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Author Topic: Balanced valves in pcp's  (Read 454 times)

Aerotulz

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Balanced valves in pcp's
« on: July 28, 2017, 06:54:45 AM »
Hi Bob, Can you explain the various ways these work and what there individual advantages might be ?
Thanks, John Hagan


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rsterne

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Re: Balanced valves in pcp's
« Reply #1 on: July 28, 2017, 01:16:39 PM »
HI John....

Good question!.... In regards to their use in PCPs, balanced valves are relatively new, although in the basic form of a SPOOL VALVE they have been around for hundreds of years.... Currently we are in the situation of a few manufacturers using them, and their designs being proprietary, so it wouldn't be proper for me to show detailed drawings, or photos, of the internals....

A conventional valve in a PCP is similar to the common poppet valve in a car engine, except that instead of being opened by a camshaft, it is usually opened by a hammer (striker) striking the stem.... The valve is held closed by a combination of the valve spring and the air pressure inside the valve....  Once opened there is a closing force provided by the valve spring and a smaller force from the air pressure inside the valve, as most of the force across the head of the poppet disappears because of the high pressure air in the exhaust port.... The greater the air pressure, and the larger the poppet, the greater the force holding the valve closed, so you need a bigger hammer strike to open it.... In the drawings below, I have left out the spring for clarity, and I will use 3000 psi in all calculations.... and I am ignoring any drag across the head of the poppet causing an additional closing force, to make the calculations easier to understand.... Where you see the letter "P" on a drawing, that pressure is variable, between atmospheric when the valve is closed, and the HPA pressure when the valve is open, but all areas which have a "P" are at (roughly) the same pressure....



In the valve above, the force required to crack the valve open is 0.375^2 x PI/4 x 3000 = 328 lbs.... Once the valve is open, the forces on the head of the poppet cancel out, and the closing force once the valve is open is 0.125^2 x PI/4 x 3000 = 36 lbs, which is the HPA pressure acting on the stem area.... This closes the valve, resetting it for the next shot....

The valve below is basically a partially balanced Spool Valve in concept.... There is a smaller force holding the valve closed, and once the valve is cracked open, the pressure in the exhaust port P forces the valve to open fully and stay open until the reservoir is empty.... Note that the small volume ahead of the poppet is vented to the atmosphere, so remains at 1 bar throughout the valve cycle....



In the valve above, the force required to crack the valve open is (0.375^2 - 0.313^2) x PI/4 x 3000 = 100 lbs.... Once the valve is cracked, the pressure in the exhaust port P rises to HPA pressure, and there is an initial net force of (0.313^2 - 0.125^2) x PI/4 x 3000 = 194 lbs. OPENING the valve.... It stays open fully until all the air in the reservoir is exhausted, and must be closed manually, or reset with a spring....

In a Balanced Valve in a PCP, there is an opposing force on the poppet that partially (or completely) cancels out the force from the air pressure that is holding the valve closed.... All Balanced Valves share in this concept, it is how they arrive at it where they differ.... You can change a Spool Valve into a Balanced Valve by using a small port through the poppet to connect the small (now sealed) balance chamber in front of the poppet to the exhaust port.... Since the pressure in the exhaust port changes during the cycle, this adds a closing force to the valve to make it shut.... The valve below is the same as the Spool Valve above, with the addition of this port....



In the valve above, the force required to crack the valve open is (0.375^2 - 0.313^2) x PI/4 x 3000 = 100 lbs.... Once the valve is cracked, the pressure in the exhaust port and the balance chamber rises to HPA pressure, and the forces on the head of the poppet cancel out, leaving only the pressure on the stem area to close the valve, the same as in the conventional valve.... The closing force is therefore 0.125^2 x PI/4 x 3000 = 36 lbs.... After the valve closes, the pressure in the small balance chamber returns to atmospheric pressure, like the exhaust port, which is vented through the barrel, and the valve is ready for another shot....

