Balanced Valves for PCP Airguns. They’re Here Today!
Today, we’ll be discussing balanced valves for PCP airguns.
In the first five articles I tried to give you a better understanding of how PCPs work, and how they are tuned. In the last article I introduced you to the “SSG”. This can help conserve air instead of blowing it out the barrel after Elvis has left the building by eliminating hammer bounce.
Several times I referred to this problem, particularly prevalent in big bore PCPs. In order to open big valves against high pressure, long enough to launch heavy bullets, there is so much hammer strike required that the limiting factor can be that the gun gets nearly impossible to cock.
Many big bores use a separate cocking handle mounted directly on the hammer to make that easier, and to take the load off the bolt handle. Even so, the cocking effort can approach 30 lbs. or more. There is a solution to such heavy cocking effort – balanced valves for PCP airguns.
Many types of valves have been invented and used over the years. Most PCPs use a simple poppet valve, held closed by a light spring and air pressure, and opened by striking the stem with a hammer or striker.
Below is an example.
Note that in all these drawings, the spring is omitted for clarity. Also, the dimensions in these drawings are for example only, and are not “real”, to protect the commercial value of these products.
When the valve is closed, as shown, the pressure P in the exhaust port is atmospheric. If there is 3000 psi of HPA in the reservoir, the force holding the poppet closed is (0.375 x 0.375 x PI/4) x 3000 = 331 lbs. plus a bit from the valve spring.
The hammer has to overcome that force just to crack open the valve.
Once the valve is fully open the pressure P in the exhaust port approaches 3000 psi, for the most part cancelling the force across the head of the poppet. That pressure acts on the valve stem area to provide a closing force. The force is much less (0.125 x 0.125 x PI/4) = 37 lbs. plus maybe 7-10 lbs. from the valve spring. It doesn’t seem like a lot, but that is enough to slow the hammer, reverse its travel, and close the valve in 1-2 milliseconds.
Some Valves Can “Blow Open”
There are valves available that are much easier to open, and once cracked off the seat will “blow open” and stay open until the reservoir is empty, or nearly so. Here is one version of the “spool valve” that does that.
Note that at the front end of the poppet is a chamber, sealed by an O-ring, that is vented to the atmosphere. The O-ring that prevents the HPA from escaping there is a bit smaller than the poppet seat, and provides a force opposing the 331 lbs. holding the poppet to the seat. In this case, that opposing force is (0.313 x 0.313 x PI/4) x 3000 = 231 lbs. so there is now only (331 – 231) = 100 lbs. holding the valve closed, instead of 331 lbs. We have reduced the force required to crack the valve by 70%, so it can be done by a much lighter hammer strike.
Now think about what happens as the valve opens and the pressure P in the exhaust port builds. That 331 lbs. closing force quickly disappears, but the 231 lbs. against the O-ring (opening the valve) does not, so the valve “blows open” and stays that way until the reservoir is empty, or the valve spring closes it.
This isn’t very practical, but with a small change we can make this idea work for us.
If we drill a tiny vent hole through the poppet, and seal the chamber in front of the poppet instead of venting it permanently to the atmosphere, a little bit of magic happens. Since the exhaust port is at atmospheric pressure when the valve is closed, there is 100 lbs. of force holding the valve closed, like in the spool valve.
However, when the valve is fully open, and the pressure P in the exhaust port approaches 3000 psi, the same thing happens in that sealed chamber. The forces on both sides of the O-ring cancel out, and the only closing force left is that 37 lbs. on the stem area.
That closes the valve like a conventional PCP valve, but it was much easier to open. This is the idea behind balanced valves for PCP airguns. It is easier to open, requiring less hammer strike, but then cycles normally.
Other Types of Balanced Valves For PCP airguns
There are many variations on this idea, some of them have been adopted by some of the leaders in PCP development.
Don Cothran turned the above idea “inside out” with his aftermarket Powerhouse Valve.
