Author Topic: Tale of THREE BRods - .25, .30 and .357 Completed, Page 12 ! (Read 26371 times)

rsterne · « **Reply #30 on:** January 13, 2017, 09:29:49 PM »

I like them, and I think they have more potential than I have seen as well.... You need a way to make very small, repeatable adjustments.... and the time to experiment, which is where my problem lies.... no time in the summer to truly test....

Bob

rsterne · « **Reply #31 on:** January 14, 2017, 04:57:59 PM »

Today I made the first SSG.... I left all the parts longer than necessary until I find out if it will work properly throughout the range of tuning I need, I can always shorten them up and make them prettier later....

The housing is a piece of 1.25" OD 6061-T6 aluminum turned down to fit in the back of the main tube, with the appropriate tapped holes to secure it.... On the top are two 4-40 screws through the receiver on 0.688" centers, and on the bottom a single 8-32 hole that mounts the trigger group.... The front just clears the back of the hammer when it is all the way back.... It is drilled through and tapped 9/16"-18 NF for the gap adjusting bolt.... I know that is huge, but it allows the spring assembly to be withdrawn out the back without any other disassembly.... That is a feature I always like to employ on my RVAs and SSGs, it makes changes a breeze.... The gap adjusting bolt has a spring seat turned on the front end, to hold the spring in alignment.... The stop rod part of the spring guide is a 4.5" long piece of 3/16" drill rod, threaded 10-32 both ends.... At the back it has quite a bit of thread for preload adjustment, which is provided by the two nuts locked against each other.... There is a # 008 O-ring on the shaft to act as a cushion when the guide comes to a halt.... The front part of the guide is turned from a piece of aluminum rod....It is 0.48" OD at the front (the same as the spring) and profiled to fit the bottom of the hole in the hammer.... Behind that is a 1" long section that is 0.355" OD to act as a spring guide.... There is enough space between the two parts of the guide to allow for more preload and cocking distance than I need.... The spring I am using at the moment is 3" long (instead of the 2.5" used in a stock MRod), with a rate of 13 lb/in.... I have 0.40" of preload at the moment, which is about 5 lbs.... If I need more, I just wind the nuts down on the shaft.... If I need less, I can shorten the gap adjusting bolt (moving the rear spring seat back), but a better solution would be a weaker hammer spring.... I just don't have anything lighter at the moment....

In the photo above, the SSG is adjusted for zero gap with the current 0.40" of preload with my valve.... With the Cothran valve, where the end of the stem sits further back, the gap adjusting bolt would be further back.... I can shorten the back of the SSG housing to allow more adjustment if I have to increase the preload much.... I'm thinking I'm getting pretty close to being able to tether the version with my valve in it.... That will answer a lot of questions about the hammer spring requirement at various pressures.... The Cothran valve will require less hammer strike than mine, at any given pressure, so I could use a lighter spring, or just increase the gap.... My valve at 3000 psi will be the test of the maximum hammer strike required for these guns....

Bob

rsterne · « **Reply #32 on:** January 15, 2017, 03:58:17 PM »

I made the Teflon transfer port today and assembled the gun and fired it for the first time.... Tethered at 1900 psi, at zero gap on the SSG, the velocity was a bit low, so I increased the preload in two 0.10" stages from 0.40" to 0.60", at which point I was able to hit the velocity plateau at zero gap.... I then installed the action in the wooden stock.... It weighs right on 7 lbs. without the bottle.... I haven't done anything with the comb of the stock yet....

I then tested the velocity with 25.4 gr. JSB Kings and 34.2 gr. JSB Heavies, adjusting the Gap from zero out until the velocity dropped below 600 fps.... and then went the other way, adding preload (negative gap) until the plateau was confirmed.... ie I knew what the maximum possible velocity was with this valve, and 0.219" ports, at 1900 psi.... Here are those results....

