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Piston valve specs and optimization?

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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Piston valve specs and optimization?

Unread postAuthor: pvcmaster » Wed Jul 22, 2009 2:22 pm

I haven't been on spudfiles in a while, but i've got a question for everyone. I'm looking to use my experience to build a really nice piston valve, starting with the design and just building it straight through with no compromises or major changes. So i need to figure out the optimum measurements for these three aspects of the valve:
X: How long should the pipe that carries the piston be? (How far into the tee should it extend)
Y: How long should the piston be? (Making sure to prevent piston cock)
Z: How far into the tee should the barrel extend?

Right now i am looking at a piston just slightly longer than it is wide, and a gap between the two pipes of 1".

The valve is inside a 2" tee with a 1.5" barrel seal
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Unread postAuthor: qwerty » Wed Jul 22, 2009 2:41 pm

the barrel shoud be half way into the T so the piston meets it in the middle and the air has something to push back.

About the piston: it should be atleast the diameter of you T so it dosent tip over.

In the picture its not a T and i know its probably not to scale but you piston is a little to small.
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Unread postAuthor: pvcmaster » Wed Jul 22, 2009 2:58 pm

It's the top view of a tee, and i threw it together just so people would get the idea.

So if the barrel is on the midline, what should the gap between it and the 2" pipe carrying the piston be?
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Unread postAuthor: jrrdw » Wed Jul 22, 2009 3:44 pm

D4 rule says 1/2" if your piston is 2" diameter. Good to see you here again, what happened with the piston valve I machined for you?
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Unread postAuthor: pvcmaster » Wed Jul 22, 2009 4:37 pm

Actually i'm still using all the parts you made for me, i just have to build a new piston and make sure i've got all the spacing in order. I took a looong break from building this cannon to work on a project for a national engineering competition, but i'm ready to finish it now :D I really can't believe i forgot about the D4 rule.. its been way too long
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Unread postAuthor: mark.f » Wed Jul 22, 2009 5:07 pm

I start off with opening distance before scaling everything else. For a 1-1/2" porting piston valve, I would use any length between 1/2" - 3/4". After that, you can move on to piston length (which obviously has to be longer than your opening distance, plus more to remain in the piston track to keep it centered), and piston carriage length.

Additional details would be helpful, like building constraints (are you trying to fit this all into a tee, or will you have external fittings?).
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Unread postAuthor: pvcmaster » Wed Jul 22, 2009 5:31 pm

It's going into a tee, i have a cut down reducing bushing as an end cap and pilot that is sealed with an o-ring and extends 3/4" into the tee. How far out can a 2" diameter piston extend past its track before it begins to lose its centering?
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Unread postAuthor: mark.f » Wed Jul 22, 2009 5:44 pm

Depends entirely on the fit of the piston. Mine have had an O-ring about 1/16" from the back of the piston, and tape wrapped around in front of that to help, so I keep about 3/8" in the piston track.
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Unread postAuthor: Technician1002 » Wed Jul 22, 2009 5:56 pm

qwerty wrote:the barrel shoud be half way into the T so the piston meets it in the middle and the air has something to push back.

About the piston: it should be atleast the diameter of you T so it dosen't tip over.

In the picture its not a T and i know its probably not to scale but you piston is a little to small.


I'm going to disagree on the first couple points and here is why.

The valve seat in the middle of the t makes sense while the piston is closed, but is an obstruction to flow when open. It makes more sense to me to position the opening between the piston and seat when open, centered in the center of the T for the highest possible flow when open.

D4 rule says 1/2" if your piston is 2" diameter.

This is pretty much true for coax pistons where the air enters from all sides. When built into a T consideration must be given in modifying it due to the air arriving from only one side.

In a coax, a piston moving 1/4 the diameter is often considered fully open. This provides a restriction the same size entering into the space between the piston and valve seat as the valve seat provides. In a T where air can come in from only one side will require a longer opening to have little restriction. This is another reason to not have the seat half way in the middle of the T. Move it over to make room. The original comment for air flow to open the valve is still true, the valve seat should not be stuck down a hole in the leg of the T.

The length of the piston will be a function of the required stroke. In a coax where the air can enter the valve from 360 degrees, the piston does not have to be longer than the diameter. A piston only 1/2 the diameter works well when the stroke to full open is 1/3 the valve seat diameter. A piston 2/3 the valve seat diameter works well, even though this length can be less than 1/2 the piston diameter. The piston in my Mouse Musket is of this hockey puck shape and works fantastically.

