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Largest piston piloted by schrader?

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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Largest piston piloted by schrader?

Unread postAuthor: metalmeltr » Wed Jan 12, 2011 6:06 pm

What is the largest piston that can be efectivly piloted by a schrader around 100psig? I tried making a coaxial of 1" and 1/2" pvc that was less than 10", the chamber was close to 4". The piston was cast bondo about 1" long. The piston sealed fine but failed to operate when piloted by the schrader. the piston fit tightly and did not have an O-ring.
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Unread postAuthor: saefroch » Wed Jan 12, 2011 6:13 pm

Did you verify that the chamber is filling?
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Unread postAuthor: Vincenzo » Wed Jan 12, 2011 6:14 pm

JSR's piston cannon is probably the largest I've seen. BUT, the size doesn't matter. As long as your piston doesn't allow backflow, you could, in theory, pilot any size piston through a schrader valve.

JSR's--
http://www.spudfiles.com/forums/3-4-exhaust-valve-cannon-t7173.html
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Unread postAuthor: metalmeltr » Wed Jan 12, 2011 6:22 pm

I belive the chamber was filling, i drilled though the piston to act as a check valve. I would imagine there was too much leakage between the piston and the chamber. The valve did close properly when the chamber was filled. I belive this little thing will work with a larger pilot.
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Unread postAuthor: Gun Freak » Wed Jan 12, 2011 6:23 pm

Just drilling through the piston doesn't make a check valve :wink:
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Unread postAuthor: metalmeltr » Wed Jan 12, 2011 6:29 pm

The hole went though behind the sealing face, air will push the face out of the way when pressured from behind and close when pressure is applied from the front. I belive that is a check valve.
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Unread postAuthor: Gun Freak » Wed Jan 12, 2011 6:33 pm

I don't think that will be very effective though. On a large scale with a bigger pilot valve, maybe, but when you're dealing with a schrader pilot... I don' think so. I would think there would still be backflow- too much for a schrader to handle.
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Unread postAuthor: metalmeltr » Wed Jan 12, 2011 6:36 pm

I would say the backflow can be blamed on lack of o-ring, also the piston can travel about a full inch so pilot volume is probley way too large.
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Unread postAuthor: Gun Freak » Wed Jan 12, 2011 6:45 pm

You're missing the point...
If you have an airtight piston, air can't get the the chamber.
If you don't have an airtight piston, then the valve will equilize too fast for enough pressure drop to pilot it.
If you have an airtight piston with a check valve, air can fill the chamber, and a valve as small as a schrader can pilot the valve. Catch my drift?
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Unread postAuthor: metalmeltr » Wed Jan 12, 2011 6:49 pm

I understand what you say, but the chamber is in fact filling and the piston is less than airtight, with a smaller pilot voulme it may pilot via schrader
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Unread postAuthor: Gun Freak » Wed Jan 12, 2011 6:52 pm

Yes, it is possible. But still, it can be more efficient.
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Unread postAuthor: Lockednloaded » Wed Jan 12, 2011 7:09 pm

theoretically, your piston would only need to move back 1/8" of an inch according to the d/4 rule which means you would only have to pilot ~1/10 of a cubic inch worth of air. i couldn't find the proper specs on a schrader valve, but I think its safe to assume it could pilot 1/10 of a cubic inch.

But for your pilot volume to be exactly 1/10 of a cubic inch you would need an o-ring seal. In your case, a tight fitting, low friction piston should work, but you didn't need to drill an EQ hole because your piston is not air tight.
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Unread postAuthor: metalmeltr » Wed Jan 12, 2011 7:16 pm

Well it too late to modify pilot volume so i guess i will just use a larger valve.
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 12, 2011 7:19 pm

2" diameter piston piloted by a schrader, with the benefit of an o-ring seal.

With larger diameters, you're pushing the limit on how much play the piston can have, and without a seal the equalisation hole is unnecessary and harmful to performance.

Best thing is to do the math ;)
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Unread postAuthor: metalmeltr » Wed Jan 12, 2011 7:35 pm

This piston is farily tight, bondo casted in the same size pipe, had to be lightly sanded to slide. covered in lithium grease
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