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Piston Valve Design

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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Piston Valve Design

Unread postAuthor: penquin » Wed May 19, 2010 12:28 am

Hey members of spudfiles! I am very new to these air cannons and was wondering if you could help me on my design of my piston. It is for a physics project, (high school) and is needed to go at least 300ft. My partner and I decided on trying to build a piston valve that could give us the edge on the competition. However, upon reading through the forum, I have seem to have run into a problem with our design.

Image

Due to money constraints and the need to fire a tennis ball, we could not order a 2.5" pipe online. So instead we decided to use 3" for the barrel and 4" for the chamber. We are using a 3" tee and will probably add some proxy or pieces of wood cemented on the barrel end to keep the piston from flying out. (Red marks.)

I have also read through the wiki but came out a bit confused. This seems like a 1 to 1 ratio valve but we were not sure because of our use of the stopper on the barrel end.

Will the piston just end up staying in place as described in the wiki or will it work?

If anything is confusing, please tell me and i will clarify. We are fully open to suggestions.

Thank you guys and gals!
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Unread postAuthor: Technician1002 » Wed May 19, 2010 12:32 am

The piston will remain closed as there is no force to open it. 1:1 ratio piston valve is a QDV or Quick Dump Valve. There is a page on it in the wiki. It does not use pressure in the pilot, but does require a mechanical force to trigger it open. Once it starts to open, the chamber pressure will finish opening it for you. The difficulty in building them is the chamber air has to be sealed from leaking to the barrel and to the pilot. This often requires o rings which is difficult to put on a peanut butter can.

Does your school have a machine shop with a lathe?

http://www.spudfiles.com/spud_wiki/index.php?title=Quick_Dump_Valve

One of the barrel sealer valves may work better for you with a peanut butter can. A reducer can be cut down to fit in the end of the 3 inch barrel to give you a 2 inch valve seat. A flat seal such as sheet rubber can be used for the barrel seal instead of o rings. A traditional trigger valve such as a ball valve or sprinkler valve can be used.

Don't forget to do the math. What pressure are you planning on using. Figure the area of the can exposed to the chamber pressure. Can the piston hold up to the pressure?
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pistondesign1.JPG
Modification to the design to permit a traditional piston.
pistondesign1.JPG (28.37 KiB) Viewed 887 times
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Unread postAuthor: MrCrowley » Wed May 19, 2010 1:18 am

I'd also like to point out that judging by your drawings, you're depicting DWW fittings, which aren't pressure rated. Be sure to read this article:
http://www.spudfiles.com/spud_wiki/inde ... sure_rated
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Unread postAuthor: nosnik » Wed May 19, 2010 2:12 am

can i use this design on a smaller piston valve or should i make one of those rubber things ( i forgot how to spell expongy or ... im an idiot) thingys?

btw i agree with MrCrowley about the DWW fittings,, not a good idea..
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Unread postAuthor: penquin » Wed May 19, 2010 7:07 pm

Thank you guys for the quick reply! :)

Technician1002
- My school does not have a lathe unfortunately.
- Thank you very much for your suggestion on using a spring and the image of redesigning our valve. We will most likely use that design.
- We are also using a 1" sprinkler valve modded with a blow gun.

MrCrowley
- Yes they were DWV fittings and so we went to return them today, the home depot staff said that DWV should hold pressure up to approx 120 psi but from reading forums last night found out that they have been known to blow at very low pressures. We have been looking every where for pressurized fittings and haven't found any yet. Thank you for saving us from being blown up. ;)

I will update once we get our pressure rated fittings.
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Unread postAuthor: MrCrowley » Wed May 19, 2010 7:49 pm

DWV should hold pressure up to approx 120 psi

DWV shouldn't do anything except what its standards define. It has no standards requiring it to hold that much pressure, only that it is strong enough to deal with the forces and low pressure of water mains etc I believe. So while it might be able to hold 120PSI on occassion, by no means should it.

