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Piston getting pulled apart, need advice

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 getting pulled apart, need advice

Unread postAuthor: FighterAce » Thu Apr 28, 2011 10:20 am

I recently finished the bolt action mechanism for my new rifle and decided to test fire it... when the pressure got over 10 Bar, the rear of the piston slammed back into the bumper and the front of the piston remained sealed on the barrel.

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The piston body and check valve are made out of polyamide, the thread joining the 2 halves is a 7mm long M10 which is obviously not strong enough to hold the piston together.
I'm not sure what to do right now... it should have been able to go up to 50 bar and it didn't even survive 10.
Should I epoxy the 2 halves together and hope the check valve inside won't need a replacement O ring or something? Or have another piston made with a different thread joining the 2 halves? Ideas?
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Unread postAuthor: qwerty » Thu Apr 28, 2011 10:46 am

I'd epoxy it, i doubt the O-ring will need to be replaced just lube it well before you glue.
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Unread postAuthor: Lockednloaded » Thu Apr 28, 2011 11:29 am

Most check-valves are not servicable, yours'll be fine if it can handle the pressure
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Unread postAuthor: Technician1002 » Thu Apr 28, 2011 11:38 am

My main suggestion would have been a solid one piece piston with a possible port drilled through it for a check valve insert. That way the force on the piston does not stress a small threaded section.
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Unread postAuthor: FighterAce » Thu Apr 28, 2011 12:22 pm

Technician1002 wrote:My main suggestion would have been a solid one piece piston with a possible port drilled through it for a check valve insert. That way the force on the piston does not stress a small threaded section.


That sounds very interesting... how would you attach the check valve insert into the port and keep it sealed but more important - keep it from flying out under pressure?
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Unread postAuthor: Technician1002 » Thu Apr 28, 2011 12:39 pm

1) I would start by drilling a small bore hole all the way through the piston.
2) Then enlarge the first 3/4 of the borehole from the barrel end to fit the check and spring.
3) Drill a small hole through the piston sideways to vent the valve into the chamber.
4) Next thread the opening of the large opening for a bolt and washer to seal the barrel end of the valve. Use the same hole that holds on the barrel seal washer and set the spring pressure.

I made a very quick drawing.
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Solid piston. Valve would be an insert in the center.
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Unread postAuthor: FighterAce » Thu Apr 28, 2011 1:18 pm

Simple as that... I don't know why I complicate things so much :D

One more thing... in your design, that bolt on the barrel end of the piston would hold all the pressure in the chamber from going out the barrel. If those threads fail... well.... it will be one hell of a AD.
I'm still not sure if I can trust those inner polyamide threads... not M10 at least. What thread do you suggest I put on the inside of the piston?
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Unread postAuthor: Lockednloaded » Thu Apr 28, 2011 1:29 pm

What's wrong with floating oranges?
EDIT: O-rings...
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Last edited by Lockednloaded on Thu Apr 28, 2011 7:15 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Thu Apr 28, 2011 1:38 pm

The pressure on the bolt can't exceed it's cross sectional area. That was the original problem with the piston. The area for the pressure was the OD of the piston. With the bolt the area is just the diameter of the bolt. Now instead of plastic on plastic, it is metal on plastic. Instead of the threaded area shorter than the diameter, the area of the threaded area can be longer than the threaded diameter for a good bite. It is unlikely to spit out the bolt. It will anchor into the plastic very well.

For a good bite, I recommend one of the coarse threads. This gives a deeper bite into the plastic. A fine thread is more likely to pull out. A sealant can be applied to the threads as it is assembled to keep leakage to a minimum.

For the bolt, I would use the same thing you hold on the rubber washer with. When firing, it has full force on it anyway holding on the washer. If it doesn't pull out when firing, it will hold when used in the check valve too.

What's wrong with floating oranges?
I tried that once. I could only get hang times of about 13 seconds. Gravity wins every time. :D
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Last edited by Technician1002 on Fri Apr 29, 2011 10:39 am, edited 1 time in total.
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Unread postAuthor: Gippeto » Thu Apr 28, 2011 2:06 pm

Skip all that and use a solid piston with a floating o-ring..or buna u-cup seal. Have used both with success.

There's really no need for the spring loaded check in the piston...it's just something to cause grief...as you found out.
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Unread postAuthor: FighterAce » Thu Apr 28, 2011 2:14 pm

Thats M5 holding the rubber gasket and washer in place... but thats way too fine. I could just make a threaded piece with coarse threads on one side for the plastic and that M5 on the other side for the rubber gasket and washer.

Sounds like a plan :D Thanks!


EDIT

Gippeto wrote:Skip all that and use a solid piston with a floating o-ring..or buna u-cup seal. Have used both with success.

There's really no need for the spring loaded check in the piston...it's just something to cause grief...as you found out.


I have used solid pistons with floating o rings but it didnt work exactly as a check valve... the U cup seal seems kinda fragile to me... I admit I never tried using one because that check valve is a proven and reliable thing for me. Did I mention I'm piloting with a schrader valve?
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Last edited by FighterAce on Thu Apr 28, 2011 2:22 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Thu Apr 28, 2011 2:20 pm

I would plan on just a larger bolt. An insert is another possible leak.
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Unread postAuthor: FighterAce » Thu Apr 28, 2011 2:24 pm

I was thinking M5 on one side and M12 on the other... leaks shouldn't be a problem with some teflon tape but I think it might not be necessary. Polyamide seals pretty good if its tight enough.
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 28, 2011 4:07 pm

Gippeto wrote:Skip all that and use a solid piston with a floating o-ring..or buna u-cup seal.


seconded
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Unread postAuthor: jhalek90 » Thu Apr 28, 2011 9:50 pm

if you still want to go the check valve route, build it like tech's first drawing, only instead of building a check valve, epoxy in a sharader (spelling) valve.

should do the trick. you can find brass sharaders (spelling) with 1/4'' NPT threads at most hardware stores.
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