As expected, adding any preload against the valve stem instantly increased the report.... I like to adjust the preload on my SSGs until I can reach the velocity plateau just as the gap goes to zero.... Any more preload and you are just making the gun harder to cock, and it means that you have to increase the gap to drop the velocity back down to the knee of the curve.... I want just a bit more power, I would like to be on the knee of the curve at about 950 fps with the 34 gr. Heavies, so with this valve I would need to increase the regulator setpoint slightly, I think 2000 psi should be about perfect.... However, before I make that decision, I want to test the other tube with the Cothran valve at the same pressure, to compare the results....

All in all, I'm delighted with the initial results.... With the SSG adjusted so that there is a tiny gap, I am at 944 fps (68 FPE) with the 34.2 gr. JSB Heavies, at only 1900 psi.... Moving the seat forward in the MRod valve so that I was able to achieve the 7/32" exhaust port was worth the effort....

Bob

oldpro · « **Reply #33 on:** January 15, 2017, 04:07:39 PM »

That stock came out pretty cool!!!! I like it!!!

rsterne · « **Reply #34 on:** January 15, 2017, 06:18:33 PM »

I'm going to add a cheek-piece.... I think a piece of 1-1/4" ABS pipe will be just about the right height, and carry the black back onto the stock.... Until I get some optics mounted I won't know for sure what height to mount it at tho.... Once again, Travis, thank you so much for all the parts.... that stock sealed the deal to kick my butt into doing this project.... so it's all your fault....

Bob

Shorty · « **Reply #35 on:** January 16, 2017, 01:24:31 PM »

Nice work Bob.

What's the barrel length for the 25 ?

Can't wait to see the difference in power level or efficiency between the 2 valve systems. This will be the first heads up comparison of the 2.

rsterne · « **Reply #36 on:** January 16, 2017, 02:14:25 PM »

The story of todays post is.... NEVER BE AFRAID TO LEARN SOMETHING NEW.... or at least open your eyes to existing problems....

I am always careful to make sure that the area between the front of the hammer and the back of the valve can't get pressurized, either from the hammer movement itself, or from an air leak through the valve stem during firing.... If that happens, you can get (in the worst case instance of a leaky valve stem) a machine-gun action where the pressure partially recocks the hammer and fires the gun again.... until the reservoir is empty (yes, I've had that happen).... Travis suggested that I flute my hammers to prevent this, but I double checked, and the sear slot in the bottom of the tube vents into the trigger, and out through the bottom.... However, I haven't been paying enough attention to the BACK of the hammer, where a vacuum can build up and slow the hammer strike.... The only gun I ever vented there, on a suggestion from Scott (Motorhead), was my B-51.... WHY I haven't been looking for this problem in other guns I don't know.... complacency, I guess.... Well, that just bit me.... in spades....

While I was triple checking the space in front of the hammer this morning, I looked at the area behind it.... Yes, there is a rather large cocking slot, with a matching one in the receiver.... but guess what.... There is nowhere for the receiver to vent, other than past the bolt at the ends, in an MRod receiver, with the rear bolt.... I had noticed some large ES values yesterday when I had the SSG backed off with a large gap, and the lightbulb finally went on.... With an SSG (or SSS or TSS), the spring quits pushing the hammer before it gets to the valve stem, and it coasts the last part.... What if there is a vacuum behind the hammer?.... Yup, that will start slowing the hammer down before it gets to the valve.... In addition, that vacuum is unlikely to be 100% consistent, shot to shot, which could increase the ES.... and the more gap you have between the spring and hammer, the worse both might be.... Time to find out....

I set the SSG for 1.5 turns of gap, and shot an 8-shot string.... The average velocity was 811 fps, but the ES was huge, with shots going from 790-834 fps.... definitely not good.... I stripped the back of the gun apart, and machined two slots in the bottom of the rear plug (SSG housing), at about 4:30 and 7:30, in the section that inserts into the tube.... The slots were 3/16" wide and 1/8" deep, and continued back 3/16" behind the rear of the tube.... That is the equivalent area of a 1/4" hole through the rear plug to prevent any vacuum forming behind the hammer on firing.... Here is what you see with the action out of the stock....

