Piston Valves Explained Visually

[cfb_rolley]

Ahh I get it. So if my barrel is 30mm i.d and the I.d of the pipe it slides in is 50mm, then a piston 75mm long that slides back 45 mm should be fine?

[Technician1002]

If the barrel is 30mm, then by most accounts the valve is considered fully open when the piston moves 1/4 that diameter or about 8mm. For a fast light piston, one slightly shorter than the diameter works well. The piston in my Mouse Musket resembles a hockey puck in shape.

The shorter the distance the piston travels, the less speed it picks up in the travel, and the less speed it hits the back of the cannon. Excess travel distance is a bad thing.

The shorter piston has less mass so it opens faster.

Here you can see a piston that is shorter than it's diameter. It works well.

[jackssmirkingrevenge]

The shorter piston has less mass so it opens faster.

True, but it's also worth pointing out that for this to work well, the piston needs to be a pretty tight fit.

[Technician1002]

The photo of this piston in place is one page back.
It is a very snug fit, but still can move by gravity.

[cfb_rolley]

Well in aus, we have pressure pipe rated by a pn number, it turns out that 25mm pn18 joiners are a tight fit inside 40mm pn12 pipe, you can squeeze it in, but there's no way it would slide. So I was thinking of trimming one down, filling in the middle and sanding the outside gradually until I get a good fit. Don't get me wrong, i love combustion cannons but these seem easier to build than combustion cannons!

[Sevenhawke]

Thanks clide. I am new to all of this and that animation gives me a better idea of what I am dealing with

[AnotherSnappedLine]

On the T-setup, instead of air filling from behind the piston, couldnt it be spring loaded and filled from the chamber? (easier to experiment with chamber volumes)

[saefroch]

Yes, it could be. However, you may lose some performance because the spring restricts the ability of the piston to retract, though that may not be a factor, depending on the spring constant of your spring as compared to the forces on the piston.

[jackssmirkingrevenge]

I made an animation showing the basic function of my pen gun designs, sticking it here as it's a coaxial piston valve:

[cowscankill]

I would hate to have this as my first post... but I have a problem that has affected 4 variations of my coaxial gun.
For some reason unknown to me, the piston never fires backwards upon opening the pilot valve. I have used a 2" and 1" piston, both with hot glue and resin. They seal the barrel fine, and air flows into the chamber as well. However, whenever I open the pilot, all of the air flows out the trigger instead of creating a vacuum to suck back the pilot. I have reduced the pilot volume and used lube, wider, smaller pistons, etc. Nothing has worked.

My gun looks very similar to this: http://www.instructables.com/id/Piston- ... /?ALLSTEPS

If anyone can help, please do. I hate to seem like a noob, but I have built 5 air rifles before and have experience with wooden stocks. I just can't make pistons.

[Hubb]

If all of the air is flowing from the pilot rather than opening the piston valve, then you probably have the equalization too large. It seems that you have a little experience, and you certainly know how to search, so I recommend trying to fit an o-ring / check valve combo into the piston.

[Technician1002]

Umm, jack? How long have you been here?

Safety first. Add the projectile, then add the air. Other than the safety issue due to the loading sequence, I like the animation.

If all the air blows out the pilot without the piston opening, the piston is allowing too much air to pass it. Too much leakage is the main reason pistons fail to fire.

[Yamroll]

EDIT

So, I'm having trouble getting my barrel sealer to seal with rubber washers (no shock there), so I'm ordering some neoprene for it.

My question is about thickness/durometer. It's a 1.9" OD piston and a 1.65" OD sealing face on the barrel.

Any thoughts?

Right now I'm looking at about 50A, 1/4"...

[Secret Squirrel]

Cowscankill, what are you using for your pilot valve? It sounds like your problem is that air can flow past your piston faster than it can flow out the pilot. A tighter fitting piston would fix this, but it might be easier to just use a larger pilot valve.

My first piston valve used a 2in. piston. With my first pilot, a 1/4in. ball valve, all the air hissed out the pilot. I replaced it with a 3/4in. valve and it worked.

[Technician1002]

With a low leakage piston larger valves can be piloted with small valves. I piloted the Dragon with a 1/4 inch ball valve. The piston cylinder contained an o ring. Even with the smallish pilot valve the main valve opened hard enough to shatter the piston. That was a 3 inch piston with a 2.5 inch barrel seat.

Unfortunately trying to use a 2.5 inch domed pipe cap as a piston left a poor surface for a bumper. I do not recommend trying to use a pipe cap as a piston. All of my attempts eventually broke.