Simplified Balanced Valve

[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

[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

[jackssmirkingrevenge]

If I could suggest a small variation, by matching the diameter of the stem to the housing you will trap air that will act as a cushion, removing the need for the bumper:

[Hobbyman2007]

If I could suggest a small variation, by matching the diameter of the stem to the housing you will trap air that will act as a cushion, removing the need for the bumper:

That would work if there wasn’t a vent to atmosphere. But then you’d basically have a cobra valve withought the air bleed.

[jackssmirkingrevenge]

That would work if there wasn’t a vent to atmosphere. But then you’d basically have a cobra valve withought the air bleed.

Perhaps you misinterpreted my suggestion, I'm talking about air trapped in the stepped portion as indicated in green:

[shorty]

Here's my latest design that I having been toying around with.

Floating valve stem with oring bumper.
Hollow poppet

The idea is that the floating valve stem pushing against the hollow poppet creates a high pressure within the  (I will call it the) "cobra" chamber when fired and when the floating stem comes off the hollow poppet, the back pressure from the pellet/barrel aides in closing.

I have been working on the left design for more available air flow and smaller "cobra" chamber.

[Hobbyman2007]

That would work if there wasn’t a vent to atmosphere. But then you’d basically have a cobra valve withought the air bleed.

Perhaps you misinterpreted my suggestion, I'm talking about air trapped in the stepped portion as indicated in green:

Yep , thought you mean the where the end of the valve stem is .

[rsterne]

Jack, the O-ring bumper is not a necessary part of the design.... but it is easy to incorporate when building the valve.... The idea is to prevent the poppet from "crashing" into the thimble if during tuning you use too much hammer strike.... It is also a VERY good idea to make sure that the stem is short enough that even if the hammer drives it flush with the back of the valve the poppet does not strike and part of the thimble.... It would be easy to drive the stem through the poppet, even though it is threaded, otherwise....

I don't know how many balanced valves you have played with designing and building in PCPs, but they can do things that are quite unexpected, particularly during development, and destroy themselves in short order.... They can blow open with huge force if the vent is too small.... slam closed if it is too large (the Cothran will destroy the seat doing that)…. and machine-gun while emptying the reservoir if you get the wrong balance between the way the valve operates and the hammer strike.... All in all, development can be pretty exciting.... 

The bumper was to hopefully avoid such destruction by providing a resilient cushion, but with a VERY high spring rate.... I don't think the small amount of air trapped as you indicate would be capable of doing that job.... and it would require precision machining of two concentric bores in the thimble and concentric surfaces on the two diameters of the poppet.... Even then, I don't know if it would trap enough air to be useful without an O-ring.... However, thanks for the suggestion....

[jackssmirkingrevenge]

I don't know how many balanced valves you have played with designing and building in PCPs, but they can do things that are quite unexpected, particularly during development, and destroy themselves in short order.... They can blow open with huge force if the vent is too small.... slam closed if it is too large (the Cothran will destroy the seat doing that)…. and machine-gun while emptying the reservoir if you get the wrong balance between the way the valve operates and the hammer strike.... All in all, development can be pretty exciting.... 

Virtually none compared to your body of work, so I defer to your experience as far as hammer valves go.  You've seen the one for the Apache elsewhere and there's also my first which was made for a 1322:



"that looks about right" design :

The bumper was to hopefully avoid such destruction by providing a resilient cushion, but with a VERY high spring rate.... I don't think the small amount of air trapped as you indicate would be capable of doing that job.... and it would require precision machining of two concentric bores in the thimble and concentric surfaces on the two diameters of the poppet.... Even then, I don't know if it would trap enough air to be useful without an O-ring.... However, thanks for the suggestion....

My feeling is that a simple o-ring bumper would be destroyed in short order and I have some experience with this, trying to arrest pistons for exhaust valves.  I've found that a carefully made air brake works wonders and indeed it's commonly used in other applications like for example pneumatic nail guns:



You can see that the "hammer" (#21) traps air when it interacts with the bumper at the end of its travel.

Once you have the valve stem supported at both ends I don't see why concentric bores would be difficult to achieve and also the bumper area does not need to be 100% airtight either so you have a slight margin of error on top of that.

[rsterne]

Nice work on the 1322 valve.... I have used O-ring bumpers in SS valves, and despite runaway hammer bounce, never found any damage to them.... Maybe they don't even touch, providing the hammer can't drive the stem far enough to force that?.... If so, just a cheap "peace of mind" kinda thing.... I did have one valve stem back out of the PEEK poppet (in which case the O-ring would have come into play), despite being a press fit and glued in with Loctite 638 on assembly.... I don't know if that was a momentum related shear failure during machine-gunning, or if the air pressure found its way around to the front of the stem, or both.... but even in that case, the bumper O-ring showed no sign of a problem.... My gut feel is that without it, the PEEK poppet would have suffered damage from the hammering it took.... but who knows.... 

Bob

[rsterne]

Mark, you mentioned the idea of plugging the front vent in an SS Valve and using a new poppet to set it up like this new valve.... The only fly in that ointment is that in some of Travis' SS Valves he uses a large front end on the poppet.... This makes the hammer strike higher, but he says makes the valve more tuneable.... In this design, a larger front end will make the valve EASIER to open, the opposite to what it does in an SS Valve.... If they are too close to the same diameter, you may have such a light hammer strike the valve may be hard to tune.... Just something to keep in mind, it may or may not be an issue....

Bob

[rsterne]

Travis and I were discussing this valve elsewhere, and I just wanted to warn anyone trying this idea that they may not want to put in a lot of effort until the idea is proven one way or the other.... He stated:

It will have a very narrow tuning field and no I dont think it will be capable of a bell curve other than the first initial shot being slightly slower than the rest.

Remember, not all ideas succeed, as I stated in my OP.... I guess we'll see what happens.... I'm with it either way, because we will learn more about what makes balanced valves tick.... and that's EXACTLY what this thread is all about.... 

Bob