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
