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Piston Valve Problem

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 Valve Problem

Unread postAuthor: clemsonguy1125 » Fri Aug 28, 2009 7:41 pm

On my 2inch piston valve, the piston seals but when I trigger the pilot it does'nt fly back. It fits in the housing perfectily. It has a 1/2 inch pilot valve.
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Unread postAuthor: c11man » Fri Aug 28, 2009 8:07 pm

seat size? pics would be the best
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Unread postAuthor: inonickname » Fri Aug 28, 2009 8:13 pm

1. Too much pilot volume.
2. Piston not good enough fit (is there visible light around it when held to a light?)
3. Pilot valve is too slow.
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Re: Piston Valve Problem

Unread postAuthor: velocity3x » Fri Aug 28, 2009 8:39 pm

clemsonguy1125 wrote:On my 2inch piston valve, the piston seals but when I trigger the pilot it does'nt fly back. It fits in the housing perfectily. It has a 1/2 inch pilot valve.
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Last edited by velocity3x on Wed Jan 13, 2010 4:09 pm, edited 1 time in total.
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Unread postAuthor: far_cry » Fri Aug 28, 2009 9:29 pm

seat size? picture of the piston and the housing will help
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Re: Piston Valve Problem

Unread postAuthor: Technician1002 » Fri Aug 28, 2009 9:35 pm

velocity3x wrote:
clemsonguy1125 wrote:On my 2inch piston valve, the piston seals but when I trigger the pilot it does'nt fly back. It fits in the housing perfectily. It has a 1/2 inch pilot valve.


It the valve seat is recessed from the adjacent chamber port and the front portion of the piston seals against the cylinder wall, a strong suction will be formed, holding the piston sealed. Much like trying to pull a rubber stopper from a drain with many feet of water on top of it. The front portion of the piston needs some lateral clearance for pressurized gas to break the vacuum and allow the piston to retract.


In the world of physics, there is no vacuum. There is lower absolute pressure.

A piston will move when the forces on one end exceed the friction and forces on the other end.

With the above in mind, attention to the area of the pilot area and pressure, the force on that end of the piston can be calculated. The portion of the other end of the piston exposed to the chamber pressure can be calculated and the force pushing it open can be calculated.

The piston will open when the force holding it closed is less than the force trying to open it.

Failure to open falls into a couple catagories.

Too much friction.. It's stuck.
Too much blow by, so the pilot area pressure remains relatively high while the chamber pressure rapidly escapes into the pilot area.

Solving the friction or pressure issues will solve the problem.

New piston builders often underestimate the space between a piston and the wall. The space may be narrow, but when long all away around the piston, the area adds up to a large eq port.
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