There happens to be a slot in my stock at the location of the vents.... If that isn't the case with a different stock, a different vent design may be necessary.... Anyway, I assembled the back of the gun again, adjusted the SSG gap to the same, 1.5 turns out, and fired a 34 gr. JSB through the Chrony.... Instead of 811 fps, it was 956 fps.... WOW !!!.... Obviously the vacuum behind the hammer was a reality, I had moved up onto the velocity plateau.... I went through the process of documenting the velocity vs gap with the vented rear cap.... On the graph below, the dotted red line is the new data, with the vent.... while the solid red line is yesterday's data, without the vent....

There are two important things to notice.... First, it takes a LOT more gap to get to the knee of the curve with the vents.... which means that without them, the vacuum behind the hammer was slowing it dramatically.... Secondly, the slope of the downslope part of the curve is a lot shallower.... It takes more gap to lose the same amount of velocity.... To drop from 944 fps down to 760 fps now takes a 3 turn increase in gap instead of two.... That makes sense, because the bigger the gap, in the unvented setup, the more the vacuum would reduce the hammer velocity.... There is absolutely NO doubt this gun needs the area behind the hammer vented....

Since I had dramatically increased the hammer strike at the same settings, I could now do two things.... I could reduce the preload on the spring, making the gun easier to cock.... I reduced the preload to my original 0.40" (which was not enough yesterday, with the vacuum), and shot the data shown by the purple line.... As you can see, the "knee" of the curve still requires 3 turns more gap, even though the preload on the spring is reduced from nearly 8 lbs. to just over 5.... Since the larger gap means the spring is not compressed as far (and starts at less preload as well), the maximum cocking force, when the gun is set just below the plateau, at 944 fps, has dropped from 19 lbs. down to just 15 lbs.... In fact, I could further reduce the preload if I wish, and that would allow me to decrease the gap, and the cocking force, even more....

These vents did one more thing for me.... They increased the available hammer strike to the point that I could now test the gun at 2900 psi.... I checked it with the 34 gr. JSB Heavies, and the results are shown as the black line on the graph above.... This was shot with the preload on the spring still at 0.40", and to get to the plateau I had to set a turn of negative gap (actual preload on the valve stem).... I didn't try it, but I am confident that by going back to the 0.60" preload I could operate at 2900 psi and hit the plateau with a small gap.... In any case, these results proved that this valve is capable of cranking out 90 FPE at 2900 psi with the 34 gr. Heavies.... I didn't have the proper sized 51 gr. BBTs for this barrel, the ones I had were sized to 0.250" (so they were likely leaking a bit in the bore).... but by cranking in some preload I managed to find the plateau for them as well.... I was able to hit 963 fps (105 FPE) at 2900 psi with those bullets.... I have ordered an NOE Mould for the 51 gr. BBTs in 0.253/0.255", and the sizing bushings for them.... so at some point in the future I can experiment with them....

In conclusion, I am extremely glad I vented the area behind the hammer to prevent the vacuum from slowing the hammer.... I am frankly shocked at how much hammer strike was lost because of that vacuum, and the horrible ES that it caused.... You can bet I'm going to have a long, hard look at all my Disco based guns.... There may be more consistency, and easier cocking, lurking amongst any of them....

Tim, the LW barrel is the standard, 23.8" length they sell....

