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Author Topic: Tale of Two BRods  (Read 8130 times)

rsterne

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Re: Tale of Two BRods
« Reply #105 on: February 23, 2017, 06:14:58 PM »
Travis sent me a replacement poppet for my Cothran valve, as I think I went too far with the 0.035" vent hole.... because I lost some of the peak velocity.... I tried this one with the stock 0.020" vent hole, but with no metering rod, and it acted normally.... The velocity on the plateau (.30 cal. 50 gr.) was the same as stock, ie removing the metering rod didn't change it.... I had no strange effects like hammer bounce, but remember, I am running a #210 90D O-ring on the back of the valve to limit how far the hammer can drive the stem to about 0.075".... and the O-ring then stops the hammer, absorbing any excess energy.... However, with the small 0.020" vent, the valve was back to being "on or off", with a flat plateau and then a sudden cliff, losing the small amount of adjustment you can get with a bigger vent....

I had previously tried a 0.028" vent hole, which worked fine, and then increased it to 0.035", which IMO is too big, because I lost velocity on the plateau.... I think this means that I cannot get enough dwell to max. out the power.... Both the previous sizes were achieved by a pointed burr in a Dremel, which is difficult to get to the correct size, requiring that you keep checking the hole with a piece of wire of the diameter you want.... A couple of days ago, in a local Home Hardware, I saw some Cobalt twist drill that were 1/32" (0.031"), which is in between what I had tried, so that is what I used to enlarge the hole in the new poppet.... Travis had told me that the shaft of the poppet was very hard, and to grind it with a Dremel.... but I picked up a pair of these drills (they were $6.49 for 2) in the hopes that the cobalt steel would be hard enough to do the job.... I used the smallest collet in my Dremel, and ran it at the lowest speed (still probably 5000 PRM), and it drilled the hole out perfectly.... You have to be careful not to break such a small drill, but it was a lot easier to do than using the pointed burr, and the hole will be the same size, every time....

I was very pleased with the results.... I haven't lost any of the peak velocity on the plateau with the 1/32" vent hole.... This compares with a 35 fps loss with the 0.035" hole.... I haven't yet had the opportunity to see how much of an adjustment range I can get with this size.... ie what the knee looks like.... but I'm sure it will be similar to the 0.028" hole, which gave me some degree of adjustability before the steep dropoff.... Plotting that curve, and checking the efficiency on it, will have to wait for another day....

Bob
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oneshot61

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Re: Tale of Two BRods
« Reply #106 on: February 28, 2017, 06:28:41 PM »
That's what I did mine with. But I set up one night and twisted it slowly with my fingers until I broke through. I still broke one of the bits, which is very easy to do. I think my stronger valve spring has taken a bit to much off the top power though. Looking forward to your results.

rsterne

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Re: Tale of Two BRods
« Reply #107 on: March 11, 2017, 07:06:56 PM »
The first trials with the new stem with the 0.031" (1/32") vent hole were a bust.... I removed the metering rod, and over a matter of a few shots at 2900 psi the velocity started to drop.... I had to keep increasing the hammer strike, and even that wouldn't put me back to where I started.... I pulled the valve apart, and found that the seat had a deep groove in it.... I can only assume that by removing the metering rod the valve was closing with such force it destroyed the rather soft seat on the poppet.... Either that, or there was a leak in the force reduction piston O-ring, which put a lot more force on the poppet.... I don't know why I never experienced this with the original poppet, it was still in good shape.... The solution was to disassemble both poppets and build one good one from the two sets of parts....

The poppet is made in three pieces, the hollow stem, the seat, which looks like a nylon washer, and the tubular front part, which is the sleeve of the force reduction system....The two metal parts are threaded together using 10-32 threads and glued, which make the poppet difficult to take apart without scarring the surfaces.... By using the two chucks on my lathe, and turning the headstock backwards by hand, I was able to take the poppets apart.... I used the new stem, with the 1/32" vent hole, and the old seal, turned over to use the flip side, which was perfect, and one of the front sections.... I reassembled it with a drop of blue Loctite and let it cure overnight before assembling and pressurizing it.... It sealed up perfectly first try.... I had installed the stock metering rod, and set about testing it....

