Author Topic: Simplified Balanced Valve  (Read 1444 times)

rsterne

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Simplified Balanced Valve
« on: December 23, 2018, 01:25:17 PM »
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
« Last Edit: December 23, 2018, 01:29:05 PM by rsterne »


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Alan

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Re: Simplified Balanced Valve
« Reply #1 on: December 23, 2018, 02:13:39 PM »
A very clever design. I trust you didn't lose too much sleep?

There is another advantage to publishing the design—no one can "steal" it, and call it their own. And, there is no way anyone can claim patent infringement.


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rsterne

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Re: Simplified Balanced Valve
« Reply #2 on: December 23, 2018, 07:06:03 PM »
That, my friend, was the plan....  8)

Bob
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tkerrigan

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Re: Simplified Balanced Valve
« Reply #3 on: December 23, 2018, 11:32:11 PM »
Could a smaller hole be drilled through the peek rather than the stem?  Then use an acetylene tip reamer to adjust the size larger if needed?  Regards, Tom
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rsterne

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Re: Simplified Balanced Valve
« Reply #4 on: December 24, 2018, 08:18:18 AM »
There is not much room between the stem and the bottom of the O-ring groove, which is in the smaller diameter part of the poppet.... If you can get a hole in there without hitting the groove, that will work fine.... You might have to shorten the stem somewhat, to before it gets to the O-ring groove, and drill a second hole on an angle from the front end of the poppet (starting in the middle), on an angle to miss the O-ring groove, and intersect the vent hole between the O-ring, and the end of the stem.... It depends on the dimensions of your poppet, and the CS of the O-ring chosen.... You could use a Metric O-ring with a smaller CS....

Good Luck....

Bob



« Last Edit: December 24, 2018, 08:28:31 AM by rsterne »
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Re: Simplified Balanced Valve
« Reply #5 on: December 28, 2018, 06:08:37 PM »
Now that Christmas is over, I got back into the shop to start on the parts for my new simplified balanced valve for my Hayabusa Mk.II…. Yesterday I roughed out the poppet and stem and assembled them.... The stem is made from a piece of 1/8" O1 Drill Rod.... It is drilled from the front end with a 3/64" drill to a depth of 1.20", which is about 1/4" behind the seat of the poppet, and about in the middle of the valve exhaust port.... It took a while to drill that deep, because the flutes on the drill are only 3/4" long, so after that I could only drill about 0.015" at a time before clearing out the chips.... otherwise the drill would have broken off in the hole for sure.... The front 5/8" of the stem is threaded 5-40, the reason for that size is that you don't have to turn down the shaft, the nominal OD of the threads is 1/8" and the die just screws on.... The new Die Holder I made worked perfectly, and the threads are nice and straight.... The poppet was turned down oversize and then drilled through with a #38 drill, which is the tap size for 5-40 threads.... It was then counterbored from the back to 0.128" for a depth of 0.300", leaving a shoulder inside where the unthreaded portion of the stem will stop.... and then tapped from there to the front of the poppet.... I then coated the inside of the hole in the poppet and the threads with Loctite 638 (green) and assembled the poppet and after wiping off the excess (and making sure there was none inside the vent hole), I left it to dry overnight....

The next step was to make the thimble.... I used a piece of 1144 Stressproof steel that is 1/2" OD, turned the back end flat, centerdrilled it, and then drilled a 1/4" hole 1.25" deep, the front of which is the spring pocket.... I then used a 19/64" drill (1 size under 5/16") and drilled in for the front portion of the poppet to a depth of 3/4", then drilled out the back to 27/64", finishing it to a depth of 0.300" using a 7/16" mill for the pocket to miss the rear part of the poppet.... I then used a 5/16" 4-flute mill to finish the front bore to a depth of 0.830", enough that the finished poppet can go just below flush with the back edge of the thimble without either the front of the poppet hitting the front of the bore, or the spring going coil-bound.... Once the bumper, which is a 7 mm ID x 1.5 mm CS O-ring (10 mm OD), is installed on the poppet, it will stop it 0.050" before anything can come to a crashing halt and get damaged....

