What is valve lock in a pneumatic gun?
by B.B. Pelletier
Two things drove me to this posting today. We had a reader from Hawaii whose AirForce Condor is not performing as it should, and another reader named baldtrucker asked what happens when a multi-pmp pneumatic like a Benjamin 397 is over-pumped. I did a search and couldn't find where I had addressed this question before; but, even if I have, it's time to do it again.
How does an impact pneumatic valve work?
The most common valve is the impact type or knock-open valve, and that's the one that has a problem with over-pressurization. When a hammer strikes the end of the valve stem of an impact valve, it forces it to momentarily lift the valve face off the valve seat. When that happens, air can flow through or past the valve stem and out into the breech of the airgun.
The valve face is held against the valve seat by a return spring. Also, any air pressure inside the reservoir where the valve face and seat are located pushes against the back of the valve face, forcing it against the valve seal. These two forces (the return spring and air pressure) are what keep the valve closed.
The hammer has to strike the valve stem with enough force to unseat the valve momentarily, allowing air to flow from the reservoir. The weight of the hammer and the strength of the spring that pushes it have been calculated to open the valve when the pressure inside is at its maximum. For most multi-pump guns made today, the valve allows all the stored air to be released. That's easy because their reservoirs are very small. But, precharged pneumatic reservoirs are larger and only a portion of air is released. The next time the valve opens, the pressure inside (pushing against the valve face) is slightly lower, so the valve remains open slightly longer. A little longer flow of air at lower pressure is released, giving the same velocity to the pellet. This is always easier to control when the barrel is longer, so long-barrelled rifles are generally the most consistent, though a valve can be tuned for any barrel length.
What happens when a pneumatic is over-pressurized?
When the air pressure inside the reservoir is higher than the design of the action can accommodate, the hammer cannot open the valve as far as it should, so less air escapes. That is exactly what is happening to the Condor out in Hawaii. The Condor valve face is HUGE! It has to be, to allow as much air as possible to move through the valve. However, such a large surface area means the valve is also EXTREMELY sensitive to air pressure. Any over-pressurization will hold the valve shut, so the pellet gets very little air to push it.
When you put air into an airgun, it is nothing like putting gasoline into the tank of a car. Even then, more gas doesn't make the car go faster, does it? What a pneumatic gun needs is air FLOW, and that happens only when the valve remains open as long as it was designed to.
Condors do not like to be filled to 3,000 psi. I have seen only a few that would tolerate it. Most like to be filled to around 2,800 psi. And I have seen a few that liked to be filled to just 2,600 psi. No matter what pressure you fill them to, as long as it is their maximum pressure, they will all give you about 20-25 VERY powerful shots when the power setting it set on high. That may seem counter-intuitive to some, but consider this: A NASCAR race is not won by putting more fuel into the car. It's won by making the most of the fuel that is put into the car.
The same thing happens when a multi-pump is over-pumped. I hear stories all the time about how so-and-so pumped up his Sheridan Blue Streak 20 times, and it cracked like a .22. I just smile and keep my thoughts to myself, and now you know why. A gun that is supposed to be pumped a maximum of eight times isn't going to crack like a firearm with 20 pumps. It isn't going to do much of anything; and if it does, that gun is already worn out and powerless.
Crosman had the answer!
In the 1950s, Crosman came out with a pneumatic valve that couldn't be over-pressurized - at least not easily. They put it in the Crosman 140 rifle and the 130 pistol and touted it as the answer to over-pressurization and valve lock, as this problem is commonly known. The valve did work as advertised, however, it had a few drawbacks. As the pressure increased, the trigger became harder to pull. It was impossible to fix that, and the triggers were always second-rate. This type of valve had the habit of opening on its own when the pressure was still low. I've had guns fire while I was filling them - so this valve type was not the solution to valve lock that it promised to be. They also had the problem of the pliable parts of the valve extruding through the valve ports under pressure.
So, that's the story on valve lock. It's pretty straightforward. As long as you operate your pneumatics within the parameters that they were designed to operate, they will serve both well and long.