Simplified Balanced Valve

[PakProtector]

Getting the stem vent hole close to the poppet face gets a shorter path down the stem....so on a resistance per length affair, it is absolutely directionally correct. I am thinking about a vent hole diameter under the poppet face from what I recall of the benefits of exhaust port shape and usual modification.
cheers,
Douglas

[rsterne]

Glad to see lots of activity on this thread.... I have similar problems converting back and forth between Metric and Imperial measurement, as although we are "officially" on the Metric system in Canada, our proximity to the USA requires a complete and thorough knowledge of how to convert between them.... I admit, having been brought up and schooled using Imperial measure that it is my default system.... I often wonder today how many people even know how to work with fractions.... Or, with cursive writing now left out of the Curriculum in schools, how many of the kids today will even be able to sign their name to legal documents.... I can just see marriage licenses and land deeds with "X's" on them, co-signed by a witness with the words "His Mark" above and below, like was common 200 years ago.... *end rant*…. 
 
Back to the topic at hand.... Yes, there is a huge mass of air behind the bullet.... At 3000 psi, air weighs 60 gr. per CI.... The 28" barrel of my .357 cal, at 3000 psi, would contain (.357^2 X PI/4 x 28) x 60 = 168 gr. of air, more than the bullet I was launching.... Basically, there goes half your input energy, or more, right there.... Speaking of which, have any of you seen the claim of 2900 fps with a 144 gr. roundball launched from a Texan with the assistance of a vacuum drawn on the barrel....



Nearly 2700 FPE of energy in the bullet, from a 3' barrel in .457 cal.... would require about 5500 psi input with NO losses, including no energy lost to the propelling air.... Yeah, I have my doubts.... 

Bob

[rkr]

Getting the stem vent hole close to the poppet face gets a shorter path down the stem....so on a resistance per length affair, it is absolutely directionally correct. I am thinking about a vent hole diameter under the poppet face from what I recall of the benefits of exhaust port shape and usual modification.
cheers,
Douglas

From flow point of view that is a low pressure area. On the other hand having entry hole in the poppet face gives air flow much less sharp turn than a tight 180 degrees turn to the inside of the valve shaft. Rounding up the entry hole in the poppet face could help.

[Brian W Cook]

In regards to the video the guy could have put a bit of distance between the aluminum foil and the chronograph.  His numbers were kind of inconsistent. 

[MJP]

That was completely bs the whole video, not showing what he put in the breech and shooting way to close to the chrony.
If making a video claming something so extraordinary then show what you are doing, and not just some part of it. Who knows he could be shooting plastic balls.
780fps gun don't get to be 2900fps gun with some vacuum, not even if loading it with hydrogen. And that would be incredible dangerous and stupid!

Marko

[Hobbyman2007]

I’m having a hart time with the vent hole creating the issue of tunability. The Evanix airspeed is using a similar vent hole arrangement to cycle the action with a huge piston in comparison . If I was to guess the vent hole in it is .065” to .085” . I would be more inclined to have a closer look at flow past the poppet as MJP suggests . I’m going to be trying a valve with no vent to atmosphere and only a stiff return spring in the ballanced chamber  for testing .

[oldpro]

I’m having a hart time with the vent hole creating the issue of tunability. The Evanix airspeed is using a similar vent hole arrangement to cycle the action with a huge piston in comparison . If I was to guess the vent hole in it is .065” to .085” . I would be more inclined to have a closer look at flow past the poppet as MJP suggests . I’m going to be trying a valve with no vent to atmosphere and only a stiff return spring in the ballanced chamber  for testing .

 Prepare for the Big Bang I tried that and it didn’t go well.

[rsterne]

The Evanix doesn't need to cycle to 3000 psi in less than 1 mSec.… A small vent is fine if the time is available for it to build the pressure it needs.... JMO....

Bob

[Hobbyman2007]

Tried the no vent valve today ( mainly because I can’t drill a small enough vent lol) what happened was basically a cobra valve . Before the thimble I would get 12 shots averaging at 900 fps from 220 to 160 bar within 4% Es . With the thimble installed the velocity started much slower at 800 up to 930 fps at 175 bar and would go back down to the mid 800’s at 140 bar . That happened over 24 shots . Right off the bat I know the ballanced / easier opening is not working . As I lost nearly 100 fps at the same fill pressure. I will try a bit of preload and see if this will get me back to the same power and improve the efficiency . No Big Bang Travis but no big gains either . This is not the best platform to test with to start as it’s a side lever and I really don’t need to reduce the cocking force . Worst comes to worst , I gain some efficiency with a non vented cobra chamber .

