GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: antithesis on May 21, 2020, 10:56:05 PM
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Now I'm sure this has been extensively covered before, and I'm sure it varies by application and numerous other factors.
But a link or some basic info would be nice.
In short, aside from making the valve close faster under low pressure situations and making it harder to close, what are some pros and cons to consider when deciding how heavy a valve return spring to use. I know it'll be easier or harder to crack, and can affect usable curve. But on an unregulated gun, basic crosman pcp setup, are there any consistency benefits, performance benefits, or efficiency benefits assuming I'm aiming for high fill pressure in an unregulated gun?
We all know a soft spring can make an easier open, could that contribute poppet or hammer bounce?
Just a few thoughts, but specific links or some sage like wisdom would be nice so I can make a decision and get this project moving finally.
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I’m on my phone now so I’ll be brief but a heavier valve spring has very little effect on how easy it is to crack the valve open (air pressure is dominant) but it does influence how quickly it closes.
That plays into the self-regulation characteristics of a conventional (i.e. unregulated) PCP, influencing how long a useful bell curve you will get. Heavier is better in this respect.
However with a regulated gun, the balance of lift and dwell do not need to adapt to a changing pressure so much, therefore you can pretty well use whatever spring you want.
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The valve opening speed will be the same with a light spring or a heavy spring, but the valve duration will be longer or shorter, depending on the spring.
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Since air pressure is closer to equal once the firing cycle starts, could it also be argued that a lighter spring may give the tune a preference for heavier pellets due to a longer lasting back pressure? I know that can be offset by preload on hammer but curious because I don't like using much if any preload
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In a regulated PCP, I see no reason to use a valve spring any heavier than what will allow you to fill the gun from zero.... and even that handicap can be overcome if you wish to run no valve spring.... When unregulated, a heavier valve spring will allow a wider pressure range to be achieved within a given ES.... Lighter valve springs make the valve easier to open (crack), and they also increase the lift and therefore the dwell for a given hammer strike.... It is simply a matter of how much hammer energy/momentum is required to overcome the force of the spring....
Hammer bounce is caused by the valve stem throwing the hammer back on closing and recompressing the hammer spring.... An SSG largely eliminates that effect.... The worst case for producing hammer bounce is very slight preload (hammer pressing on the stem lightly when uncocked).... If you combine that with a weak valve spring, when the pressure drops you are designing in the ability to machine-gun from runaway hammer bounce.... (yes I have done it)....
Bob
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Regarding how easy it is to crack open the valve, I mentioned earlier that the valve spring plays an insignificant role, that instead the air pressure is the dominant force. A quick calculation will give some perspective. Consider that most valve springs will measure in the single digit pounds. Contrast that with a 0.25" dia. valve seat at 2500psi operating pressure. That works out to 123 lbs of air pressure holding the poppet closed. Therefore a few pounds more from the valve spring has very little influence over how easy it is to crack open.
To substantially reduce cocking effort, the following options are considerably more effective than a weak valve spring:
1. A hard poppet material like PEEK. Less of the hammer energy is dissipated trying to "unstick" a compressible plastic.
2. Reducing the size of the valve seat. Not so easy to accomplish for the DIYer, and may unacceptably restrict airflow. However for valves with a broad flat seat, you can cheat so to speak and reduce the poppet diameter so it only seals against a small ring. May still need a harder material to keep the poppet from mushrooming or wedging down into the throat due to cold flow (plastic creep).
3. Installing a balanced valve. Tuning with a balanced valve introduces other complications and generally isn't worth the trouble with small calibers/typical pellet power levels.
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I disagree with regulated rifles not necessarily needing valve springs dialed in for them.
Just as much as a valve spring helps create a bell curve in an unregulated rifle, they can help just as much in a regulated rifle to help create a bell curve while coming off regulator. While lighter springs make it easier to open the valve, they create more sensitivity to variation in hammer strike due to increased difference holding the valve shut as pressure varies both while on reg (reg creep or variance just from how many air molecule packed into your plenum prior to regulator sealing off) and while off reg.
The higher your valve spring return rate the lower the difference in force holding the valve closed from shot to shot once pressure holding the valve shut drops. Although they have to be 'tuned' to operate correctly and create the correct lift over the pressure range...
Having a valve spring be around 10% of the force holding your valve closed seems like a healthy benefit that doesn't require much more hammer strike to overcome...just my 2c.
