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first 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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first piston valve

Unread postAuthor: kotabuuki » Sun Apr 10, 2011 7:53 pm

Hey guys, I'm a senior in high school, and am doing a physics project on how pressure (psi) effects the distance of the projectile. Additionally I have to give a live demonstration during class (outside of course) so everything has to be safe. There are several other students that are doing potato cannon related projects (spray and pray cannons for the most part), and I want my cannon to crush the competition. So I have decided to build a cannon with 2” piston valve with 3” chamber, with removable barrel. The ammo for the demonstration will probably be a mini vortex for safety reasons. I've made pneumatics before, but this is my first with a piston valve, so I need a little help.

Proposed dimensions
...............Dia..........Length
Barrel_____2” _______48”
Chamber___3” ____ 25” or 30”
Piston____1" 7/8______1.5”

1 Do you think my overall design will work? I don't have the reducer/bushing for the 3” chamber I plan on making.

2 Is my pilot volume large enough? I measured the plug and it's volume is about 3.5 I plan on using a ¾ sprinkler valve to dump the air, but I haven't bought that yet.

3 Do you think my piston design will work? It weighs 48 grams. The spring is to aid in absorbing the some of the shock, because the black piece of plastic can slide up and down the bolt, however the other piece is secured in place. The bumper is going to be made out of some dense tempur-pedic foam covered in some rubber like stuff. If this design won't work I can just make a new one.

4 Is my piston going back far enough? It fits rather well in the 2” pipe, but I’m wondering if I should add some o-rings, or just make a new piston. I've read that the piston only needs to go back d/4nfrom the sealing face, but am unsure if that is correct.

I know that it may seem like I just want answers , but I have done some research, but I just want a final opinion before I start construction. Any and all helpful/ constructive criticism is appreciated.


Oh and side note, the 3” pipe is not what I intend on using,it is cell-core, and is for mock-up. And I don't have the sprinkler valve yet.
-Kota

mock up assembly
Image

disassembled to show individual pieces
Image

piston
Image

valve closed
Image

valve open
Image

rear view of piston. I removed the bushing and put a peice of 2" pipe to show how well it fits.
Image

as I said earlier, Any and all helpful/ constructive criticism is appreciated. this is my first piston valve and it's for school so I want it to work well and safely.
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Unread postAuthor: Gun Freak » Sun Apr 10, 2011 9:17 pm

It appears your piston is servicable out the front, no? Rather uncommon way of doing it, but certainley not unheard of. Most people use a female adapter in the back where the piston is to take it out.

1) yes as long as you get the coupling and bushing. NOT a reducer!

2) yes it looks fine.

3) upon close inspection of your piston... it doesn't look like the sealing face is very strong. You might want to consider remaking on our of a bolt and some washers (Example, along the lines of it but shorter). You could also add o-rings on the proposed design, as seen in the picture.

4) your piston is moving enough.

These are some basic answers. If you need clarifying on any of them just ask. I answered straight up so you didn't have to read too much (also so I don't have to type as much :D )
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Unread postAuthor: saefroch » Sun Apr 10, 2011 9:23 pm

Vast dead volume is going to really hurt your performance. Try to make that barrel assembly shorter, or just go with a permanently attached barrel. The less volume between the projectile and the valve that the air can expand into without propelling the projectile, the better. Ideally your projectile can touch the piston.

Pilot volume cannot be too small. If it's too large, the piston will not pilot. The smaller, the more responsive the valve is and the less gas will leak from the chamber out the pilot.

Your piston should retract so that the area of the rectangle formed by the diameter of the port and the distance from the piston to the port is equal to or greater than the area of the port itself.

In regards to the actual construction of the piston, I'd try it out first before scrapping it, just be sure everything is constructed such that you can get the piston in and out of the valve when construction is complete. The piston doesn't look incredibly promising to me, but function comes before form with internals.

EDIT: I'm not nearly a concise as Gun Freak. Oh well...
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Unread postAuthor: Technician1002 » Sun Apr 10, 2011 9:30 pm

I highly recommend using a valve orifice at least 2/3 the diameter of the barrel if you want to smoke the competition. Find a 3 or 4 inch pressure rated T and build a valve with a 2 inch valve seat if possible using a 2.5 to 3 inch piston.

A 2 inch QDV works well too. My ABS test cannon smoked a spray and pray of the same size in side by side competition.

We were pretty much matched if I lowerd the pressure to about 35 PSI.
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Unread postAuthor: saefroch » Sun Apr 10, 2011 9:38 pm

This really seems like double-posting, but for a valve this large a QDV may offer some significant performance benefits. You might want to consider that, except you've already purchased a lot of the components for a normal piston valve.
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Unread postAuthor: kotabuuki » Sun Apr 10, 2011 10:32 pm

Thanks guys,
[Gun Freak]
1) yes as long as you get the coupling and bushing. NOT a reducer!

do you mean like a 3" - 2" bushing? for some reason i don't see where the coupler comes into play.
For the piston, i want it to be as light as possible correct? because if my current piston doesn't work, i can make a new one out of several more disks of the black plastic. it is very light, yet durable. The current sealing face is a piece of acrylic, so it is liable to crack.

