Piston Valves Explained Visually

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MrCrowley
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Sun Jan 03, 2010 6:05 pm

bobsaget wrote:If i am molding a piston out of the connector that the piston will slide in, i will need to make a slight equalization hole because the piston is the same exact size as the gun itself, correct? And how large should my equalization hole be if the piston is 3/4 in wide or about two CM. I am planning on making a very small hole and making it larger untill the piston moves back. The piston should move back if i have a very small hole right, something around 2 mm?
You don't need an equalization hole unless you seal the piston using o-rings. Even if it looks like a tight fit, air will flow through quite easily.
mrsteel wrote:im building a piston valve airgun and i want to know if the diameter of the exhaust port should be bigger or smaller than that of the barrel, whichone woold give more power to the barrel?
    It shouldn't need to be as large as the barrel in large bore guns, but in small bore guns it should be larger. For most small bore guns with small pistons (anything up to 3/4"), a blowgun should work if the piston is built well. But you will be hindering performance, I always recommend using 1/4" ball valves as they do improve performance quite noticeably in most cases. When it comes to exhaust valves, bigger is always better (in correlation with speed).
    bullrees
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    Mon Jan 11, 2010 8:45 pm

    Thanks for the visual. It makes it much more easy to understand.
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    Labtecpower
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    Tue Feb 23, 2010 3:10 pm

    Thank you for the explanation!
    it made the construction of a piston valve much more clear for me.
    :salute:
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    jackssmirkingrevenge
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    Thu Apr 15, 2010 11:51 pm

    This was one of the first explanatory animations of how piston valves work I ever saw all those years ago, it's just as valid today (in spite of the excess pilot volume :roll:) and deserves a place in this thread :)

    Image
    hectmarr wrote:You have to make many weapons, because this field is long and short life
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    Crna Legija
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    Fri Apr 16, 2010 12:42 am

    thats cool, love the piston design
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    someguy314
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    Tue Jun 08, 2010 8:11 am

    This helped me TONS! Thanks!
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    Technician1002
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    Tue Jun 08, 2010 10:22 am

    The animation is great for beginners. In real life the valve is a little more complex. The animation shows the piston begins to move after the pilot is fully vented. Depending on the ratio of the valve, this does not happen in real life.

    Often the piston will start to open well before the pilot is fully vented and the chamber may vent faster than the pilot area is venting and re-close before it fully opens. This causes some valves to go put put put.

    The animation shows a valve with a wide area ratio. The area of the piston sealed inside the barrel seal is a small portion of the total piston cross sectional area. Those large ratio pistons require a fast large pilot to get good performance.

    A narrow ratio piston remains closed longer and pops open faster when they do pop open because there is less pressure in the pilot area when they do pop open.

    If you view the piston face on, a very large area is exposed to the chamber pressure. Only a slight pressure drop in the pilot is required to start this valve open. Once it opens, the chamber quickly drops pressure. This may happen faster than the pilot area drops pressure. If this happens, the pilot pressure pushes the valve back closed. As the pressure in the pilot continues to drop this repeats. This creates the put put put of some valves, or HONK if a diaphragm.
    kwhalley
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    Mon Jul 19, 2010 8:48 pm

    I have read a lot, and appreciate the high quality descriptions of the piston design. My question is how snug should the piston be?
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    clemsonguy1125
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    Mon Jul 19, 2010 9:05 pm

    It should slide easily but if you shine a flashlight through one end you should not see light out of the other
    That is all.
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    Technician1002
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    Mon Jul 19, 2010 9:25 pm

    Here is a photo of a good piston fit. It is loose enough to close by gravity when tipped on end.
    Image

    With too much of a gap, excessive leakage can prevent them from working.
    kwhalley
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    Mon Jul 19, 2010 9:55 pm

    Thanks for the fast reply. I pu the final touches on my piston and was able to get a successful test by blowing into the valve system with my mouth. I know hats odd... But then again I am 30 and making an air cannon. Thanks again and j will test it for real tomorrow. Kwhalley
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    Technician1002
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    Mon Jul 19, 2010 11:16 pm

    Don't feel odd. The Mouse Musket fills from the chamber. I have blown into the pilot valve a couple times to close the piston as I am not using a spring. Most of the time it can be closed by gravity, but when it gets a little dust or someting, it sometimes needs help. Once sealed, I fill it from the chamber side.
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    Tue Jul 20, 2010 1:36 am

    kwhalley wrote:I have read a lot, and appreciate the high quality descriptions of the piston design. My question is how snug should the piston be?
    It depends on various factors.

    When you trigger the piston by opening the pilot valve, air is flowing out of the pilot chamber through the pilot valve, but at the same time air from the main chamber is flowing into the pilot chamber.

    If more air is going in from the main chamber than is going out of the pilot valve, then the pressure will never drop and it won't fire.

    Say you have a 4 inch chamber and you're piloting with a 1/8" ball valve.

    So, chamber inner area: 12.568 square inches
    Ball valve maximum flow area: 0.012 square inches

    Say your piston is 3.999 inches in diameter, so it has an area of 12.561 square inches. The area through which air can flow from the main chamber is therefore 12.568 - 12.561 = 0.007 square inches, less than the pilot valve flow so it will fire.

    Say your piston is 3.995 inches in diameter, so it has an area of 12.537 square inches. The area through which air can flow from the main chamber is therefore 12.568 - 12.537 = 0.031 square inches, greater than the pilot valve flow so it will never fire.

    If your pilot chamber is too big, pressure will drop at a lower rate and performance will be reduced.

    So er... yeah, in general, as snug as possible without too much friction ;)
    hectmarr wrote:You have to make many weapons, because this field is long and short life
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    cfb_rolley
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    Tue Sep 07, 2010 10:13 pm

    I'm plannng on making my first pneumatic this weekend, and this thread is incredibly helpful. Is there such a thing as having too little pilot volume? Also why is there sometimes a bolt in the piston, I Noticed in one diagram it was labeled as a "lag bolt". And what about piston length, does a longer piston mean better performance?

    Edit: also, what safety considerations should I take into account? Pressure rated pipe is obvious...
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    Crna Legija
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    Tue Sep 07, 2010 10:31 pm

    cfb_rolley wrote:I'm plannng on making my first pneumatic this weekend, and this thread is incredibly helpful. Is there such a thing as having too little pilot volume? Also why is there sometimes a bolt in the piston, I Noticed in one diagram it was labeled as a "lag bolt". And what about piston length, does a longer piston mean better performance?
    yes you can have to little pilot, your piston should move at lest 1/4 of the ID of you barrel. if it doesn't then you wont have a the max performance that you could. (someone els will probably explain it better than i did)

    the bolt is used to hold your sealing face to the piston.

    A longer piston would would have less performance because it is heavier and take longer to speed up, you should try to keep it a bit bigger then the ID of where it is sliding to stop it from spinning around.
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