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piston valve/ blow gun actuation

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/ blow gun actuation

Unread postAuthor: Major Collins » Sat Dec 18, 2010 2:37 am

What type of piston is needed to be actuated with a blow gun ?... i have one but i don,t know why it works so well.

eg light , heavy , hollow , solid piston etc

i just need some clarification

my piston is a rubber sealing face with a washer behind it and has hollow light plastic body and is held together by a nut and screw just for those who are wondering
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Unread postAuthor: MrCrowley » Sat Dec 18, 2010 6:05 am

Without a doubt the two biggest factors are:

1: Tolerance. The gap between the piston and the housing the piston moves inside. JSR (jackssmirkingrevenge) has a good diagram on the forum somewhere that shows even the smallest gap between piston and housing adds up substantially in area. The bigger the gap, faster air from the chamber can fill the pilot area, slowing the pressure drop, delaying the valve opening time (even as much as not opening the valve at all).

2: Pilot volume. Similar to the above. The more volume of air you have behind the piston and in front of the pilot valve, the more the pilot valve has to vent. Big volume means a slow pressure drop and slow valve opening time (or even no opening at all).

Make sure your piston is tight (but can move under force), lubricant is almost a requirement. Keep pilot volume as small as possible, fill up excess pilot volume with foam/wood/whatever to reduce the volume of air behind the piston.

Weight doesn't really matter so much.

edit:

While, technically, one could actuate a 2" diameter piston with a blow gun I wouldn't recommend it. Generally, when it comes to pilot valves, the bigger they are the more performance you will get. While a blowgun may actuate the piston, it could limit performance whereas a 1/2" ball valve would be better suited. Personally, I use 1" sprinkler valves to actuate 2" diameter piston valves. Not only does this assure I get maximum performance, but it also allows me to use a blowgun as the trigger (blowgun pilots the sprinkler valve which pilots the piston valve).

Piston diameters 3/4" and below should be able to use a blowgun pilot no problem if they're built decently.

edit 2:

I'm impressed by this PUI reply. Perhaps one of the all time best PUI's on this forum,
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Unread postAuthor: jackssmirkingrevenge » Sat Dec 18, 2010 2:00 pm

MrCrowley wrote:JSR (jackssmirkingrevenge) has a good diagram on the forum somewhere that shows even the smallest gap between piston and housing adds up substantially in area. The bigger the gap, faster air from the chamber can fill the pilot area, slowing the pressure drop, delaying the valve opening time (even as much as not opening the valve at all).


More than a diagram it's a calculation, but given the PUI circumstances, good effort! And thanks for feeling the need to clarify who JSR is :D
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Unread postAuthor: Labtecpower » Sat Dec 18, 2010 4:29 pm

After how many beers does "PUI" occur?

I made a 22 mm piston gun, (without O-rings).
It actuates great with a blowgun.
First I made a piston out of PVC-piping. It broke down after every couple of shots.
Now I machined a piston out of nylon at my school. It's a very relyable gun now.
It was very annoying when shoing it to friends, I could shoot it one time, and come to the conclusion that it broke down again :evil:

In my opinion you should be fine with a blowgun op to one inch.
But bigger is ALWAYS better.
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Unread postAuthor: Major Collins » Thu Dec 23, 2010 3:20 am

hrmmmmm thinking about all the factors , can i get a drawn diagram of the factors of a piston in the housing please :)
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Unread postAuthor: jackssmirkingrevenge » Thu Dec 23, 2010 7:23 am

I want to do a series of animations for the Wiki illustrating the various factors affecting performance, I'll try and do them tonight.
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