SS Valve Stiction



Author Topic: SS Valve Stiction  (Read 175 times))

Offline rsterne

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SS Valve Stiction
« on: December 24, 2018, 02:47:33 PM »
I think I have figured out a possible mechanism that is responsible for the "stiction" reported by some users of the SS Valve after it has been left to sit for a while.... Some have reported a low velocity on the first, or first few, shots after sitting before the velocity returns to normal.... In the SS Valve, the two poppet O-rings are loaded in opposite directions when the valve is closed.... The area between them is at atmospheric pressure, so the front O-ring is loaded towards the rear, while the rear O-ring towards the front.... When you fire the gun, the poppet moves forward, so the rear O-ring, even if it is "stuck" to the inside of the thimble, moves back into the center of the groove in the poppet.... However, the front O-ring, which is already jammed into the outer, rear corner of the groove, gets pressed even harder into that corner, like this....



The arrow shows the direction of movement of the poppet, relative to the thimble, on firing.... It doesn't take a lot of imagination to figure out what may be happening in this circumstance.... I think the front O-ring is wedging into the corner of the O-ring gland when first fired after sitting for a long time.... Any lube present could easily have been squeezed out from between the outside of the O-ring and the inside of the thimble from the pressure being applied over time, and in fact the O-ring over time could be taking a bit of a "set" into the corner....

After firing once or twice, with the pressure the same on both sides during the valve cycling, and the O-ring moving in the groove, the lube is redistributed, and the valve returns to normal operation until the next time it sits with air pressure against it for a prolonged period of time.... That's what I think may be causing the problem, anyways.... I figured this out while working on my new simplified balanced valve.... https://www.gatewaytoairguns.org/GTA/index.php?topic=152413.0 . in which I hope to have cured the problem....

Bob
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Offline oldpro

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Re: SS Valve Stiction
« Reply #1 on: December 24, 2018, 03:02:59 PM »
 Weve known that was the problem for awhile its just very hard to combat it and making a TUNABLE valve without two orings is impossible, sure We could make a non tunable balanced valve but Don Cothran already has that on the market with his power valve. Its not hard to tune around it in high power applications its usually noted in low power applications where the hammer strike cant over comb the stiction on the first strike.
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Offline rsterne

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Re: SS Valve Stiction
« Reply #2 on: December 24, 2018, 03:54:51 PM »
I'm surprised you never mentioned that the wedging of the front O-ring was the issue, it would have kept us all from scratching our heads.... Perhaps I didn't have that problem with my first SS Valves because they were only used at 2000 psi, and the 1.2mm CS Metric O-rings, being smaller, didn't wedge in the same manner?.... I'm pretty sure I am seeing it in my 500 FPE .457 Hayabusa, though....  :(

Lloyd's original balanced valve, with the piston, was tunable to a bell-curve, as is the valve in the Slayer.... so you don't need two O-rings for that.... I guess I'll see what happens with my new valve, your prediction is that it won't produce a bell curve then?.... and won't be tunable for velocity via hammer strike?.... Maybe I'm wasting my time then....  ::)

There is no wedging action of the O-ring on firing in any of the other balanced valves I have looked at, including my new one.... The static load from the HPA when the valve sits, is in the opposite direction to the movement of the poppet on firing, so even the first shot moves the O-ring back out of the corner.... The only one I see where the wedging happens is on the front O-ring in the SS valve.... Of course I only looked at a half-dozen different designs in my HAM article, there could be some I haven't seen.... The Cobra and Python valves (which are not balanced) have the same pressure on both sides of the O-ring, so no wedging action is possible.... ;)

Bob
« Last Edit: December 24, 2018, 03:57:17 PM by rsterne »
  • 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 oldpro

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Re: SS Valve Stiction
« Reply #3 on: December 24, 2018, 04:25:51 PM »
I'm surprised you never mentioned that the wedging of the front O-ring was the issue, it would have kept us all from scratching our heads.... Perhaps I didn't have that problem with my first SS Valves because they were only used at 2000 psi, and the 1.2mm CS Metric O-rings, being smaller, didn't wedge in the same manner?.... I'm pretty sure I am seeing it in my 500 FPE .457 Hayabusa, though....  :(

Lloyd's original balanced valve, with the piston, was tunable to a bell-curve, as is the valve in the Slayer.... so you don't need two O-rings for that.... I guess I'll see what happens with my new valve, your prediction is that it won't produce a bell curve then?.... and won't be tunable for velocity via hammer strike?.... Maybe I'm wasting my time then....  ::)

There is no wedging action of the O-ring on firing in any of the other balanced valves I have looked at, including my new one.... The static load from the HPA when the valve sits, is in the opposite direction to the movement of the poppet on firing, so even the first shot moves the O-ring back out of the corner.... The only one I see where the wedging happens is on the front O-ring in the SS valve.... Of course I only looked at a half-dozen different designs in my HAM article, there could be some I haven't seen.... The Cobra and Python valves (which are not balanced) have the same pressure on both sides of the O-ring, so no wedging action is possible.... ;)

Bob
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.
  • USA, CA, Mt. Shasta
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Evanix Speed .22
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Evanix max ML .45

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