Another way to balance the forces on the poppet is to use a small piston in a separate chamber behind the poppet.... This piston is mounted on the valve stem, and travels with it.... There is a small chamber behind the piston which is connected to the HPA reservoir by a small port, so that when the valve is closed, the forward force on the piston acts against the closing force on the poppet seat, reducing the force required to crack the valve.... In this drawing D = 0.313"....



In the valve above, the force required to crack the valve open is (0.375^2 - 0.313^2) x PI/4 x 3000 = 100 lbs.... Once the valve is cracked, the pressure in the exhaust port enters the chamber on the front side of the piston through another small port (in the case of the drawing above, around the valve stem, which at that point is in an oversized hole).... the forces on the piston cancel out, leaving only the pressure on the stem area to close the valve, the same as in the conventional valve.... The closing force is therefore 0.125^2 x PI/4 x 3000 = 36 lbs.... After the valve closes, the pressure in the front of the small balance chamber returns to atmospheric pressure, like the exhaust port, which is vented through the barrel, and the valve is ready for another shot....

I hope this primer in Balanced Valves helps everyone understand the basics.... You can change the balancing force, and hence the force required to crack the valve, by changing the diameter of the balance chamber.... If it is the same diameter as the valve seat (3/8" in the above drawings), then the forces balance out, and you are left with only the valve spring holding the valve closed.... In this situation, that force will not change with air pressure, which is likely to result in a valve which cannot "self-regulate" (open less at higher pressures), which is the key to unregulated PCP bell-curves.... In the opposite case, the conventional valve, the opening and closing forces are mostly related to air pressure (with a minor component from the valve spring).... so they are readily tunable to produce a bell-curve.... If you make the balance chamber too big, the valve is easier to open, but harder to get to make a bell-curve....

Therein lies the rub.... If you want a Balanced Valve for a regulated PCP, then you can use a larger balance chamber, and make the valve really easy to open.... but it already is, because you are running less pressure from the regulator.... So yes, you can reduce the hammer strike even further, but the only way you can tune the velocity is by changing the regulator setpoint.... That is what happens with a Cothran valve, it's very easy to open, but the velocity is 100% dependant on pressure, you cannot tune it for a bell-curve.... The Balanced Valve used in the Slayer, which is similar in concept to those made by Lloyd Sikes (and my last diagram) can produce a bell-curve, but they have to sacrifice the easiest possible opening to achieve that.... The key lies in the proportions between the piston and poppet seat diameters.... and also in the diameters of the small ports connecting to both sides of the balance piston....

We still have a lot to learn about Balanced Valves, and I keep seeing new designs all the time, some good, some not so good.... One of the biggest problems is that some of the really sophisticated ones are nearly impossible to build accurately enough to take advantage of their theoretical advantages.... As with any mechanical device K.I.S.S. applies (keep it simple, stupid).... The ideal Balanced Valve, IMO, will be one that reduces the required hammer strike by 50% or more, will produce a bell-curve, and is simple enough to make that I can do one with my limited skills and equipment.... So far, I have only seen one, and I hope to try my hand at making my own version soon.... but I have to wait until the designer announces it first, so as not to steal his thunder....

I decided to "Sticky" this thread, and will leave it unlocked, but please stay on topic with any questions.... If it gets too cluttered, I will remove those posts and lock it....

Bob
« Last Edit: July 29, 2017, 08:23:21 AM by rsterne »
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Christopher

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Re: Balanced valves in pcp's
« Reply #2 on: July 28, 2017, 02:08:48 PM »
Wow Bob, really good info! Kudos to you for taking the time to post such a detailed post....pictures and all. 


Thanks,
Chris
« Last Edit: July 28, 2017, 02:32:30 PM by Christopher »
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Aerotulz

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Re: Balanced valves in pcp's
« Reply #3 on: July 28, 2017, 10:46:01 PM »
Bob, I knew you were the man to ask. Thank you so much. It makes perfect sense and now I have work to do!!
Thank you again!  John
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Monkeydad1969

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Re: Balanced valves in pcp's
« Reply #4 on: July 29, 2017, 11:50:40 PM »
Hey John,

I have several Cothran valves.  You can always PM me and we can get together for you to shoot them.  I'm only in Aurora.  Easy enough to hook up to go shoot.  Let me know.

