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Piston bounce?

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 bounce?

Unread postAuthor: ALIHISGREAT » Wed Sep 17, 2008 1:38 am

well i have just been thinking of piston bounce and i was wondering if it can be caused by the piston hitting a hard surface and bouncing back, or is it just because of pilot valves not being able to keep up?

in my copper coax i have a 26mmx3cm HDPE piston that hits a brass reducing bushing that i glued a one layer ring of bike tyre rubber as a muffler as opposed to a bumper..

so will it bounce?
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Unread postAuthor: BlackH » Wed Sep 17, 2008 4:10 am

there has been a few times i shot my gun and realized the piston was against the sealing face again but the air escapes so fast that by the time the piston bounces back all the air is out. keeping in account pilot size and piston size and chamber size.
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Unread postAuthor: VH_man » Wed Sep 17, 2008 5:43 am

piston bounce is more the effect of suction. In(my) theory, if you have a 100% sealed piston and you can vent your pilot well below halph the pressure in the chamber by the time the piston has moved SLIGHTLY back, then you should be fine.
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Unread postAuthor: Gippeto » Wed Sep 17, 2008 9:00 am

My .02 :)

If you model your launcher using ggdt, you can see the effect of pilot volume/valve size by checking the valve position.

Decrease the pilot valve size too much, and watch the piston bounce.

Put simply, according to my interpretation of ggdt results, piston bounce is caused by RECOMPRESSION of the pilot volume.

When things are in "harmony", ggdt predicts the piston not bouncing.

It is my belief, that the closer you get to having control of airflow within the valve, the closer you get to the "ideal" model on ggdt.
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Unread postAuthor: jrrdw » Wed Sep 17, 2008 9:30 am

If you have a pressure gauge on your chamber it will show pressure still in the chamber if your piston is bouncing back to sealed possion.
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Unread postAuthor: jimmy101 » Wed Sep 17, 2008 10:24 am

Before worrying about piston bounce (honking) you probably should decide if it is actually a problem. It may or may not affect the performance of the gun.

If the piston bounces after the ammo has left the barrel, because of the rapid depressurization of the chamber which makes the residual pilot pressure greater than the chamber/barrel pressure, then the bounce has no affect on the performance of the gun. The bounce may be an annoyance but it is not a performance problem.

If the piston bounces before the ammo has left the barrel then you are choking flow and the gun's performance will suffer.
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Unread postAuthor: clide » Wed Sep 17, 2008 3:13 pm

Gippeto wrote:If you model your launcher using ggdt, you can see the effect of pilot volume/valve size by checking the valve position.

Decrease the pilot valve size too much, and watch the piston bounce.

Put simply, according to my interpretation of ggdt results, piston bounce is caused by RECOMPRESSION of the pilot volume.

When things are in "harmony", ggdt predicts the piston not bouncing.


Indeed it is caused by a combination of compression of the pilot volume from the rapidly moving piston as well as the rapid pressure loss from the chamber of the open valve.

The dimensions of your valve play a key role in the likelihood of the piston bouncing. The more pressure left in your pilot area when the piston begins to open then the more chance you have of it bouncing. The pressure left in your pilot area when the piston opens depends on your ratio of seat diameter to piston diameter as well as you configuration (barrel or chamber sealing).

As an example say you have a 2" piston with a 1.9" seat, and lets say the piston seals perfectly. In the barrel sealing configuration you could pilot it with a schrader valve and without a barrel and you won't get a bounce because the pilot pressure is very low before the valve actually starts to open. However if you change it to a chamber sealing configuration you are going to end up with serious bouncing issues if you are using a small pilot valve. This is because the pressure in the pilot hardly has to drop at all before the valve starts to open, so unless you are using a pilot valve big enough to get the remaining pressure out faster than the chamber pressure is falling then you will end up with a bouncing piston.

Other factors that affect piston bounce are pilot volume size, piston weight, and projectile weight. In some cases a bigger pilot volume can reduce the tendency to bounce because it is compressed to a lower pressure from the rapidly moving piston. However it is still usually the best idea to minimize the pilot volume. A heavier piston will also reduce the effect of piston bounce because it takes longer for that piston to change direction and bounce back, which gives more time for the pilot valve to do it's thing. Projectile weight is also important, if you are using a very light ammo or no ammo at all then the chamber pressure is going to fall very rapidly which means it is more likely that the pilot pressure will be higher than the chamber pressure and send the piston back the other way.

In my experience GGDT is quite accurate at predicting piston bounce.
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