Back before the hackening, we had a nice long (very long) thread called Project Leprechaun where Nish challenged everyone to come up with ways to modernize the unregulated Nelson valve train.
One of the items that came up was a "sail" (basically a flat washer) added to the back of the valve stem such that recharge flow into the chamber would help close the valve.
While I liked the concept, I feel like it is a step back from what I really wanted, which is to make the flow over the valve poppet itself more interactive with the flow rate. I am less concerned with efficiency than I am consistency. If I'm playing with 25 to 30 shots, I want each shot to be as consistent as possible. The goal is to get the flow forces to be a useful part of the valve closing, so that it closes faster at high pressure, and slower at low pressure, which means a flatter velocity curve as you shoot through a 12 gram. This means a high-drag flow design. Ideally, a freshly pierced 12 gram should not give a significantly higher velocity than a half full 12 gram. Same thing for changing temperature from hanging out in the sun or taking a few shots in succession.
My first attempt was to put a sail on the seal of a JT ER2. The first sail did very little, being about 1/8" smaller than the inner bore diameter of the chamber. A second attempt with 1/16" smaller than the inner bore diameter appeared to make a significant difference, leveling out the first shot, down to the last few shots at the end of the cartridge. The shot drop-off at the end of cartridge was limited to the last couple of shots (not completely a "feature" when playing,
). While successful, it was a PITA to make, install and play test with any consistency.
Since then, I have been 3d printing various seals and poppets with ninjaflex (captive seals) and Prusaflex 98A (poppets). I made a successful poppet for an SL68. I have made a seal for the Cooper-T Back Bottle velocity adjuster on my Piranha. I made some regulator seals for a couple of Azodin regulators. The printed TPU seems to work well. Sometimes it needs to set under pressure for a 12g or two before it seats up completely tight.
During the damndemic, I picked up a Piranha long barrel for a project. It already had a light hammer and Cooper-T adjuster. After setting up some detents and fixing other fiddly items, I set about trying to play with it. I found the velocity with 12 grams just as ugly and varied as I had in the 90s with my first one. Hot first shots, large multi-shot drop-off, slowly drooping velocity in the last half of the cartridge, etc. Thinking back to Project Leprechaun and the JT ER2, I decided to replace the poppet. Looking at the original, the rounded edge on the face side is actually defeating turbulence, making the poppet almost immune to changes in flow. I designed a fat, square-edged face seal, the first iteration was 0.484" in diameter which was very tight for flow. The print has 6-32 threads to fit on the stainless stem I made for it (not my prettiest work). This was apparently too tight, as I could only get between 140 and 180 fps with my adjuster. My second seal is 0.440" in diameter. It is shooting 260 fps with the adjuster backed all the way off. Consistency has been very good on the first 12g, +/-3. The only thing changed was the poppet seal diameter.
When I installed the seals, I used a little bit of blue loctite. When I changed the seals, I put the stem in a vise and used a pair of vice grips to twist the seal off. Afterward, I'm not seeing any marks on the old seat; that TPU is tough. The seat is Prusaflex 98A (98A being the approximate durometer). I narrowed the waist of the valve stem to increase the flow area and hopefully the drag forces. I still have to check the efficiency numbers in play, but I expect they will be slightly better once I learn to count off the right number. I've always changed early when it started to droop.
tldr;
1) 3d printed seats and seals are very workable.
2) Poppet/seal diameter can be varied to balance springs and hammer forces in the valve train.
3) Valve drag can be increased to compensate significantly for pressure variations out of 12gs, just like in a sweet-spotted autococker.
One of the items that came up was a "sail" (basically a flat washer) added to the back of the valve stem such that recharge flow into the chamber would help close the valve.
While I liked the concept, I feel like it is a step back from what I really wanted, which is to make the flow over the valve poppet itself more interactive with the flow rate. I am less concerned with efficiency than I am consistency. If I'm playing with 25 to 30 shots, I want each shot to be as consistent as possible. The goal is to get the flow forces to be a useful part of the valve closing, so that it closes faster at high pressure, and slower at low pressure, which means a flatter velocity curve as you shoot through a 12 gram. This means a high-drag flow design. Ideally, a freshly pierced 12 gram should not give a significantly higher velocity than a half full 12 gram. Same thing for changing temperature from hanging out in the sun or taking a few shots in succession.
My first attempt was to put a sail on the seal of a JT ER2. The first sail did very little, being about 1/8" smaller than the inner bore diameter of the chamber. A second attempt with 1/16" smaller than the inner bore diameter appeared to make a significant difference, leveling out the first shot, down to the last few shots at the end of the cartridge. The shot drop-off at the end of cartridge was limited to the last couple of shots (not completely a "feature" when playing,
). While successful, it was a PITA to make, install and play test with any consistency.Since then, I have been 3d printing various seals and poppets with ninjaflex (captive seals) and Prusaflex 98A (poppets). I made a successful poppet for an SL68. I have made a seal for the Cooper-T Back Bottle velocity adjuster on my Piranha. I made some regulator seals for a couple of Azodin regulators. The printed TPU seems to work well. Sometimes it needs to set under pressure for a 12g or two before it seats up completely tight.
During the damndemic, I picked up a Piranha long barrel for a project. It already had a light hammer and Cooper-T adjuster. After setting up some detents and fixing other fiddly items, I set about trying to play with it. I found the velocity with 12 grams just as ugly and varied as I had in the 90s with my first one. Hot first shots, large multi-shot drop-off, slowly drooping velocity in the last half of the cartridge, etc. Thinking back to Project Leprechaun and the JT ER2, I decided to replace the poppet. Looking at the original, the rounded edge on the face side is actually defeating turbulence, making the poppet almost immune to changes in flow. I designed a fat, square-edged face seal, the first iteration was 0.484" in diameter which was very tight for flow. The print has 6-32 threads to fit on the stainless stem I made for it (not my prettiest work). This was apparently too tight, as I could only get between 140 and 180 fps with my adjuster. My second seal is 0.440" in diameter. It is shooting 260 fps with the adjuster backed all the way off. Consistency has been very good on the first 12g, +/-3. The only thing changed was the poppet seal diameter.
When I installed the seals, I used a little bit of blue loctite. When I changed the seals, I put the stem in a vise and used a pair of vice grips to twist the seal off. Afterward, I'm not seeing any marks on the old seat; that TPU is tough. The seat is Prusaflex 98A (98A being the approximate durometer). I narrowed the waist of the valve stem to increase the flow area and hopefully the drag forces. I still have to check the efficiency numbers in play, but I expect they will be slightly better once I learn to count off the right number. I've always changed early when it started to droop.
tldr;
1) 3d printed seats and seals are very workable.
2) Poppet/seal diameter can be varied to balance springs and hammer forces in the valve train.
3) Valve drag can be increased to compensate significantly for pressure variations out of 12gs, just like in a sweet-spotted autococker.
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