The Monkey
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Aerotulz

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Re: Balanced valves in pcp's
« Reply #5 on: August 11, 2017, 06:07:55 PM »
Hi Bob, I noticed in your force formulas the valve was only represented by the seat diameter. Why would'nt  the surface area that the air pressure is contacting on the whole valve head not used?
Thx, John
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rsterne

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Re: Balanced valves in pcp's
« Reply #6 on: August 11, 2017, 07:36:55 PM »
By seat diameter, I meant the OD of the sealing surface of the head of the poppet.... If that is 3/8", and the poppet head is 1/2" OD, but undercut so that the sealing surface is only 3/8", the 1/16" wide ring on the bottom of the poppet head (1/2" OD - 3/8" seat)/2  is exposed to pressure, pushing the poppet away from the seat, and offsetting the poppet OD.... Only the OD of the sealing surface of the seat is important in calculating the force holding the poppet closed....

Bob
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Shorty

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Re: Balanced valves in pcp's
« Reply #7 on: Today at 11:08:53 AM »
Bob,
I think I understand the idea of (using air pressure to reduce valve cracking) but can this be done mechanically with let's say wave springs loaded on the valve stem ? What would be the difference between the two if possible.

Here's my latest thoughts on a "pressure" reducing valve. It uses the "hollow" valve stem to deliver the HPA to the secondary poppet to balance. And, since the secondary poppet is in the exit port, it allows for additional closing force when the main poppet opens.

The design is actually small enough to fit into (slugged Mrod valve) and is scaled in the drawing without dimensions.
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rsterne

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Re: Balanced valves in pcp's
« Reply #8 on: Today at 02:14:17 PM »
If your used Bellevilles, or wave springs to counter the force holding the valve closed, you would not be able to fill the gun from empty without finding a way to hold the valve shut.... and if the air pressure dropped below the spring force the valve would dump all remaining air in the reservoir.... Balanced valve change the "counter balancing force" in proportion to the air pressure, your idea of a mechanical spring would have a fixed force (the spring) countering a variable force due to air pressure.... It could be made to work in a regulated gun, subject to the problems mentioned above....

Functionally, your "hollow poppet" works the same as the 4th drawing above, the "Balanced Valve", except that the vent used to pressurize the balance chamber is through the poppet instead of through the valve body.... The Balanced Valve as drawn also has increased closing force while cycling because of the oversize hole around the stem between the balance piston and exhaust port.... There are lots of ways to physically come up with the same result, yours is yet another variation.... You will need an O-ring on the balance piston to prevent the HPA from leaking past it into the exhaust port when the valve is closed....

Bob
« Last Edit: Today at 02:16:01 PM by rsterne »
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Shorty

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Re: Balanced valves in pcp's
« Reply #9 on: Today at 06:19:43 PM »
Lets say I found a way to open up a valve mechanically using "spring-method" and be able to tune it to be fully balanced at any pressure. Anything lower would just leave it open like some other "pressure" reducing valves. And can be filled from empty.

I just don't know if it would be safe or advantageous to other balanced valves. Theoretically, it will be easy to open but I can not see it closing as efficiently unless you think otherwise.

The drawing is complicated to me but, you may understand it. The hammer rides (slides) on a stem that is connected to a rigid hollow stem connected to the cap which has a screw to increase or decrease valve stem load.
For tuning,
Decrease valve stem pressure to neutral and charge the gun to 200psi higher than what you want it to fully balance at. Screw in the screw to load the valve until air is released and finally stops at desired "balance pressure". This is the pressure that you should never go below before dump.

Just don't know if it would close fast enough to be efficient. What do you think?
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