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Piston design with a built in check valve

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: saefroch » Tue Nov 30, 2010 1:58 pm

Indeed there was not enough "meat" :wink:. I do not have a lathe, I have access to a drill press.
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Unread postAuthor: FighterAce » Thu Dec 02, 2010 2:53 pm

I recently acquired mechanical properties of materials... I was wondering who could make sense of it and tell me witch one to pick since there are so many variations? I have yet to translate it to English so I didnt post it here... and theres quite a lot of it... at least it seems like it to my untrained eye.
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Unread postAuthor: saefroch » Thu Dec 02, 2010 3:33 pm

Well which mechanical properties are we talking? Just give us a list, and I'm sure we can explain to you the significance of each.

We can't simply tell you which one to pick, since there are so many variations. For example, UHMWPE has very low density and very high impact strength, but is almost useless for many high-stress applications because of its low shear and tensile strength. Here is the table I use for plastics.
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Unread postAuthor: FighterAce » Thu Dec 02, 2010 4:00 pm

Hm... it seems like I was wrong... on the website they say they have 2 types of UHMWPE but in the document there are only PE, PP, L and teflon. So whats the darn difference between this PE 500 and 1000.

Really sorry I have to do it like this... I was denied from uploading the .doc file.

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Heres the website... http://www.ekoplastika.hr/detaljno1.php
And in case you cant read Croatian :D

PE-500 The material is suitable for producing elements exposed to stress at low temperatures, elements, particularly resistant to abrasion, elements exposed to sliding friction and for making boards for meat and food industries.

PE-1000 High molecular PE, very resistant to wear, is used for making guides, and transport line elements in industrial production. Does not absorb moisture and is suitable for use in temperatures from -273 ° C to 80 ° C.

What about teflon? Is it any better then the PE in that list?
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Unread postAuthor: Technician1002 » Thu Dec 02, 2010 4:30 pm

PE 500 is the normal white cutting board material also known as HDPE.

PE-1000 is also known as UHMWPE

I've used both as pistons. HDPE is easier to machine.

I haven't used Teflon other than some wire I have uses it for insulation. Teflon beats both for high temperature. A soldering iron directly touching Teflon won't melt it.
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Unread postAuthor: FighterAce » Thu Dec 02, 2010 4:39 pm

Ah so... I had no idea, thanks Tech!

I'm not really interested in how easy/hard it is to machine but how it will perform when machined and in use... I'm not the one machining it :D
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Unread postAuthor: FighterAce » Mon Dec 06, 2010 5:57 pm

There wasnt any PE so I had to take PA (polyamide). They say its even harder then PE...

I just need to finish the drawings on paper for my machinist and hopefully it will be done this week :D

But I still need to get a brass T piece to finish the valve because the threads on the regular ones go into a light cone witch prevents me from screwing in the piston seat. And that might be a problem because I cant find them anywhere anymore... looks like everybodys out of stock :evil:
Time to get really creative...
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Unread postAuthor: saefroch » Mon Dec 06, 2010 6:34 pm

Get a steel hex part and just keep taking it in and out, using it like a tap. You'll make little shards of brass and eventually be able to screw something in all the way.

Alternatively, you could buy an appropriate tap from McMaster and make a part yourself...
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Unread postAuthor: FighterAce » Tue Dec 07, 2010 12:25 am

Its the steel ones that have the problem, not the brass ones. Thats why I said I need a brass T piece :D
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Unread postAuthor: Technician1002 » Tue Dec 07, 2010 1:26 am

In performance if you took a hammer and beat both to death, the HDPE seems to be harder and more likely to crack. The UHMWPE seems slightly softer, but more scratch resistant and impact resistant.

For a strong stiff piece of plastic, use HDPE. For machined bearings and very high impact pistons use UHMWPE. For high pressure pistons that need to not deform, and are protected from high impact damage, use HDPE.

If your concern is weight, UMHWPE is slightly lighter.

This is my 2 cents worth from using both. Your results may differ.
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Unread postAuthor: FighterAce » Tue Dec 07, 2010 8:38 am

Technician1002 wrote:In performance if you took a hammer and beat both to death, the HDPE seems to be harder and more likely to crack. The UHMWPE seems slightly softer, but more scratch resistant and impact resistant.

For a strong stiff piece of plastic, use HDPE. For machined bearings and very high impact pistons use UHMWPE. For high pressure pistons that need to not deform, and are protected from high impact damage, use HDPE.

If your concern is weight, UMHWPE is slightly lighter.

This is my 2 cents worth from using both. Your results may differ.


Here you talk about HDPE and UMHWPE even though I previously said I got polyamide not polyethelyne.
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Unread postAuthor: FighterAce » Fri Feb 11, 2011 5:16 pm

I got the piston 2 days ago... both sides of the piston and the check valve are made out of Polyamide PA-11.

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Piston all the way forward

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Piston all the way back - yes I know it moves too much, I'll shorten the piston housing to fix this.

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View of the seat from the pilot side.
To make the seat I took one 3/4 connector with outside threads and hammered in a 3/8 connector with inner threads. Then I grinded the inner threads for better flow. No leaks so far.

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Back of the piston, check valve, spring, front of the piston

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Note how its tapered in the middle. It was a bit too wide in the middle so it was getting stuck... while fixing this I over did it... doesnt cause any problems tho

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Piston housing and solid rubber bumper

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Piston centering - view from the barrel side.

The check valve works excellent! Its letting air through into the chamber when filling but stops the airflow during firing so I get a 100% seal.
I tested this by putting a ball valve on the barrel end. Then I pumped it up to what I think was about 15bar. Closed the ball valve and depressed one of the schraders. The pilot vented and the piston slammed back but the chamber air was not leaking out. Then I opened the ball valve and all of the air from the chamber went out.

I still need to attach a pressure gauge.... I was thinking about tapping 1/4NPT threads on the side of the chamber and screwing the pressure gauge there (pressure gauge has threads on the bottom).
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Unread postAuthor: FighterAce » Sat Sep 03, 2011 11:41 am

Finally got all of the bugs out of the design.... Switched to a one piece HDPE body and an aluminum check valve piston. I was planning to install 2 O rings but they weren't needed because of the tight tolerances :roll:

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The second piston will hopefully be installed into a brand new M4 replica... the old M4 is falling apart due to low quality wood.

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Unread postAuthor: Gun Freak » Sat Sep 03, 2011 11:42 am

Very nice! Lightweight and efficient :) Good work!
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Unread postAuthor: mattyzip77 » Sat Sep 03, 2011 12:14 pm

I dont even know what to say, unbelievably awesome!!
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