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Sealing 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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Sealing Problems

Unread postAuthor: DYI » Sat Sep 29, 2007 7:19 pm

The insanely powerful pneumatic that is mentioned in my sig is essentially complete, but I have ran into a problem that will sink the entire project if it is not solved.

The launcher utilises a pressure differential triggered burst disk system that I have used to great effect before. One of the steel unions has to be sealed on both sides, and doing so has proven to be very difficult. I use thin sheet aluminum (i.e. soda can aluminum) for burst disks. Please do not suggest a different disk material, as you will be ignored.

Some things that I have tried are: O-rings, which didn't work (obviously). Gaskets, attached to one/ both sealing faces of the union, and gaskets augmented by generous amounts of teflon paste on both the threads and sealing face of the union. None of my methods so far have worked, and I am running short on ideas.

I have narrowed down my course of action to two possible options, neither of which is simple, or easy to work with.

The first option is to grind down the tapered sealing face of the union so that it is flat, and the disks won't crumple anymore. Apparently SB15 has used this method effectively. The downside is, if it doesn't work, I will be down a union, and back to the drawing board.

The second option is to have a flange custom make by a machine shop, which I know would seal. The downside of this is the obvious expensive of having it made. Although the reload time would be unpleasant, for obvious reasons, it wouldn't be any worse than it currently is.

Cam-locks are not an option, so don't even suggest them.

I am open to suggestions for better ideas, or better ways to implement the existing ideas.
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Unread postAuthor: LikimysCrotchus5 » Sat Sep 29, 2007 7:31 pm

Is this your supersonic cannon that you want to make? Oh yea and i forgot to mention on your other post trying to make a supersonic cannon, if it goes supersonic you may need to wear earmuffs because it will make a sonic boom. You better be careful if that happens.
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Unread postAuthor: mopherman » Sat Sep 29, 2007 7:49 pm

how did you seal the last one? I would glue gasket rubber on both sides.
And on a semi-related note, I have been pondering a 1000fps+ nail gun. What kind of pressures do you think are reqired for this? It will have an overlarge chamber for cb testing.
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Unread postAuthor: williamfeldmann » Sat Sep 29, 2007 8:01 pm

What exactly, to the best of your knowledge is happening?

Are the faces of the unions not flat, or is the union just not able to close tightly all the way?

If the faces themselves are not flat you could try to gring the faces, however, speaking from experience, that is not so easy to do if the union is already welded to the corresponding pieces. If they are still seperate, you could try the sandpaper trick (its a woodworking trick, used in getting hand tools perfectly flat work faces). Using very fine grit sandpaper, preferably self-adhesive, attached to a perfectly flat surface (I use my table saw surface, or if I have some, a sheet of glass which works better with wet sanding). Then place the flage face on the sandpaper, and sand the face without really pushing on the surface, rotating as you go. As long as you have something softer than carbide, this will give you a perfectly flat face with time. Depending on how far out of plumb your face is this could take some time, if you just have a pitted or not finished face this could be done real quick. Wet sanding has worked better for my steel bedded hand planes that I tuned up with this method. Brass is also easily finished this way as well.

If you have a problem with the actual unions, you might be better off having a flange made. Yes, you will end up eating the cost, but you get a quaranteed part. I don't know of anyway to make a union close up. Hopefully, someone here will know a good way to do so.
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Unread postAuthor: DYI » Sat Sep 29, 2007 8:42 pm

First off, this isn't the planned supersonic gun, although it will fire a golf ball at slightly over 1000 fps. The possibly supersonic one has been delayed for cost reasons, but will be built. This is the gun that you see in the preview on my sig.

Alu. foil disks sealed fairly well in this union, so I believe that the problem is a compound of massively increased pressure, plus disks that wrinkle when they are put in the union, which makes sealing difficult.

There is nothing welded on this launcher, and there will not be anything welded on this launcher, since I can't take it over 500 psi anyway.

The usual way to seal a union is with a gasket, or gaskets. I agree that a flange would seal more effectively, and , come to think of it, the few high pressure burst disk guns that I have seen all use flanges. The only real difficulty with a flange would be getting the bolts tight enough to seal properly, without air tools (no, I'm not hooking up the N2 tank to an air wrench, despite the obvious entertainment value). It isn't impossible, just time consuming.

I have wondered if normal hardware store steel flanges would be adequate, but I really don't feel like pushing my luck, especially with such high pressure.

@Mopherman: GGDT is saying that for a .25" barrel and a 10 gram projectile, you're probably going to need in excess of 1000 psi. I would suggest using a larger bore barrel and a sabot, to reduce the required pressure down to something more feasible. Failing that, you could always make a 1000 psi steam gun, which would likely be the scariest thing to ever grace these forums. On the topic of steam, have you ever seen the 3500 psi steam cannon that MIT built?
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Unread postAuthor: clide » Sat Sep 29, 2007 8:42 pm

Sounds like his problem is the disk wrinkling on the edges. This may be a stupid question, but have you tried using no gasket and just cranking the union down really tight? The theory being that the steel may press against the softer aluminum and create a seal and leave no room for wrinkling. I've never used a metal union so I'm not really sure if that idea is plausible or not.

Edit: Posted at the same time as the post above, just pointing that out so I don't look like a moron repeating what he just said about the wrinkling.
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Last edited by clide on Sat Sep 29, 2007 8:47 pm, edited 1 time in total.

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Unread postAuthor: DYI » Sat Sep 29, 2007 8:45 pm

The aluminum disks aren't thick enough for the effect that you mentioned clide, although it is a good idea. And yes, the problem is almost definitely the disk wrinkling on the edges.
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Unread postAuthor: noname » Sun Sep 30, 2007 12:13 am

That's strange; a rubber gasket on one (or both) of the sides should seal it. How big is the union that you're using?
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Unread postAuthor: SpudBlaster15 » Sun Sep 30, 2007 12:24 am

Grind down the taper on the union, and use a rubber gasket on both sides of the disk. This will eliminate the wrinkling problem that occurs when using pop can disks in a tapered union. A gasket isn't much use when large grooves are forming in your disk when the union is tightened.
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