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Clear Plexiglass Piston Problems. FIXED!!! WORKING GUN NOW

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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Unread postAuthor: Matheusilla » Wed Apr 11, 2007 5:59 pm

still, from what I can see, put O-rings and a tiny equalization hole. If that don't do it, then yer piston's too small for the barrel size (I doubt it though, but that's all I see at the moment).
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Unread postAuthor: demilus » Wed Apr 11, 2007 7:03 pm

I've tried o-rings, and made the piston seal better. I've oiled and greased, but to no avail. I really think that the piston isn't overlapping over the barrel enough. Oh well, I'll fix it. Expect to see a WORKING piston cannon from me soon :D
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Unread postAuthor: Hawkeye » Wed Apr 11, 2007 7:19 pm

Can you get a reducer? You could just put one on with Epoxy to see if you can get it working. Just clean the T and barrel of any grease or oil.
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Unread postAuthor: rad14701 » Wed Apr 11, 2007 7:40 pm

demilus wrote:I've tried o-rings, and made the piston seal better. I've oiled and greased, but to no avail. I really think that the piston isn't overlapping over the barrel enough. Oh well, I'll fix it. Expect to see a WORKING piston cannon from me soon :D


I think you're correct in that the piston is too close in diameter as that of the barrel... You simply don't have enough piston face to create the pressure differential needed to force it off the barrel seat...
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Unread postAuthor: chaos » Fri Apr 13, 2007 4:07 am

so wats the go with this thing man, gettin it to work yet??
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Unread postAuthor: demilus » Fri Apr 13, 2007 4:34 pm

I got all the parts. I just have to get some time to solder them together. I'll post pics when I finish it.

EDIT: All the parts are now soldered together!! Talk about a sh!tty solder job... Oh well, I can use my scratched cornea as an excuse. I squeezed a 5" spring all the way down and it popped out of my fingers and smashed me in the eye. That hurt really bad and you could see a huge gouge in my eye. Hooray 3 hours in the emergency room!

Now I just gotta wait for my dad to get home, he works till 9 PM and will be bringing my 1" brass cap home.
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Unread postAuthor: demilus » Wed Apr 18, 2007 4:40 pm

Hehe Bumpage.... :D

So I got my cap and decided to try it. It didn't work. Again. I'm beginning to wonder if the seal around the piston isn't good enough.

About how tight does the piston have to be in the pipe in order for it to work?
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Unread postAuthor: rad14701 » Wed Apr 18, 2007 5:06 pm

The piston doesn't, contrary to popular belief, need to be tight... Pressure is what holds it against the barrel or, rather, the difference between the atmospheric pressure in the barrel and the chamber/pilot area pressure... The biggest issue is how much more piston area there is that extends past the area of the barrel... This area is what is responsible for forcing the piston back when the gun pilots... If this area is too small then, regardless of the fit of your piston, the pressure between the chamber and the pilot area will continue to equalize and exhaust out the pilot valve port... The piston won't leave the barrel seat until gravity causes it to fall away or it might not move at all...

As an example of the theory described above, a 1/2 inch copper barrel inside a 3/4 inch copper chamber would probably equate to just about the least amount of additional piston surface area required to achieve proper piloting... And this small amount would be a very bare minimum considering the OD of the barrel would be 5/8 inch which would only yield 1/8 inch of piston face inside the 3/4 ID chamber... This additional area needs to exceed the area of the barrels outside OD...

I hope this information helps...
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Unread postAuthor: spudthug » Wed Apr 18, 2007 5:48 pm

ok dem since i know what ur going throguh i ahd the same problems with my comp gun...well first of all buy a 1 1/4 tee and put it to ur chamber.. then jsut find a different piston (hotglue) and put the pilot like u have it only on a 1/4 ball valve and scrap the pressure washer....for the barrel side instead of 1/2 use 3/4 and then ur piston will work...right now it probably isnt working for these reasons

ur piston is in to small of a tee and the barrel istn sealing right on the half inch barrel..

ur pilot is to small thats jsut like a blowgun use a ball valve

your piston is really long

the air is going around the piston and probably not going to your chambers because ur tee is to small...

before u buy the tee and remodel it try a diferent port as a stated before...put a ball valve on it...
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4" piston valved cannon-half done..( i spilt my cement...)

