Tuning Regulator pressure VS Hammer Spring tension

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With regulated airguns, the relationship between the regulator's setpoint and the hammer spring tension is really quite simple. There is only one hammer spring setting for a given regulator setpoint that will yield the best consistency (extreme spread or standard deviation).

To find the optimal hammer spring setting, keep increasing the tension until the velocity no longer increases. Then back it off until the velocity is somewhere around 95% - 97% of that maximum. So for example, if your max velocity was 900fps, dial the hammer spring back until the velocity drops to about 855 - 873fps.

When adjusted in this manner, your airgun will be somewhat insensitive to the little inevitable variations in pressure or hammer strike. It is akin to how an unregulated PCP operates in and around the top of its bell curve (the sweet spot). It will also be reasonably efficient. Better efficiency would be possible by backing off the hammer spring tension further, but that puts the airgun operating at a state of partial valve lock. It will take little sips of air on each shot but the extreme spread suffers. For short range shooting (e.g. 10m practice in the basement), that may be fine....preferable, even. But it's no good for 50+ yards unless your accuracy standard is a soup can. 

Accuracy on the other hand can't be so easily correlated to something as simple as the balance between regulator and hammer spring. Many, many things play into it. Having a stable velocity certainly helps but it's just one piece of the jigsaw puzzle.
 
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Very good info from nervoustrig. I also learned a lot from Bob Sterne's PCP tuning articles at Hard Air Magazine.

Index -- https://hardairmagazine.com/ham-columns/the-definitive-index-to-bob-sternes-ham-technical-articles/

Regulator tuning -- https://hardairmagazine.com/ham-columns/tuning-regulated-pcp-airguns/

I'm no expert, but following Sterne's regulator article, here's how I recently set my .22 FAC HW100K. I wanted to increase the number of shots from 40 and was willing to give up 100 fps. (from 900 to 800).

  1. Pick an initial regulator pressure. Ideally, by the end of a full shot string, cylinder pressure should drop to about the same as the regulator pressure. That's to make sure all shots are regulated. I initially set regulator pressure to 100 bar. This is down from 120 which is the beginning of the 'yellow' zone on the Weihrauch cylinder. To get more shots, I figured I had to go to a lower final pressure.
  2. Pick an initial pellet velocity. I picked 800 fps. Increase hammer spring tension until velocity levels off. That's what Bob Sterne calls the 'knee' or sweet spot. It's the max your rifle will produce with that regulator setting.
  3. Back off the velocity about 3% to 5%.
  4. If this doesn't meet your target velocity, bump the regulator pressure a little, then re tune the velocity.
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    I found the knee was around 830 fps and set velocity to 800 fps. It now gets 60+ regulated shots @ 800 fps before cylinder pressure hits 100 bar. I didn't map the shot string after that (garage was too cold and I was tired of pumping the Hill Mk 4).

    Bob Sterne discusses tuning on the downslope of the curve, i.e., backing off even more on the hammer, then increasing regulator pressure to achieve your target velocity. He says it can improve efficiency but may result in erratic velocity jumps, unless the gun is exceptional. This is probably what nervoustrig is referring to when he mentions partial valve lock. The theory seems to be that a light hammer doesn't open the valve as much, thereby conserving air. Also a lighter spring may reduce hammer bounce. 

    My HW100's original regulator setting was 145 bar (way above the yellow zone) so I suspect the factory tuned it on the downslope. The gun originally shot a beautifully even string, maybe it's an exceptional gun. Now that I have a new EC3000 and a warm garage, I may go back and tune the HW100 on the downslope. 

    HW100 owners may be interested to know that I got the regulator gauge and two full o-ring sets from the UK site HW100tuning.com. Just to be safe, I took the o-ring specs and ordered 100 of each from oringsandmore.com. A lifetime supply for about $25.

    I was concerned that my HW100 might have an 'anti-tamper' setup, where the hammer adjustment screw is bonded into the aft hammer block to prevent people from raising power above 12 fpe. There are several youtube videos about this. I even ordered a replacement spring, screw and block from the UK (ebay). However, it was not necessary. The hammer screw came out easily after removing the little locking grub screw. I was able to re-use the original parts.



 
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With regulated airguns, the relationship between the regulator's setpoint and the hammer spring tension is really quite simple. There is only one hammer spring setting for a given regulator setpoint that will yield the best consistency (extreme spread or standard deviation).

To find the optimal hammer spring setting, keep increasing the tension until the velocity no longer increases. Then back it off until the velocity is somewhere around 95% - 97% of that maximum. So for example, if your max velocity was 900fps, dial the hammer spring back until the velocity drops to about 855 - 873fps.

When adjusted in this manner, your airgun will be somewhat insensitive to the little inevitable variations in pressure or hammer strike. It is akin to how an unregulated PCP operates in and around the top of its bell curve (the sweet spot). It will also be reasonably efficient. Better efficiency would be possible by backing off the hammer spring tension further, but that puts the airgun operating at a state of partial valve lock. It will take little sips of air on each shot but the extreme spread suffers. For short range shooting (e.g. 10m practice in the basement), that may be fine....preferable, even. But it's no good for 50+ yards unless your accuracy standard is a soup can. 

Accuracy on the other hand can't be so easily correlated to something as simple as the balance between regulator and hammer spring. Many, many things play into it. Having a stable velocity certainly helps but it's just one piece of the jigsaw puzzle.
Click to expand...

NT: Very good explanation.

As an experiment, I tried this with my .25 caliber Veteran Long a while back. Naively, I was hoping to achieve optimum accuracy at 50 yards with the JSB .25 25 g pellets. My calculation ( ie., 95-97% of max HST ) was around the 940 FPS range. Interestingly, my ES values and groups were not nearly as good as to when I backed off the HST to 890-920 ranges. 

Your last paragraph says it all. Lots of variables to achieving desired accuracy.

Tom 
 
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