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QDV bumper question

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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QDV bumper question

Unread postAuthor: c9d » Fri Jul 08, 2011 9:51 pm

I have researched many threads by Technician about his QDV projects, but on thing I would like to know is can a QDV be used without using a bumper for the piston?
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Unread postAuthor: saefroch » Fri Jul 08, 2011 10:02 pm

Yes, but a bumper is suggested for all piston valves, or the piston tends to just crush itself into whatever it hits.
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Unread postAuthor: Technician1002 » Fri Jul 08, 2011 11:55 pm

Due to the speed and force on the piston, it does tend to break things without a bumper. Trust me, use a bumper.
Image

An early version had PVC and it did not last long. This was later upgraded to a very robust bumper and ABS construction which has lasted well.
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Unread postAuthor: c9d » Sat Jul 09, 2011 3:40 am

Ok... so I have to use a bumper... how about a spring with a rubber bushing? Basically I am looking for a way for the piston to reseat itself without me having to manipulate anything like you have in your videos. Is it possible? Would the construction materials play an important role, ie aluminum vs ABS?
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Unread postAuthor: Technician1002 » Sat Jul 09, 2011 9:56 pm

Self resealing is possible even without a spring. If you make the leakage small by the rod exit hole and allow some leakage through the piston, the air behind the piston can be enough to reset it after the shot. I get this sometimes with my large launcher. In my video on you-tube on o rings the shot at the end of the video shows the almost seal on the low pressure shot. After the shot the remaining pressure can be heard bleeding off from an imperfect seal. On a high pressure shot, I sometimes find up to about 10 PSI captured in the tank from the piston bounce off the bumper and air in the pilot.

Link to the video. Turn up the sound and fast forward to 5:10. This low pressure shot did not fully seal, but it tried.
http://www.youtube.com/user/Technician1002#p/a/u/1/BbDiZiRVrnk
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Unread postAuthor: c9d » Tue Jul 12, 2011 2:47 am

do you think a spring could be used to cushion the piston AND return it back to its original position on the rod? I understand that whatever the rod is connected to would have to be made of very sturdy materials and properly braced.
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