balanced valve goes full auto under pressure

[Rob M]

its 70 percent balanced.. the balance chamber is not leaking that i can tell , its designed like a blow open valve where atmospheric gas is vented to the hammer , not the transfer port.. I turn on the gas and bam bam bam , slow , maybe 2 shots a second , no trigger pull needed. is this simply gas pushing the poppet off the seat ?? then recycling the spool and doing it again?

[rsterne]

Not sure, would have to see the design....

Bob

[Rob M]

same basic design as the one i posted on GTA,with a tiny 4 mm stem as opposed to the 6mm in the picture  ill add link below .  i tested a few theories , the only thing that resolved the issue was unbalancing the valve by removing the spool oring.( obviously adding a 150 lbs of seating force. adding a heavier spring t the balance chamber didnt resolve it , it seemed to increase the cyclic rate.. anyway

https://www.gatewaytoairguns.org/GTA/index.php?topic=162137.msg155806905#msg155806905

[jackssmirkingrevenge]

Is the larger diameter of the stem the same as the inner diameter of the tube it's riding in?

[Rob M]

yes..( if you mean the very back of the valve where the stem rides )
4mm and 4mm reamed hole

if you mean the front balance chamber.. the spool area is .004 smaller than the hole its riding in , but it carries an oring which seals the atmospheric air

[Rob M]

i have an idea as to whats causing this.. and it may be lack of seating forces..
basically as the gas enters the valve it goes all directions.. including up the balance chamber where it hits the oring that seals off the atmospheric air from the co2.. well , that co2 pushing that oring creates a force.. ( whatever that may be even if its small) .. and that force exceeds the seating force of the valve.. this causes it to fire , and then the spring in the balance chamber compresses pushes back hard to shut the valve again.. and again it fires when the spring force returns to less compression.. causing a machine gun effect  where the gas fights the spring over and over.
just a guess i have no idea.
the problem with my theory is that the equal amount of force would be pushing on the poppet end also..

[rsterne]

When the poppet is open (not seated) there is no closing force on the poppet, just an opening force due to the balance chamber.... However I suspect you are correct, the issue is the poppet not seating 100%....

Having the vent hole through the stem all the way back to the hammer should make the valve dump, rather than have a closing force from the pressure inside it.... I think we talked about that before, no?.... I my Reply #6 in the previous thread I calculated that the chamber should dump until the pressure dropped to ~ 68 psi, at which point the spring would reclose the valve.... What I don't know is what mechanism is opening the valve in the first place, to start each cycle....

Bob

[Rob M]

When the poppet is open (not seated) there is no closing force on the poppet, just an opening force due to the balance chamber.... However I suspect you are correct, the issue is the poppet not seating 100%....

Having the vent hole through the stem all the way back to the hammer should make the valve dump, rather than have a closing force from the pressure inside it.... I think we talked about that before, no?....

Bob

yes.. i wanted it to dump the plenum, that was the plan ... but instead it fires full auto.. it does hold air and the poppet seats  when i remove the balance chamber seal.. only when in balanced config will it go full auto ..its just soo odd since im doing nothing to trigger the sequence.. were i firing in some fashion , then it happened , id have more theories. this just has me stumped.. my next experiment will be reducing spool diameter making the valve less balanced , perhaps 50% and see if it operates as expected

[Rob M]

the reason for the dump valve design is that i have a strong affinity for larger bore co2 rifles. were this a 177 pcp , id just follow the instructions shared on the forums and vent to the transfer port.

NOw that i reread what i said.. its back to what you said.. the lack of seating force on the poppet allows some gas to escape, lifting the poppet just a hair, allowing the gas force on the spool oring to start the full auto effect.

[rsterne]

Possibly the balance chamber diameter is just too large, and you don't have enough seating force to hold the valve closed and it's leaking, which reduces the closing force and the balance chamber overcomes the closing force and the valve fires.... Once the pressure drops to ~ 68 psi the spring closes it, the pressure builds until the poppet leaks again and it repeats.... The fact that it will work if you remove the O-ring kind of proves that, no?....

Bob

[Rob M]

Possibly the balance chamber diameter is just too large, and you don't have enough seating force to hold the valve closed and it's leaking, which reduces the closing force and the balance chamber overcomes the closing force and the valve fires.... Once the pressure drops to ~ 68 psi the spring closes it, the pressure builds until the poppet leaks again and it repeats.... The fact that it will work if you remove the O-ring kind of proves that, no?....

Bob

yes.. thanks BOB.. i had just edited my previous comment to say i think we found the culprit.. I will be reducing my spool diameter and increasing seating force tommorow.. thank you .

[lloyd-ss]

Rob, yes, it looks like your design has the balance forces on the hairy edge, and a very slight leak past the seat upsets that balance. So your valve has  a special "feature" in that it can also go full auto, LOL.

