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Help: Can't get my 4" piston to seal against 3" ba

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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Help: Can't get my 4" piston to seal against 3" ba

Unread postAuthor: JDP12 » Tue Sep 16, 2008 7:08 am

In my t-shirt gun, i have a barrel sealing piston with a 3" inch barrel. The end of the barrel is square, but the piston doesn't seal that great. Occasionally it will start to seal, then i hear a big BANG and i lok down the barrel, and the sealing face has "wrinkled." The sealing face is a piece of neoprene, that was originally bolted onto the piston. This cause it to wrinkle and not seal, so i JBwelded it down last night and will test it when i get home tonight.

But even when it did partially seal and i could get it to fire, some air was still leaking out the barrel, so I would appreciate some help solving this.

Thanks,

ILTBTU

<img src="http://i157.photobucket.com/albums/t79/justhavinagoodtime/DSC00262.jpg?t=1221567028">

<img src="http://i157.photobucket.com/albums/t79/justhavinagoodtime/DSC00260.jpg?t=1221567095">

<img src="http://i157.photobucket.com/albums/t79/justhavinagoodtime/DSC00261.jpg?t=1221567115">
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Unread postAuthor: CasinoVanart » Tue Sep 16, 2008 7:16 am

Is that neoprene woven? It looks like it in the pics, i can't imagine that stuff ever sealing properly. The weave would make the air leak past.
That still doesn't explain the bang you hear.

I have used woven burst discs and they wrinkle too. End up in a square shape.
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Unread postAuthor: JDP12 » Tue Sep 16, 2008 8:48 am

Yea i think it is actually..its from a mousepad.

the bang is it starts to seal, then the seal breaks and all the air goes out.

SH!T... now i need some sort of real neoprene...

can anyone please point out a good type of rubber on MCMASTER that i can buy- this is my third try of rubber, and it failed, so i need a good thing i know will work!

or would home depot have some sort of gasket or rubber sheeting i could get that would seal?
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Unread postAuthor: mark.f » Tue Sep 16, 2008 9:26 am

Before you try more neoprene, take a look at your washer.

Now, take a look at the washer on a Supah Valve.

Image

Much bigger, right?

If you have too small of a washer, it will allow too much air to get under the piston between the edges of the washer and the edges of the barrel, which will forces the edges of the neoprene into the barrel.

Try a bigger washer. It should be as big as possible (maybe a 1/4" maximum space on the edges between it and the sealing face). Of course, you said your sealing face was square, so this changes things a bit. By the way, how the hell did you manage to get a square sealing face???

EDIT - and here is a picture snapped from a video Velocity made about his AmeriCannon. You can see that the washer is pretty large, and you can also see clearly the bulging of the neoprene around in, where air is getting under it. If the washer were too small, the neoprene would pop-up into the barrel.
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Unread postAuthor: JDP12 » Tue Sep 16, 2008 9:28 am

hahahahaha..

by a square sealing face, i meant the barrel angle was 90 degrees- its not cut at an angle or anything lol..

so a bigger washer?

do you think the weave could have anything to do with it? for some reason i dont think so, because its glued down to the piston so air shouldn't be able to get under it.

I will definitely make a bigger washer when i get home from school


so mark- in that picture of the Americannon- the washer is too small?
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Unread postAuthor: mark.f » Tue Sep 16, 2008 9:39 am

I don't know. The weave may or may not affect how long the sealing face lasts before it finally gives up the ghost. How thick is it, and where did you get it from?

Try it with the sealing face just glued on when you're able. If that doesn't work, then try the bigger washer. Be careful, though, because those sealing faces turn into projectiles if they get loose when they're only glued on. :wink:
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Unread postAuthor: JDP12 » Tue Sep 16, 2008 1:57 pm

its from a mouse pad.

I tried to glue on the sealing face last night and will test it when i get home.

If that doesn't work i'll use a big ol' washer
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Unread postAuthor: LikimysCrotchus5 » Tue Sep 16, 2008 3:54 pm

Basically what everyone else said.

Dont use mousepad as a sealing face, use a neoprene from mcmaster.

A bigger washer as well, i know that was mentioned. Make sure the lip of the pipe that the piston is sealing to is flush to the sealing face.

And just out of curiosity, isnt a 3 inch piston overkill for a T shirt launcher?
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Unread postAuthor: Velocity » Tue Sep 16, 2008 4:12 pm

I used McMaster Part# 9013K262 for the rubber sealing face on the Americannon. It is expensive at $15 for only a square foot, but it works...never had a sealing problem. It is also extremely durable (for spudgunning uses).

You can get some nice big washers off of McMaster.com as well...I used a washer that was 1.25" in diameter, but I still had problems in terms of the rubber sealing face popping out. I bought 2.5" washers subsequently, which work great.

<a href="http://s94.photobucket.com/albums/l112/rmich732/?action=view&current=DryFire30PSI.flv">Here</a> is the video that markfh11q was talking about...I don't know if it will help, but just in case it does.
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Unread postAuthor: jrrdw » Tue Sep 16, 2008 4:20 pm

Just about any kind of rubber will work as long as the washer is all most the inside diameter as you barrel, leaving only the barrel wall thickness amount to do the sealing. That tiny washer holding down the large area down will never hold.
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Unread postAuthor: JDP12 » Tue Sep 16, 2008 6:20 pm

hahahah, umm well let's see...YES it is overkill

but then again, thats the way i do things- the whole gun is around 5 feet tall or so... its a beast and its overkill

but this is going to fire other projectiles other than t-shirts...lol
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Unread postAuthor: jook13 » Tue Sep 16, 2008 6:29 pm

just to let you know, I have found rubber gasket sheets at home depot or any other hardware stores in my area. They are usually in the plumbing sections.
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Unread postAuthor: McThunderThighs » Tue Sep 16, 2008 10:05 pm

Hey,

I recognize that stuff like the back of my hand. 10 bucks says you are using a mousepad!

Mousepads are made of foam rubber, and very cheap stuff at that. Don't use it.

Head over to a hardware store and ask for gasket material. Most places sell it by the foot(inch).
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Unread postAuthor: blind909 » Tue Sep 16, 2008 10:51 pm

Another thing you could try is the latex swimming caps. I used that as a sealing face on my Piston Cannon. Use about 5-7 layers. It seals EXCELENTLY.
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