[oldpro]

Tried the no vent valve today ( mainly because I can’t drill a small enough vent lol) what happened was basically a cobra valve . Before the thimble I would get 12 shots averaging at 900 fps from 220 to 160 bar within 4% Es . With the thimble installed the velocity started much slower at 800 up to 930 fps at 175 bar and would go back down to the mid 800’s at 140 bar . That happened over 24 shots . Right off the bat I know the ballanced / easier opening is not working . As I lost nearly 100 fps at the same fill pressure. I will try a bit of preload and see if this will get me back to the same power and improve the efficiency . No Big Bang Travis but no big gains either . This is not the best platform to test with to start as it’s a side lever and I really don’t need to reduce the cocking force . Worst comes to worst , I gain some efficiency with a non vented cobra chamber .

sound like its not balanced and you have basically a standard valve. I would drill the vent hole and see if you start leaking air.

[PikeP]

Added quite a bit. Now calculating some pressure drop during the shot and using the average pressure for flow into chamber along with other calcs as itll better model what's going on during the shot cycle with the balanced valve. Here is the updated one of Bob's current valve.

[rsterne]

Looks good, Mike.... I don't know where you are getting your valve lift figure from.... but it seems very low to me.... Measurements of several PCPs show the actual lift to vary from about 1/8 the throat diameter when sipping air to 1/2 the diameter when the valve is open until the pellet leaves the muzzle.... With a shot such as we are getting here, I would guess the valve lift is about 1/4 the throat diameter, or about 0.094".... plus or minus a bit....

BTW, the hammer travel available is 1.09", but the SSG propels the hammer less than that depending on the gap setting.... With 7 turns of gap (the greatest I can use and still get maximum velocity, and the best efficiency) you would subtract (7 x 0.050") from 1.09", so the distance the hammer is being accelerated is only 0.74".... The hammer can only drive the poppet stem 0.20" before it hits the back of the valve body.... However, if the valve is blowing open (which I suspect), it could travel a bit further than that before the poppet bounces the O-ring bumper against the inside of the thimble.... maybe 0.23", I haven't measured it precisely.... When I am building valves, I always allow the stem to travel slightly more than 1/2 the throat diameter to enable the gun to reach maximum velocity (valve open until pellet exits the muzzle)…. and I have yet to fail to reach the plateau (maximum velocity) doing that....

Bob

[PikeP]

Good to know. Those little details add up. Is the hammer spring exactly 11 lb/in and under zero preload when uncocked? I added a few changes that raised the valve lift to .076", still not quite where you have it but getting there

Once developed if this spreadsheet helps build a balanced valve I'll certainly release it here.

I added ideal barrel length and approximate valve close for the intended energy output for efficient conditions, the estimation was based off Bob's current estimation himself at the valve closing at 16"~ down the barrel.

The optimal barrel length is surprisingly easy to calculate for a given energy level. I tested it with various calibers at different power levels and it scaled surprisingly well. For example a 65 fpe 25 cal at 2000 psi is recommended a 23" barrel where as a 500 fpe. 45 cal at 4000 psi only needs  the same 23" of barrel. This calculation is based entirely on intended fpe, pressure, plenum size, and caliber. It does not currently take into account port size which is assumed to be suited for the caliber. I also calculate the optimal port size for a provided intended fpe at the given pressure, caliber, plenum and barrel length.

Also added optimal plenum size at current pressure and set point based on current plenum volume. Values are based on keeping pressure drop between 7% and 10% which I can certainly widen to 7% to 14% comfortably.

Also scaled recommended hammer spring ratings to stem size where as before it was scaled only to force holding poppet shut. Surprisingly I estimate the .1563" stem only needing 11 to 12 lb spring with 78~ grams of hammer. Will be interesting to validate or refine that calculation although my tests up to this point seem in line.

[rsterne]

Hammer spring is 10.9 lb/in. and the preload is 0.31" when uncocked, but mounted on the SSG.... I didn't measure the valve lift on this gun, but I have on some others, using a lightweight indicator rod through the RVA with a siding O-ring.... Unfortunately, you can't do that with an SSG.... The 0.094" is just a guess, your 0.076" is likely in the ballpark....

