GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on February 25, 2014, 02:07:46 PM
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I've always been curious about the affect changing the valve spring has over the shot string, in particular how long it is in terms of the pressure range.... This has been brought to a head with my FD-PCP which only fills to 1500 psi and only has a 400 psi usable pressure range, which severely limits the number of shots.... I tried a lighter spring and it didn't do any good.... so then I tried the bstaley O-ring buffer between the hammer and the valve at while it dropped the power, as I expected, it doubled the length of the shot string and sent the efficiency through the roof, to 1.57 FPE/CI in a .22 cal at 20 FPE.... However, it still only extended the pressure range to just under 500 psi....
bstaley made a comment which got me to thinking we need to discuss this.... He views the buffer as a VERY STIFF AND PROGRESSIVE valve spring, and it makes a lot of sense to think of it that way.... He reasons that this prevents the valve from opening as much at the middle to low pressure end of the curve, flattening the shot string and extending the pressure range.... I did some experiments using it on a Disco and the results were poor, unless you were drastically detuning the gun, but it is very successful on an MRod.... One of the differences is that the MRod operates at 3000 psi and has a 1/8" stem, while the Disco operates at 2000 psi but has a 5/32" stem.... Therefore the MRod has a much smaller closing force on the valve, relative to the opening force.... The FD-PCP has only a 0.118" stem.... Could the small stem require a stiffer valve spring?.... and work better with a bstaley buffer?....
Bob
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Somehow I think this is going to get interesting... lemme grab a bowl of popcorn and settle back to learn something new! :P
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That was a thought I had about your clipped 2200 spring. When you posted the results, I was tempted to ask "why not use the whole spring to bring the power down?" Anybody know where to get tapered wire springs?
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The whole spring was too long to fit inside the valve.... and I was trying to duplicate the spring force of the stock setup in any case....
Bob
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I always like to run relatively light valve springing so the valve head size and pressure will be the dominant forces deciding the return forces. We want to have a gun that slams shut quick when at the higher end of the pressure spectrum so it runs a low volume of higher pressure or shut slow with lower pressure at the lower end.
I don't think I have noticed the Stem size do anything but restrict flow more the bigger it is. Running small stems and opening up the Porting to accommodate the larger ones is a good way to go so they are not the most restrictive part of the flow. With a 5/32" stem it can easily be your throttle and I like to have the throttle where it is affecting straight(less chaotic flow)line flow at the Bolt probe or transfer port restrictor.
If you get the flow dynamics right the spring is there to hold the valve shut in the absence of any pressure and does little else but get in the way of the flow. For that reason alone I like thin wire so it affects the flow less. I see no reason on my systems to change hammer or valve return spring tensions no matter the build power. Hammer weight, port restrictor settings, valve head size and to a lessor degree hammer spring preload are the primary tuning parameters.
One tuning parameter that I've seen that few use is the length of the section of valve head major diameter. If I measure from the spring perch to the sealing point which is the large diameter of the exhaust valve head in my systems the gun likes far lower pressure the shorter that length is. BY making it longer it likes higher pressure and higher pressure has more energy stored so running higher pressure will generally yield better shot count at the same power. It won't necessarily extend the pressure range useable but it will increase shot count.
I like the flow and pressure to make the call as to how fast they slap the valve shut, not some heavy spring, fighting with the striker spring. In my mind heavy springs show someone gave up and stood on it to get what they want.
TimmyMac1
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I always like to run relatively light valve springing so the valve head size and pressure will be the dominant forces deciding the return forces. We want to have a gun that slams shut quick when at the higher end of the pressure spectrum so it runs a low volume of higher pressure or shut slow with lower pressure at the lower end.
I don't think I have noticed the Stem size do anything but restrict flow more the bigger it is. Running small stems and opening up the Porting to accommodate the larger ones is a good way to go so they are not the most restrictive part of the flow. With a 5/32" stem it can easily be your throttle and I like to have the throttle where it is affecting straight(less chaotic flow)line flow at the Bolt probe or transfer port restrictor.