Here are a couple of suggestions that might help modify the special "feature." They are both easier than making new parts, but the new parts might be necessary anyway.

Is there a pressure, either high, or low, at which the valve becomes more stable and doesn't go in auto mode so easily? I am assuming full auto is what has sometimes been called "machine gun?" First, try lapping the plastic valve against the seat. (I sometimes use automotive paste Rubbing Compound, which is coarser than Polishing Compound, but finer that regular valve lapping compounds. That might prevent the low pressure leak past the seat and thus stop the self-initiation of the auto mode. But the balance will still remain on the hairy edge. (what does hairy edge mean, anyway?)
I also see a short stiff spring in the picture in the valve build thread you linked to. Maybe going  a little stiffer on that spring will increase the static seating force just enough to make a difference. Sometimes, making that spring that it goes to coil bind at the desired valve opening gap can be helpful. Like an over-travel limit.

Changing an air passage diameter will sometimes change the speed at which the state of the valve changes. I have tried putting a piece of wire in a hole to reduce the dia, or peening the entrance to the hole with a sharp center punch. You are probably pretty close and just need a tweak in the correct direction.
Lloyd

[Rob M]

thanks lloyd.. I did a little process with the poppet being held in the tailstock.. then , with the valve seat in the 3 jaw chuck i lubed the valve seat, turned on 600 rpm and gradually worked the poppet into the seat.. It makes a good seal in the non balanced state, but i think seating force is a hair too low .. Part of that is the spring force returning the poppet to closed  as you mentioned ,, and part of it i suspect is that in a lower pressure application Im on the brink of over -balanced. In other words, the gas seating force on a valve is influenced by the pressure of the application.. So co2 acting on that small unbalanced area is going to create less seating force than even a regged pcp running on E at 1400 psi .. Ive been trying to get the best of both worlds essentially.

[rsterne]

IMO, you have to be careful using the valve spring, or any other feature acting on the plastic poppet head, to limit the valve lift.... I have had poppets that stopped solidly fail by having the stem shoved through the plastic, or break the plastic, from the hammer impact.... B50/B51s were infamous for having the poppet shatter if fired with no air pressure in them.... The culprit was a combination of the valve spring going coil bound and the hammer trying to keep moving, causing the stem to break the poppet where the diameter reduced at the spring seat....

Bob

[lloyd-ss]

IMO, you have to be careful using the valve spring, or any other feature acting on the plastic poppet head, to limit the valve lift.... I have had poppets that stopped solidly fail by having the stem shoved through the plastic, or break the plastic, from the hammer impact.... B50/B51s were infamous for having the poppet shatter if fired with no air pressure in them.... The culprit was a combination of the valve spring going coil bound and the hammer trying to keep moving, causing the stem to break the poppet where the diameter reduced at the spring seat....

Bob

Bob, I agree 100%. I should have been more clear in stating that the coil bound condition, could be used (with caution), to stop an over-travel condition, where the valve stem is all the way into the valve body.

[MJP]

Flow over the poppet head aids the closing, not just the spring.

Marko

[anti-squirrel]

The scenario here is exactly how a Vacco reducer lift valve will fire- picture n automatic shotgun basically going "Brah-ah-ah-ah-ah-ah" at about 135 decibls.  Basically, one the valve unseats, the re-seating pressure is such that the poppet "bounces" one side to another until pressure drops enough on one side so the rebound doesn't lift.  The momentum of the poppet is the balance point.  Bob, Lloyd, Rob and all the others know all the trickery involved to really tune.  I do some basic stuff but I'm at the point where air an CO2 "mostly stock" meet my needs.

I think there's a fine line with balanced valves- a range of mass, if you will- where things can go awry.

[Rob M]

The scenario here is exactly how a Vacco reducer lift valve will fire- picture n automatic shotgun basically going "Brah-ah-ah-ah-ah-ah" at about 135 decibls.  Basically, one the valve unseats, the re-seating pressure is such that the poppet "bounces" one side to another until pressure drops enough on one side so the rebound doesn't lift.  The momentum of the poppet is the balance point.  Bob, Lloyd, Rob and all the others know all the trickery involved to really tune.  I do some basic stuff but I'm at the point where air an CO2 "mostly stock" meet my needs.

I think there's a fine line with balanced valves- a range of mass, if you will- where things can go awry.

good news is now if i ever want to make something full auto , ive found a good formula (-'

[jackssmirkingrevenge]

good news is now if i ever want to make something full auto , ive found a good formula (-'

sounds good, doesn't work 

[Rob M]

good news is now if i ever want to make something full auto , ive found a good formula (-'

sounds good, doesn't work 

i know youve  built some full auto designs , ill take your word for it.