Lloyd's Internal Ballistics Spreadsheet, and my personal experience, dictate MUCH longer barrel lengths than you are indicating.... By optimum, I mean the barrel that can produce maximum FPE in the projectile for a given air release by the valve.... The expansion of the air after the valve closes can add a LOT of velocity, while adding to the efficiency as well.... all without using 1 mg. more air.... Compare the Texan with its 34" barrel to the Texan SS or Carbine with their 23".... HUGE difference in velocity with the same bullets....

Bob

PS - Additionally, I think you will find that the larger the plenum the less pressure drop at the pellet during the shot.... I can't see any reason for there to be an "ideal" plenum volume (ie where the pressure and/or FPE and/or efficiency peak.... Bigger is better, IMO, providing you don't lose primary reservoir volume to achieve it.... It's just that the gains above 1 cc/FPE are minimal.... Actually, the higher the overall FPE/CI achieved, the smaller a plenum you can "get away with", without losing too much power or needing increased plenum pressure....

When I remove my "tiny tank" and replace it with the 500 cc bottle, the reservoir/plenum volume becomes 460 cc (instead of 108 cc)….

RBS

[rsterne]

I DID IT !!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Adjustable, efficient, low hammer strike.... everything I wanted.... Details to follow....

Bob

[rsterne]

First of all, the changes I made, and final dimensions.... I pulled the valve and disassembled it and drilled the through vent out to 0.052 (#55 drill)…. that went really easy, no chance of breaking the drill.... I then drilled another cross vent closer to the seat of the poppet on the opposite side of the stem, and enlarged it to 1/16" and angled it about 30 deg. to ease the flow.... I left the previous 0.040" vent there, 0.1" further out.... The vent length from the large cross hole to the end is 1.0".... I turned the spring seat down a few thou to accept a spring made from heavier wire that was still 7/32" OD, but 0.028" wire and 1" long.... It has 5 lbs. of force when the valve is closed, and a spring rate of about 14 lb/in.... The travel depth available to the poppet is 0.250", but the O-ring bumper reduces the travel by 0.030" to 0.220".... However, the stem only protrudes from the back of the valve by 0.210", so the hammer cannot even drive the poppet far enough to hit the O-ring bumper.... The balance chamber is therefore 0.315" diameter (I measured it) and 0.250" long.... In addition there is a spring chamber 0.250" diameter and 0.40" long.... You can subtract from that volume the spring seat on the poppet of 0.160" diameter by 0.100" long, and the volume occupied by the spring, which weighs 0.72 g.... The poppet c/w O-rings weighs 4.32 g. so including 1/3 the weight of the spring, the poppet mass for calculations would be 4.56 g.... The gun was reassembled and tested for leaks, and had none....

Now to put all my results into a spreadsheet and produce some charts.... Stand By !!!

Bob

[mann]

Derlin finally made it so I'll try to make a new poppet tonight it will probably end up being .375 and .218 or what ever drill bit I have close to .210

[tkerrigan]

Good news Bob, I hope they end up being simple enough that I can make some, I have six DAQ pistols that are a bear to cock.  Thanks for the effort on this project, we really appreciate it.  Regards,Tom

[oldpro]

Bob here’s that simple balanced valve tune it was buried 50 pages deep into my tune sheets lol.

Created: 12/12/17 7:45 PM
Description: SS 2 .25 cal Mrod
Notes 1: 2700 tether poppet 250/352
Notes 2: 1/4 turns
Distance to Chrono(FT): 1.00
Ballistic Coefficient: 1.000
Bullet Weight(gr): 34.00
Temp: 59 °F
BP: 29.79 inHg
Altitude: 0.00
#          FPS   FT-LBS       PF
9     932      65.59    31.69   
8     561      23.76    19.07   
7     420      13.32    14.28   
6     446      15.02    15.16   
5     403      12.26    13.70   
4     336      8.52     11.42   
3     302      6.89     10.27   
2     283      6.05     9.62   
1     255      4.91     8.67   
Average: 437.6 FPS
SD: 208.3 FPS
Min: 255 FPS
Max: 932 FPS
Spread: 677 FPS
Shot/sec: 0.1
True MV: 438 FPS
Group Size (in): 0.00

[rsterne]

I can see why you gave up on it, Travis.... I'm glad I didn't.... 

With the changes detailed about, the valve is now adjustable, and incredibly efficient.... Here are the results, using the SSG with the 11 lb/in spring and 3.4 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.... 

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.... 

Bob