GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: sb327 on September 28, 2020, 11:09:25 AM
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I posted this in my sidelever breech thread and would like to break it off as it’s own question/discussion.
Some generalizations from reading about bv’s
1. They are more tune able by vent and bv chamber size than by hammer strike.
2. They work within a smaller tune window than hammer tuning but have the advantage of opening large valves with less hammer force.
3. Maximum poppet lift is dictated by travel within the balance chamber then hammer bounce/crash occurs.
4. A bv can ‘blow open’ and ‘hold’ open, but offers nothing to closing force
5. The tuning of the ‘hold’ open is what gives us dwell
With these things in mind, could we limit the travel within the chamber to just the cracking distance, run a large vent to speed the neutralizing of the bv, extend the poppet stem back out to allow hammer movement (dwell) AND MOST IMPORTANTLY ‘float’ the balance chamber not only a touch radially (for machining tolerance) but also allow it to travel with the poppet away from valve seat. If the bv gets negated as soon as valve cracks, then we might possibly have a larger tuning window by giving dwell control back to the hammer without altering the valve itself.
Dave
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I posted this in my sidelever breech thread and would like to break it off as it’s own question/discussion.
Some generalizations from reading about bv’s
1. They are more tune able by vent and bv chamber size than by hammer strike.
Primarily there change in power output by PRESSURE changes. over striking and too heavy of hammer creates over dwelling very quickly
2. They work within a smaller tune window than hammer tuning but have the advantage of opening large valves with less hammer force.
Not necessarily, they can be tuned over very broad range, but not by hammer strike alone
3. Maximum poppet lift is dictated by travel within the balance chamber then hammer bounce/crash occurs.
Lift can be controlled via HEAVIER poppet spring preventing blow open, Larger or Smaller venting path to the balance chamber along with not having excessive hammer weight as stated above.
4. A bv can ‘blow open’ and ‘hold’ open, but offers nothing to closing force
If balance ratio gets to lopsided they turn into a DUMP valve. Stem venting from valves threat pressurizes balance chamber and help shut the valve.
There is the KEY and trick to manufacturing and tuning a BV .... There are design parameters that must be in harmony for the "System" to work well.
5. The tuning of the ‘hold’ open is what gives us dwell
Not exactly a Hold ... but a stalling of the open / close cycle from the initial opening and final closing. A correctly built BV that is not operating as a DUMP valve, never a point in the cycle where poppet just "Hangs" and flows being always in motion on it's way to max lift and then returning towards closure
... This is how dwell is created by increasing or decreasing this time period.
If you truly have read many of the threads here on the GTA on the SIMPLIFIED BALANCE VALVE you know there is a whole lot more going on than simple cause and effect of what i have skimmed over in the above responses. A whole lot more !!!
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That’s why I said ‘generalizations’. I did read the threads.
I can throw out all the generalizations. No hard fast rules, I get that.
I guess my question should have been more direct.
Would allowing the balance valve to float once the crack has happened cause issues with closing?
Dave
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I don't think a balanced valve ever "floats", the poppet is always in motion, unless overbalanced in which case it becomes a dump valve.... The volume of the balance chamber and the vent size dictate whether and for how long there is an "opening assist" occurring.... Generally, the more opening assist there is, the less tunable the valve with hammer strike, and the more the valve acts like a Cothran where it has a plateau and a cliff only (at constant pressure).... This type of valve tends towards a declining velocity as the pressure drops....
The ratio of balance chamber area to poppet seat area governs how much the cracking force is reduced.... It is after that the vent size and balance chamber volume (and their relation to hammer energy and momentum) take over the rest of the valve cycle....
Bob
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I am referring to the balance chamber floating with the poppet once the crack has happened. Giving the poppet the ability to move the chamber out of its way after say..0.020” of movement. Or less even.
Basically, negate all the effects of the balance chamber EXCEPT the lowering of cracking force.
Then tune as conventional with hammer strike.
Dave
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Looking at your sketch, I could see that working if you had a small spring inside the balance chamber along with the spring you have drawn. If you can build it so it can be assembled and fitted in the valve could be difficult. I think I have seen a poppet design that had a smaller diameter section that opened a 1/32 before pushing the main poppet open that does the same as what you desire. Don’t know if has been used successfully or an idea.
It may be Bob’s idea ?
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Your sketch did not show earlier, which is why I did not understand what you were trying to do.... The idea the Charles referred to was discussed in another thread, not my idea but I had some input in the discussion.... Nothing ever came of it....
I am pretty sure that as soon as the poppet touches the inside of the thimble, the force on the stem goes from "reduced" for cracking to "normal" during the remainder of the shot cycle.... By that I mean the valve changes from balanced to conventional, which is I guess what you are after?.... Only a small amount of the hammer energy/momentum is used up by the cracking force, and the residual energy creates lift and the residual momentum creates dwell.... Therefore, you would only be reducing the required hammer strike (for a given FPE result) by a small percentage....
Don't forget that the force forward on the thimble while the valve is closed is equal and opposite to the balancing force on the poppet (so considerable, possibly hundreds of lbs.).... If the contact between the poppet and thimble occurs so early that the force on the thimble has not "disappeared" (by the HPA entering the balance chamber), whatever force that is left will decelerate (or even stop) the opening of the poppet....
Bob
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I see. So I’ll need to play with that distance a bit probably to get it right. That could be achieved by placing shim washers underneath the head of that screw up front.
Dave
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Assuming that you can flow enough air through the vent during the first 0.020" (or whatever) of travel to equalize the pressure in the balance chamber.... As an example, with a 1/4" balance chamber at 3000 psi, the force towards the seat on the thimble is 147 lbs.... Even if you got rid of 90% of that pressure difference, the thimble would "act" as if it weighed 15 lbs., which would drastically slow the poppet down when they collided....
Bob
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Yep, I can see that Bob. Thanks for pointing that out.
I may need to use two large vents if it’s ever gonna have a chance to work the way I would like.
Dave
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I am working on this idea here
https://www.gatewaytoairguns.org/GTA/index.php?topic=178164.msg156032715#new (https://www.gatewaytoairguns.org/GTA/index.php?topic=178164.msg156032715#new)
Dave