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Mike, thanks for adding those comments about the benefits of a suitable valve spring in a regulated setup. That's the sort of thing I had in mind last night when I replied "with a regulated gun, the balance of lift and dwell do not need to adapt to a changing pressure so much". Lots of things can cause the pressure to be different than intended...reg creep, temperature rise, recovery time, input regulation characteristics, etc., and of course what happens as the pressure begins to fall below the setpoint. Granted, for many uses these effects may not significant enough to notice so for the purposes of a concise answer, it's not unreasonable to ignore them. Regulators work pretty darn well most of the time, enough that they will usually give great results without having to obsess over the fine details but if you're interested in really optimizing things for the best possible pressure agnosticism, so to speak, it's useful to consider mitigating them. Simply having a modest valve spring is an easy way to help.
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Jason, you are of course correct, the spring is usually a small percentage of the total force holding the valve closed on the seat.... However, it is a much larger percentage of the CLOSING force on the poppet, once the valve is open and flowing air.... If, for example, your poppet has a 1/8" diameter stem, at 2500 psi (to use the same pressure as your example), the closing force on the stem area is only 30 lbs.... A Disco valve spring is 7 lbs on the seat, a bit more (7.5 lbs.? ) when open, so it adds another 25% to the closing force on the poppet during the shot cycle.... Doubling the spring force to 15 lbs would add 50% to the closing force on the stem, or 20% to the total compared to the 7.5 lb. spring.... which will cause an extension to the pressure range that can be used for the same ES.... The spring becomes a larger percentage of the total closing force if you increase it....
Mike, you are correct of course, a regulated gun running below the setpoint (unregulated) will have a different curve when off-reg. change if you change the valve spring.... However, it is then no longer regulated, right?.... Sorry I didn't specify a regulated PCP while running above the setpoint, but I though that was implied....
Bob
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Jason, you are of course correct, the spring is usually a small percentage of the total force holding the valve closed on the seat.... However, it is a much larger percentage of the CLOSING force on the poppet, once the valve is open and flowing air.... If, for example, your poppet has a 1/8" diameter stem, at 2500 psi (to use the same pressure as your example), the closing force on the stem area is only 30 lbs.... A Disco valve spring is 7 lbs on the seat, a bit more (7.5 lbs.? ) when open, so it adds another 25% to the closing force on the poppet during the shot cycle.... Doubling the spring force to 15 lbs would add 50% to the closing force on the stem, or 20% to the total compared to the 7.5 lb. spring.... which will cause an extension to the pressure range that can be used for the same ES.... The spring becomes a larger percentage of the total closing force if you increase it....
Mike, you are correct of course, a regulated gun running below the setpoint (unregulated) will have a different curve when off-reg. change if you change the valve spring.... However, it is then no longer regulated, right?.... Sorry I didn't specify a regulated PCP while running above the setpoint, but I though that was implied....
Bob
FWIW my experience with squeezing out as much hammer energy into budget bullet shooter builds hasn't exactly fit this. As long as the spring isn't creating the choke point within the valving I have seen zero difference between a light to medium valve spring and no spring. Not trying to stir the pot, but my hunch is that there is a more complicated relationship of closing forces than just a simple area times pressure solution. I have no experience with stiff springs, so i cant comment there.
Just my .002 cents :)
Shane
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By "zero difference" do you mean in every conceivable way?
For example, I wouldn't expect there to be any difference in terms of the ability to tune to a particular energy level, just the amount of hammer spring preload necessary to achieve it. Or substitute hammer stroke or hammer weight in place of hammer spring preload...anything that influences the lift and dwell.
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By zero difference, i mean no difference in hammer energy requirements, ie exact shot strings with and without spring installed using removable ssgs set to the same preload/gap settings. I guess i should clarify, i did some pretty extensive testing with one rifle in particular, a .224 making about 100 fpe maxed out. I had a hard time getting the hammer energy up enough to shoot 55+ grain bullets out of it. Removing the spring didnt gain anything for me
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Per my regulated rifles(Impacts), if you have the means to to tune it to a specific velocity several different ways; I believe the valve return spring isn’t necessary or as influential. Most run springless in order to not take up unnecessary volume within their plenum/air tube or to minimize valve cracking force.
I run absolutely no return spring in both rifles and as they are tuned, when they come off regulation it’s a very slow gradual(descending) drop in velocity. The ES is a mere 3-5fps for the next 8-10 shots pass the regulator set point. So, although the pressure is dropping in the tank; everything is so in harmony you barely notice a difference in POI or sound. The final outcome depends on your tune ability of your specific airgun.
Peter