As I found previously at 1900 psi, the velocity was right back up to where it was with a stock vent hole.... In .25 cal, there was very little adjustability, basically a plateau and a cliff.... In .30 cal, there was a slight rounding of the edge of the cliff, but the valve was still rock stable just above the dropoff.... With 70 gr. bullets, I did lose a few fps, so there is some indication that this is the largest vent hole you can use in .30 cal at this pressure without losing performance.... I would not go larger, because with the 0.035" vent I lost about 20 fps.... I did some short strings (1 magazine) tethered to my 500 cc tank with the output regulated at 1900 psi, and the efficiency was slightly better than with either the stock vent hole, or with the 0.035" vent, so I think I hit the sweet spot for the vent size, at least when regulated at 1900 psi.... Here are the results....

.25 cal. 34.2 gr. JSB.... 987 fps (74.0 FPE) @ 1.00 FPE/CI, with an ES of 13 fps....
.25 cal 46.4 gr. Cast RN.... 878 fps (79.5 FPE) @ 1.04 FPE/CI, with an ES of 7 fps....
.30 cal 44.8 gr. JSB.... 971 fps (93.7 FPE) @ 1.11 FPE/CI, with an ES of 12 fps....
.30 cal 49.3 gr. Daystate.... 949 fps (98.6 FPE) @ 1.17 FPE/CI, with an ES of 12 fps....
.30 cal 70.0 gr. BBT.... 824 fps (105.7 FPE) @ 1.35 FPE/CI, with an ES of 11 fps....

The .25 gr. pellets required 6 turns of gap on the SSG, the .30 gr. bullets required 5 turns, and the other three all were shot at 5.5 turns of gap.... I can probably get a bit smaller ES by using a lighter hammer, a weaker hammer spring, or a bit less preload on the SSG, to reduce the gap.... I was running the 51 g. MDS hammer with the steel core....

I then took the gun all apart, stripped down the valve, and replaced the metering rod with one I had sanded down 0.003" smaller.... I had heard this was something worth trying, and it produced some interesting results.... First of all, it created a curve where the plateau used to be.... With no gap in the SSG, the velocity dropped about 15 fps with the .25 cal 34 gr. JSBs, and about 40 fps with the .30 cal 50 gr. JSBs.... It then increased until there was 3 turns of gap, and then curved downwards to about 6-6.5 turns and was in the basement when 7 turns out.... I think the lower velocity at heavy hammer strike may be because of the hammer bouncing off the rubber O-ring, although why it should show up only with the undersized metering rod, I have no idea.... The total adjustment range between peak velocity (at 3 turns) and when it rolled over to head into the basement was about 100 fps, so I was quite excited that I would finally be able to get some adjustment in velocity with the Cothran valve without having to play with the pressure.... Unfortunately, for nearly all of that 100 fps range, the velocity was unstable, with the ES increasing as you approached the cliff.... In other words, with the undersized metering rod, I got a slight knee, but not a usable one, as the ES quickly became unusable.... Just before the velocity took a dive, the ES could be as much as 130 fps from one shot to the next.... I fiddled with it, trying various adjustments, but could never get the ES as low as with the stock metering rod, so I took the gun apart and put the stock rod back inside the valve stem....

I tried a few more tests with the stock metering rod and the 1/32" vent hole, and I am very pleased with this combination at 1900 psi.... As long as you stay just above the cliff, in the area where the velocity is 10-20 fps below the maximum at that pressure, you are rewarded with a narrow ES, lots of power without maximum report, and decent efficiency.... I don't think there is anything I can do to get a better tuning range than I have for the Cothran valve, in .25 or .30 cal, when regulated at 1900 psi....

