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Piston Valve Problems

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 Problems

Unread postAuthor: bobgengeskahn » Fri Apr 16, 2010 1:29 am

Ok, so, I decided to stop lurking and try to build something. I will post pics later in this post (once I get the camera working), but this is the gist of it:

So I am making a chamber sealing piston valve in a "T", the chamber is a 1/2"x4" nipple, attached to the T, with a 2" nipple narrowed down to 1/8" for my valve. Coming off the short end of the T is a 1/2" to 3/8" M-M into a 3/8" fitting. I could not figure out how to get a hose barb to fit, but in looking around found a 1/2" M to 1/2" F flare nut fitting and fit that into the T and the chamber into that. I made the piston out of about a 2" piece of acrylic rod that I turned down on a lathe and put an O-ring on and drilled a 1/16" hole through the center.

The problem: First I attempted a number of sealing faces and glues, the seals I tried worked, but kept blowing off because the glue kept failing. After a while I just molded about 1/4" of hot glue to the end of the piston that stuck really really well, but I have a re-occurring problem, that is that when I pressurize it I might get one or two good pops when it is firing, then nothing. The piston gets stuck in the forward position, but gas is leaking straight out the barrel. I've taken it apart and lubed and re-assembled it and then there isn't even a pop when it should fire. I am testing it around 180 to 200 psi off of an air compressor, but am planning to run it off of regulated CO2 and would like to run it up to ~500psi. Any ideas??? Like I said, I will post pics some time tomorrow if my explanation is a little confusing.


Edit:
Pictures!!! There are more in the album on photobucket if you follow the link, I think these are the important ones...

So here is the complete system:
Image

System taken apart:
Image

Chamber fitting:
Image

Chamber installed:
Image

Chamber installed from the pilot side:
Image

Piston assembly: (note: i know that the 2" nipple is jacked up, I plan to replace it once I get the system working before I take it to higher pressure)
Image

Complete Piston:
Image

Pilot side:
Image

Sealing face:
Image

Sealing face side without the hot glue on it: (I cut it off since it wasnt sealing anyway, figure if I have to it was easy enough to put it on)
Image


If its still confusing just ask because I really want to get this thing working. The piston is tight, when I installed the piston and ran low pressure and took the chamber off it worked fine, as in there were no leaks around the piston itself...[/img]
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Last edited by bobgengeskahn on Fri Apr 16, 2010 12:24 pm, edited 2 times in total.

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Unread postAuthor: chinnerz » Fri Apr 16, 2010 3:44 am

some questions first:

so when you pressurize it does it hold pressure? and when you fire does any air come out of your barrel?

also i wouldn't recommend using hot glue with that high pressure.
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Re: Piston Valve Problems

Unread postAuthor: SpudFarm » Fri Apr 16, 2010 8:22 am

bobgengeskahn wrote:drilled a 1/16" hole through the center


Have I misunderstood you or have you drilled a hole straight through your piston?

Edit: Sorry about that, I didn't see the chamber sealing part.
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Last edited by SpudFarm on Fri Apr 16, 2010 12:06 pm, edited 1 time in total.
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Unread postAuthor: Crna Legija » Fri Apr 16, 2010 9:20 am

its a chamber sealing piston thats how that are made
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Re: Piston Valve Problems

Unread postAuthor: Ragnarok » Fri Apr 16, 2010 10:12 am

bobgengeskahn wrote:So I am making a chamber sealing piston

Chamber sealing valves require a lot more precision and care in their production than barrel sealing valves. They require two airtight seals, rather than just one.

There are reasons to build them, but they're rather fiddly to get right and are not something I would recommend as a good first build.