The poppet head is hollow and runs over a solid stem carrying the O-ring that seals the chamber, simplified in this drawing. I chose dimensions on all these diagrams to keep the “cracking force” of the valve at 100 lbs. just like the two preceding ones.
Don spent a lot of time developing his valve, and there are a few “tricks” he uses I will not disclose here that give the valve a crisp and distinct “cycle”. The Powerhouse Valve is not intended to be tuned down, it is designed to deliver maximum FPE from a very light hammer strike, and it does that well. It is almost impossible to make it produce a “bell-curve”, the velocity is varied by changing the pressure.
The very first balanced valves for PCP airguns I ever saw was designed by Lloyd Sikes. It used a separate “balancing piston” to reduce the cracking force, and then used the increasing pressure in the exhaust port to cancel that force to allow it to close again. Like all balanced valves for PCP airguns, the devil is in the details…
Tom Coston at American Air Arms uses a similar concept in the “Slayer” big bore PCPs. According to Tom, the proportions of the balance piston and the vent passages are key to proper operation, and this can be said of all balanced valve designs. By careful attention to detail, Tom has been able to produce a “bell-curve” when desired, or to max. out the power as is often desired in big bores, yet with feather light cocking.
Lloyd Sikes was also involved in the latest development in the design of balanced valves for PCP airguns. Travis Whitney gave Lloyd the concept as a “back of the napkin sketch”, and Lloyd did the prototype work. Travis then continued the development, and I was privileged with enough “inside information” to allow me to build my own version. Thank you both for that opportunity!
This aftermarket valve is made commercially by Jefferson State Air Rifles, and sold as the “SS Valve”, in varieties to fit many existing PCPs. It looks quite different, but functions much the same.
When closed, the forces on the large diameter section of the poppet cancel out. The pressure P in the middle chamber of the poppet is atmospheric, so the force holding the valve closed comes from the small diameter portion of the poppet, (0.206 x 0.206 x PI/4) x 3000 = 100 lbs. just like in the other balanced valves above.
Remember, these are “fake” dimensions, chosen for consistency in this article.
When the valve is wide open, the pressure in the small chamber between the two poppet O-rings rises like the pressure P in the exhaust port to nearly 3000 psi. This cancels out all the forces except that 37 lbs. closing force on the valve stem area, and the valve cycles like a conventional PCP valve. Well, almost!
Travis has added a little twist, allowing the SS Valve to be “fine-tuned”. He uses replaceable jets from Mikuni carburettors to control the diameter of the vent on the inlet side of the HPA poppet chamber.
In my versions I use quite a large hole, about 1/16-inch diameter, because I am using my SS valves in big bores where I am primarily interested in easier opening to allow a lighter hammer strike. Travis has shown that by restricting the size of that inlet port, you can decrease the valve dwell, because that chamber acts like an air spring.
Truthfully, I don’t fully understand this “jetting” system, but it sure seems to work!
I can tell you that the diameter of the vent through the poppet is, once again, quite critical. If it is too large, the pressure between the poppet O-rings rises so rapidly, before the poppet has the chance to clear the seat by more than a few thou, the closing force rises and slams the valve shut, and you lose the ability to get that nice light hammer strike.
It still works, you just have to hit it harder…
On the other hand, if that vent is too small, the valve tends to blow open, like a spool valve, and it gets hard to tune. It has a definite “cycle” (like a Cothran Valve), and tiny changes in hammer strike can make it “work” or “not”.
Once you get that vent about the right size, the valve can be tuned over a decent range of velocity with just hammer strike, and can develop a normal “bell-curve” in unregulated PCPs, but much easier to knock open. It also works great in regulated PCPs.
I find this to be an exciting time in the development of PCPs. Balanced valves for PCP airguns have made big bores a lot more practical, in that you don’t have to be a gorilla to cock them. The reduced hammer strike required makes the use of an SSG very practical to control hammer bounce.
I am looking forward to the days when the major manufacturers combine these features to produce a whole new generation of PCPs.
Balanced valves for PCP airguns are here and they can help us a lot!