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Assisted Pilot

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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Assisted Pilot

Unread postAuthor: Secret Squirrel » Wed Dec 01, 2010 2:03 pm

This is a spin off of my thread about piloting piston valves.

I had an idea to get a piston valve to pilot quickly with a very small pilot valve. By adding another sealing face to the rear of the piston, and using it to seal the pilot, two things happen:

1. the valve's seat to piston ratio increases significantly, causing a smaller change in pressure to trigger the valve

2. as soon as the piston starts to move, the rear of the pilot opens exhausting the remaining pressure and causing the valve to open without honking or piston bounce

I can see it being a bit difficult to get both sealing faces to seal well at the same time, but it looks like it's at least worth a try.

[edit:picture wasn't working]
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Unread postAuthor: Lockednloaded » Wed Dec 01, 2010 2:23 pm

It looks like a toolies piston with a spool valve, it ought to work fine if you drill an eq hole with a check valve and build it correctly
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 01, 2010 2:28 pm

Brilliant, with that level of porting it should give you practically this levelof piston speed.

Secret Squirrel wrote:I can see it being a bit difficult to get both sealing faces to seal well at the same time


As soon as there is some pressure in the pilot it should seal it shut, a small spring will help proceedings without having too much of a negative effect on performance.
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Unread postAuthor: Technician1002 » Wed Dec 01, 2010 3:27 pm

A close ratio piston simply remains firmly closed until the pilot pressure is very low. I took that to an extreme with my latest valves which have a 1:1 ratio and a large vent behind the piston.

It is an interesting concept. I would like to see if there is an easy way to seal it.

Having the valve seat a lot smaller than the barrel will hurt performance. A smaller barrel or larger valve and chamber is recommended.
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Unread postAuthor: Fnord » Wed Dec 01, 2010 3:43 pm

Something about the physics of this doesn't look quite right... unless the barrel port is somewhat larger than the pilot port wouldn't the forces sealing it balance out, resulting in a piston that can't possibly hold pressure?

Though it does seam interesting, and kind of like what SB15 used on his piston hybrid. I think seamless tube with orings would work better for the pilot seal.
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Unread postAuthor: Secret Squirrel » Wed Dec 01, 2010 5:03 pm

Technician1002 wrote:Having the valve seat a lot smaller than the barrel will hurt performance. A smaller barrel or larger valve and chamber is recommended.


I know the reduced seat is going to choke the flow into the barrel, but I don't think it's going to be as bad as it looks in my drawing, which is not to scale. I plan on using this valve with a 1.25" seat and a 2" barrel, so it's not that big of a difference. The barrel will (most likely) be interchangeable anyway, So I could easily switch to a smaller barrel for some real performance tests.

Fnord, the reason it looks like a balanced valve, is because when it is charged with air it nearly is. However there is a small net force holding the piston closed, even if both the barrel port and pilot port are the same size. The barrel seal is sealing against the outer diameter of the pipe while the pilot seal is the inner diameter. I plan on using 1.25" pipe for both, which has an actual inner diameter of about 1.36 and an outer diameter of 1.66. insert brief calculator session here The area of the barrel seal is 2.163 sq.in. while the pilot seal is 1.452 sq.in. So the area on which pressure is acting on to close the valve is .711 sq.in. Since force is pressure times area, there should be over 70 lbs. of force holding the piston closed when charged to 100psi.

And JSR, a spring to reset the valve is unnecessary, I could just push it closed by hand.
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Unread postAuthor: Fnord » Wed Dec 01, 2010 5:15 pm

I plan on using 1.25" pipe for both, which has an actual inner diameter of about 1.36 and an outer diameter of 1.66.


Mmm... I thought it was really the ID of the barrel port that was what counts. I could be wrong but I guess we'll find out :)
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Unread postAuthor: Gun Freak » Wed Dec 01, 2010 6:55 pm

This is how the piston hybrids work, except they dont have a pilot valve. I though about doing something similar in a piston cannon. If you get the right dimentions, it should work.
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Unread postAuthor: Technician1002 » Wed Dec 01, 2010 7:19 pm

Secret Squirrel wrote:I know the reduced seat is going to choke the flow into the barrel, but I don't think it's going to be as bad as it looks in my drawing, which is not to scale. I plan on using this valve with a 1.25" seat and a 2" barrel, so it's not that big of a difference.


The valve orifice area to barrel area turns out to be fairly large. I find the power starts to drop off rapidly when the barrel diameter is much larger than the valve orifice. For example I have screw on barrels for the Marshmallow Cannon. The power difference when going from the 1 inch valve into a golf ball barrel is huge.

The math for this not that big of a difference turns out to be;
Valve 1.25 inch = 1.23 sq inches
Barrel 2 inch = 3.14 sq inches

The barrel has about 2.5 times the area of the valve orifice.

Remember Area = Pi times Radius Squared. Radius is 1/2 Diameter.
The valve is choking the maximum flow into the barrel.
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Unread postAuthor: Secret Squirrel » Wed Dec 01, 2010 8:49 pm

I guess 1.25" does choke the flow more than I thought, but I didn't arbitrarily pick that size. As the seat size increases, so does the sealing face. In a Toolies valve, air must flow around the sealing face to get out the barrel, so for a given chamber size the seat can only get so large before the sealing face becomes a flow restriction. For a two inch chamber I found the sweet spot size wise was about 1.3" and 1.25" pipe was the closest standard size.

I know you're going to ask why not just use a larger chamber? Well despite the performance gains, I'd rather have the entire launcher one diameter for it's entire length, and be full bore on the end, so it looks like a semi-realistic AT weapon.

I'm not looking for maximum velocity and damage with the two inch barrel. I modeled this gun on GGDT and came out with a result of around 180 fps for a vortex at 100 psi. Which seems like more than enough for airsoft use. It needs to be safe to be hit with it. Not pleasant, but safe.

One of the advantages of the Toolie's design is that the barrel's are easily made interchangeable. If I want to do some real damage I can just swap out for a smaller, more efficient barrel.
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