The next step was to finish machining the poppet.... I ground the stem off flush with the front end of the spring seat in the poppet and cleaned up the end of the vent hole with a tiny center drill.... With the stem of the poppet in the lathe chuck, and the front running on a live center in the tailstock, I finished the poppet to size and machined the O-ring groove for the 5 mm ID x 1.5 mm CS 70D O-ring (8 mm OD).... It's nice doing it this way as it ends up machined true to the stem.... The last thing on the poppet was to drill the cross vent hole in the stem on the exhaust port side of the seat.... I used a 1/32" drill for that.... I have a selection of small drills coming, as the next size up I have currently is 0.040".... I can go all the way to 0.047" to match the 3/64" hole in the stem if necessary.... That is about 2.3 times the area of the current 0.031" vent hole....

The last thing to do on the internals of the valve was to finish the thimble.... I finished the length to 1.90", the same length as the previous SS valve thimble I made, so that if this new valve doesn't work I can make SS internals to fit the valve body (which I still have to make)…. I then drilled the front end to a depth of 1/2" and tapped it 8-32.... I turned the OD of that portion to 0.290" OD so that it will have lots of clearance in the 5/16" milled shoulder I will machine in the perforated front mounting wheel.... The center hole in that wheel will be 3/16", which will be a loose fit on the 8-32 screw used to mount the thimble.... I turned off the last 1/8" of threads on the mounting screw, so that it can be tightened up against the bottom of the threaded hole without damaging the threads.... When it is tight there is 1/4" of space between the head and the end of the thimble, so that I can set the end play to just a few thou and yet have lots of radial play so that the thimble is self-aligning with the poppet.... The last machining on the thimble was to cut a 10 deg. taper joining the two ODs.... Here is what all the parts look like....



I know for a fact that the O-ring doesn't leak, at least at low pressures, and yet slides easily.... If I slide the poppet into the thimble without the spring in place (with the vent hole plugged), it compresses the air in the thimble and the poppet pops back out from that tiny bit of pressure.... With the bumper O-ring in place the poppet stops with about 1/32" showing behind the back of the thimble.... I will set the back of the thimble 1/4 from the seat of the valve and that will give me about 0.220" of lift before the bumper touches inside the thimble.... The stem of the valve, however, will only protrude 0.200" behind the back of the valve body, so the hammer cannot drive the poppet far enough to make it crash inside the thimble....

Next step is to make the valve body, and the perforated wheel that mounts the front of the thimble....

Bob
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rsterne

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Re: Simplified Balanced Valve
« Reply #6 on: January 03, 2019, 06:23:17 PM »
I finally got back into the shop today and finished the new valve.... Here is the front end, showing the perforated mounting wheel for the thimble....



The thimble is free to move radially about 0.020" where it fastens to the center of the wheel, so that it can self-align with the poppet.... The back end has a much larger port than the previous valve.... You can see the vent in the stem down inside the exhaust port....



The throat is 3/8", and the stem is 1/8", so the area is the equivalent of a hole measuring 0.354", or almost bore area for the .357 cal and larger than all the other calibers that can fit on my Hayabusa Mk.II…. The exhaust port is 0.328", and the transfer port for the .357 will be the same size.... For the smaller calibers I will match the transfer port to the caliber.... The OD of the transfer port has increased to 7/16", so I had to drill the hole in the main tube out to match that....

On first assembly I had a small leak, so I took the valve apart, ran a 1/2" end mill (by hand) up against the seat, undercut the PEEK poppet by 3 deg. so that it only touches around the outer edge, and then lapped it with Solvol Autosol, cleaned everything and then burnished the poppet against the seat.... It sealed up just fine.... I started at 500 psi, there were no issues getting it to seal, and I dropped the hammer down the tube a few times to get it to burp.... It still wasn't leaking, so I filled it to 1000 psi, then 1500, 2000, 2500 and finally 3000 psi, which is the MSWP of this PCP.... Each time I dropped the hammer down the tube to fire the valve, and as expected as the pressure increased it took more hammer strike to get the valve to "pop".... There was no tendency for the valve to burp at pressures over 1000 psi, just a nice clean pop, and if I launched the hammer a bit harder, a very loud "BANG".... I am delighted that the valve seals up fine, and look forward to getting the gun built up so that I can test further....