As for pressure rated fittings, you should fine them in 2" and below at Home Depot or Lowes, sometimes even in diameters larger than 2". Apart from that, look for businesses which deal in plumbing. They're usually cheaper as well. Personally, I go to a place that does bathroom plumbing, fittings and fixtures. Out back they have the "trade store" for all the plumbing stuff.
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Unread postAuthor: nosnik » Wed May 19, 2010 8:33 pm

my DWV endcaps for air chamber exploded at 80 psi.. im not sure about 120 psi ...
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Unread postAuthor: grock » Wed May 19, 2010 10:04 pm

keep in mind that you might not need to build a piston valve, especially one of that size. I've got a TB gun thats only ~5' of 2" chamber and 2' of 2.5" barrel, with a modded sprinkler valve for a valve. That shoots about ~80 yards at 100 PSI. A 2" sprinkler or piston valve would probably be enough, and would be a hell of alot easier than making a 3 or 4 inch piston valve.

on a totally unrelated note, if you shave/ burn the fuzz of the tennis balls, they go alot farthur.

and @ JSR, before you make a joke, you already have on the last post of that page :D
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Unread postAuthor: Technician1002 » Wed May 19, 2010 10:28 pm

If you must use DWV because you can't find sched 40 PVC in sizes over 2 inch, may I recommend ABS? I test mine every year at over 80 PSI before the season of use at 40-65 PSI. I tested to 100 PSI this year trying to get a failure. It held.

Many big box stores don't carry pressure rated PVC in sizes over 2 inch. All PVC in larger sizes is DWV. Pressure rated PVC in sizes over 2 inch can be hard to find.

See the ABS cannon in my sig for details. Before the ABS VS PVC pressure rated war erupts, remember, pressure rated PVC is not rated for air pressure, so both are not rated for air pressure.
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Unread postAuthor: MrCrowley » Thu May 20, 2010 12:49 am

Isn't ABS even harder to find than pressure rated PVC in America?
Though I do agree that non-rated ABS is better than non-rated PVC.

pressure rated PVC is not rated for air pressure

I'd say more like it isn't insured for air pressure/covered by warranty. Although a 17 year old can drive a car, without a licence he wont be insured because he is more likely to make one helluva accident.
If the PVC can take 120PSI of water pressure, no reason it can't take 120PSI of air pressure, main difference is the failing characteristics, no?
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Unread postAuthor: Technician1002 » Thu May 20, 2010 1:00 am

Cellular core ABS DWV isn't too hard to find. It's what I made my test cannon from. It passed a 30 minute 100 PSI safety test this year.

The main difference is the failure mode. ABS DWV isn't rated for pressure, but it seems to hold up better than a soda bottle. Some day I'll burst test a piece.

30 minute 100 PSI safety test in progress on the ABS cellular core cannon.
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Test pressure
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Unread postAuthor: penquin » Thu May 20, 2010 11:11 pm

Thank you for your suggestion about abs Techniciain1002 but we have found a place to buy the "sch40 Portable water" fittings. Yay! So we will start working on our piston tomorrow.

We were also wondering if a check valve was necessary, or will it work with out a check valve. I have tried using search but it seems to run into an error when i have to many letters so it is hard to find something on the check valve.

Thanks!
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Unread postAuthor: Crna Legija » Fri May 21, 2010 1:24 am

you dont need a check valve if you piston isnt air tight.
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Unread postAuthor: penquin » Fri May 21, 2010 7:08 pm

Yeah we were originally going to not use one. however, just thinking about how if it isnt air tight the air would just flow straight out of the pilot valve. Is this right? We werent really sure.
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Unread postAuthor: Gun Freak » Fri May 21, 2010 7:11 pm

The point of using a check valve is for an airtight piston. When you fill the gun, it can get by, but when you pilot it, no air leaks around it because of the check valve, giving you the most effective piston. However, you can make a tight fitting piston with a small equilization hole so minimal amounts of air are lost upon piloting.
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