Image
Mouse Musket piston is shorter than the diameter.

As far as the size of the piston, it depends on if it is a barrel sealer or chamber sealer what size is best.

On barrel sealer pistons, the ratio of the piston OD to valve seat diameter greatly affects performance. Too small an OD makes the valve difficult to get to fire as there is too little chamber pressure to start to open it, but when they open, the low pilot pressure and lighter piston (smaller diameter and thus shorter possible length) will snap open quickly when the chamber pressure hits the full face of the piston. A larger diameter OD makes them less fussy, but due to the higher pressure in the pilot area when it opens, they tend to re-close as the chamber pressure rapidly drops and the heavier piston is slower due to the larger volume in the pilot for these.

In a chamber sealer, a close ratio piston is the slow valve as it opens early with high pilot pressure and tends to re-close as the chamber pressure drops. A large diameter piston again has the disadvantage of the larger pilot volume again, but has better snap action as the rising pressure in the valve when it opens increases the force opening it. The large diameter is discouraged in chamber sealers due to the large dead space, huge mass of the piston to have a decent size valve seat size, and as a result a longer piston. Most sprinkler valves are of the chamber sealing configuration. They are noted for the dead space between the valve seat an outlet.

To get good performance with most chamber sealer valves, a large fast pilot is required to keep ahead of the rate of chamber pressure drop to prevent re closing. This is why sprinkler valves are modified. Without the modification, they re-close several times when fired producing the telltale honk.

I hope this helps. A coaxial valve can be shorter than one built in a T. Ones built in a T need a longer stroke to provide the same opening to flow. Ones built in a t need the opening space centered on the T, not the closed valve seat. There is a balance between too large a piston OD and one too small that balances performance against reliability. A valve seat that is 0.8 and 0.9 the diameter of the piston diameter is a good starting point for a barrel sealer. If using a large piston OD in relation to the seat diameter such as the seat 1/2 the piston OD, you will want a large fast pilot to get decent performance. :D

In valves that require a large fast pilot, a small pilot area is highly desirable. In valves that do not require a large fast pilot such as a close ratio barrel sealer, the pilot needs to be larger. The reason for this is the sudden compression of the air in the pilot when the piston snaps back. The piston snapping back changes the volume and thus the pressure in the pilot area. A larger pilot area provides less pressure rise and thus faster opening speed.

Since good design keeps the close ratio valves closed until the pilot are pressure is low, even a piston halving the volume while doubling the absolute pressure is still providing much less force than the newly exposed piston face is providing to open it.

For example, a 7 bar chamber pressure opening a close ration valve at 25% of the chamber pressure will snap open. Here is why. 7 bar is 8 Atm. 25% of the chamber pressure is about 2 bar or 3 atm. When opening the force on the piston face increases 3X due to the increased exposed area. When the piston is all the way open, the force on the back has only increased 2X. This baby was still opening with force when it lands in the bumper. I hope this makes sense. :D
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Unread postAuthor: pvcmaster » Mon Jul 27, 2009 8:20 pm

Thank you very much for all that information Technician, i think i'm ready to begin now, i'll post it as soon as i finish :D
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Unread postAuthor: pvcmaster » Thu Jul 30, 2009 11:22 am

Alright two more questions. Will removing material around the center of the piston with a lathe to save weight affect performance? And could two rubber washers attached to the pilot end of the piston serve as a good bumper?(essentially another sealing face setup)
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Unread postAuthor: Technician1002 » Thu Jul 30, 2009 1:01 pm

pvcmaster wrote:Alright two more questions. Will removing material around the center of the piston with a lathe to save weight affect performance? And could two rubber washers attached to the pilot end of the piston serve as a good bumper?(essentially another sealing face setup)


Two part question..
1 shaving to save weight.. Yes. On a drill press it's a little hard.. Is the performance significant.. yes. especially the mass hitting the bumper..

2 in regards to 1 above.. why add to the weight of the piston.. put the doughnut shaped bumper in the other end of the pilot with a hole the size of the pilot valve. The center won't have support in either case so use a ring shape bumper.
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Unread postAuthor: randompkguy » Thu Jul 30, 2009 11:46 pm

The information that you provided should most definitely be put in a sticky, thanks to you I have learned tons more about piston design.

