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1'' Custom Piston Valve CAD

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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1'' Custom Piston Valve CAD

Unread postAuthor: Mr.Tallahassee » Sat Aug 04, 2012 10:58 pm

Hey guys, I haven't had a post up in a while now. I just got Autodesk Inventor 2012 to play around with and came up with this. I really want some scrutiny here. I took a Drawing Design and Production class in my high school sophomore year and used Autodesk 2010. I want to see how rusty I got. :lol: I seriously intend to have this made within a year. I give myself a year because I'm fresh out of high school and need the job and money first.
I intend to use full length bolts to hold the whole thing together. The bolts will lock into threads at the rear of the valve body and the back plate will be held on by nuts on the end of those bolts if I can drill it right.

If you want to see close ups of any individual parts then ask and I will post them.
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 04, 2012 11:29 pm

Piston is far too big and heavy, grooves cut in the front part of the piston will improve airflow and four bolts on a faceplate aren't the best way to contain pressure.

Otherwise good :)
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Unread postAuthor: Mr.Tallahassee » Sat Aug 04, 2012 11:38 pm

I wasn't too sure about the four bolts. I'm really not too sure on how to secure it without making it too much larger. I'm not quite sure what you mean by grooves cut in the front of the piston. Oh, I completely forgot to mention that the rear of the piston is hollowed out for a spring to dampen the piston. I'll try to find the calculations for the mass they hid away in the program, too.
EDIT: I found the material properties and it turns out that the piston will weigh .44 pounds when made of high strength low alloy steel. Maybe I should make it from UHMW? UHMW would only weigh .052 pounds (.8 oz) for the same piston.
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 04, 2012 11:58 pm

I would make it from alumium alloy, but much leaner - heavy pistons reduce performance and increase stress on the launcher body.

Something like this:
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Unread postAuthor: Mr.Tallahassee » Sun Aug 05, 2012 12:05 am

I was just worried about the integrity of something that thin against the forces I'll probably be dealing with. I was hoping to get this up to high pressures, possibly 300+ PSI. Of course, that means fixing the issue with the four bolts. So many things to do. This should be along night. :lol:

Actually, I just thought about it, my seat OD is the same diameter as the OD of the piston. I have a better idea though so I'll draw up another.
EDIT: I drew up another piston from 6061 Aluminum. It has a .98'' OD and the narrow part is .25'' in diameter. This piston weighs just shy of an ounce.
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Piston remake.png
Piston remake 2.png
The blue is the inner retainer for a bumper spring. If I use a stiff spring It should hold up to the impact.
Last edited by Mr.Tallahassee on Sun Aug 05, 2012 12:36 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Sun Aug 05, 2012 12:35 am

Mr.Tallahassee wrote:Actually, I just thought about it, my seat OD is the same diameter as the OD of the piston. I have a better idea though so I'll draw up another.


That's what I meant by "grooves", you should still have the full diameter for support otherwise the thing will jam up, but it should be perforated to let the air through.

Of course it needs to be strong but make it too heavy and it's a vicious circle.
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Unread postAuthor: Mr.Tallahassee » Sun Aug 05, 2012 12:39 am

I changed it up a little. That piston isn't finished yet but it's much lighter. I want make sure it's stronger than a basic "rod and plate", too.

Update: I'm a cluster lately. :lol: I finished a rendering with genuine pipe dimensions. The front will fit 3/4'' pipe threads and the rear will fit a 1'' plug. This is a real close ratio. I had to adjust the length and play with the seat diameter to prevent a seal blowout. This is how it looks at the moment. I'm happy with the valve body personally but if anyone sees any flaws please do point them out.

Also, I did the calculations and there is 95 pounds against the sealing face at 300 PSI. The ratio actually turned out to be .5929. I'll be running GGDT in the morning. It's just to late for me.
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Unread postAuthor: silentkill12345 » Sun Aug 05, 2012 1:14 pm

nicly done, i have one sugestion :) i should make the hole right in the middle of your square, it is much easy-er to mike this way, you can just put it in a lathe with a 4chuck head and just drill your hole.

oh one more, i should use metric tread for that hole in the bottom and just make an fitting with G tread, with metric tread you need less treads to connect your homemade fitting.

good luck
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Unread postAuthor: ramses » Sun Aug 05, 2012 1:36 pm

Looks pretty good. A solid cylindrical UHMWPE piston would be much easier to make, less likely to jam, and probably about the same weight. The body of the piston (and therefore the valve body) could be shorter, further reducing weight. That would apply to the aluminum piston, too.

You should also calculate the force trying to pull the piston apart at 300 PSI. With an oring in the front, the piston might actually experience this condition (300 psi trapped between the two ends of the "spool). Depending on the flow from the pilot to the chamber during filling, you might also want to look at the chances of the small section of the piston buckling. It is unlikely, but probably worth looking at.

If you're filling from the pilot, you would want to make sure make the oring floating and slot the groove to make it act as a check valve. I would put a floating oring at the front, too, just to make sure it doesn't bind. Tight tolerances could work as well. You also need to make sure you have something to retain the tube to the chamber.

From a maneuverability standpoint, you have fully threaded blind holes, which is quite tricky to make without several taps of the same size, or a thread-mill and CNC milling machine. Make sure to drill the hole for the "chamber" before you finish boring the space for the piston.

You also have the spring retaining nub of the piston part of the piston. We turn face grooves like that where I work, but not inside of holes, so I suggest you instead drill and tap for a short socket head cap screw, and use that instead.

The bolt holes in your original design would have been tricky as they look to be at least 30 times their diameter. A carbide drill would help, but the runout in your drill press would have likely snapped it. Carbide drills are also incredibly expensive.
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Re: 1'' Custom Piston Valve CAD

Unread postAuthor: velocity3x » Sun Aug 05, 2012 2:06 pm

Mr.Tallahassee wrote:I intend to use full length bolts to hold the whole thing together. The bolts will lock into threads at the rear of the valve body and the back plate will be held on by nuts on the end of those bolts if I can drill it right.

You wont be able to drill without the drill walking in such a long distance and miss-aligning at the other end of the block.

As JSR stated....4 bolts wont hold (especially with any significant pressure.)

Without a bumper or cushion, the piston will quickly self-destruct.

The valve body and piston are much longer than they need to be.

The internal bore at the piston face needs to be slightly larger diameter (.125 +/-) than the piston. If not. the piston will not easily retract. It's much like slowly pulling the plug in a bathtub full of water. The water rushing to get out tends to pull the plug back into its seat.

With any significant pressure, the piston will fracture at its waist. It doesn't need to be that thin or long.

I built the same valve style a couple of years ago and I learned the hard way........
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Unread postAuthor: Mr.Tallahassee » Sun Aug 05, 2012 2:09 pm

I didn't calculate that force because I gave the piston a .01 clearance all the way around and there isn't a front o-ring. I probably should though. There is a groove in the rear for a floating o-ring to act as a one way, too. The possibility of the piston buckling is why I originally thought of UHMW but I second guessed myself and went with 6061 Al. I keep forgetting to add details that can't be seen. I may scratch the whole original idea and start from the beginning. It seems to have a lot more flaws than I anticipated.
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