Help Please: Upgrading My First Cannon to a Piston Valve

[pomplemus]

Hey everyone, allow me to introduce myself:

I'm Adam, and about a week ago I created my first pneumatic cannon using 3/4" PVC for both the chamber and the barrel, as well as a 3/4" sprinkler valve. This was to test out my construction techniques, and give me some experience with a small cannon before moving on to bigger things

It lasted about a day before the barrel and chamber were both upgraded to 4 feet of 2" PVC.

I have previous experience with water rockets (my personal record is 460') so migrating to cannons wasn't too huge a step for me.

I now have two goals:
1. Make the cannon more compact (the current configuration is about 9 feet long)
2. Make the cannon more efficient/powerful without making it bigger

I discovered the wonderful world of Spudfiles a few days ago, and have been reading as much as I could since then. When I stumbled upon piston valves, I knew this was what I needed.

I designed it in paint, and I think that it should work, but I want to run it by the experts before I actually make it.

I plan on the piston being a duct tape wrapped caulking tube filled with hot glue, and the seal being attached with a nylon bolt and super glue. The bumper will be made out of some foam insulation.

By some miracle, my local Home Depot has all the materials in stock, at least when I check online. I did not expect them to have half of what I needed, but perhaps it was meant to be.

I based my design on Mr. Crowley's instructable, many thanks to him and all who answer.

Thanks again for the help! 8)

-Adam

[mark.f]

Just to verify, is the bottom or the top piece of 2" pipe in that diagram the chamber? Spudfiles is going all wonky on me and won't give me the larger view of the diagram, so sorry if you mentioned there.

[pomplemus]

Oops, should have labelled that.

Its a barrel sealing piston, so the top blue pipe is the chamber.

Thanks for catching that.

[mark.f]

Then yes, it should work fine. You will have to perform some modification on that bushing to allow it to pass farther into the tee and allow space for the 2" barrel, though.

[clemsonguy1125]

Diagram looks good except for a couple things. I would take a look a gun freaks superna. Your pilot volume is a bit big as well as that piston will not work up to the guns full potential. I personally make a version of hubbs piston valve.
http://www.spudfiles.com/forums/superna ... 20950.html
http://www.spudfiles.com/spud_wiki/inde ... yay0yzvdqt

[Gun Freak]

If you plan on keeping that massive pilot volume I would move up to a 1" sprinkler valve. However I recommend just configuring the pilot a bit differently to make less volume like I did on Superna (link above). Oh and also I don't know if the fitting to the left of the tee is a female adapter or a slip coupling, but just for clarification it should be a female adapter which enables the use of a threaded bushing so you can service the piston. Other than that... it looks like the run of the mill 2" piston valve Good luck! Keep us posted, and if you run into any problems or aren't sure on anything, just ask one of us

[Technician1002]

Many excellent points are already pointed out. An additional item is in regards to the location of the valve seat where the piston and barrel breech meet. The diagram shows the valve closed centered in the T. It looks good on paper but it is not the best location. You want the gap of the OPEN valve centered in the T for best flow when open. Since the piston moves and the valve seat does not, this would place the valve seat off center in the T so it is near one side.

If for example the piston opens so the gap between the piston and valve seat is 1/2 the valve seat diameter, then when open the piston would extend 1/4 the way into the T from the breech and the barrel seat would extend into the T 1/4 the way from the other side. When closed the piston would reach 3/4 the way through the T to the seat.

If possible use a larger T so the valve seat is the full barrel diameter so the valve does not choke the flow when open.

[pomplemus]

Thanks for the suggestions so far.

I'm going to try to reduce the pilot area. If I can find a 1/4" NPT Schrader valve, I'll thread it directly into the sprinkler valve like Gun Freak did. If I cant find one, I'll just attach my current fill valve into a 3/4" tee between the back of the piston housing and the sprinkler valve.

I really like hubbs piston, simple and efficient. However, I don't think any local stores carry the 1-1/4" slip plug. Currently, its not worth it for me to order that part online just to make hubbs piston. I like it, but I think I'll do that later in the future after I have experience with a simpler piston. 8)

Technician, I think I understand what you are saying. I can't use a bigger tee, because none are available locally. Ordering online is currently beyond what I want to do.

Regarding the piston position, is this what you mean? (New diagram includes reduced pilot area and larger bumper)

[Gun Freak]

I'll thread it directly into the sprinkler valve life Gun Freak did.

If you go with that, try threading it in where the attached picture shows.

I don't think any local stores carry the 1-1/4" slip plug.

Really? It is a very common part.

[pomplemus]

Correction: they may carry them, but all stores in a pretty large area are out of stock. I'll check to confirm when I'm actually in store, but I think for my current purposes/ability the simple leaking piston design will suffice. Definitely in the future though.

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)

[Technician1002]

Regarding the piston position, is this what you mean? (New diagram includes reduced pilot area and larger bumper)

Umm, somewhat. You overshot the postion. Push it back into the T so the closed gap is off center but still in the T and not in an end.

[pomplemus]

So the 1.5" PVC should stick into the 2" tee a measure between the original and the edit? Like This?

The way I understand it, the gap between the piston sealing face and the barrel sealing face when open should be 1/2 the diameter of the interior of the tee. (2.375/2=1.1875")

EDIT: Wait, I think I get it now. You don't want the open gap to be smaller than the barrel seat, because that would reduce flow, At the same time, you don't want it to be bigger because that would introduce needless dead space. In my case, that is 1.5". (The diameter of the piston seat)
Am I Right?

[Technician1002]

The new drawing is correct. The big problem with it too far forward is when the piston is opening, it is blocking flow to the barrel until the piston pulls out of the end of the T.

[mark.f]

I really like hubbs piston, simple and efficient. However, I don't think any local stores carry the 1-1/4" slip plug.

I've actually made a piston from 1-1/4" fittings in the same situation. I just substituted the 1-1/4" plug for a 1-1/4x3/4" bushing and a 3/4" plug, which I trimmed to length and ground the lip on the outside down so it fit flush in the bushing.

It would be a good idea to make a piston out of 1-1/4" fittings because it allows you to easily create an o-ring groove, which will make piloting the valve much, much easier. I could pilot mine quite effectively with a 1/4" ball valve and quick-disconnect.

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Here's the piston I told you about. The 1-1/4"x3/4" bushing was also ground down and trimmed to length so it sit flush in the 1-1/4" coupling.

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And here's it piloting through the aforementioned 1/4" ball valve and quick disconnect...

[youtube][/youtube]

[pomplemus]

Thanks for the alternative, Mark.
Well, it seems everyone is really pushing Hubbs piston design even though its my first. Alright, assuming I find all the required materials, I'll build it.

I'm having trouble finding the o- ring though. If i'm correct, the 3/16" x 1-1/2" o ring required is the #322- which I cant find anywhere. Is there a common o ring size that could work in its stead that I could find in a Home Depot?

I suppose I can test fit in the store, but I'll have to do it sneakily becasue I don't like the looks the HD people give me when they see me doing that.