Simplified Balanced Valve



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Offline jackssmirkingrevenge

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Re: Simplified Balanced Valve
« Reply #620 on: February 18, 2019, 05:30:52 PM »
It was a guess on my part but on the other hand, if the thing is loose in the stem and there isn't a return spring, won't the rod move forward and backwards depending on the attitude of the rifle?  It seems to me that the de facto rest position would be determined by gravity more than anything else... or am I missing something?
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Offline rsterne

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Re: Simplified Balanced Valve
« Reply #621 on: February 18, 2019, 06:00:16 PM »
Could very well be.... I guess the only thing reasonably certain is that is would be forward during the early part of the shot cycle, when the balance chamber is filling, and back during the latter part when that pressure exceeds the pressure in the exhaust port.... I likely doesn't matter where it is between shots.... I still think describing is as a "leaky check valve" is the best way to think of it....

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline jackssmirkingrevenge

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Re: Simplified Balanced Valve
« Reply #622 on: February 19, 2019, 09:19:01 AM »
Something like that?

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Offline mackeral5

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Re: Simplified Balanced Valve
« Reply #623 on: February 19, 2019, 10:10:35 AM »
That is very much how I envisioned the rod functioning.

i have a question around running the Cothran with and without the rod.  I read Bob's previous posts around destroying a poppet without the rod.  I am curious as to why this happens with the Cothran vs. other balanced valve designs.  Is this due to the volume of the stem bore?  Could this be mitigated by decreasing volume of the stem bore, by reducing its diameter?

I have a Cothran with approximately 100 or so shots with the rod removed and haven't noticed any changes in performance.  It has a .032 stem vent.  What I do notice is a very nice snappy shot cycle and very consistent velocity.  It is also somewhat responsive to preload changes.  It is a relatively low power (55-70fpe) application with a 19" barrel.  With the rod it kept the valve open way too long and wasted a ton of air.  I've yet to have a reason to disassemble so I can't speak to the condition of the poppet seal, but its not leaking, no preload changes have been required, and velocity is consistent---at least for now. 
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Offline rsterne

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Re: Simplified Balanced Valve
« Reply #624 on: February 19, 2019, 12:49:04 PM »
The Cothran valve uses a very soft sealing washer, softer than Delrin.... It got a groove pounded into it in just a few shots with the rod removed, and Travis had the same thing happen I believe.... The poppet failure is likely related to the soft seal.... On the other hand, the low cracking force means a low sealing force, so a soft seal helps do that....

The lack of a tuning range is more than likely due to the large balance area of the Cothran valve, compared to the seat diameter.... The closer to zero force you get for cracking the valve, the more likely it is to "blow open" and provide only a plateau and cliff.... I was able to get some tunability with a Cothran valve, but it was accompanied by a very large ES when you operated close to the cliff.... I had some tunes that were only 100 fps below the plateau, but the shot to shot variation was 200 fps.... eg. plateau at 900, and when I decreased the velocity to about 800 fps, some shots were as low as 600 fps.... so in practical terms, that was not "tuneable", it was simply "unstable" operation....

I'm not saying that you can't get a Cothran valve to operate with an acceptable ES at 10% below the plateau velocity.... or get one to produce a bell curve.... just that I have not been able to do either....

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline mackeral5

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Re: Simplified Balanced Valve
« Reply #625 on: February 19, 2019, 06:34:09 PM »
Thank you for the additional explanation Bob.  So far my poppet hasn't shown any signs of failure, at least not yet.....I"ll be watching it.  Eventually i'm sure I will have to replace it based on your and Travis' experience. 
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Offline jackssmirkingrevenge

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Re: Simplified Balanced Valve
« Reply #626 on: March 13, 2019, 05:13:31 AM »
Some thoughts on this based on recent experiences:

- The vent hole from the balancing chamber to the transfer port is absolutely necessary if you don't want the valve to lock open from the overpressure on firing

- having the balancing chamber on a separate part isn't a terribly good idea for sealing because it's more difficult to guarantee that all parts are on the same axis inside a tube.  My latest prototype was built with the configuration illustrated in the first diagram, however I think the configuration in the second diagram would work better:

« Last Edit: March 13, 2019, 05:17:19 AM by jackssmirkingrevenge »
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Offline rsterne

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Re: Simplified Balanced Valve
« Reply #627 on: March 13, 2019, 11:47:34 AM »
The thimble (part that encloses the balance chamber) is mounted via a single screw at the front to a perforated wheel that allows air to enter the valve.... The mounting hole is oversize, to allow radial movement of the balance chamber, but I use a shouldered screw (SHCS with a spacer on it) to prevent more than a couple of thou of axial movement of the thimble.... This arrangement (which is also used by the Cothran valve) allows the thimble to self-align with the poppet.... Simple and effective....

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline jackssmirkingrevenge

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Re: Simplified Balanced Valve
« Reply #628 on: March 13, 2019, 12:02:04 PM »
Thanks for the tip Bob, I'm going to re-make the "thimble" and see if leaving it loose will solve the intermittent sealing issues I've been having.
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Offline skorec

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Re: Simplified Balanced Valve
« Reply #629 on: March 13, 2019, 02:20:56 PM »
With what we have learned since I introduced this thread, I thought I would put all the ideas together in a drawing, and add one thing I thought about a week or so ago.... Instead of having the thimble extend down over the OD of the larger part of the poppet, I wondered about shortening it and fitting a spring over the outside, instead of having the spring inside the balance chamber where it increased the volume.... Note that this has NOT yet been tried, but it should work just fine.... I also incorporated the idea of a shorter balance chamber, with enough depth to safely allow the poppet to travel 1/2 the throat diameter.... In addition, I shortened the stem, since it is no longer needed to create a spring seat on the front of the poppet, and drilled a hole in front of the stem, slightly larger than the vent diameter, to keep the resistance to the airflow filling the balance chamber, and the chamber volume itself, to a minimum.... This stepped hole helps avoid too little room between the OD of the stem and the bottom of the O-ring groove.... Here is the drawing with all those changes.... It is "generic" in that the proportions are basically correct, but no dimensions are shown....



