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Piston Actuating Late

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 Actuating Late

Unread postAuthor: spudkilla224 » Mon Jan 26, 2009 5:29 pm

Hello, I built a piston cannon, and when i exhaust the piston it takes a second or two for the piston to acuate. What could be the problem?


My exhaust valve is a 1/4" ball valve. Is this too small to vent a 2" piston valve?

Or should i upgrade to a 1/2" ball valve?
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Unread postAuthor: Brian the brain » Mon Jan 26, 2009 5:41 pm

Well there's your problem...

You need a larger, faster exhaust valve.

A 1/2"ballvalve would help, a sprinkler or QEV would be even better.
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Unread postAuthor: jonnyboy » Mon Jan 26, 2009 5:46 pm

How about a 1 inch piston to pilot the 2 inch then a 1/2 piston then a blowgun trigger. :D
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Unread postAuthor: Brian the brain » Mon Jan 26, 2009 5:53 pm

You forgot the 3/8"and 1/4".

But the idea isn;t half bad.
You could build a 1"piston valve for it.
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Unread postAuthor: Gippeto » Mon Jan 26, 2009 8:14 pm

The 1/4" ball valve should be adequate to get it working.

Based on previously seen problems, you have one or more of the following problems.

1) Piston is too loose, allowing excess blowby.

2) Hugely excessive pilot volume/piston travel.

3) A lack of exposed frontal area on your piston.

4) Your bumper material is partiall blocking your pilot exhaust.

Check and correct as required. :)
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Unread postAuthor: MrCrowley » Mon Jan 26, 2009 8:39 pm

Adding to Gippeto's points, to find out if you should check those off or not, i'll explain them a bit more...

1: Loose piston. How well does the piston fit in the piston housing? It should only be able to move forwards and backwards, not side to side. You shouldn't be able to see a gap either. The piston should fit perfectly inside the piston housing, but not so tight you can't move it with your fingers.

2: A) Pilot volume. This should be kept to a minimum, look at other 2" piston valved cannons for ideas on how much pilot volume you should have.

Here are two of mine:
http://www.spudfiles.com/forums/v-a-l-p ... 14702.html

http://www.spudfiles.com/forums/viewtop ... tml#100616

B) Piston travel. In a barrel sealing piston valve, the piston really only needs to travel about half of the seat's diameter. The seat is what the piston seals against (that leads to the barrel). So for a 2" piston, it really only needs about an inch of room to travel from as far back as it can go, to sealing against the barrel.

2-3" shouldn't hurt performance too much though.

3: This shouldn't matter that much either, but it could be a contributing factor. The 'exposed frontal area' is area on the sealing face of the piston that is not sealed by the seat. This is the area that is exposed to pressure and forces the piston back when the pilot is exhausted.

4: You should understand this one.

:)
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Unread postAuthor: Ragnarok » Mon Jan 26, 2009 8:51 pm

MrCrowley wrote:3: This shouldn't matter that much either, but it could be a contributing factor. The 'exposed frontal area' is area on the sealing face of the piston that is not sealed by the seat. This is the area that is exposed to pressure and forces the piston back when the pilot is exhausted.

It should be said that although a low percentage of the piston's front is exposed to pressure increases the delay before the valve starts to open (as more air needs to vent from behind the piston to trigger it), it will improve the time it takes the valve to go from closed to full open.

To tell the truth, the problem is probably a mix of 1 and 2, but...
Although as Don Gippeto says, a 1/4" BV can do the job, personally, I would recommend stepping up the valve to a 1/2" one, which will heavily cut the piston delay.
Although, that said, this is coming from the nut who decided to use a 1/2" QEV to vent a 7/8" piston valve, a record in total venting overkill. I'm told that people with 1/2" QEVs as main valves still have a contract out on me for it.
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