I will be building a new SSG for this gun, it never had one.... Once that is done I will be able to find out how tuneable the valve is.... I have my fingers crossed....

Bob
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rsterne

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Re: Simplified Balanced Valve
« Reply #7 on: January 04, 2019, 07:06:56 PM »
The valve is still holding air fine after 24 hours, so I guess I got the leak fixed.... I spent the day today making/modding the rest of the parts to assemble the .357 version of my Hayabusa Mk.II using this new valve.... The first thing to do was to modify the receiver to accept the new 7/16" OD transfer port, and then drill out the port in the receiver to 0.328" to match the exhaust port in the valve and make the new transfer port from a piece of Delrin rod.... I then milled out the barrel port, leaving it 0.280" wide but increasing the length to 0.380" and blending that into the diameter of the transfer port.... This insures that the barrel port is at least as large as the transfer and exhaust ports, and also the same area as the chamber minus the bolt probe.... The ports are now all 0.328", compared to the 0.281" they were previously....

I have two hammers for this version of the Hayabusa…. The .224/.257 hammer weighs 123 gr. including cocking handle, and had a stoke of 1.2".... The .284/.308/.357 hammer weighs 147 gr. including cocking handle, and had a stroke of 1.45".... The new valve is 0.10" further back in the tube, so I lose that much stroke, and I decided to try the smaller hammer, even though the valve is larger than the .357 valve I had before.... I am confident that the balanced valve will at least make up that much difference.... I had a look at all the hammer springs I have to choose from, and decided to use the original spring from the .357 for my initial testing.... It is a McMaster Carr spring that is 3" long, 0.36" OD, made from 0.051" wire and rated at 15 lbs/in.... although once I set it, the length decreased by 0.1" and the rate was only 14.1 lbs/in.... The .357 originally took about 23 lbs to cock with this spring, at the 1.45" cocking distance.... With the SSG, using this spring, I have set the preload at 0.30" (4.2 lbs), and with a small gap between the SSG and hammer, and the new 1.10" cocking distance, the maximum cocking force will drop to about 19 lbs.... I suspect this will still be more hammer strike than I need for the new balanced valve, but I have two 3" springs with lower spring rates, of 0.047" and 0.049" wire, to reduce the hammer strike further if required.... I also have some 3.5" springs, but the spring rates are similar, and I have enough cocking distance without reaching coil bind with the 3.0" springs, so unless I need to increase the cocking distance, I should be able to stay with the 3.0" springs.... Here is what the new SSG looks like....



If I end up needing a heavier hammer with more stroke, I can revert to the other one I have.... but for now I will use this one from the .224/.257 version....



You can see the new, larger transfer port in that photo as well.... This is as big as I can go with the current valve and bolt probe design, it is 92% of the caliber, at 0.328".... I put the .357 barrel into the receiver before dinner.... and tomorrow I hope to finish the assembly and test this new valve in the .357 Hayabusa….

Bob
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rsterne

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Re: Simplified Balanced Valve
« Reply #8 on: January 05, 2019, 01:13:04 PM »
Before removing the trigger group and stock from my .457 Hayabusa Mk.III (those are shared between all the versions), I thought I would perform a test to see if "stiction" was an issue with the SS Valve I recently built and installed in it.... The gun has been sitting for weeks, and still had over 3200 psi sitting in it, but I topped it up to its usual 3600 psi fill and fired 4 shots through the Chrony…. They were 962, 973, 978 and 973 fps.... That means the first shot was less than 2% below the peak, so stiction is NOT an issue for that valve....

I then assembled the .357 Hayabusa Mk.II, using the new simplified balanced valve I have been working on.... The first shot was with the 0.051" spring with 4 turns of gap, and with a 131 gr. bullet it screamed across the Chrony at 955 fps.... That is more power than I have ever had before, which is attributable to the larger port, of course.... I backed out the gap a couple of turns at a time, and the velocity didn't drop until I was 12 turns out, at which point it tanked to 275 fps.... I then reduced the gap to zero, and the gun was only slightly louder than with 10 turns of gap, and absolutely NO sign of hammer bounce or machine-gunning.... So, the initial test was successful on every count but one.... the valve acts like a Cothran valve, either it cycles or it doesn't....

