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Piston Seal / Bumper Info

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 Seal / Bumper Info

Unread postAuthor: velocity3x » Wed Jul 20, 2011 4:20 pm

I know that some members have trouble with front seals and bumpers on their piston valve designs. This is a piston assembly I run is a cannon which has a working pressure of 1kpsi and the piston seems to be indestructible. I posted it thinking it might be of some help to others. This works quite well for me but.....you'll have to decide what's best for your situation.

I found that front seals fail for a number of reasons:

1. Front seal retainer diameter is to small or the retainer is to thin.

2. Seal material is to soft and ergo permeable, especially under high pressure. A soft seal will explode internally under high pressure.

3. Front seal material is to thin ergo flexible, allowing the seal to rip / blow away from the piston face when fired. A soft, thin seal has very little structural integrity.

4. Bumpers seem to suffer from the same problems that plague the front seal.


The piston assembly renderings:
This assembly is a full retract (1.800") unit w/ a working pressure of 1kpsi.
The piston is black acetal,1.998" dia.
The front seal material is neoprene, 91 durometer (shore "A" scale).
The front seal retainer is carbon steel and has a major diameter 0.010" less than the barrel ID.
Rather that a conventional "bumper", it uses a plunger on the rear of the piston that acts as a dashpot. The plunger OD is 0.005" less than the hole diameter for the pilot valve.
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Unread postAuthor: Gun Freak » Wed Jul 20, 2011 4:26 pm

Good post. Can you explain further the plunger?
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Unread postAuthor: Secret Squirrel » Wed Jul 20, 2011 5:21 pm

I'm just going by the diagram, but it looks like the plunger on the rear of the piston blocks the port to the pilot valve. The trapped air is compressed and acts as the bumper. Do I have the right idea Velocity3x?
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Unread postAuthor: velocity3x » Wed Jul 20, 2011 5:36 pm

Secret Squirrel wrote: I'm just going by the diagram, but it looks like the plunger on the rear of the piston blocks the port to the pilot valve. The trapped air is compressed and acts as the bumper. Do I have the right idea Velocity3x?


Yes, however the plunger doesn't block the port.....it only limits the gas flow like a needle valve.

Gun Freak wrote:Good post. Can you explain further the plunger?


A plate seals the rear of the piston bore. The plate has a pilot vent hole of 0.500". Total piston travel is 1.800". The piston retracts at high speed for 1.300". At that point, the plunger begins to enter the pilot vent hole. The plunger OD is 0.010" smaller than the pilot vent hole. Because the plunger causes a restriction in pilot gas flow, the rearward speed of the piston is greatly retarded in the remaining 0.500" of piston travel.
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Unread postAuthor: ramses » Mon May 28, 2012 12:15 pm

I like it! How did you analyze the design to determine the leakage of the dash pot? I am also very impressed that acetal is holding up at 1ksi.

Is the pilot port (.500" dia) crowned in any way? What material is the plate on the back?
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Unread postAuthor: velocity3x » Mon May 28, 2012 1:49 pm

ramses wrote:I like it! How did you analyze the design to determine the leakage of the dash pot?

I didn't analyze. I knew that under 1ksi, even a small clearance would pass a large volume of gas. I just kept the clearance as tight as comfortably possible so as not to cause a strike on / in the pilot port.


I am also very impressed that acetal is holding up at 1ksi.

My first design used a heavy rubber pad for a bumper attached to the piston. The pad usually disintegrated within a few shots due to explosive decompression. If the pad did manage to hold up long enough, the piston always suffered a broken neck after 5-6 shots. Never happened again with the air cushion design.


Is the pilot port (.500" dia) crowned in any way? What material is the plate on the back?

Other than a .03125 chamfer to remove any sharp edge, the pilot port is a straight cylinder bore. The back plate is 6060 T6, 4 x 4 x 1.0.
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Unread postAuthor: ramses » Mon May 28, 2012 7:28 pm

velocity3x wrote:Other than a .03125 chamfer to remove any sharp edge, the pilot port is a straight cylinder bore. The back plate is 6060 T6, 4 x 4 x 1.0.


Is that the material of the back of the pilot, or the back of the piston?
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Unread postAuthor: velocity3x » Mon May 28, 2012 11:10 pm

ramses wrote:Is that the material of the back of the pilot, or the back of the piston?


The plunger on the back of the piston is PVC. I'd post solidworks drawings but their in my shop. Here's an old pic that might help.

The 4x4x1 back plate seals the 2" piston bore. The back plate has a .500 pilot hole bored completely through the plate. The ball valve is shown bolted to the backing plate. When the piston retracts, the PVC plunger enters the .500 pilot hole in the back plate and slows the discharge through the ball valve....thus slowing the pistons retract speed.
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