Donation

All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Simplified Balanced Valve

<< < (149/166) > >>

PikeP:

--- Quote from: rkr on December 28, 2021, 02:37:16 PM ---
--- Quote from: PikeP on December 28, 2021, 11:37:27 AM ---Fully balance the valve, use minimum spring rating with a softer material to achieve your seal (this would take trial and error as it depends on material used, sealing surface diameter, and spring used)

Then play with larger valve stem sizes, since that is, as you put, primary closing force (which it is in smaller calibers that move little air). If you're talking about a 10-30 fpe rifle with a balance valve, opposed to where they're really needed (150+ fpe rifles), the air movement through the valve is far less, and not as concerning.

Essentially, no one has the answer except trial and error. If you have 120 lbs holding a poppet shut right now, and 25 lbs of closing force,
 your hammer strike is absorbed by both, but no one has a true answer as to what % of that strike is left fighting the 25 lbs of closing force in a very small caliber, probably marginal when compared to the 120 initial #'s holding the valve shut...introducing a larger valve stem diameter thru the valve body would re-introduce hammer strike sensitivity.

Is it possible? Sure. Is it practical? Maybe? Does anyone currently have all the answers? No.

My 100# recommendation at pressure holding a valve shut is mostly for practical purposes and safety precautions.


You also have to consider, if you make a valve open too easily, the hammer strike needed to open it becomes, slow, lazy, weak, and with that more sensitive itself to variation, this is where you find yourself only lightening the hammer itself while keeping the same spring rating, and the end result? A ms or two shaved off hammer lock time which is already in the sub 10 msec range....

Also consider, what is the lightest hammer + spring you would want in your rifle. IMO its vital to find the ideal combo here, where the hammer is not too light, nor heavy, and the spring not too stiff, nor too light... I have all the formulas to approximate that personally, which makes it easier to have my perspective. If one wanted to do a lot of work for near nothing, I suppose they could run a short throw, stiffly sprung, light to medium weight hammer to achieve an incredibly low lock time with a near fully balanced valve, seems like a lot of work to me to cut off a few MS of hammer lock time though...so, to me, that is about the only reason to go this route...


--- End quote ---

A light hammer spring would be good for my build, I was thinking of converting one of my BSAs to a balanced valve and to use this "Caselman" principle for cycling: https://www.gatewaytoairguns.org/GTA/index.php?topic=155499.0

--- End quote ---

Very interesting and I can see a very light hammer strike benefiting a semi auto action, but again be mindful of what MJP stated about being safe and covering any bump fires or odd scenarios that could cause a mis-fire on a valve thats too easy to open.  I haven't done testing in that department and can only give a number I've safely lowered to which is that 100# mark, maybe others have gone further?

PikeP:
Another thing to consider is the fill rate, depending on the balance ratio, the higher you go the larger vent hole you'll want to help increase your balance chambers fill rate. (larger vent hole = better imo)

Ultimately that is what allows this balance valve to be adjustable via hammer strike, is countering the balancing via fast enough fill rate to return to 'conventional' mid valve opening.

A fully balanced valve with just a strong spring should theoretically still be tunable via hammer strike with a large enough vent hole, because the valves "tunability" occurs after the poppet is cracked off its seat, which takes a fraction of ms compared to the 1-2 ms of dwell you obtain while off the seat and decaying residual "tunable" hammer energy via fill rate. I still believe fully balanced valve with a heavy spring would be quite inefficient compared to one that's balanced 'just right' or compared to conventional, better to use the air that's already present then a stronger valve spring.

sb327:
It is not the thread to discuss it but the pilot valve I’m using in my l2 inspired build is super easy to open and showing tunability with the venting. And it is easy to make with an inherent stictionless aspect.

Dave

mackeral5:
Long-ish read, but the primary subject is balance chamber vent sizing.

I have resumed use of my Cothran-Based simplified balanced valve conversions.  These involve converting to a delrin valve seal material and increasing stem vent diameter. 

A key learning....

The large diameter of the Cothran stem hollow effectively adds excess volume to the balance chamber.  The "vent" is at the far end of the hollow stem, creating somewhat of a plenum.  I believe this contributes to a very narrow tuning range.  Setup like this the valves would be very easy to open, very snappy shot cycle.  However they wanted to run where the wanted to run so to speak----they would only function efficiently, without bounce in a very narrow range of hammer strike.  If the output yielded desired energy levels--awesome, I got lucky.  If the output did not achieve desired energy levels it became a frustrating experience of trying to force something to do what it doesn't want to do. 

I can be extremely hard headed, so this part of the journey was long and wasteful.  A brief period of using peek seats (bad idea!) Lots of nitrogen and lead.  A decent amount of damage ranging from sucking the valve spring out the valve exhaust, wrecking the valve seal beyond repair to destroying a light kit and chronograph with a tank dumping machine gun.  There were many more, but those were the highlights that I wish were on video.

I stumbled across the "fix" by accident.  Without going into a long drawn out story, I ultimately moved the vent to the top of the stem, to the base of the balanced chamber.  I did this by plugging the upper end of the stem and redrilling it, creating a jet/vent of sorts.  This mod seems to be key to tuneability. This particular combination allowed a broad tuning range, using hammer strike, and could be shifted up/down in energy like a normal unregulated knock open valve. 

There was a sweet spot of hammer strike where the valve became almost self-regulating, resulting in a very flat bell curve across >1kpsi of pressure.

All of my previous testing/development was with short barrels---16", 12", and 9" in a .357, at energy levels of 50-100fpe-ish.  Almost like the big bore .357 equivalent of a 12fpe .177.  All that was needed was a very short burst of air.  For this testing I ran a relatively large vent of 1/16 or a bit larger. 

Recently I have begun using these valves with longer barrels---in excess of 24".  The current project is a .30 Brod.  My current poppet modifications do not provide enough valve dwell.  I can create a very efficient tune, but it is at relatively low energy levels, those that you would expect out of a well tuned short barreled pcp, not taking advantage of the longer barrel.

I believe I need to decrease stem vent size and will soon test that. 

Bob, Scott, others that have real world experience-----anything to add?  thanks in advance.

MJP:
Smaller vent to get more dwell, or larger volume in the balancer with same port size.
Transfers can be bigger than caliber, no matter what some say about wasted volume.

Marko

Navigation

[0] Message Index

[#] Next page

[*] Previous page