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How far should a piston atleast move backwards?

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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How far should a piston atleast move backwards?

Unread postAuthor: POS » Tue Oct 31, 2006 5:54 pm

The title says it all. How far should a piston move back in a barrel sealing piston valve ?

If I think logicaly, I would say at least half of the ID of the barrel. Can anyone confirm this ?
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Unread postAuthor: jrrdw » Tue Oct 31, 2006 8:00 pm

This is a good question, i think there are to many verieables (sp) to generalize it, you know - air pressure/side clerances/over all size/ect...
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Unread postAuthor: squeaks » Tue Oct 31, 2006 8:02 pm

I'd say as far as you possibly could allow the piston would be the best. Wait, never mind that, because if it moves farther then it adds space for the air to go, so I guess you wouldn't want it to go too far.
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Unread postAuthor: djt » Tue Oct 31, 2006 8:19 pm

i would say a little more than the diameter of your barrel. like if you have a 1 1/2 barrel then make it move back 1 1/2. sounds logical to me anyways
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Unread postAuthor: SquishY » Tue Oct 31, 2006 8:22 pm

Math says that it should move atleast 1/4 of the sealing face, so for a 2" sealing face it should go back .5" for max flow. Suppose a little more wouldn't hurt.
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Unread postAuthor: mark.f » Tue Oct 31, 2006 10:39 pm

SquishY's got it. 1/4 of the barrel diameter is all you need for maximum flow. Any more is just wasting energy.
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Unread postAuthor: drac » Tue Oct 31, 2006 10:49 pm

No, it's 1/4 of the barrel's cross sectional area if i recall correctly.
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Unread postAuthor: Brian the brain » Wed Nov 01, 2006 12:34 am

Yes that's right.More will hurt performance, and so will less..

You could make it open a little more to account for the neoprene flap blocking flow a little as it bends with the airflow, as we did with Chappy..
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Unread postAuthor: SquishY » Wed Nov 01, 2006 1:56 pm

I'm not sure how far mine will open but I will try to compensate for it by playing with tthe piston length before sticking it together. I how ever can not take credit for the original math but I have since applied it, Gort and I were talking and that came up so yeah.
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Unread postAuthor: POS » Sat Nov 04, 2006 1:28 pm

Thanks for the answers. I let mine move half of the inner diameter of the barrel. That should be good.

In the MB2 I made, the piston moves back 4 cm, the barrel is 28 mm. The piston got dammaged all the time, cause it smashed against the stopper. Got a steel stopper now, the piston itself has a steel bolt in it.

So, with this new information, my fourth gun, also the first PVC gun, will be a lot bether.
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Unread postAuthor: SquishY » Sat Nov 04, 2006 4:25 pm

As we've posted, to much movement can hinder the performance, go by what the math says and you will have a much better valve.
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Unread postAuthor: sgort87 » Sat Nov 04, 2006 5:36 pm

Cuz knowledge is power!
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Unread postAuthor: squeaks » Sat Nov 04, 2006 6:02 pm

Wait, one quick second here. Is it different for chamber sealing vs. barrel sealing valves or is it the same?
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Unread postAuthor: boilingleadbath » Sun Nov 05, 2006 2:38 pm

It might change it some... but it shouldn't hurt preformance (or saftey) noticably to allow it to move back twice as far as calculated, so you might as well let it. Anyways, it makes sure it moves back far enough to allow full flow.
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Unread postAuthor: POS » Sun Nov 05, 2006 3:51 pm

SquishY wrote:As we've posted, to much movement can hinder the performance, go by what the math says and you will have a much better valve.


And what does math tells us ?

I think it is logical the piston moves about 1/2 of the diameter of the barrel. If math tells otherwise, how come ? How do you calculate it ?
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