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Help Please: Upgrading My First Cannon to a Piston Valve

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Gun Freak » Wed Apr 27, 2011 6:51 pm

Gun Freak, thanks for the pointer. However, I just took apart my Lawn Genie sprinkler valve and have determined that the best place for the valve is in the center of the bottom. (Its flat)

Don't drill there... I'm pretty sure that just leads to the output, not the chamber. Make sure that where you drill is in the chamber.
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Unread postAuthor: pomplemus » Sat Apr 30, 2011 4:52 am

Well I made it yesterday after a surprisingly short trip to Home Depot. It went together pretty smoothly, but today when I began to test it, some problems came up. It appears the the 1-1/4" end caps I got for the ends of the piston are slightly convex on the inside, so the neoprene check valve in Hubb's plans leaks no matter what. Not too big of an issue, just some lost efficiency. It can be fixed later.

However, there is a bigger problem. I can't get the piston to make a good seal for the life of me. It leaks slightly until about 20-30 psi, and then it just lets go, dumping the chamber in a few seconds. I think that the neoprene seal is deforming enough at rising pressures that one of its edges just slips beyond the inside of the 1.5" PVC sealing face. I think this is due to a combination of factors.
1. The seal is currently juuuust big enough to seal at 0 psi, and I need to make it a little bigger.
2. Stupidly thinking "Oh I'll save some weight! :idea:" and proceeding to shave off the knurled edges of the 1-1/4" end cap behind the seal. This loss of support area for the seal my be allowing it to deform too much and start to leak.

I will experiment again tomorrow after I buy some more neoprene. I might just have to totally start over on the piston. I am REALLY glad that I made the piston serviceable, even if it did increase the pilot area.

If anyone has some helpful hints regarding piston sealing, please let me know! :?

Until then, some pictures of my progress. The second Schrader valve sticking off the chamber is there just because I didn't have a threaded 3/4" plug handy.
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Attachments
DSC05978.JPG
Close up of the fill valve and pilot area
DSC05978.JPG (16.6 KiB) Viewed 395 times
DSC05977.JPG
3 and a half hours later, the final product
DSC05977.JPG (16.13 KiB) Viewed 395 times
DSC05976.JPG
The parts I started out with
DSC05976.JPG (20.49 KiB) Viewed 395 times

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Unread postAuthor: Technician1002 » Sat Apr 30, 2011 8:04 am

To get a seal it is essential that the valve sealing surfaces are flat and true. Other sources of problems are leakage along the bolt that holds the washer on the face of the piston. This can cause the problem bubble under the rubber and resulting bubble in it.

The better the valve faces are flat and true, the easier it is to get it to close and seal. This is why valves are ground in gas engines and then the valves are lapped in so the seat can seal properly against the valve.

All my launchers get extensive sanding to make the valve surfaces flat and true.
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Unread postAuthor: mark.f » Sat Apr 30, 2011 9:14 am

So...

What tech is trying to say is, take out your piston and a flat surface (glass, sheet steel (new), etc.), slap some stick on sandpaper, and start sanding the face of your piston (minus rubber and washer) until it is flat. You can use a square (the tool) to check squareness as you go. Use progressively finer grades of sandpaper.

After this, glue the same stick on sandpaper to the face of your piston (minus rubber and hardware), and find a way to attach it to something you can grip outside of the tee. Some people like to use drills for this, but you can do it by hand quite easily, especially with PVC. Use progressively finer grades of sandpaper and square up the sealing face with the piston track.
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Unread postAuthor: Technician1002 » Sat Apr 30, 2011 10:26 am

Image

Example, 1 inch pipe cap sanded flat. You can see where it seated on the rubber o ring seat. The seat was lightly sanded too.
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Unread postAuthor: pomplemus » Sat Apr 30, 2011 1:06 pm

I sanded the piston before putting on the seal, and the 1.5" PVC seat was factory cut; already as smooth and true as it could get. Thanks for highlighting the importance of this. :) I may have to do some more sanding.

The reason I think the issues are due to an undersized/deforming seal is that when I pull out the piston, I see the ID of the 1.5" PVC imprinted all around the seal. However, the OD shows only about 2/3 the way around, before slipping off the edge. The seal isn't entirely circular, I'm not the best with scissors. :roll:

The edges of the seal are also totally unsupported, extending beyond the 1/1/4" end cap.

