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Gun jammed

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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Gun jammed

Unread postAuthor: limbeh » Mon Jan 06, 2014 9:45 am

I operate coaxial piston cannons for my college. We get the freshmen to shoot them every year as part of a student project -



I know, the above video's lame, this is more interesting.



Anyway, 2 years on and the cannons are still going strong, just that we've changed a couple of piston seals due to wear and tear. Only that maybe once every 50 shots the gun wouldn't fire. We would have to depressurise the gun and open it up to re-position the piston.

I'm not sure what causes this problem; but my suspicion is that the piston doesn't seal. We have no pressure gage on the gun itself and our pistons do not have O-rings. Any of you have experienced this before?

Thanks!
Limbeh
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Last edited by jrrdw on Mon Jan 06, 2014 3:49 pm, edited 1 time in total.
Reason: Fixed video's.
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Even the Aussies for all they complain have it good compared to us.
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Re: Gun jammed

Unread postAuthor: POLAND_SPUD » Mon Jan 06, 2014 1:13 pm

nope - never happened... though you're probably right that it is caused by no pressure in the gun/piston not sealing properly
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Re: Gun jammed

Unread postAuthor: jrrdw » Mon Jan 06, 2014 3:44 pm

Try any lube?
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Re: Gun jammed

Unread postAuthor: Technician1002 » Mon Jan 06, 2014 8:07 pm

This looks similar in design to my Mouse Musket. Friction is your enemy. An audio recording of it firing is useful as the fime of the hiss of the pilot can be evaluated for several common problems. A short hiss followed by main valve operation is normal.

Failures include constant hiss indicating too much piston blow by. Hiss indicates pilot pressure was unable to drop low inough to unbalance the piston. Solutions include larger pilot valve and reducing piston blow by. Consider an o-ringed piston with a drilled EQ port to have repeatable blow by rates. The expansion coeffecient of plastic is horrible for close tolerance plastic parts.

Stuck pistons would have the sound of a normal pilot, but failure of the main valve opening. This would have a short hiss as the piiot vented and then near silence as pilot pressure dropped and eq leakage internally is muffled.

Clean stuck pistons and keep lubed with water based lube for plastic cannons. Liquid dish soap works well. Oil based lube can cause parts to swell.

If you have to replace worn seals, the seals are deforming too much. Consider reducing the peak PSI on your seal with a wider valve seat.

Do you have good photos of your valve seat and piston seal showing wear?
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Re: Gun jammed

Unread postAuthor: limbeh » Tue Jan 07, 2014 11:23 am

For some reason I can't post pictures, but here's the dropbox links to the pictures;

The piston. https://www.dropbox.com/s/nhmkcauz87gssoe/P1150485.JPG

Piston seals tear beacuse the waasher concentrates stress on the seal at the boundaries. We have filed down the washer, but it still occurs from time to time. https://www.dropbox.com/s/mmhdk6twge6zj ... 20tear.jpg

It seems like the problem we had was the piston didn't seal and all the air leaked out. Didn't have air coming out of the pilot valve when we opened it.

So far all my existing audio recordings have crappy sound. I'll have to go wait for a good time when I can take out the gun and re-record the firing sound.

Thanks!
Limbeh
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Re: Gun jammed

Unread postAuthor: jrrdw » Tue Jan 07, 2014 4:37 pm

Use the "Upload attachment" link in full editor to upload pictures from your computer.

sf upload.jpg
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Re: Gun jammed

Unread postAuthor: Technician1002 » Tue Jan 07, 2014 7:43 pm

In regards to your piston photos, there is a large area of seal exposed allowing air behind the seal resulting in tearing at the washer. The washer is too small so all the force holding the seal on is concentrated on the small permiter of the washer. Visit a hardware store or visit the school machine shop and build a washer that is only slightly smaller than the valve seat opening. This will reduce the stress on the seal and reduce deformation when the valve opens. The seal buldging down the barrel before firing is not good.

Here is an example of a small piston with a large washer covering most of the centeral portion of the seal to prevent this.

This example shows use of an o ring to control piston piolt leakage. Not shown in the photo is the small EQ port from the side of the piston to the rear past the o ring to balance the pilot. Use about 1/16th inch bit for this or smaller on small pistons.

Image

Making the EQ port for my Mouse Musket replacement piston. Drilled from the side to the rear for controlled leak rate into the pilot. On this launcher the seal is on the valve seat instead of the piston for a lighter faster piston.
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Re: Gun jammed

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