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Need help getting Semi consistantly.....

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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Need help getting Semi consistantly.....

Unread postAuthor: BearMan » Fri May 28, 2010 9:11 pm

Guys,

Just played with my first build..... a cannon very similar to http://www.spudfiles.com/forums/my-second-generation-piston-valve-cannon-t20966.html

It is for shooting marshmallows in the yard with my son. We have been having a blast tonight with it. However, I have one problem....

It will Semi-auto without any ammo but if shot for real, I has to have the barrel sealed to allow the piston to slide forward. I made sure the sliding area was really slippery and smooth and the felt fits just perfectly I think. The piston will slide under its own weight.

I am going to make another one similar but want to work out my bugs first.....


Help?????


Barry
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Unread postAuthor: Gun Freak » Fri May 28, 2010 9:12 pm

I don't understand what you're asking... can you clarify the question?
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Unread postAuthor: BearMan » Fri May 28, 2010 9:30 pm

I am sorry....

When I "dry fire" the gun, the piston will re-seal the barrel every time and "semi-auto" but when I put in ammo and fire it, the piston won't seal the barrel. I have to place my hand over the end of the barrel and build up pressure and release for the piston the seat.

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Unread postAuthor: Gun Freak » Fri May 28, 2010 9:37 pm

Oh ok... I had the same exact problem with my coaxial launcher, but whatever you do, don't load it then cover the muzzle with your hand! Especially with a faulty piston. It may just be a marshmallow, but Technician puts them through car doors. I never found out exactly what the problem was, but what is the piston made of?

EDIT Have you tried adding a weak spring? That will help seal it semi-automatically.
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Last edited by Gun Freak on Fri May 28, 2010 9:39 pm, edited 1 time in total.
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Unread postAuthor: clemsonguy1125 » Fri May 28, 2010 9:39 pm

Do you have a spring, if not that may help
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Unread postAuthor: irisher » Fri May 28, 2010 9:39 pm

I have to place my hand over the end of the barrel and build up pressure

I could see where that could be problem. Are you filling it fast enough? On a burst disk of mine it would leak at low pressure but would seal at higher pressure. So I had to create a pressure spike on mine, this could be solution to your problem.
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Unread postAuthor: BearMan » Fri May 28, 2010 9:54 pm

I could come up with a spring but I don't know if that is my problem. I will try it if need be.

The one I am basing it on works down to 20 lbs PSI and I have been playing at 40-60 PSI today. I am running with a constant connection to my air compressor.

Maybe I need a larger chamber or flow from the chamber. I am currently using a 2L. bottle for the chamber glued to a 1" fitting.

Would a larger chamber and connection fittings help the piston to "blow" closed easier? Maybe this is my problem. The cannon I based this on used a 3L bottle and I bet that has a larger mouth and greater flow...... Am I on to something here?


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Unread postAuthor: BearMan » Fri May 28, 2010 9:59 pm

Gun Freak wrote:.....but whatever you do, don't load it then cover the muzzle with your hand! Especially with a faulty piston. It may just be a marshmallow, but Technician puts them through car doors. I never found out exactly what the problem was, but what is the piston made of?.



I always cover the muzzle first and then carefully load it. I have plenty of real guns and play safely....... I didn't get to 40 years of age by being stupid. :wink:

The piston is made from a 1 1/4" coupling with a small block of wood glued in one end. The seal is 2" rubber glued to the end after the face was sanded flat and true. The travel is just under 1" from open to closed.
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Unread postAuthor: chinnerz » Fri May 28, 2010 9:59 pm

does your ammo seal in the barrel??
if so, i think your pilot and chamber are equalising too fast, and pressure is the same on both sides which results in little piston movement.... i think..

If this is the case, maybe a tighter fitting piston, a higher initial pressure spike (a few fast pump form a hand pump) or a weak spring may help.
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Unread postAuthor: Gippeto » Sat May 29, 2010 12:31 am

Sounds like the piston is bouncing closed without the added back pressure of the projectile. ie. recompressing pilot volume combined with rapid depressurization of the chamber, is reversing the direction of piston travel.

The piston should not slide under its own weight.

What are you using for a pilot valve, and where are you introducing the air from your compressor?
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Unread postAuthor: BearMan » Sat May 29, 2010 8:45 pm

Gippeto wrote:Sounds like the piston is bouncing closed without the added back pressure of the projectile. ie. recompressing pilot volume combined with rapid depressurization of the chamber, is reversing the direction of piston travel.

The piston should not slide under its own weight.

What are you using for a pilot valve, and where are you introducing the air from your compressor?


Well.... Actually the piston doesn't slide when the pilot side is sealed. The felt seal around the piston and the vacuum hold it in place. When it was just in the housing, it would slide under its own weight. I was trying to make it a smooth and slippery fit. I can re-apply the felt seal and get a tighter fit.

My pilot valve is a Rain Bird 1" sprinkler valve.

I have a chamber on a 90 deg. elbow that screws into the bottom of the 2" Tee Piston valve. The air inlet is a air hose barb that is screwed into the bottom of the elbow.... just below the chamber.

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Last edited by BearMan on Sat May 29, 2010 8:55 pm, edited 2 times in total.

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Unread postAuthor: BearMan » Sat May 29, 2010 8:45 pm

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Last edited by BearMan on Sat May 29, 2010 8:56 pm, edited 1 time in total.

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Unread postAuthor: BearMan » Sat May 29, 2010 8:47 pm

Gippeto wrote:Sounds like the piston is bouncing closed without the added back pressure of the projectile. ie. recompressing pilot volume combined with rapid depressurization of the chamber, is reversing the direction of piston travel.

The piston should not slide under its own weight.

What are you using for a pilot valve, and where are you introducing the air from your compressor?


Well.... Actually the piston doesn't slide when the pilot side is sealed. The felt seal around the piston and the vacuum hold it in place. When it was just in the housing, it would slide under its own weight. I was trying to make it a smooth and slippery fit. I can re-apply the felt seal and get a tighter fit.

My pilot valve is a Rain Bird 1" sprinkler valve.

Look at the pics here....

http://www.spudfiles.com/forums/my-second-generation-piston-valve-cann%20on-t20966.html

I have a chamber on a 90 deg. elbow that screws into the bottom of the 2" Tee Piston valve. The air inlet is a air hose barb that is screwed into the bottom of the elbow.... just below the chamber.

Mine is very similar to this..... My air barb is on the bottom of the elbow pointing toward the piston and tee......

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Unread postAuthor: Gun Freak » Sat May 29, 2010 8:49 pm

Why the tripple blank post?
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Unread postAuthor: BearMan » Sat May 29, 2010 8:51 pm

Well.... Actually the piston doesn't slide when the pilot side is sealed. The felt seal around the piston and the vacuum hold it in place. When it was just in the housing, it would slide under its own weight. I was trying to make it a smooth and slippery fit. I can re-apply the felt seal and get a tighter fit.

My pilot valve is a Rain Bird 1" sprinkler valve.

Look at the pics here....

The air ishttp://www.spudfiles.com/forums/my-second-generation-piston-valve-cann%20on-t20966.html

I have a chamber on a 90 deg. elbow that screws into the bottom of the 2" Tee Piston valve. The air inlet is a air hose barb that is screwed into the bottom of the elbow.... just below the chamber.
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