I took the SSG apart and installed the lightest spring I have of that size, made from only 0.047" wire, with a spring rate of only 11 lb/in.... The velocity was essentially unchanged (a few fps less), and it still showed no signs of adjustability.... With 10 turns of gap it was 950 fps, 940 fps at 11 turns, and 270 fps at 12 turns of gap.... I spun the adjuster back in to 4 turns of gap and fired a few 154 gr. bullets through the Chrony and they were 892-894 fps (273 FPE).... Did I mention that this valve is INCREDIBLY STABLE for velocity?....  ::)

So, I have drained the air from the reservoir, and will be pulling the valve and drilling the vent out to 0.040" to see if that makes any difference.... If not, then I will go all the way to 0.047", which is the size of the hole through the stem.... Bottom line at the moment is that I am quite pleased with the new valve, it does 90% of what I wanted.... I have increased the size of the valve throat from 0.328" to 0.375", and the poppet from 3/8" to 7/16".... The hammer weight has been reduced from 145 gr. to 122 gr. (and can obviously be much less) and the hammer stroke from 1.45" to 1.09" (much less with the large SSG gap)…. The cocking force has gone down from 23 lbs. to about 9 lbs.... The shot cycle sounds crisp, with no sign of hammer bounce.... The ONLY thing I have not achieved so far is tunablity…. We'll see if that can be achieved or not.... If not, at least I have still produced a simpler balanced valve that yields big power for low hammer strike....

Bob
« Last Edit: January 05, 2019, 08:01:43 PM by rsterne »
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rsterne

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Re: Simplified Balanced Valve
« Reply #9 on: January 05, 2019, 06:18:23 PM »
This valve is obviously "blowing open" like a Cothran valve does.... I tried increasing the vent diameter from 0.032" to 0.040" (a 56% increase in area, possibly double the flow because of friction affects)…. and it made NO difference to the way the valve operated.... Same velocity, same cliff, at the same SSG gap.... I then made an MDS hammer to replace the steel one.... I kept the same 1.09" stroke but the hammer with handle weighs 1/3 as much, 42 g. instead of 123 g.... The only way the gun would fire with the light 0.047" spring was with the SSG gap set to zero.... With just 1/2 turn of gap the velocity dropped to under 200 fps.... BTW, as Travis requested, I tried some preload with this setup, by turning the SSG in a couple of turns, to give 4 lbs. of preload.... The velocity remained the same as with zero gap, the bark was a little louder, and there was still no sign of hammer bounce....

I tried the original 0.051" spring with the light hammer and it would fire properly (955 fps) with 1 turn of gap, but at 2 turns of gap it was inconsistent in velocity, shots varying from 450-950 fps.... (I saw similar things with the Cothran valve when it was right on the edge of the cliff, the ES went through the roof.... So, for that matter did the original SS valve I got from Travis, which had only about 100 fps of adjustability and then a cliff).... Anyways, I can either use the light hammer and the heavy spring with 1 turn of gap on the SSG.... or the heavy hammer and the light spring with about 8-10 turns of gap.... With the light hammer and heavy spring set to 1 turn of gap I checked the pressure drop.... With the tiny tank fitted my total volume is only 6.6 CI (108 cc) way too small for proper performance at 265 FPE (0.4 cc per FPE)…. Filled to 3000 psi, the pressure drop was 498 psi (measured with a digital gauge) for a shot of 956 fps with the 131 gr. bullet (266 FPE)…. That works out to 1.17 FPE/CI.... pretty respectable for this much power with way too small a plenum.... The normal volume available for a shot is 28 CI, which means that I should only see a pressure drop of about 120 psi instead of 500, and with the average pressure increasing from 2750 to 2940 the FPE should increase nicely as well, hopefully with an increase in efficiency too....

Obviously, neither current hammer solution is the best.... The next time I have the valve apart I will drill the vent out to 0.047", even though I don't expect that to make any difference in the tunability.... The next step is to get the hammer weight somewhere in the middle, so that I can use the light spring (hopefully) but with about 4 turns (0.20") of gap before it falls off the cliff.... To do that, I  will make a brass or steel insert for the MDS hammer, drill it out and press in the insert.... I think about double the weight I am now, which is 2/3rds what I started with, should be about right, but that is really just a guess, so I will try and hit about 80-85 g.... With 3 springs to choose from, I should be able to get it about right....

