Piston Problems

[Suedo]

Hey guys, I'm having trouble with not only piston design but piston actuation as well. Every piston I have designed so far has failed as instead of pressurizing the chamber all the air leaks out of the barrel. The pneumatic cannon I have built uses a 1.5" tee that reduces to a 1" barrel. Below I have attached pictures of some of my failed piston designs and a picture of my cannon. How can I build a working piston? I have seen many forums on how to machine a piston but I don't have access to machine equipment or a lathe. Is it just an issue of getting the perfect fit? Are there any PVC parts I could buy that may create that perfect fit inside the 1.5" tee? Thanks.

[fenrir]

Make sure whatever piston material you're using is soft enough to seal at whatever pressure you are using (don't use electrical tape). Also, try to keep your pilot volume (the space behind your piston) low. I think 1 1/4 inch Pvc will fit into 1 1/2 inch, but you should double check. Lastly, your piston should have a snug fit, but still be able to move, when you gently push it with your finger.

[jackssmirkingrevenge]

I think the basic issue is that your piston is awful. Well, as you're in Syria I would think supplies are rather hard to come by

Have you considered casting one out of epoxy?

[Suedo]

Fenrir, what do you mean by soft enough to seal? I intend to use the cannon to it's full potential which from what I understand is 100-120psi. Also what are the physics behind having the pilot volume small?

Jack, Do you have any experience with Bondo? Would it be similar to that of a piston made out of epoxy as you suggested? Would it be able to withstand 120psi?
P.S. I am originally from Syria, I immigrated to the states a while back.

[jackssmirkingrevenge]

P.S. I am originally from Syria, I immigrated to the states a while back.

Lucky you

Bondo piston you say?

Have a look.

[fenrir]

I just meant to use rubber for sealing. A small pilot volume means the piston will respond faster when your cannon is piloted, which means better performance.

[Suedo]

I just casted my first piston out of Bondo it's fresh out of the mold I put it in so it needs some filing and sanding. I'm going to epoxy rubber to the top of the piston for it's sealing face. Is there any recommended thickness for the sealing face? I have really thin rubber from an inner bike tube that's probably 1mm thick. I also have a block of rubber that's around 1/2in thick. What would be better for piston? Btw the 1/2" thick rubber is really hard to shape and cut so it wouldn't be as precise as the 1mm rubber.

[jackssmirkingrevenge]

I also have a block of rubber that's around 1/2in thick. What would be better for piston?

Use this! Also, don't just epoxy it to the piston face, put a bolt and washer there (obviously using a washer of smaller diameter than the inside of your barrel).

Btw the 1/2" thick rubber is really hard to shape and cut so it wouldn't be as precise as the 1mm rubber.

Doesn't really matter as long as it is big enough to seal against the whole outer diameter of the barrel.

A good compromise is truck tyre inner tube, usually about 1/8" thick.

[Suedo]

Well, after hours invested on the Bondo piston it broke when I was tapping the hole for the bolt. I'm glad it did I guess because if it was structurally weak then I would not want it in the cannon anyways. It was probably because of air bubbles trapped within the Bondo. Any tips for preventing air bubbles or just casting Bondo in general? Also, would I be able able to just insert it in the mold and let the Bondo cure around it? Or does the rubber need to be bolted in?

[jrrdw]

Fiberglass resin when mixed using manufacturer's instructions can be drilled and tapped. Ask the Corvette people! Bondo isn't your best option because it is brittle but if it's your only option then yes put your sealing face set up (seal + center bolt) in the mold then pour the Bondo.

After pouring the Bondo stir in the mold with something thin slowly to work out air bubbles. You know your way from there but I do reccomend using fiberglass instead.

[jackssmirkingrevenge]

Also, would I be able able to just insert it in the mold and let the Bondo cure around it? Or does the rubber need to be bolted in?

I have done that in the past but nothing beats having a bolt through it. You can do both, put a bolt through the rubber then stick in into the bondo as it sets. Make sure the rubber is perpendicular to the mould walls though or you will have sealing problems.

I would search for a more liquid epoxy though, it makes casting much easier - or search for a friendly neighbourhood machinist who will quickly spin one up on a lathe for you for a couple of bucks

[Suedo]

Sigh, the piston has defeated me again. The piston in the picture below moves nicely with a 2 finger push and I was satasfied with my work until I tried filling it. Air leaked out of the barrel again, not an extreme amount of air flow like last time, but to the point where pressure won't build up.

[Anatine Duo]

I've had the same problem more than twice...

Use softer barrel sealing material? Might not last as long but has taken my launchers from "doesn't fill or only fills with rapid air" to "works"

FIll with air faster?

Wrap with some tape just to see if the pilot pressure will seat the valve better? Annoyingly this has worked for me a few times, annoying because it shows shortcomings in my construction and I normally hate tape.

[MrCrowley]

Sigh, the piston has defeated me again. The piston in the picture below moves nicely with a 2 finger push and I was satasfied with my work until I tried filling it. Air leaked out of the barrel again, not an extreme amount of air flow like last time, but to the point where pressure won't build up.

You might have to epoxy the bolt and washer on the sealing face. That's a fairly common problem but not always necessary, better than chucking the piston out however.

[Suedo]

Taking a break from this build and looking at it with fresh eyes really helped me out. Playing around with it I discovered that the piston was just slightly too loose and when pressurized the piston would not seal the barrel because too much air would leak around instead of forcing it up against the barrel. Using Anatine Duo's fix, I applied a single piece of electrical tape around the bottom of the Piston. I lubed up the area in which the piston slides and went to pressurize it and I heard it slide up like a charm. However, the back of the pilot valve ejected off the gun. Of course, it is not yet glued all together because of my trouble with the piston. So, my final question is: Am I ready to put it all together permanently? Or should I be concerned about the pressure in the back of the cannon? My piston fits the two-finger push criteria so I don't believe I have much to worry about.
Thanks.