You'll probably need to provide photos. It's not easy to visualise an explanation of how the parts have been put together.
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Unread postAuthor: bobgengeskahn » Fri Apr 16, 2010 11:59 am

@ chinnerz: The first one or two cycles with the hot glue face held pressure and fired when i opened the check valve and there was a nice "pop" when it cycled, both when the piston moved forward and back and air did come out of the barrel fitting when it was actuated. after the second "fire" when the piston should have moved back to re-fill the chamber there was a slow flow out of the barrel hole (not as much as was going in) and no cycling when i actuated the check valve. Once I opened it up the piston was in the "sealed" position, but obviously was not sealing lol.
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Unread postAuthor: Gun Freak » Fri Apr 16, 2010 3:24 pm

Maybe you need some better lube.
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Unread postAuthor: Ragnarok » Fri Apr 16, 2010 5:45 pm

Gun Freak wrote:Maybe you need some better lube.

Posts nondescript comment in response to this to try and stop Jack from making a descript one.
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Unread postAuthor: Gun Freak » Fri Apr 16, 2010 5:51 pm

Ragnarok wrote:
Gun Freak wrote:Maybe you need some better lube.

Posts nondescript comment in response to this to try and stop Jack from making a descript one.


That's worth a sig :D
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Unread postAuthor: bobgengeskahn » Fri Apr 16, 2010 10:40 pm

You need lube!!!!?!?? lol jk, I'm using a silicone spray that I have for airsoft... what do you guys normally use for piston valves? Come to think of it, I haven't really seen many threads on this topic....
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Unread postAuthor: bobgengeskahn » Sat Apr 17, 2010 11:59 pm

Bump please? still hoping that this can work...
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Unread postAuthor: Crna Legija » Sun Apr 18, 2010 12:46 am

i use lithium grease for my piston
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Unread postAuthor: chinnerz » Sun Apr 18, 2010 1:32 am

what Lube do i use you say??
well first some art from the shelves in my garage.....

Image

...This is roughly where i keep my lubrication products and stuff like that.
i would usually start by putting a thin layer of the epl2 around the piston, this stuff gets hard after a while and ends up filling in all the little cracks deformities on the surface of my piston. Then, depending on what the piston is made form, i would add lube on top of that. for plastic i would dab some of that handy oil, it is very light and does the job well, if it was metal or something else i would use INOX or dry glide depending on speeds (inox burns our or something when used in high speed applications) and at the moment i dont have any WD-40 but that is good too for metals.
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Unread postAuthor: Technician1002 » Sun Apr 18, 2010 1:39 am

I have a couple comments.

To get it to stop leaking out the barrel, the sealing surfaces must be true. When the surface looks like a hand carved piece of rubber without a smooth flat surface, the irregularities will provide ample room for air to leak through the low places. Placing the piston in a drill press and dressing the face to be true to the valve seat may help this issue. I have hand lapped some pistons to a valve seat in some of my cannons. Two that I have built with hand lapped piston faces include the Dragon and the Mouse Musket. They are 1 inch and 2.5 inch valve seat PVC piston coaxial cannons.

How tight is the piston and o ring? If they are quite tight, there may be too little force to overcome the friction of the o ring on the wall. A floating ring with some compression room can still provide a seal while reducing the pressure against the wall. This can reduce the stuck piston problems.

If you search youtube under my user name you can find some info I have put on there showing some tips for using o rings on pistons to prevent stuck pistons.
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Unread postAuthor: bobgengeskahn » Sun Apr 18, 2010 2:14 am

Technician1002 wrote:I have a couple comments.

How tight is the piston and o ring? If they are quite tight, there may be too little force to overcome the friction of the o ring on the wall. A floating ring with some compression room can still provide a seal while reducing the pressure against the wall. This can reduce the stuck piston problems.


This is actually something I was thinking of as well. The O-ring is tight on the piston and, for the most part is just barely larger than the OD on the piston itself when it is on. What should the tolerance be on the piston in relation to the barrel? I have looked at some of your videos Technician (it was actually your video on your QDV that got me interested into pneumatic spud guns in the first place), and I noticed that your O-rings are very loose. I understand the idea of a free floating O-ring, I am just afraid of taking the piston down too far when trying to reduce friction over all.

I was talking to a friend and he too recommended lithium grease as well, so I will give that a shot first. I like what Chinnerz had to say about the epl2, if I end up having to take down the piston I will probably give that a shot too.

Thanks for the advice so far! Hopefully I will have some time this week to put some of it to use and follow up on this thread.
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