If you get the flow dynamics right the spring is there to hold the valve shut in the absence of any pressure and does little else but get in the way of the flow. For that reason alone I like thin wire so it affects the flow less. I see no reason on my systems to change hammer or valve return spring tensions no matter the build power. Hammer weight, port restrictor settings, valve head size and to a lessor degree hammer spring preload are the primary tuning parameters.
One tuning parameter that I've seen that few use is the length of the section of valve head major diameter. If I measure from the spring perch to the sealing point which is the large diameter of the exhaust valve head in my systems the gun likes far lower pressure the shorter that length is. BY making it longer it likes higher pressure and higher pressure has more energy stored so running higher pressure will generally yield better shot count at the same power. It won't necessarily extend the pressure range useable but it will increase shot count.
I like the flow and pressure to make the call as to how fast they slap the valve shut, not some heavy spring, fighting with the striker spring. In my mind heavy springs show someone gave up and stood on it to get what they want.
TimmyMac1
I agree with your line of thought which is why I run my Evanix without a valve spring. What interests me is that length of the wide area of the valve seal. If you make it shorter, don't you also reduce the volume that the valve seal or head occupies? During my Evanix tuning efforts I noticed that one reason for low pressure inefficiency was the inadequate valve chamber volume and too small inlet. By opening up the inlet I got more air in and out at low pressure keeping the velocity higher. Now the same effect could be had by decreasing the volume that the valve seal or head occupies in the chamber, so could that be the reason for what you state?
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I always like to run relatively light valve springing so the valve head size and pressure will be the dominant forces ......In my mind heavy springs show someone gave up and stood on it to get what they want.
TimmyMac1
I agree with your line of thought which is why I run my Evanix without a valve spring. ...........Now the same effect could be had by decreasing the volume that the valve seal or head occupies in the chamber, so could that be the reason for what you state?
You had to ask why. I have to admit I get to a theory point and I'm reluctant to generalize. What I observe in my system is yet to be determined as a peculiarity of my valve design or it may be a tuning parameter for people who want to use higher pressure. I learn a lot when I make mistakes and I was trying to save some parts that had been made wrong and in the process I learned another tuning tool.
All I can say is more volume gives higher average pressures so it makes sense that is the valve is somehow being held open for a longer duration at high pressure because of the profile of the head in concert with the air flow charachteristics. It may have to do with the head mass and momentum but it seems to be related to the Fluid dynamics of a more abrupt convergence and divergence.
I don't know why and I can theorize but I have a repeatable tuning parameter and that is a desirable thing to have when I want a gun to like a higher or lower PSI. I have found many desireable charachteristics that we can use in tuning that are repeatable and work awesome so we keep executing the same type tune because it works, not because we understand why it works. That may take more science and math than I have in my kit. I just know I can get the same result when I do a certain thing. I don't need to know why. Just how.
The same result can come from lowering the valve spring tension as raising the striker spring tension so that is my choice normally. To me when you force a major striker spring in there you have backed yourself into a corner and need to have a heavy return spring as well. I find heavy springing to be the opposite of elegant/clever, as anyone can stand on it.
TimmyMac1
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The biggest thing I see an oring tune doing is increasing efficiency. The orings are very lossy compared to an air valve slamming shut so will reduce bounce energy in the hammer and also will impede the second strike from burping as much. It has proven to flatten the peak out of a string if you get it working right in your particular airgun but I don't see how it can help but always make the bottom end of the curve drop away too quickly as it does in my uptuned Crosman Challenger. One other comment I can make from my experience is that if you go too stiff with the orings you tend to get only a descending string with no flat spot. As you would expect if you were to fit a solid block instead of orings which takes away any of the self regulating aspects of the valve. I am using 2 90D orings in it right now but can't get a decent string so will end up taking them back out. I had 2 70D orings in there at first but wanted to try the 90's since I was told that stiffer is always better but it definitely not. I might cut the spacer that the Challenger requires to try 3 70D orings in order to keep my excellent efficiency. Or just go back to no orings since I am running the Altaros regulator.