I'm buying some new materials in a few hours, I'll let everyone know how it went. :shock:
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Unread postAuthor: Technician1002 » Sat Apr 30, 2011 4:59 pm

Sometimes the T or coupler is not perfectly straight due to injection molding and manufacturing tolerance. This is why I sand the end of the barrel after the coupler is installed. The breech must be square to the piston travel.

On the Mouse Musket, a coupler was shoved into the heated and softened end of the chamber. I got it close, but not perfect. The end of the barrel was sanded once more after assembly for a good seal.
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Unread postAuthor: clemsonguy1125 » Sat Apr 30, 2011 6:02 pm

Make sure you have a really good bumper, I forgot to mention that earlier.
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Unread postAuthor: pomplemus » Sat Apr 30, 2011 9:48 pm

Well I redid about half the piston. After a couple of attempts and the subsequent readjusting, I finally got the piston to seal. I pumped it up to 60 psi aaand... nothing. Just the hiss of the pilot valve for a second or so, and then BANG!!
It scared the crap out of me, I wasn't expecting any thing at all! :shock:

After getting over the extremely loud sound, I took a look at the piston. The seal pulled away from the piston a bit, nothing some glue can't fix. I'm writing this as I am waiting for the glue to dry. :D

In conclusion, it worked once so far. I'm still expecting a few more bumps in to road to reliability, but hey, IT WORKED!!

Clemsonguy, my bumper is a few inches of foam followed by half a tennis ball. Is that enough?

Thanks everyone for the help so far, I expect I will be needing more. 8)

EDIT: Well after an hour of trying I can't get the piston to seal again. :x
I haven't lost all hope. I can see a small scratch/indent on the seal, so that might be the issue. I will make another seal (I bought plenty of neoprene) but I tried something else first. I slathered the seal with a Permatex gasket maker, and pushed the piston onto the sealing face. My hope is that the softer gasket material will create a seal more easily. I doubt it will work, but its worth a try.
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Unread postAuthor: Vincenzo » Sat Apr 30, 2011 11:05 pm

Is there a hole in the half tennis ball?
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Unread postAuthor: pomplemus » Sat Apr 30, 2011 11:08 pm

Yes, several. A 1/2" in the middle, and four 1/4" holes around that.
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Unread postAuthor: Technician1002 » Sun May 01, 2011 12:08 am

I've had better luck with seals that are flat and not too soft. Soft seals get crushed and cut, then don't seal a second time. To prevent the seal from peeling off the face use a bolt or washer that covers most of the washer inside the barrel so it has little exposed area to bulge into the barrel.

Almost all of the rubber in the center is covered by metal. Only the edge that touches the valve seat is exposed.
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Unread postAuthor: pomplemus » Sun May 01, 2011 3:07 am

I will definitely do that, Technician. I'll probably grind it out of a metal plate; gives me an excuse to use my Dremel. :lol:

Well after the gasket maker cured, I slathered the whole thing in grease, physically pushed the piston onto the seat, and closed it up. It still leaked! This was getting ridiculous. There was almost no way that air could possibly be leaking past the seal with all that I did. With a sneaking suspicion, I poured some water down the barrel and peered in with a flashlight. Bubbling was happening only when the water covered the bolt in the middle. Suspicion confirmed.

Its the hole around the bolt that's leaking, not the seal! I'm kicking myself for not checking this earlier, let alone sealing it up in the first place. :oops:

I covered the bolt threads, nut on the outside, hole in the plug, and all with my trusty gasket maker. No way air is getting by that bolt on my watch! If the piston still leaks when I check it in the morning, I'll be all out of ideas. I'm feeling pretty good about it though. 8)

On a related note, my bike pump heated up so much from me furiously pumping and trying to get a good seal, that it melted a hole in the plastic. :shock:
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Unread postAuthor: clemsonguy1125 » Sun May 01, 2011 10:44 am

I would add a spring behind the piston along with your other bumpers. I will eliminate the furious pumping needed to seal the piston. Also look again at superna. Your piston is like his and he used a tennis ball, hose, and racket ball I think. I know the racket ball got tore up after a few shots.
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