Saefroch,
when you say dead space, do you mean the distance from the valve to the projectile or do you mean the distance from the chamber to the projectile? because i can always get rid of the second T and end cap, that is just there for aesthetics.
And in order to reduce the volume between the projectile and the valve, I could remove the male and female adapters closest to the T and then glue the first bushing directly into the T, and then put a female adapter on the back so that I have access to the piston, however that increases the pilot volume. Thoughts?

Finally Technician,
That is similar to what i wanted to do in the beginning, but i cannot find pressure rated pipe that size in my area. But in regards to your QDV, does the chamber have to be inline with the barrel or could i modify what i have? and ironicly I was looking at your marshmallow gun just the other day. If i can go the QDV route for less that $20, then i'll probably do that, but you have to understand that i have limited resources and money so i'm trying to make do with what i have available.


once again any feedback is appreciated.
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Unread postAuthor: hi » Sun Apr 10, 2011 10:45 pm

I recommend adding bumper to cushion the piston for when it fires. Use a piece of hose something soft. I had a piston gun that worked great for a year or so and withstood several hundred shots, but that one shot was too much and busted the tee.

The weight of the piston doesn't effect the performance a whole lot. Obviously you don't want a piston made of solid lead, but in general, the lighter the better. Piston I am talking about that broke my gun was rather heavy, I bet if it was lighter it would have been fine.

As for the reducer, but don't just a bell shaped reducer. Make sure it is pressure rated and does not say"DWV" on it. you need a coupler to attach a bushing to a piece of pipe.
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Unread postAuthor: kotabuuki » Sun Apr 10, 2011 10:50 pm

I'm going to have a bumper, i state that under number three, the foam is dense and very absorby (yea, i just made that up). i've read somewhere about using part of a tennis ball for a bumper also. ideally i would use a donut of rubber, but i don't have any sheet rubber around the house.
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Unread postAuthor: Technician1002 » Mon Apr 11, 2011 12:14 am

Finally Technician,
That is similar to what i wanted to do in the beginning, but i cannot find pressure rated pipe that size in my area. But in regards to your QDV, does the chamber have to be inline with the barrel or could i modify what i have? and ironicly I was looking at your marshmallow gun just the other day. If i can go the QDV route for less that $20, then i'll probably do that, but you have to understand that i have limited resources and money so i'm trying to make do with what i have available.


once again any feedback is appreciated.


Pressure rated pipe in the larger sizes is hard to find in a city. Big box stores simply don't carry it. In farming areas, an irrigation supply may have it for sprinkler systems.

In the city you need to find where the Public Works and Contractors go. It is generally commercial sales only, but if you explain the project in detail and it's for school, they may talk to you. I got my T Shirt barrel that way. I bought a 20 foot length of 2.5 inch pipe and split it with the school team I mentored. In the US if you are here look for suppliers such as Furgeson's Plumbing. At first they wanted to charge me for shipping. I told them I wasn't in a hurry and they included it in another shipment the following week no charge to their warehouse when another order was shipped. It pays to be flexible and ask.

The valve can be built inside a T. A 3 inch T should work fine with a couple of bushings filed or sanded out so the pipe can be pushed all the way through to make the valve.

The disadvantage with doing that is the larger sizes get expensive quickly.

Small sizes in comparison are dirt cheap. Other than the stack of various barrels for the Marshmallow cannon, the total cost of purchased materials was less than $20. Purchased materials was a 10 inch long pipe nipple, two pipe caps (one PVC broke and one metal to replace it). 2 o rings, scrap sheet of UMWHDPE at about $1.75/lb paint, brazing rod, welding gas, and sandpaper.

The tank was trash, an empty propane bottle, and the rod from an old fawcet for the drain. (pop up rod) The old fawcet was replaced so I kept the rod from the old one.

I'm glad you like the Marshmallow Cannon. It and it's big brother are my favorites in my collection. Making the 2 inch even with scrounged parts still cost about 5 times as much.

A QDV is a very ambitious first piston project. With 2 o rings that need to seal and ports that can't chew up the rings for lunch, and the bumper requirement, it might not be the best for a first piston build. A single o ring traditional piston may be much easier to get built and working.

May I recommend a 3 inch T with a reducer with 2.5 inch pipe for a 2.5 inch piston to go inside to seal a 2 inch barrel. Much easier to build and get it working properly. This is especially true if you don't have a lathe. Getting o rings to seal 100% that a QDV requires without a lathe is difficult at best.