Bob

rsterne · « **Reply #37 on:** January 16, 2017, 05:00:04 PM »

After writing the previous thread, I changed all the parts over to the tube with the Cothran Powerhouse valve in it.... This is the best way to compare it to my modded MRod valve, by tethering it at the same two pressures, 1900 psi and 2900 psi, and using the same pellets.... Here are the results.... The horizontal axis on this graph is the same as the one above for my valve, and I used the same colours for the 34.2 gr. JSB Heavies at the two different pressures.... In both cases, the SSG had the same 0.40" preload, and "Zero" is the point where there is just barely a gap in the SSG setup.... The end of the valve stem in my valve is further forward than that of the Cothran valve, so my setup has 0.88" of maximum hammer stroke, whereas with the Cothran valve it is only 0.79".... but that suits the two valves, because mine needs more hammer energy to open.... This means that the SSG has to be adjusted to a different point (about 1.5 turns further out) with the Cothran valve to end up with zero gap.... but zero on both graphs has the same meaning, minimum gap on the SSG....

You can see at once the huge difference in the response of the Cothran valve to the gap setting.... Basically this valve is either "ON" or "NOT".... It exhibits the typical "cliff" which we have seen in other tests of the Cothran valve.... The most efficient point to operate the Powerhouse valve is just above the point where it no longer opens properly and consistently.... This may only be a matter of one flat (1/6 turn) on the SSG gap adjuster.... I didn't explore the exact location of that point yet, all I was after was the maximum velocity available at the two test pressures, and the range of SSG gap where the cliff started.... Relative to zero gap, at 2900 psi the cliff was at about 2 turns out, compared to requiring actual preload with my valve.... at 1900 psi, the cliff was at about 4 turns out, fairly similar to where the knee was with my valve.... As expected, with the larger ports, the Cothran valve was able to produce a bit more power than my valve was.... Here are the maximum velocity and energy achieved with the two valves at the two test pressures, with the pellet and bullet tested....

The port size on my valve is 0.219".... The Cothran valve has a 0.257" exhaust port, I used a 0.250" transfer port, and the barrel port is equivalent in area, but there is a 3/32" diameter probe in the chamber.... This reduces the effective area to that of a 0.234" port.... so it doesn't hurt my valve, but probably handicaps the performance of the Cothran valve slightly.... However, the only way around that would be to just a probeless bolt with a flat face, that retracts flush with the back of the barrel port in the firing position.... This would require milling a "J-slot" in the MRod receiver for the bolt lockup and cocking pin to retract into.... Since I don't plan on using anything heavier than my 51 gr. BBT, and I can already push that to nearly 1000 fps, I simply don't need to bother making that modification to the receiver.... However, if you wanted to use an even heavier bullet, as you might if you fitted a .257 cal barrel, you would be well advised to take the trouble to use a probeless bolt arrangement to achieve full barrel size porting with the Cothran valve....

Bob

Motorhead · « **Reply #38 on:** January 16, 2017, 08:58:58 PM »

... Air the often overlooked drag contributor to those parts put into motion QUICKLY
Great observation and grasping of this frequently overlooked area.

oldpro · « **Reply #39 on:** January 16, 2017, 10:44:50 PM »

That brings me to another thought. If we made the front portion of the hammer diablo or bullet shaped would it travel faster if the weight was the same? Instead of the flat frontal area? Or is the speed and distance traveled too short to take any advantage of the better BC?

Motorhead · « **Reply #40 on:** January 16, 2017, 10:51:41 PM »

Would think because we're operating within the "Bore" sort of speak ... with no real air flow around the hammer it's dynamics is nearly pure displacement of frontal and rear air re-positioning and those pressures involved.

rsterne · « **Reply #41 on:** January 17, 2017, 08:21:59 AM »

I would agree, Scott.... the air doesn't flow around the hammer because of the tube.... Simple flat sides, like a PRod/Challenger hammer may be the best.... unless you need the weight.... As with most airgun stuff, Lloyd has a spreadsheet for hammer data.... and the hammer velocities at valve impact are quite low.... For the one in my gun, with 2 turns of gap and 0.40" of preload, with the hammer and guide mass, the hammer velocity at impact is only 12.3 fps.... After leaving the guide behind, the hammer energy is 0.54 FPE.... assuming it doesn't lose velocity before hitting the valve stem.... The hammer is only moving the same velocity it would if you dropped it a distance of 28"....

