Donation Donation
All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"
Simplified Balanced Valve
rsterne:
You are a decade too late to take part in those discussions.... Here is a simple chart from 2012 showing the relationships, developed using Newtonian mechanics.... It was presented to many, including Steve in NC on the Yellow and Green forums, and nobody (until you came along) disagreed with the science behind it, nor the real-world observed effects....
V is the residual hammer velocity after the valve is cracked off the seat, and M is hammer mass.... The closing force on the poppet is assumed to be a constant.... That force slows the hammer/poppet combination and returns it to the seat, with the time required to do that being the dwell....
Bob
Motorhead:
--- Quote from: rsterne on April 17, 2023, 02:34:30 PM ---Thank you for destroying a very valuable thread with your rant.... >:(.... I see that your "Watched" label is still required, something I will be requesting....
I, Scott, Travis, Lloyd, and many others have built way more custom PCPs, valves and hammers than you could ever hope to achieve, There is your middle ground.... Accept the fact that you are not always right, or leave the Workshop, please....
Bob
--- End quote ---
Fancy charts, talking louder, slighting others work etc .... THIS THREAD should be LOCKED.
If Matt feels he has something to teach us all, Let him create his own thread and then see who contributes or pays attention.
rsterne:
One more chart, for those who do not understand what I mean by flow limiting (the "curtain effect") when the valve lift exceeds 1/4 of the seat diameter.... The flow through the valve is represented by the area under the curve....
This occurs because the throat area is PI/4 x D^2, while the area between the poppet and the seat is PI x D x L, where L is the lift.... Those areas are equal when....
PI x D x L = PI/4 x D^2 .... simplified to D x L = D^2/4 .... which is L = D/4
This is a well known effect in any poppet style valve (eg. auto engines), and part of camshaft design....
I have done a lot of direct measuring of valve lift, and have found that in efficient PCPs (1.0 FPE/CI or greater) the lift seldom exceeds 1/4 of the diameter.... However, if that is not the case, which could easily occur in a balanced valve, then the only way to vary (or maintain) the area under the curve is to work with the dwell.... If you fit a lighter hammer, you need to increase the spring force and/or travel to get back to the same dwell, and hence the same area under the curve, and FPE of the PCP.... While the lift may be changing (or not, if there is a mechanical stop), you don't see the effect, because of the flow being limited by the curtain effect....
Bob
oldpro:
--- Quote from: Motorhead on April 18, 2023, 09:36:33 PM ---
--- Quote from: rsterne on April 17, 2023, 02:34:30 PM ---Thank you for destroying a very valuable thread with your rant.... >:(.... I see that your "Watched" label is still required, something I will be requesting....
I, Scott, Travis, Lloyd, and many others have built way more custom PCPs, valves and hammers than you could ever hope to achieve, There is your middle ground.... Accept the fact that you are not always right, or leave the Workshop, please....
Bob
--- End quote ---
Fancy charts, talking louder, slighting others work etc .... THIS THREAD should be LOCKED.
If Matt feels he has something to teach us all, Let him create his own thread and then see who contributes or pays attention.
--- End quote ---
Matt had to take a vacation. David took him to the train station ;)
mackeral5:
I've shared this elsewhere but since this thread has seen little activity as of late decided to add it here....
In toying around with Cothran-based versions of the simplified balanced valve I was able to achieve good results and with a bit of trial and error on vent size could tune for fairly tight ES across a 1000-1200psi pressure range. The key was placing the vent at the base of the balance chamber, in turn reducing volume and allowing dwell to be more responsive to vent size.
The later developments of this thread took a while for me to digest, but once I did I implemented some of it on my Bulldog's Cothran-based valve. The key change from my previous implementations of this valve is the variable balanced chamber volume achieved by adjustable piston height, and the piston being allowed to move away from the valve seat with the poppet when less blow open is desired.
This was the final version of the valve components, here you can clearly see the modified Cothran poppet and balance piston. Piston height is adjusted by loosening the 6-32 nyloc serving as a jam nut screwing the 6/32 screw in or out of the Cothran piston.
Piston height can be adjusted in from an extreme of blowing open like a Cothran, with very little tuneability, to the other extreme of almost no balanced chamber volume and closing very much like an traditional knock open valve.
Balance ratio was not defined by any desired specification, it would be whatever the available combination of parts would yield as I do not have the ability to fabricate such precise components. The delrin poppet sealing washer has a sealing margin of approximately .420 over the .375 valve throat. Piston is .250".
The vented disc/bushing/spider whatever we want to call it rests in the Bulldog's tube adapter. As Matt tried explaining to me on many previous occasions, there is no requirement to firmly attach this to anything. the same closing force that is removed from the sealing margin of the poppet is transferred to the piston, thus the vented disc will stay located rearward as long as the gun is pressurized. I put an oring around the disc to keep it from rattling around loose, should the gun depressurize. Filling from empty is a simple process, to date I have experienced no problems with this particular detail.
A bit of damage from the development process.......
Here the closing forces sheared a piece of threaded brass tubing I used in the first version, as a threaded coupler, to facilitate piston height adjustment. In the current version the brass tubing is just a sleeve, a bearing surface if you will. The 6/32 ss screw is solid and piston height adjustment occurs in the piston itself.
Here the stem sheared where it threads into the balance chamber of the Cothran poppet. Please understand this is not any indicator of lack of robustness of the Cothran parts. These parts have seen thousands upon thousands of rounds of abuse and in conditions they were never designed for. I take full credit for the demise of this component.
Based on all of my reading around balance valves and tuning, some type of adjustable valve lift is good to have.
This came in the form of a brass cored MDS hammer with peek striker. Here are a couple versions of the hammer as well as the crudely made proof of concept/prototype.
The Bulldog valve stem sits inside a valve body retention nut, the protruding striker is required due to this. Adjusting striker protrusion reduces/increases hammer induced valve lift. Unfortunately this arrangement also varies hammer travel, so the adjustable striker is not influencing valve lift solely in the area of hammer induced lift, but also hammer strike due to increasing/decreasing hammer travel....
Pics of a spare bulldog valve body/retention nut, showing why the protruding stem is required. Note this particular peek poppet has an oring, it is for a cobra chamber, another valve configuration developed during my bulldog journey. IMO 3-5 shot for unregulated, max energy applications, this valve/poppet is hard to beat. Swap barrels/bolt probes to .357 and to a 2024t3 unregulated tube and it will do a 3 shot tune a little over 380fpe on 3500psi. Not the most pleasant shooting gun at this tune, but dialed down to 350fpe it is a sweet shooter.
Later I drilled the hammer side of the valve retention nut with 6 holes and inserted/glued cut sections of 90 duro -118 oring. the rubber protrudes approximately 1/16 from the nut's face, serving as a hammer buffer. I tried the bstaley oring buffer, but I cannot get those to stay in place....at some point I will try to get pics of this area....
Navigation
[0] Message Index
[*] Previous page
Go to full version