Hybrid- 75% done. need to build propane holder and drill/tap sparkplug hole..
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Unread postAuthor: demilus » Wed Apr 18, 2007 6:40 pm

Ooops.

I forgot to write what new parts I added on to the cannon. Instead of a 3/4" tee, I stepped it up to a 1" copper tee, 1" male adapter, and 1" brass cap.

I don't think that the problem lies in the venting, as the pressure washer handle I have dumps more air than a 1/4" ballvalve.

I put a new 1" piston in and it fits slighty looser than the old one. It rattles a bit if I shake it.

From the 1" tee, I go to the 1/2" barrel with a 1" to 3/4" bushing and then to a 3/4" to 1/2" bushing. The barrel extends midway into the tee through the bushings.

The piston seals the barrel fine, but when I exhaust, the piston doesn't jump back at all until gravity pulls it back.

I think that the piston doesn't seal tight enough. I don't have any access to a lathe. How would you suggest that I make the piston seal better? Also, how do you make a hot glue piston? Just cast one in 1" copper pipe?

Do you think that my "venting area," the area between the piston and exhaust valve, is too big? How could I cut down on that? Longer piston?
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Unread postAuthor: spudthug » Wed Apr 18, 2007 7:18 pm

ok ur tee will work but i very very highly doguht that a pressure washer opens more than a 1.4 ball valve...try moving the piston with ur mouth when it is on the gun...if u can it will be good...about the piston not moving back the whole way. truely i think ur piston it to long but maybe your seat where the piston sits is to small for the piston to move back alot...that can be solved with a smaller piston..no ur venting area is not to long mine on my comp gun is like 1 1/2' longer than urs... making a hot glue piston is simple...just spray a bunch of pam into a small peice of 1 inch tube and then melt a bunch of hot glue into it...


oh and btw put a 1/4 or 1/2 bv on there and tell me if it works....
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4" piston valved cannon-half done..( i spilt my cement...)

Hybrid- 75% done. need to build propane holder and drill/tap sparkplug hole..
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Unread postAuthor: demilus » Wed Apr 18, 2007 7:24 pm

Aww crap. I think it isn't venting enough. It works without the washer handle, if I just cover the exhaust and then move my hand. With the washer, it doesn't. I just kinda figured that it was fine. Oh well, I'll put on my modded sprinkler, and if it can't exhaust it then I'll try something else. Sonnovabitch. I'll post back in a few.
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Unread postAuthor: hi » Wed Apr 18, 2007 7:29 pm

@spudtug, i see you have learned a lot. (wink wink, cough cough)


I didnt read 2 full pages, so this may have been brought up.

the barrel need to be half way in the tee.

you piston needs to have about 1/2'' cut off.

you need to oil it.

those have probably all been said, but i thought i would remind you. you also need a sealing face. i didnt see one in the original picture. to attach the sealing face, you need to drill a hole through it and bolt on a piece of rubber, try to find a washer that fits in the barrel perfectly, this will prevent the sealing face from being sucked into the barrel.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: spudthug » Wed Apr 18, 2007 7:31 pm

hehe i said all of those things...

and btw for everyone that wants to know hi taught me all i know about pistons pretty much and helped me alot with my new competition gun...

thanks hi :wink:
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4" piston valved cannon-half done..( i spilt my cement...)

Hybrid- 75% done. need to build propane holder and drill/tap sparkplug hole..
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Unread postAuthor: rad14701 » Wed Apr 18, 2007 7:34 pm

Ah, yes, I can see where you could be having problems... The washer handle might be causing compound problems... First, the air nipple is the small type which is causing a restriction... I used to have to replace those on some of my air tools when I was working as a mechanic many years ago due to CFM problems... I had to use larger nipples and couplers... The other possible problem relates to air flow in a reverse direction from the original design of the valve within the handle as the reverse flow could also cause CFM issues... An alternate air gun might yield better results if you opt to try something other than the sprinkler valve in the future...
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