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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Simplified Balanced Valve

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rsterne:
There is a lot of interest right now in balanced valves for PCPs.... Part of this is because of my recent article in HAM, where I describe a few of the types that have been tried.... https://hardairmagazine.com/ham-columns/balanced-valves-for-pcp-airguns-theyre-here-today/ …. Reading that over will give you a pretty good idea about how they work to reduce the hammer strike required.... Some of these designs, or variations of them, are used commercially, which is why I haven't given any specific details.... However, one design, and perhaps the simplest one to build, I have never seen used commercially....

As you all know, it is my desire to share things publicly, to further the sport of airgunning…. In that spirit, I have drawn up the concept for the next valve I plan to build for my Hayabusa…. I hope to have it work properly from .224 through .357 cal, although it is sized for the .357.... This may be an ambitious project, but I wanted to publish the concept drawing from the beginning, so that you can all share in this project, be it a success or failure.... Here is the drawing....



The poppet is made from Delrin or PEEK, and is threaded internally with 5-40 threads.... The stem is a piece of 1/8" O1 Drill Rod, threaded 5-40 at the front (the threads secure the poppet to the stem in shear), and drilled lengthwise back into where the exhaust port is, with a small cross-drilled hole, to vent the forward balance chamber of the valve into the exhaust port of the valve.... This means that the balance chamber will start out at atmospheric pressure between shots.... when the valve lifts from the seat and the pressure in the transfer port rises, so will the pressure in the balance chamber.... and when the valve closes again, that pressure will fall back to atmospheric gradually as the pellet moves down the barrel and departs the muzzle.... Since the area of the balance chamber is half the area of the valve seat, it will only take 1/2 the force to crack open the valve, allowing it to open quickly with less hammer strike.... Once open, the rising pressure in the balance chamber will help to shut the valve quickly, and as it takes a short time for that pressure to bleed out through the vent in the stem, hopefully resist any second hammer strike which might cause valve bounce.... At least that is the idea....

Most likely I will drill the lengthwise hole in the stem with a 3/64" (0.047") drill, partly because using a smaller drill it would be so easy to break.... The metering of the airflow through the vent will be done by changing the size of the single cross-drilled hole in the exhaust port area.... Since that is easy to get at, I can start small, at 1/32" (0.032") and go larger if necessary.... I want the hole to be large enough to prevent the valve from "blowing open" on its own, which it can do if the pressure in the balance chamber rises too slowly.... When a balanced valve blows open (like the Cothran valve does), it is impossible to tune with hammer strike, as the valve either works or doesn't.... ie it has a definite "cycle" nearly independent of hammer strike.... I don't want that, I want to be able to tune the valve over a wide range of velocity using only hammer strike.... This will (hopefully) produce a valve that can be used either regulated, or produce a bell curve when used in an unregulated PCP....

For this valve I plan to use a 7/16" OD poppet, made from PEEK, with a 5/16" front section.... That means the front diameter is 71.4% of the rear, so the area is 51%, about as close as I can get to my 50% goal.... The valve throat will be 3/8", which when you subtract the area of the 1/8" stem will leave the throat area the same as a hole of 0.354" (nearly .357 caliber sized).... The exhaust port will be 21/64" (0.328"), which is the same area as the chamber in my .357 Hayabusa which has a 9/64" (0.141") bolt probe.... I will make sure the transfer port and barrel port are also the same area, to insure the maximum possible flow....

One of the advantages of this valve design is that it only needs one dynamic O-ring instead of two like the SS Valve, so less potential for leaks.... The SS Valve is prone to "stiction" problems when the guns sits for a while between shots, and I have a gut feeling that is because at rest the O-rings are loaded in opposite directions and tend to "wedge" inwards towards the center balance chamber.... The front O-ring in an SS Valve would therefore be wedged towards the seat, and opposite to the direction of travel of the poppet when firing.... When you fire the gun after it has been sitting, the front O-ring, which is already wedged into the back corner of the gland, gets pushed even tighter into that corner by the motion of the poppet.... That may be the cause of the low velocity first shots sometimes experienced.... In this design, at rest the O-ring is pushed into the front corner of the gland, and as soon as the poppet moves on firing, it moves back away from that corner into the center of the gland.... I hope that will avoid any "stiction" issue on the first shot after sitting.... Only time will tell....

Anyways, there is the idea for a simple balanced valve, with minimum parts, that is relatively easy to machine.... By putting this out in the public domain, I hope to encourage others with the skills and equipment to jump on the train and let's get these balanced valves out of the station, and roaring down the track, into the mainstream of PCP airgunning….

Bob

mann:
You could fit a new poppet into the ss valve and plug the jet hole . And use only one oring on it like you have in the drawing

rsterne:
Yes, that is quite possible.... You would have to make sure that the front jet hole was sealed 100%, as any leak there would escape out the barrel.... If the idea works as it should, that would be an easy retrofit, and prove if it is the front O-ring "wedging" that is causing the stiction.... You might need to increase the clearance between the rear portion of the poppet and the thimble to allow the HPA to pass through easily.... The poppets are vented differently, of course....

Bob

mann:
The wedging like your saying would definitely cause stiction and this way might just get around that

rsterne:
Yeah, I was trying to figure out the "stiction" problem, and that is what I came up with.... O-rings migrate under pressure to the corner where the air is trying to escape.... in this case the outer, low pressure corner of the gland.... That means forward on the rear O-ring and towards the rear on the front one on a SS Valve.... The movement of the poppet on firing would tend to increase that wedging effect on the front O-ring, possibly causing the stiction problem on the first shot, until it cycles and recenters in the gland for subsequent shots.... Once you are shooting, the pressure fluctuation in the center balance chamber, and the movement of the poppet on firing, would tend to keep both O-rings moving around in the glands.... until the next time you let the gun sit for a while.... Just my thoughts on the matter....   ;)

Bob

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