the volume to seal the piston. Try a compressor if you can.
your equalization hole in the piston must be drilled outside of the barrel diameter. I just mention this as it it could be on of those "Doh" moments.

Good luck with it.

[pizlo]

Could someone clarify to me and him what the ideal piston travel distance is?

[benstern]

"the full flow potential of the valve is realized when the piston has moved back 1/4th of the barrel's inside diameter."

[pizlo]

so for me thats 1/4 of 1/2 of an inch... Umm, I don't want to have to use my brain until tomorrow.
1/16 of an inch?

[fireman565]

I believe that's an 1/8".

[jackssmirkingrevenge]

Could someone clarify to me and him what the ideal piston travel distance is?

The math has been done for you here , and you might also want to take a look at the footnote.

[aelous]

I was able to fix mine. I still will try and get pics, but here's what I did: Picture my piston from before, it was loose in the chamber, and quite hollow. I filled it in with hot glue, and put a washer on the back as well as the front. Then another little modification I made was to fill in the sides of the piston with a marine lubricant, and the front of the piston head. It shot fine like that for a little while, and then the piston snapped (the detergent plastic lid). Luckily we had another floating around, so as soon as I put that one in, I added a layer of 1/8" plumbing gasket as well as a small layer of plastic from a Tupperware lid. Picture this in this order: 1/8" rubber, plastic, washer, hot glue, detergent lid (all mentioned before are stuck in the lid), washer, 1/8" rubber, 1/4" rubber, washer (all with a bolt running through and a nut holding it together). The 1/4" rubber side is the side that seals the barrel. I've pressure tested it to 80 psi, and was able to shoot a foam ball across the street. I will get pics up as soon as I can. It works though. I still have a problem with the marine lubricant running thin around every 20th shot, so I have to re-open it and lube it up again. Anyone know any fixes to this? Thanks again.

[dongfang]

Hi

I love silicone grease, also called O-ring grease. It lasts very very long .. much better than that spray-on stuff. Others swear to lithium grease, which might be just as good or better.

Wow that´s a complex piston... my pistons are one part only

Regards
Soren

[aelous]

I've pretty much given up on the permanent fix to my greasing problem. I have found though that heavy duty wheel bearing grease stays put longer than the marine grease, so that's what I now use. I have also now pressure tested my cannon up to 95 psi, and it held. I only had a small leak at the end with the screw on end cap. A little grease in the threads fixed that nicely.

[dongfang]

Hi,

OK great you had some success with it, but ... hmm some mineral oils and greases deteriorate some plastics.

Regards
Soren

[aelous]

I just checked and the grease is a fibrous sodium grease, anyone know if that'd cause problems?

It went through a rigorous test this weekend when I took it to a Nat'l Guard training weekend and used it to fire foam footballs at Humvees as a makeshift RPG. It worked fine, I just found a slight problem with my end connection that I'll need to fix with some epoxy. I'm thinking about exchanging my 3/8" ball valve for a solenoid and putting on a switch later down the barrel to make it resemble an M72 LAW. I'll get pics up as soon as I get it back from the armory, during the last iteration they took it with them, and I didn't realize that it went with them until after the training.

I've still got piping, and I'm going to try and build another one as soon as I can put together the money for pipe fittings.