So I have accomplished all the goals, except tunablity…. This valve would be a sweet setup tethered to an adjustable regulator.... Since this version of my Hayabusa will primarily be shot tethered, I will likely leave this valve in the gun until I can come up with something better.... The alternative would be to make an SS valve with the same proportions and try that.... Before I go that route, I think I will fit the .224 cal barrel and receiver to this gun and see what happens, and what changes I might have to make for that huge caliber change.... If that works out OK, then I will complete the conversion for the other 3 calibers, .257, 7mm and .308.... Lots of fun ahead....  ;)

Bob
« Last Edit: January 05, 2019, 06:20:59 PM by rsterne »
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Re: Simplified Balanced Valve
« Reply #10 on: January 09, 2019, 02:24:15 PM »
I rebuilt this valve yet again, drilling out the vent to 0.052" and fitting a bit heavier valve spring.... The valve is now adjustable, and incredibly efficient.... Here are the results, using the SSG with the 11 lb/in spring and 3.5 lbs preload, with the gun tethered at 3000 psi....



The valve is working about the same as an SS valve in terms of adjustability.... The hammer spring is perfect for the 131 gr. bullet, it is solidly on the plateau at over 950 fps with 2 turns of gap.... With the 154 gr. bullet it needs a slightly heavier spring or more preload to reach the plateau, which would be at about 890 fps.... However, since I never tune my guns to shoot on the plateau because it is quite inefficient, this SSG spring setup is just fine.... The cocking force is just 13 lbs. instead of the 23 lbs. with the smaller, conventional valve.... I tried a couple of short strings just to prove that I can get a bell curve, but bear in mind this is with the tiny tank fitted, and a total reservoir volume of only 108 cc....



With the 154 gr. bullet I had the SSG adjusted with 2 turns of gap, for about 3% below the plateau, and got a declining shot string, although the first 2 shots were within 15 fps (2% ES)…. With the 131 gr. bullet, I used 5 turns of gap on the SSG to produce a nice bell-curve at about 9% below the plateau.... The first three shots had an ES of only 8 fps, which is less than 1%.... Bear in mind that with the 500 cc tank fitted, the total air capacity is 460 cc, which will give over 4 times the shot count within the same ES....

I need to brag about the efficiency for a minute.... What I got for each of the 4 shots with the 131 gr. bullet is plotted on the dotted line above.... The first shot only used 284 psi from my 108 cc reservoir, or just 129 CI of air to produce 216 FPE, for 1.67 FPE/CI.... That is definitely off the scale compared to any other Big Bore PCP I have ever shot, or am aware of.... The average over those 4 shots was 1.53 FPE/CI....  :o

To say I am delighted with where I am would be a complete understatement.... It was difficult to get here, requiring several changes to the valve.... but it turns out that the large vent through the stem was the key.... We have to get a high enough flow rate through the vent to keep the valve from blowing open.... Once that is achieved, the valve is definitely tunable, and capable of producing a bell-curve.... I am just over the moon the way this worked out.... My thanks to all those who provided input along the way, and in particular to Mike, who by quantifying the fill time for the balance chamber led me in the right direction....  8)

Bob
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Alan

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Re: Simplified Balanced Valve
« Reply #11 on: January 09, 2019, 03:14:34 PM »
And the saga continues!

I have often wondered what airgun engineers go though when designing something new. I'm sure they think their efforts were rewarding, at least monetarily. But you, Mr. Sterne, it isn't money which drives you. Rather it is the perfection of the art, and the thrill of accomplishment we all strive for. Thank you for showing the rest of us, how it is done!
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I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

