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Need Help With CBV Design

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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Need Help With CBV Design

Unread postAuthor: grumpyoldman » Mon Nov 03, 2014 8:53 pm

Hey everybody. I've got a problem that I hope you guys can help with.

I've been trying to design a counter balanced valve and ran into a problem that I can't figure out. When I push the handle on the blow gun, all the air in the chamber comes out the blow gun. It's probably something simple but I can't think of what it could be. All help is appreciated. Thanks.

Specs:
Chamber I.D.: 2.87 in. / 47.498 mm
Front Piston: 1.3125 in. O.D. / 33.337 mm
Back Piston: 1.5 in. O.D. / 38.1 mm
Port to Barrel: 1.25 in. / 31.75 mm
Bleed Hole in Back Piston: .0625 inch / 1.5875 mm
Vent Diameter: .2 in. / 5.08 mm

photo_1.jpg
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Re: Need Help With CBV Design

Unread postAuthor: lockmanslammin » Tue Nov 04, 2014 12:58 am

I might be able to help if I can understand how this is intended to work. Is it supposed to momentarily open similar to a hammer valve?

Is the yellow part just there to guide the right piston, and has holes all oround the perimeter for air to go through?

I'm assuming you have an actual valve that you have made, that is exactly like the drawing since you have said that all the air leaks out through the blow gun. You must have a lathe, this isn't exactly a simple design. Just curious.

Is there something that prevents the red piston on the left from traveling to the right beyond its position in the drawing?

Is the pressure differential caused by the different sized pistons greater than the friction of the piston o-rings?

Could you put a tiny check valve in the bleed hole of the left piston? Like a grease zirk with the spring removed or something like that.

Why are there bleed holes in the green cylinder?

I might not be as much help as I hope to be , but if you help me understand how it is intended to work, I might be able to help.

Chris
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Re: Need Help With CBV Design

Unread postAuthor: grumpyoldman » Tue Nov 04, 2014 7:41 pm

Thanks for responding, Chris. I figured out what was wrong. It was something super simple. I didn't turn the air off when I tried to fire it. The pilot area was filling up as fast as the blow gun was letting the air out. :oops: Now I need to make a few adjustments and I'll be ready to make my t-shirt launcher. I'll make a video later showing how it works. Thanks again.
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Re: Need Help With CBV Design

Unread postAuthor: jackssmirkingrevenge » Wed Nov 12, 2014 8:18 pm

Why the two-part piston?
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Re: Need Help With CBV Design

Unread postAuthor: grumpyoldman » Wed Nov 12, 2014 10:12 pm

Hey, Jack. I wanted to experiment with using counter-balanced valves like BTB's pilot valve but make it a main valve. I also wanted a small pilot volume.

To explain how it works (if anyone wants to know), I'm going to refer to the back piston as the pilot piston and the front piston as the port piston.

The pilot piston goes back just far enough to unseat the port piston but not far enough for the air to have good flow around the port piston and into the barrel. So a spring is put behind the port piston so it will go back far enough to allow good flow to the barrel.

That cuts down on the backward force of the pilot piston when the pilot volume is emptied because the pilot piston travels back only about 1/4 inch. The rest of the force is transferred to the port piston which pushes back on the long spring. I think this is what is happening.
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Re: Need Help With CBV Design

Unread postAuthor: Technician1002 » Thu Nov 13, 2014 7:45 am

You might be OK with the port piston O ring in the smaller diameters if you don't use a floating o ring.

My initial tests of a 2 inch floating o ring in that configuration resulted in loss of the ring on shots above ~60 PSI. Non floating ring designs tend to get stuck and fail to fire reliabily.
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Re: Need Help With CBV Design

Unread postAuthor: Brian the brain » Thu Nov 13, 2014 1:02 pm

I can't see why the portpiston would contract into the pilot-piston.
Or is the green part airtight?
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Re: Need Help With CBV Design

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Re: Need Help With CBV Design

Unread postAuthor: grumpyoldman » Thu Nov 13, 2014 6:43 pm

Technician1002 wrote:You might be OK with the port piston O ring in the smaller diameters if you don't use a floating o ring.

My initial tests of a 2 inch floating o ring in that configuration resulted in loss of the ring on shots above ~60 PSI. Non floating ring designs tend to get stuck and fail to fire reliabily.


I also had trouble with the o-ring flying off the port piston on ealier versions. I solved that problem by adding a type of wind breaker behind the o-ring. The diameter of the piston is larger behind the o-ring so as to divert the air flow above the o-ring. It also guarantees the piston can not escape through the port hole.

1.jpg
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Brian the brain wrote:I can't see why the portpiston would contract into the pilot-piston.
Or is the green part airtight?


The green part is not airtight. I think the backward force of the pilot piston is transferred to the port piston when the pilot piston abruptly stops.

001.jpg


002.jpg


Below is 200 PSI test.

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Re: Need Help With CBV Design

Unread postAuthor: Gippeto » Sun Nov 16, 2014 1:53 am

Another method to keep the o-ring in it's groove, is to drill a series of holes from the face through to the base of the o-ring groove...during firing, pressure acts on the groove, pushing the o-ring out of the groove...venting the base of the groove gives the pressure a path of lesser resistance, and the o-ring stays put.
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