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Designing piston valve (need a little advice)

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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Designing piston valve (need a little advice)

Unread postAuthor: niglch » Mon Jan 28, 2008 12:04 am

This weekend I decided I was going to try to make my first piston valve so I went to Home Depot to get some fittings, epoxy, neoprene washers, etc. Unfortunately, I couldn't find any 2" plugs or 2" T's that didn't have threads. Impatient as I was, I instead bought the T fitting anyway and decided to go with a female adapter to end cap design for a plug. I actually found the square part of the end cap to be advantageous for design purposes. I'm going to try partially filling it with epoxy and placing a metal spring in the center which will be attached into a piston on the other end. I also want to try to use epoxy to attach a 2" coupling to the back of the cap. From there I can just buy some pipe and fittings to make a nice stock.
I have a few questions though. As far as the threaded fittings, could I just use PVC cement on the threads to make a secure permanent attachment? Also, because I want to put a stock directly behind the piston, could I drill the holes for a Schrader valve and hose barb (to attach a blow-gun) through the side of the female adapter after I cement everything together? This way, the air would be entering and exiting behind the piston itself but the valves wouldn't have to stick directly out of the back of the end cap (so I can make my stock there). Also, will a thin epoxy piston (like 1/4") be ok? The epoxy tube I bought contained less than I thought :oops: .

On a side note, would anyone happen to know any stores that sell fittings that are actually pressure rated? The guy at home depot said that all of their PVC (including fittings) was SCH-40. However, the only fitting that I found there which said "SCH-40 NSF-PW" was a 1.5"x1" bushing. All of the other fittings I needed were labeled "NSF-dvw" which are pretty much not recommended at all for pneumatic cannons especially. Are PW fittings common or will I need to order them?
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piston valve.png
Here's a diagram of what I'm planning on using for my piston.
piston valve.png (25.01 KiB) Viewed 768 times

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Unread postAuthor: jackssmirkingrevenge » Mon Jan 28, 2008 12:21 am

Your piston's going to have to be wider than that in order to be stable in the chamber, I would say at least 1" long - remember, it only has to move back 0.25" in your case for maximum efficiency.
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Unread postAuthor: mark.f » Mon Jan 28, 2008 6:52 am

And there's still one glitch I see... nearly all your parts are not pressure rated. That's especially bad for a valve, due to the forces in addition to the pressure.
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 28, 2008 7:27 am

In order to save on epoxy, you can use a block of wood in the middle of the piston casting to make up the bulk of the volume, and just use the epoxy to fill the space between the wood and the chamber.
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Unread postAuthor: SpudFarm » Mon Jan 28, 2008 10:40 am

do not use wood in a piston i did it and it cracked after 5 shots.
get a aluminium plate and cut rings to fit the piston housing then make so you get epoxy in front of the ring. that`s what i use now and i have about 300 shots on it. you can also use just the plate and a sealing face
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Unread postAuthor: bluerussetboy » Mon Jan 28, 2008 11:34 am

spudfarm wrote:do not use wood in a piston i did it and it cracked after 5 shots.


wood will work in a piston, especially as JSR recommended as a filler. larger diameter pistons made of nothing but wood have a greater chance of failure due to variety,grain and available dimensions.

in the US most wood that your average spudder is going to come across is dimensional douglas fir or hem fir. 2X2,2X4,2X6,2X8,2X10 and 2X12 these are roughsawn dimensions, after final milling the dimensions are 1 1/2" X 1 1/2", 3 1/2", 5 1/2", 7 1/4ish", 9 1/4ish", 11 1/4ish" respectively. considering most of this lumber is taken from young trees it still has a fair amount of moisture and will dry out and become brittle and split fairly easy. even if it is kiln-dried it will still split, it just isn't going to shrink much. this stuff is cheap and should not be used for pistons >1". occasionally you might get lucky and find some dowel material that is tough, not usually.:cry:

now if you were to get into fancy grades of lumber such as Brazilian hardwoods(several to choose from) or American hardwoods(maple,oak,walnut,teak,mahogany,cherry, and many others) and to cut your piston flowing with the grain they should last for quite awhile. this would be quite expensive seeing you would require special milling for larger than standard dimensions.

microllam variants should work fine also. these are layers of wood glued together alternating the grain and then compressed. cutting against the grain should provide a strong piston. the smallest microllams i've seen are 1 1/4" thick. larger variants 5 1/2". scraps can usually be found on jobsites. these contain glue that can be toxic if burned. :wink:
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Unread postAuthor: SpudFarm » Mon Jan 28, 2008 1:05 pm

it is just as easy to find a little piece of 1/4" alu plate on the place you find the wood.
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Unread postAuthor: mark.f » Mon Jan 28, 2008 4:50 pm

I support the wood idea because it can be easily worked with. Two different sized hole saws, some O-ring cordstock, a bolt, some polyurethane, wood glue, and some good quality 3/4" oak or birch plywood would work great. Here's what I'm thinking.