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I am also getting a highish shot to shot deviation of 1% with or without the regulator which has me wondering if I am getting a consistent strike through the orings since the hammer has a slightly rounded corner. It might be wedging in more one time than another.
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Tim, I agree with your comments about the thick stem restricing the airflow in the valve throat, and find it better to slim it there.... However, the MAJOR part of the valve closing force comes from the pressure differential between the pressure inside the exhaust port and ambient pressure pushing on the back of the stem, so the diameter, or more correctly the area of the stem greatly affects the closing force for any given pressure.... to a much greater degree than the spring....
RE the O-ring buffer, my results with the Disco also favoured the three 70D O-rings, the stiffer/shorter I made the stack the less plateau I got.... However, I never had any success unless the buffer was used to drastically detune the gun (as I just did in my FD-PCP, from 32 FPE down to 20).... and then it was brilliant....
Bob
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so the diameter, or more correctly the area of the stem greatly affects the closing force for any given pressure.... to a much greater degree than the spring....
Right. The VOLUME of the larger valve stem is being forced back out of the the higher pressure area back toward the hammer.
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Personally have found outstanding results USING THE SPRINGS to set valve flow.
It is JMO that using a poppets head diameter within a confined area to control motion of the closing sequence is too limiting in power range for any application where one wishes too or requires dialing power up or down at will.
These type poppet/valve configurations are like being REGULATED working within a narrow range pending hammer strike & weigh applied opening them up. ( this is fine on a finely tuned FIXED power application tho )
My own theory and belief on subject is once a poppet is open and its seat flow area is nearing or exceeding it's throat minus stem area, Only pressure differential and flow around it effects if motion to get back on the seat. As it open area decreases so is the force it feels to get closed ... this a given.
The "HANG TIME" of the poppet being at max lift and flow IMO relying on air flow and pressure differential only causes excessive dwell.
The forces effecting poppets resistance to open .... Effort to lift it .... and hold it open have such skewed energy levels threw out this sequence.
In spite the math presented in other threads of the poppets spring force relative to the pressure exerted from the storage air passing threw valve is of a very low percentile, The "HANG TIME" where valve is at its most stagnate having a strong spring forcing closure eliminates this trend of valve being lazy to reverse direction.
Understand all this happens in fractions of a second, but the dynamics of cause and effect are too obvious to overlook IMO :o
My take on it ... nuttin more.
Scott
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Basically the valve spring needs to be in proper balance with the rest of the system. In the case of your FDPCP, the stock shot string was very peaky. This indicates to me that valve dwell is too sensitive to pressure changes in the reservoir to achieve good self regulation. The increase in dewll with each subsequent shot is more than is desired. There are of course several ways to approach the problem, but stiffening the valve spring is a fairly simple one. By using a stiffer spring you increase the ratio of the unchanging forces, due to the spring rate and pre-load, relative to the changing forces due to dropping pressure. This will decrease the sensitivity of the valve dwell to pressure changes.
There are of course problems with a stiffer spring, but they can be minimized by proper selection of spring rate AND pre-load. A stiff spring with little or no preload could change the rate of change of dwell relative to pressure, without adding too much to the initial opening force.
As I mentioned in the other thread, the o-ring buffer is similar in fucntion to a return spring, and works best in over powered/heavy hammer designs. One of it's main limitations is the high stiffness and progressive rate. This limits the increase in dwell possible over the string and is probably the main reason that it works better, the more it detunes the gun. Stiff return springs with little or no pre-load might be able to achieve similar results at higher power levels.
If only valve springs were easier to change, or there was a way to adjust their pre-load externally, it would be a lot easier to experiment with them. >:(
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If only valve springs were easier to change, or there was a way to adjust their pre-load externally, it would be a lot easier to experiment with them. >:(
Actually that would be very easy using the balanced valve design discussed elsewhere. The valve spring could be external to the valve, acting on the secondary valve stem where changing and adjustment could be made simple!