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

Simplified Balanced Valve

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mann:
If my trying of the python chamber doesn't work then I will make a different poppet and give this a try should be easy enough to do  . What size vent hole should one try was thinking I'd drill the vent hole threw the poppet and that way the drill rod wouldn't have to be drilled or threaded

rsterne:
I would use either 1/32" or a #60 drill.... 0.032" to 0.040".... Make sure you don't hit the bottom of the O-ring groove with the vent hole, not a lot of room between that and the stem.... You might have to drill from both ends, with the inner end on an angle?....

Bob

mann:
Ok thanks for idea Bob

Hobbyman2007:
I’m just about ready for my first attempt in a ballanced valve . Not entirely sure what design I’m going to use or which rifle to build it for . I have a Cothran valve equipped .257 rifle and to be honest I don’t shoot it often at all . Bob in your drawing you’re using 50% of the poppet area to reduce it to 50% of cracking force . What I’m mostly interested in is being able to tune a bell curve to a shot string . What is the biggest factor for tunability ? Is it the relationship between poppet and balance chamber or is it the size of the vent to atmosphere that has the biggest effect on being able to tune the valve . Unlike the Cothran valve I don’t want to use available pressure to adjust the velocity .

rsterne:
I don't yet have 100% proof, but I think the thing that causes a balanced valve to cycle "on or off" is too small a vent between the balance chamber and the exhaust port.... What happens in that case (I think) is that the pressure in the balance chamber is too low, and once the poppet lifts off the seat, it opens like a sleeve valve.... That is exactly what happens if the balance chamber is just vented to atmosphere, instead of connected to the exhaust port.... The best SS Valve I have had, that was the easiest to tune, was one that had a 3/8" poppet with a 1/4" front chamber, and quite a large 0.040" vent.... However, it did require a heavier hammer weight than the light hammer that would work with a smaller vent....

I have found that a larger vent requires a heavier hammer strike, and a heavier hammer, but once you have that they seem to be more tunable.... If the vent is large and the hammer strike too light, the valve seems to want to slam shut quickly, before the poppet is open enough to release much air.... That makes sense to me, because the pressure in the balance chamber builds before the valve is open more than a few thou.... When you increase the hammer strike enough that the momentum gets the poppet open far enough, that slamming closed effect seems to be overcome.... and then the valve becomes more tunable....

As I said, I'm not 100% sure of this relationship yet, more testing is required.... That is one of the reasons I published this design, to get more versions built, by more guys, so that we can learn more about what makes balanced valve tick.... If my idea is correct, once the vent is large enough to prevent the valve from "blowing open" (cycling), and the hammer strike is great enough to push the poppet past the "slamming closed" point.... I have a gut feeling that an even larger vent won't make any difference.... and the valve will be as tunable as it can get with those proportions between small and large diameters.... Logic tells me that the bigger the valve, the bigger the vent also....

I wish I could give you a hard answer.... but at this stage all I can do is say "try it".... and don't be afraid to experiment.... With the idea of using the cross hole in the stem to vary the venting, with a set of carb jet drills you should be able to work up a size at a time until you get past the "blow open" stage.... and eventually find the size where larger makes no difference.... Again, my gut tells me that is where you want to be for maximum tuneability....

Bob

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