I feel that the range of balance chamber diameters that will work OK are from 60% to 72% of the diameter of the outer sealing edge of the poppet.... With all the changes we have made, you might even get to 75%.... I would still use 71% as my goal, which means that the cracking force of the valve is reduced by half.... The back of the poppet is undercut slightly (about 5 deg.) to insure a narrow seal, right at the outside edge.... Here is what the cracking force is, as a percentage of what it would be without the balancing chamber, relative to the diameter of the balance chamber as a percentage of the seat diameter....

60% balance chamber diameter reduces the cracking force by 36%
66% balance chamber diameter reduces the dracking force by 44%
71% balance chamber diameter reduces the cracking force by 50%
75% balance chamber diameter reduces the cracking force by 56%

Here are some examples for the dimensions you might like to try for a valve for a full power .257 cal. bullet shooter.... (could also be used for smaller PCPs).

Throat ID 0.312"
Stem OD 0.125" (using 5-40 threads on the forward 1/4" to provide shear strength)
Vent Hole ID 0.052" (hole forward of stem 0.062" ID)
Exhaust Port ID 0.257"
Poppet OD at Valve Seat 0.375"
Balance Chamber ID 0.250"
Balance Chamber Length (ahead of poppet) 0.170"
Poppet Travel Before O-ring Bumper Contact 0.156"
Stem Protrusion Behind Valve Body 0.150"
The length of the larger part of the poppet would be about 0.25"
The length of the smaller part of the poppet would be about 0.50"
The front OD of the thimble is 0.250" and threaded inside 8-32
The overall length of the thimble would be about 1.00"
The center of the perforated mounting wheel is drilled 0.187" and counterbored 0.281" for the front of the thimble
This assures a loose fit between the thimble, thimble screw, and the supporting wheel to allow some radial movement for self-alignment
The thimble is secured to the perforated wheel with an 8-32 screw cut to length to give minimum end float when tightened
The OD of the thimble, the poppet at the spring seat, and the spring would be about 0.42" minimum (matching the spring OD)
A good spring would be the Century #71332, which is 1"LOA, 0.42" OD, 0.038" wire, 0.344" ID, 11 lb/in, and 0.26" length at coil bind
Nearly identical is the McMaster Carr # 9657K307, same specs except the wire is 0.039", 0.342" ID, and 0.29" length at coil bind
The ID of the valve would be about 0.625" minimum, and larger is better, to provide double the throat area past the poppet, thimble and spring
You would want 8 holes of 0.156" min. ID in the front mounting wheel, to provide double the throat area into the valve
That means the mounting wheel needs to be 0.75" OD or larger, it is retained with a circlip
That circlip groove OD in turn dictates the minimum OD of the valve body, in the case of a 0.75" front wheel, the valve must be at least 0.875" OD
The overall length of the valve would be about 2.75"

I hope this will give you a basic "road map" to follow if you want to try a balanced valve....

Bob

Bob, nice work as always .
Am I  right if I am thinking that larger vent hole and smaller balanced chamber increase velocity of valve closing ?

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Offline jackssmirkingrevenge

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Re: Simplified Balanced Valve
« Reply #630 on: March 13, 2019, 06:26:08 PM »
Mr. Sterne I owe you a beer, I machined a looser fitting cup at it worked like a charm!  I was focusing too much on the poppet itself and neglecting alignment as the primary cause of my woes.  Now it seals on the first stroke from a hand pump.
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Offline rsterne

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Re: Simplified Balanced Valve
« Reply #631 on: March 13, 2019, 08:16:51 PM »
Peter, I don't think you can state that directly.... What they do is prevent the valve from blowing open as quickly/easily.... and make the valve more "tuneable".... If you have no vent hole, the valve would blow open and stay open until the reservoir was empty.... As you make the hole larger and the balance chamber smaller, it takes less time for the pressure in the balance chamber to rise.... That increased pressure make the valve act more like a conventional valve.... If you did nothing to the hammer strike, then indeed the valve would have less dwell, and likely less lift as well.... You would definitely get less power from the gun, because less air would be released.... However, this allows you to add hammer strike to get the dwell back, making the valve act more like a conventional valve once it is cracked.... and giving you a larger tunable range....

Jack, sometimes the simplest solutions are the best ones....  ;)

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline skorec

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Re: Simplified Balanced Valve
« Reply #632 on: March 15, 2019, 11:48:57 AM »
A bit different approach, poppets I can make - stems I need to buy as spares:

balanced-valve by abbababbaccc, on Flickr

After making that hole through the poppet I can say that it would be easier to file a flat on the stem.

I also want to make venting hole easier way ( channel across the stem M3,5 spire )  see my picture.
https://www.gatewaytoairguns.org/GTA/index.php?topic=155953.0

But I am  not sure if  air turbulences  across the M3,5 nut  spire will not delay filling of balance camber too much.

« Last Edit: March 15, 2019, 11:54:13 AM by skorec »
  • EU, Slovakia
AA S510 Carbine .177+ Delta Optical Titanium 6-24X42 AO,
Combo (CO2 and PCP ) QB78D .177 + Hawke Vantage 4-12x40 AO,
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