A 3 inch air chamber on a 3 inch t with a 2.5 inch piston and full flow 2 inch barrel will be a real performer.
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Unread postAuthor: kotabuuki » Mon Apr 11, 2011 5:46 am

Technician1002 wrote:
Pressure rated pipe in the larger sizes is hard to find in a city. Big box stores simply don't carry it. In farming areas, an irrigation supply may have it for sprinkler systems.

In the city you need to find where the Public Works and Contractors go. It is generally commercial sales only, but if you explain the project in detail and it's for school, they may talk to you. I got my T Shirt barrel that way. I bought a 20 foot length of 2.5 inch pipe and split it with the school team I mentored. In the US if you are here look for suppliers such as Furgeson's Plumbing. At first they wanted to charge me for shipping. I told them I wasn't in a hurry and they included it in another shipment the following week no charge to their warehouse when another order was shipped. It pays to be flexible and ask.

The valve can be built inside a T. A 3 inch T should work fine with a couple of bushings filed or sanded out so the pipe can be pushed all the way through to make the valve.

The disadvantage with doing that is the larger sizes get expensive quickly.

Small sizes in comparison are dirt cheap. Other than the stack of various barrels for the Marshmallow cannon, the total cost of purchased materials was less than $20. Purchased materials was a 10 inch long pipe nipple, two pipe caps (one PVC broke and one metal to replace it). 2 o rings, scrap sheet of UMWHDPE at about $1.75/lb paint, brazing rod, welding gas, and sandpaper.

The tank was trash, an empty propane bottle, and the rod from an old fawcet for the drain. (pop up rod) The old fawcet was replaced so I kept the rod from the old one.

I'm glad you like the Marshmallow Cannon. It and it's big brother are my favorites in my collection. Making the 2 inch even with scrounged parts still cost about 5 times as much.

A QDV is a very ambitious first piston project. With 2 o rings that need to seal and ports that can't chew up the rings for lunch, and the bumper requirement, it might not be the best for a first piston build. A single o ring traditional piston may be much easier to get built and working.

May I recommend a 3 inch T with a reducer with 2.5 inch pipe for a 2.5 inch piston to go inside to seal a 2 inch barrel. Much easier to build and get it working properly. This is especially true if you don't have a lathe. Getting o rings to seal 100% that a QDV requires without a lathe is difficult at best.

A 3 inch air chamber on a 3 inch t with a 2.5 inch piston and full flow 2 inch barrel will be a real performer.


I think i'll save the QDV for a different time. I know that just looking at your post will put me over budget. i've already bought most of my parts, and money is tight. so maybe my next project will be like your 1" QDV.
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Unread postAuthor: kotabuuki » Wed Apr 13, 2011 8:14 pm

Technician
I Did some research and I found a plumbing store, YAY! However since it is a specialty shop, and the parts they stock are good quality, they are some what expensive.

saefroch wrote:Vast dead volume is going to really hurt your performance. Try to make that barrel assembly shorter, or just go with a permanently attached barrel. The less volume between the projectile and the valve that the air can expand into without propelling the projectile, the better. Ideally your projectile can touch the piston.

Pilot volume cannot be too small. If it's too large, the piston will not pilot. The smaller, the more responsive the valve is and the less gas will leak from the chamber out the pilot.


I re-arranged the parts, and I reduced the dead space significantly. however at the expense of pilot volume. but I plan on using a 3/4 sprinkler valve, so i don't think it will be that big of a problem.

I just got all the main parts today, so I will begin construction soon. so here are some pics of what I'm planning on doing.

Is there an optimal C/B ratio for piston valves?


heres an overall view of what it's going to look like, however the barrel and the chamber are not the length i am going to use. Also, i got bored and instead of playing xbox or watching tv i decided to actually do something productive and made the barrel shroud. It doesn't look too bad does it?

Image






here's just another view of all the components.
Image



and as always, any input is appreciated.
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Unread postAuthor: Gun Freak » Wed Apr 13, 2011 8:23 pm

Looks much better now! Also the shroud is badass. Looking good man.
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Unread postAuthor: kotabuuki » Wed Apr 13, 2011 8:29 pm

Gun Freak wrote:Looks much better now! Also the shroud is badass. Looking good man.


Thanks, :) I'm going to paint it once its completely assembled. I want the Fittings and pipe to be contrasting colors, so I was thinking about making the barrel and chamber black, and all the fittings and shroud gold.


also, I want to be able to shoot golf balls out of it, but since the barrel is 2", they don't fit very snug. any suggestions on a sabot?
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Unread postAuthor: Gun Freak » Wed Apr 13, 2011 8:33 pm

Paint it flat black, trust me it will looks awesome.

Golf balls fit well in SDR 21 1.5" PVC. If you can't find that, a more common type is called SDR 26 1.5", which is slightly a looser fit but I use it and it works very well.
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Unread postAuthor: kotabuuki » Wed Apr 13, 2011 8:40 pm

can i get that at lowes /home depot or do i have to go to a plumbing supplier?
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