Determine the overall length you want the "body" of your piston to be.

Choose a good size of cordstock. If you are using 1/4", choose hole-saw sizes that are approx. 7/16" apart from each other. Cut the two smaller diameter circles from a single sheet of plywood. If you other parts are to be longer than 3/4", layer the plywood, and glue. Set overnight. Then, cut the other parts, (larger diameter), from that. There should already be a hole going through each disk if you used an arbor bit, so simply widen them to the bolt size. Cut, file, or plane the two smaller diameter disks to 1/4" thickness. Sandwich them all together, and bolt in place. Coat the thing in multiple coats of polyurethane, (mostly to protect the wood from any lubricant you will use in the valve), and then install the sealing face. Install the O-rings in the formed grooves and you're ready to roll. O-ring groove depth can be adjusted with a square file and some teflon tape. Drill an equalization hole through the piston or nick the O-rings in a spot.

Of course, depending on the piston size, this can be accomplished with PVC fittings and other parts as well.


Also, work on the DWV thing. That parts you currently have are not pressure rated, and should be scrapped.
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 28, 2008 11:53 pm

spudfarm wrote:do not use wood in a piston i did it and it cracked after 5 shots.


Could be a result of sh!tty wood, too much pilot space allowing the piston to accelerate too much and improper use of a bumper to deaded the piston impact.
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Unread postAuthor: niglch » Tue Jan 29, 2008 7:43 pm

Thanks for the feedback!

I'm definitely going to get some pressure rated fittings as soon as I can find them. Hopefully another local hardware store will offer a better selection.
I am also assuming that my proposed Schrader valve and hose barb placement won't damage the integrity of the valve since I will be drilling through the side of the cap and female adapter instead of through the back of the cap. This was really my biggest concern. Theoretically I'm guessing it will work the same as long as the air enters and exits the chamber from behind the piston's range of motion.

Also, for the piston, would it be better to use a tense spring which offers a good amount of resistance (enough that it is tough to fully compress with the hands) or a looser spring that can be compressed relatively easily? I'm guessing a more tense spring could improve opening time because it would allow more of a pressure difference build up between the chamber and pilot volume before it would actually start to move. Then when it does, the increased surface area of the piston exposed to the high pressure of the chamber combined with a lower pressure pilot pressure would slam the piston back more quickly than usual. I could be wrong though.

Last, would anyone happen to know the approximate flow coefficient of a standard piston valve of this size for use in GGDT? I'm guessing its a good improvement over the 25% sprinkler valves offer...

Thanks again!
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Unread postAuthor: MrCrowley » Tue Jan 29, 2008 7:52 pm

For the spring you need something in between, too tense, i.e too resistant, will bounce the piston back and limit flow and/or send more energy through the spring and into your end cap or what ever, which is bad.

Too little resistance will have little effect as a bumper, which again, is bad.

So you need something that works effectively as a bumper but doesn't make the piston bounce back and limit flow.

I vote no spring, just make a bumper or cut up some foam.
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 11:22 pm

MrCrowley wrote:I vote no spring, just make a bumper or cut up some foam.


Seconded, use a proper bumper (like several layers of neoprene) and keep the piston travel to 0.25" and you'll be fine.
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Unread postAuthor: niglch » Sun Feb 03, 2008 8:25 pm

Ok, I made some improvements to the design so you can check them out. Instead of using a spring glued directly in the piston, I decided that I could cut a couple springs into 1" sections and attach them to the back of a threaded plug. This plug could then be glued into a female adapter. The springs are relatively strong so I think they will be an effective bumper.

I have finally got my hands some pressure rated fittings, but it turns out that my local Home Depot only sells SCH-40 fittings for very small diameter pipes. I'm going to try looking up a plumbing supply store in my area or ordering them online.
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newer (hopefully improved) design

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Unread postAuthor: jackssmirkingrevenge » Mon Feb 04, 2008 12:20 am

Looks good but again, I would replace the spring bumper with a thick neoprene pad (with appropriate holes for filling and exhaust), as that would fill in your pilot chamber, reducing the volume and increasing efficiency.
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Unread postAuthor: niglch » Tue Feb 12, 2008 10:27 pm

Since I need the extra room in the back of my piston valve in order to fit my blowgun, would it be smarter to use a part like this instead of a female adapter and a threaded plug? So far plumbingsupply.com is the only site that I have found it on (it's called a spigot x internal spigot). Of course, if I could somehow use threaded parts to make my piston servicable, that would be even better, but I'm not sure if it would be safe since I haven't seen it done before. Also, if I use 4 layers of 1/4" neoprine as a bumper, would I need to somehow glue the layers together?

Edit: Here's a few pictures modeled of the gun
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Attachments
gun.png
side view
gun.png (14.48 KiB) Viewed 443 times
gun2.png
the whole gun from the front
gun2.png (22.92 KiB) Viewed 443 times
PVC Sniper Render.png
the piston assembly
PVC Sniper Render.png (26.87 KiB) Viewed 442 times

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