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Piston Valve Sealing, but not Filling?

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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Unread postAuthor: Mr.Tallahassee » Sun Nov 06, 2011 2:11 am

vakimo wrote:Correction: the gap is smaller than that, but i cant fit calipers in there. to give you an idea, it will barely slide with 2 dollars wrapped around it .Ill go ahead a make another piston with closer tolerance just in case though. I think oil/grease would be good, any recommendations on what type? like gun oil maybe?

again the screws hold up to the pressure at 200psi, well above the working pressure at 125. my only concern is the affect of repetitive piston impact on the rear set of screws, even though thats cushioned by a half inch of rubber and silicon.
Just try O-rings. Floating O-rings act as one-way "valves" which allows more tolerance for piston size since you don't need to worry about having a very tight fit. Also, just rebuild the rest of the gun because the short socket DWV fitting is dangerous on it's own without holes drilled into it. The screws actually reduce the structural integrity of the PVC.
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Unread postAuthor: Gippeto » Sun Nov 06, 2011 10:35 am

You've raised my curiosity....

The published tensile strength for pvc varies widely...is it safe to assume that you obtained the information from the manufacturer?

Would you post the manufacturer and the tensile strength? It's good information to have.

Did you calculate for bearing stress and tear out? Shear strength of the screws?

Did you consider the effects of temperature?

You said you "calculated"...thus the questions. :)
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Unread postAuthor: vakimo » Sun Nov 06, 2011 11:45 am

The manufacturer is Charlotte Pipe, but i could not find the manufacturer specs. Theyre out there somewhere, i just got tired of looking so i found this resource from engineering toolbox to have a nice estimate. I tested the pull out of the screws with weights, then calculated the psi required to create the same effect using the surface area of the screw holes. I wasnt concerned about their shear strength since it had PVC cement to help it, but i guess it couldnt hurt to go back and do the math.

I will remake the pvc parts of the gun without screws and with long socket fitting. The short ones were only used because i couldnt find the long, but i did find a supplier recently that has them.

So would floating o-rings be the best solution to the problem?
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Unread postAuthor: Gun Freak » Sun Nov 06, 2011 11:46 am

So would floating o-rings be the best solution to the problem?

Yes.


Charlotte 1.5" is rated to 330 psi.

...

Scrap your chamber, please.
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Unread postAuthor: Gippeto » Sun Nov 06, 2011 11:56 am

Okay, 250lbs force to "pull" the screws. A nice definetive figure. :)

If you consider the surface area exposed to pressure and the force required to "pull" the screws....you'll find that "pulling/pushing" the screws out isn't going to be possible with any sane pressure.

Bearing stress, tear out and shear strength are the things you need to calculate.
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Unread postAuthor: vakimo » Sun Nov 06, 2011 11:16 pm

what do you mean by tear out? if not the screws, as in the fitting coming off? And yes, the screws do blow out at 250psi (not force) since they are not fully 'screwed' in. the holes were drilled first with 2 different sized bits so that the screws would only have a few threads to grab onto, putting almost no stress on the pvc. maybe this is a source of confusion? i think some of you may have assumed that i just went to town with a drill and screws like it was a high school shop project, but even though im relatively new to potato guns i do have some basic structural knowledge :wink:
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Unread postAuthor: Mr.Tallahassee » Mon Nov 07, 2011 12:22 am

At 125 PSI, there is 254.5 pounds pushing against the screws. That's quite a lot of force. The stress of threading in screws is minimal in comparison to the stress that the end of the gun is under. If the bond between the coupling and the pipe breaks, the screws will just be more shrapnel.
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Unread postAuthor: vakimo » Mon Nov 07, 2011 8:16 am

The Proof strength of the screws is roughly 25,000psi. The shear strength tends to be 60% of that with most steels, so with the diameter of the 1/8" screws they can each hold about 184lbs in shear force before they start to yield. times that by four, and they have enough to withstand the 628lbs of force on the rear (@200psi, w/ 2" bore). Granted, this does not account for the twisting of the screws that might happen in the event of the coupling bond breaking before force was applied to the screws, but this was also to answer one of Gippeto's questions.
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Unread postAuthor: vakimo » Mon Nov 07, 2011 8:56 am

i am going to remake it, but here is an example of many more pins being used effectively. http://www.youtube.com/watch?v=75O71AHTIr4
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Unread postAuthor: Gippeto » Mon Nov 07, 2011 10:40 am

Yup....but much more neatly done. :wink: :P

Tear out is a failure of the material. In essence, it "tears out" (actually a shear failure) from the edge of the screw hole to the end of the "tube" (fitting in this case).

Machinery's Handbook has information on and the formula to calculate it. Searching the engineering websites will also turn up the information.

Nothing wrong with using screws in shear to hold things together...have used it several times myself, and at significantly higher force levels. :wink:

My bad on misinterpreting your previous post re: the pulling the screws...skimming again. :roll:
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Unread postAuthor: vakimo » Mon Nov 07, 2011 7:16 pm

True :wink:
Alright, good to know. Thanks for the advice!
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Unread postAuthor: MrCrowley » Mon Nov 07, 2011 7:41 pm

vakimo wrote:the holes were drilled first with 2 different sized bits so that the screws would only have a few threads to grab onto, putting almost no stress on the pvc

The screws themselves may not put stress on the PVC but, to quote the member Ragnarok,: "Even a round hole with no sharp corners drilled in pipe increases the stress around it by three times".

People do/used to use screws to hold plugs on the back of large piston valves to make them serviceable and most survived without any cracking but it has been known to happen.

Anyway, I'm sure you get the point by now :D
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Unread postAuthor: Technician1002 » Mon Nov 07, 2011 7:43 pm

Danger Will Robinson! Danger!

I am highly disturbed by this photo. A green tank and a green regulator. Using Oxygen instead of air for compressed air tools is considered a bomb unless the tool is oil and grease free.

Image

Improper Use of Oxygen

*

Never use an oxygen tank in place of a container of inert gas, air or compressed air. Oxygen can be very dangerous when used for the wrong purpose. Don't use oxygen to inflate vehicle tires, power pneumatic tools, or for any purpose except where it is necessary.

Improper use of oxygen tanks can cause fires and explosions. Oxygen can be ignited by small sparks or hot machinery. It can also react with oil, grease and chemicals, causing them to ignite or explode. Many people have been injured and even killed by improper use of oxygen tanks.


Read more: Hazards of Oxygen Tanks | eHow.com http://www.ehow.com/list_7580020_hazard ... z1d4PGGZrD


http://www.ehow.com/list_7580020_hazards-oxygen-tanks.html
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Unread postAuthor: vakimo » Mon Nov 07, 2011 8:11 pm

MrCrowley, that's pretty interesting actually.. and yes i do :P

The tank was bought new(empty) and was filled with compressed air from a local scuba tank supplier. (on a side note, this is a nice way of filling a tank to 2000psi for 10 bucks, compared to paintball fillers)
This is not the first time this has come up :D
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 07, 2011 11:50 pm

vakimo wrote:The tank was bought new(empty) and was filled with compressed air from a local scuba tank supplier. (on a side note, this is a nice way of filling a tank to 2000psi for 10 bucks, compared to paintball fillers)


Might I suggest therefore it's a good idea to relabel the tank as "compressed air" in order not to cause alarm ;)
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