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Re: Simplified Balanced Valve
« Reply #12 on: January 10, 2019, 04:28:00 PM »
After the gun sat overnight, I shot the pressure down using 68 gr. roundball, starting from 3000 psi.... It took 11 shots to get down to 270 psi.... The first shot was supersonic at 1160 fps, the 2nd shot was 1108, and it progressed down to 446 fps for the 11th shot, taken at only 315 psi.... There was NO sign of hammer bounce, regardless of how low the pressure got.... the gun just got quieter and quieter as the velocity dropped.... I then refilled it and cranked in 2 turns of preload on the SSG, so that there was 4.7 lbs. of preload against the stem.... I was finally able to get the valve to dump the tank down to about 300 psi in 1 shot by doing that.... Now, you would never tune the gun this way, because you are wayyyyyyyyyyy up on the plateau and the efficiency would be terrible.... but there you go, Travis, I forced it to machine-gun for you....  ::) …. I really doubt it would ever do this with any kind of a tune you would ever use, but if you insist on running lots of preload against the stem, you may be able to create a tank dump....  ;D

After draining the tank I pulled the valve out yet once more, because I had a smaller poppet spring I wanted to install, and then sleeve down the spring chamber to reduce the volume and speed up the pressure rise in the balance chamber.... I made a Teflon sleeve to fit inside the 1/4" bore of the 0.40" deep spring chamber, and drilled it out to 13/64".... This reduced the volume of the chamber from 0.55 cc to 0.44 cc, and I was curious what effect that would have on the tuning range.... I put the gun back together, and this is what I got....



NOTE: When calculating the efficiency, I forgot to include the air in the hose, gauge and valve.... I have estimated that at 12 cc, making the total 120 cc instead of 108.... This reduced the FPE/CI numbers to more realistic values, shown on the corrected graph above.... I still won't have the actual numbers until I install the 22 CI bottle....

The maximum velocity stayed the same at 955 fps, but the velocity dropped off faster as I increased the gap, as I expected it might.... Instead of a range of about 120 fps before the valve quit working, that range increased to 180 fps.... The velocity was still very stable when tuned below 800 fps, whereas with the larger spring chamber I couldn't even tune it that low.... Being able to tune it down that far allowed the efficiency to reach even higher than yesterday, I got to 1.57 FPE/CI at 864 fps (217 FPE)…. With my old conventional valve at the same power I was getting 1.13 FPE/CI, which I thought was pretty good at the time.... I'm now using 2/3rds of the air for the same power....  :o

Decreasing the balance chamber size by 20% certainly widened the tuning range.... However, it really only made about 1 turn difference in the SSG gap for the same velocity.... That means the valve is taking the same hammer strike to open, but with the pressure in the balance chamber rising faster, a bit more hammer strike is needed to get the same dwell.... It was definitely worth changing to the smaller poppet spring and making the chamber smaller.... Once again, it seems obvious that you want the smallest balance chamber you can achieve, with the largest vent.... That definitely seems to be the key to getting this valve tuneable....

Bob
« Last Edit: January 12, 2019, 03:09:00 PM by rsterne »
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rsterne

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Re: Simplified Balanced Valve
« Reply #13 on: January 12, 2019, 03:49:58 PM »
OK, so first of all, I have an admission to make.... Although the plenum is 108 cc, I forgot to allow for the volume in the microbore hose, gauge and valve, which I now estimate at about 12 cc.... Therefore the volume dropping in pressure when I was calculating the efficiency was wrong, it is about 120 cc, which is the reason the numbers were higher than the should have been.... They are still very respectable, but that 1.74 FPE/CI I thought I had achieved is a much more believable 1.57 FPE/CI.... or thereabouts.... I won't have a final figure until I install the 22 CI bottle, of course, because that will increase the plenum from 108 cc to 460 cc.... I went back and replaced the graph above with a (more) correct version....  :-[

Anyways, on to todays exciting news.... First thing was I checked a "cold bore shot" of the .357 after storing over night at 3000 psi.... The gauge showed no indication of a pressure drop, and with a 131 gr. bullet the first shot was 914 fps.... I refilled it to 3000 psi and took another shot, and it was 927 fps.... That is only a 1.4% difference, and it is quite possible the gun may have lost 50 psi or less overnight.... So score another victory for the new valve.... stiction is non-existent....  8)