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #108 on: March 12, 2017, 11:52:07 AM »
I took a few shots today tethered at 2900 psi, with the following results....



As you can see, the pellets that were in the mid 900s at 1900 psi are Supersonic at 2900.... I tried some heavier bullets, sized down to fit, and the biggest surprise was the 66 gr. in 25 cal breaking 120 FPE.... I have a feeling that the enlarged vent hole in the stem is hurting the performance of the 109 gr. in .300 cal, as I would have expected more FPE from it instead of less.... After all, the porting is the same as my DAQ, which shot over 180 FPE with the same bullet at the same pressures.... If you correct for my shorter barrel lengths, my .25 cal is about on a par with Lloyd's original experiments with the MRod with the Cothran valve, as is the 80 gr. bullet in the .30 cal, but the 109 gr. is lagging.... It could also be the O-ring bumper on the back of the valve that I have used to limit the hammer travel, I have, after all, done several things to throttle back the valve to work better regulated at 1900 psi.... I guess it's not to surprising that some or all of those changes have combined to knock the top end off the power at the top end.... in fact it would have been surprising NOT to see that....  ::)

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #109 on: July 31, 2017, 08:26:30 PM »
I know it has been a while since I posted in this thread, but summer is an impossible time for me to work on airguns because the Motel is too busy.... During the winter, I worked with Travis and Jim designing an adjustable regulator to go inside their drop down tank block, as used in the new COBRA.... I just received one in the mail today, and it is gorgeous.... It threads right into the WAR tube for the FLEX/WARP/COBRA....



I got both tank adapters, one for 5/8"-18 UN threads, and one for 18 mm x 1.5 mm threads, so I can use any tank.... The burst disc is on the output side, and is a 3K, as I won't be using more than about 2200 psi.... However, I understand the output can be adjusted higher than that, depending on the Bellevilles installed....



It has room for two gauges, the upper one reads output pressure, and the lower one tank pressure.... The adjusting screw is an allen set-screw on the bottom of the block.... It moves the HP seat towards the piston (CW) to reduce the output pressure and away from the piston (CCW) to increase it.... I can't wait to try it out on the tube I have the Cothran valve installed in.... It will be the perfect arrangement for that, because you must adjust the pressure to adjust the velocity....

Bob
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Alan

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Re: Tale of Two BRods
« Reply #110 on: August 01, 2017, 04:20:17 AM »
I have one of these adjustable regulators on the Cobra version I bought from Travis earlier this year. The results of the team effort are rather evident, and thanks to all who made the device possible.

Once you play with it a bit, it is really easy to readjust the pressure—handy if you shoot different weight pellets. The only issue, and Travis warned me about this, is when you do adjust the regulator, GO SLOW! Itsy-bitsy movements is all it takes! Even a 5° rotation can make the adjustment too much!

I want to add something here. Bob, you truly are an unsung hero. I've never met you in person (I'd like to sometime), but your dedication and thoroughness in answering questions is legendary here, and on just about every other shooting-related web site out there. If I could bestow a gift on you, it would be a Doctorate of Letters in airgun technology! You sure have earned it!
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

rsterne

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Re: Tale of Two BRods
« Reply #111 on: August 01, 2017, 08:28:49 AM »
Thanks for the kind words, Alan.... When I started in airguns a decade ago, there was very little GOOD information on the Forums, people were not documenting and sharing their ideas, for the most part.... I struggled trying to get helpful information (with a few important exceptions, by people I will forever be in debt to).... so I decided early on to do the best research I could, and share all my results, good and bad, as accurately and completely as I could.... I am delighted that effort has been valued, and I hope it will continue to survive, despite Photobucket's recent efforts to derail the knowledge sharing on our Forums....

Bob
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oldpro

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Re: Tale of Two BRods
« Reply #112 on: August 01, 2017, 11:56:19 AM »
 Photobucket arrrrgggghhhhhhh!!!!!!!! :( >:(
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