I have a question as well (sorry to hijack the thread). I'm planning to build a high pressure piston gun that uses unregged C02, (I think C02 is only 800 psi or so at room temp) or possibly regulated nitrogen for an operating pressure of 1000 psi . Is it feasible to have a super small Seat/OD ratio at these pressures? I was thinking somewhere around 0.95 which would give an initial opening force of about 21 pounds for a 1/2 inch seat (ignoring pipe wall thickness for the time being)

Also, as floating o-ring seals are only spec'd for 200 psi max, what type of sealing between the piston and cylinder walls do you recommend? I was thinking of machining the piston to - .0005 the cylinder diameter, but at those pressures I'm sure leaks from the chamber into the pilot area are still very apparent and will affect performance greatly.

Finally, With the 1/2 inch seat and a 0.95 Seat/OD ratio the full opening force on the piston would only be about 218 pounds, so would it still be possible to use conventional materials such as HDPE for the piston, or would the otherwise high pressures cause such materials to shrink?
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Unread postAuthor: Technician1002 » Fri Jul 31, 2009 1:35 am

randompkguy wrote:The information that you provided should most definitely be put in a sticky, thanks to you I have learned tons more about piston design.

I have a question as well (sorry to hijack the thread). I'm planning to build a high pressure piston gun that uses unregged C02, (I think C02 is only 800 psi or so at room temp) or possibly regulated nitrogen for an operating pressure of 1000 psi . Is it feasible to have a super small Seat/OD ratio at these pressures? I was thinking somewhere around 0.95 which would give an initial opening force of about 21 pounds for a 1/2 inch seat (ignoring pipe wall thickness for the time being)

Also, as floating o-ring seals are only spec'd for 200 psi max, what type of sealing between the piston and cylinder walls do you recommend? I was thinking of machining the piston to - .0005 the cylinder diameter, but at those pressures I'm sure leaks from the chamber into the pilot area are still very apparent and will affect performance greatly.

Finally, With the 1/2 inch seat and a 0.95 Seat/OD ratio the full opening force on the piston would only be about 218 pounds, so would it still be possible to use conventional materials such as HDPE for the piston, or would the otherwise high pressures cause such materials to shrink?


I hate to wimp on you. Most of the question are best answered with a simple I simply don't know..

I'll give my best shots.

As far as the floating o rings, the forces on the rings needs to be understood. I threw up a very quick drawing showing a ring in a piston groove. I showed air from the chamber pushing the ring into the tiny corner where the pressure is lower.

From the drawing it should become obvious as the pressure increases, the force and therefore the friction increases. Large rings have large area, small rings have less area and thus less force. Unfortunately to get less force, tighter clearances are required to prevent the piston ring from squeezing out the gap.

Back to the very narrow ratio problem above.. It boils down to the piston may fail to open under high pressure. I haven't done any above 110 PSI so I can't speak for actual results.

As for the floating o rings being spec'ed for only 200 PSI.. Where did you find that? That's news to me.

Unless there is some way to retain the rings in the grooves, there is a very good possibility the pressure sealing the o ring against the cylinder will lift it off the piston as the cylinder wall drops away as the valve opens. Remember, air pressure is under the ring. For high pressure, consideration for a way to retain the rings on the piston should be given.

My valve has the 3 bars joining the front part of the pipe to the rear for both alignment and to keep the o rings from blowing off.

Instead of a sticky, I'm working on a wiki entry.
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Valve ports for QDV valve. The 3 metal bars keep the ends in alignment, the piston straight, and keep the o rings from coming out of the piston groove.
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Unread postAuthor: pvcmaster » Fri Jul 31, 2009 1:12 pm

Floating designs offer a number of advantages, including greatly reduced breakout friction and longer seal life. Floating pneumatic piston seals are suited for applications in which the air pressure does not exceed 200 psi (or in hydraulic designs where a small amount of leakage is permissible). Floating O-rings are NOT suitable as rod seals. http://www.rlhudson.com/O-Ring%20Book/designing-dynamic.html


I found this while researching Technician, i hope it helps.

I can move the bumper to the valve itself instead of the piston, but i am concerned about how to keep it in place, or if a rubber washer or two is even sufficient to cushion the impact.
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