I cleaned the .224 and .257 cal. barrels, installed the .224 cal on the receiver, and made a tapered transfer port.... I used a milling cutter with a 7 deg. taper, and made a 7/16" OD port that is 0.328" ID on the bottom and 0.257" ID on the top.... I got the length right for a slight compression fit, and installed the .224 barrel and filled the gun to 3000 psi.... I still had the SSG gap set at 2 turns, loaded a 47.6 gr. bullet that was sized to 0.223", and took the first shot across the Chrony…. The resulting 1022 fps (110 FPE) put a pretty big grin on my face....  ;D …. That is more power than I have ever had before from the .224 cal.... The best I did previously was 106 FPE, but to be fair, that was with a 41.3 gr. swaged RWS bullet at 1077 fps.... Later in the day I tried one of those at this setting and it did (are you ready for this) exactly the same 1077 fps.... This pretty much shows that the gun is (and was) maxed out, because the most restrictive point in the porting system (the chamber where the 0.088" diameter bolt probe restricts the area to the equivalent of a 0.198" hole) hasn't changed.... More to come in a while....

Bob

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Re: Simplified Balanced Valve
« Reply #14 on: January 12, 2019, 04:39:11 PM »
So here is the meat and potatoes of today's testing of the .224 cal Hayabusa Mk.II based on the .357 cal. version.... Remember, all I have done is replaced the upper, nothing in the lower tube has been touched, and the SSG gap settings are still the same as on the .357 version.... I was using a 47.6 gr. "Bowman" bullet, which is a scaled down version of the Lyman 257420, made by Arsenal, which is designed to be stable in the 14" twist TJ's barrel.... With 2 turns of gap on the SSG it shot at 1020 fps (110 FPE) and peaked at 1035 fps (113 FPE) at zero gap.... Here is a plot of velocity and efficiency vs. SSG gap.... I had a usable tuning range of about 140 fps with this (admittedly oversized) valve in the .224 cal....



I also tested some lighter bullets, just a few shots to see what velocity they reached.... At 2 turns of SSG gap, the bullet I used previously in this gun, the 41.3 gr. swaged RWS that is virtually identical to a .22LR bullet shot 1077 fps (106 FPE).... I have some of the NOE version of the Lyman 225107 which weigh 38.3 gr. and they shot at 1116 fps (106 FPE).... and were just supersonic at zero gap.... I also have some of the swaged NAA 30.6 gr. bullets which are similar to a .22 Short, and they went 1195 fps (97 FPE), still at 2 turns of gap on the SSG.... The two cast bullets I tested I sized to 0.223", the swaged were nominally 0.224" and I did not touch them.... Now that I have NOE sizing bushings in various sizes, I find that the 0.223" chambers a lot easier than the 0.224".... At 0.225" you nearly have to beat on the bolt handle to chamber bullets in this barrel....

The next step was to see what kind of shot strings I could get at various SSG gap settings.... Remember, I only have a 108 cc plenum (120 cc including the valve, gauge and hose), so these shot strings are only about 1/4 as many shots as they will be once I install the 22CI bottle.... Anyways, here is the results, all shots over 96% of the peak velocity are shown....



At zero gap I have a declining shot string.... With 2 turns of gap, the first 2 shots were within 1 fps, and the velocity then dropped 2.5% for the 3rd shot, and the 4th was down more than 4%.... At 5 turns of gap, I was starting to get a nice bell-curve, the first 4 shots were within a 15 fps ES (1.5%), so the gun is tuned right at the top of the bell curve.... The average over those 4 shots is 983 fps (102 FPE)…. At 7 turns of gap the first shot was about 6% below the peak, so it isn't shown, and the bell-curve starts at 2850 psi.... 7 shots later the 4 % ES ends, the average velocity is 924 fps (90 FPE), and the pressure is down to 1870 psi.... That is nearly a 1000 psi pressure range within a 4% ES, which is very good.... With the 22 CI bottle installed I should get about 4 times the shot count.... although that may be slightly less as I expect the FPE to increase because of the higher average pressure during the shot with 460 cc on tap instead of just 108 cc.... The string I didn't shoot, at 6 turns of gap, looks to be the one that will start right at 3000 psi and still be within a 4% ES.... I'm guessing it would have been 6 shots at about 950 fps average (96 FPE) at around 1.25 FPE/CI.... If that comes to fruition, that will be more than a 50% increase in shot count from my previous .224 cal version, with the conventional valve....

I couldn't be happier with the results.... I had hoped I could just swap calibers without any other change, but there were times I thought that was a pipe-dream.... It turns out it wasn't a dream at all, but a reality....  8)

Bob
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