Bob

rsterne · « **Reply #42 on:** January 17, 2017, 05:03:26 PM »

I pulled my modded valve out of the tube today and bored out the exhaust port to 0.234", and drilled out the transfer port to that diameter as well.... I inspected the MRod poppet, and it has a nice shiny seat, just indented a few thou.... It is not showing any sign of extruding into the port, as I have seen happen with a 0.281" throat.... The throat in my current valve is 0.266", which I have also used before without issue at 3000 psi.... However, I don't want to go any larger.... Unless you change the poppet material, going larger on the port, or using more than 3000 psi, is just asking for trouble.... The throat (when you subtract the 1/8" stem), the exhaust port, the transfer port, the barrel port and the bore (when you subtract the 3/32" bolt probe) are now all the same area.... I consider this the maximum practical for a modded MRod valve, unless you change the poppet, and no point in doing that unless you also go to a probeless bolt....

The larger ports increased the maximum velocity at 1900 psi by 24 fps, from 956 to 980 fps.... Here is the comparison, before and after enlarging the ports....

I want the ability to achieve 950 fps on the knee of the curve, and with the 0.219" ports I would have needed to increase the regulator setpoint to 2000 psi.... Now I can use 1900 psi and get there.... I adjusted the gap to 3.5 turns out, and shot the following 10-shot string using the 34.2 gr JSB King Heavies, tethered at 1900 psi....

960
959
956
959
959
957
957
960
955
957

ES = 5 fps = 0.5 %
958 fps average
69.7 FPE average

This achieves my goal with these pellets.... I don't have an efficiency yet, that will have to wait until I install the bottle.... I have not yet decided if I will use my valve, or the Cothran, for the regulated .25 cal, either will work perfectly.... The edge for the Cothran valve over mine is now just 10-20 fps at 1900 psi.... depending on how I adjust the SSG gap.... To adjust the velocity on the Cothran valve, you have to change the pressure, as for best efficiency you need to operate it just above the cliff.... It would make sense to use whichever proved to be the most efficient at the velocity I end up shooting.... That has yet to be determined....

Bob

rsterne · « **Reply #43 on:** January 18, 2017, 01:22:30 PM »

I've been hearing all about these MDS Nylon lightweight hammers, and this gun, with the current hammer spring, needed a large gap on the SSG, so it seemed a good candidate to try one.... I made one of identical dimensions to the steel one I had, but used a 3/4" diam. steel core and the rest is MDS.... I simply knurled a piece of 3/4" steel, drilled it for the hammer spring, and then bored out a piece of MDS of the correct length to be a press fit, and pressed them together.... I then machined the MDS to the correct diameter, cut off the excess steel at the front and turned it to length, and drilled and tapped the 8-32 hole for the cocking pin through both, then machined the angle for the sear in the MDS.... The finished weight was 51 grams, almost exactly 50% of the original steel one, which was 104 gr.... I installed it in the tube with my valve in it, and tethered the gun at 1900 psi, to duplicate the test I did yesterday of the 0.234" ports.... I used the same 34.2 gr. JSB Heavy pellets, so the ONLY change was the hammer.... Here are the results....

With zero gap, the velocity was already starting to fall below the maximum, at 963 fps.... This is virtually identical to the velocity I got with the heavy hammer at 3 turns of gap.... The velocity at 1 turn out with the light hammer was almost the same as at 4 turns out with the heavy hammer as well.... Therefore, reducing the hammer weight by 50% required a 3 turn decrease in gap to obtain the same velocity.... The hammer spring I am using has a rate of 13 lb/in, and 3 turns of the gap adjuster, which is 18 TPI, is 0.165".... That means that the light hammer requires just over 2 lbs. more force to cock the gun, and 0.165" more hammer travel, to obtain the same results....

Using Lloyd's hammer spreadsheet, I calculated the hammer velocity at the instant it left the SSG, and from that the hammer energy and momentum.... To do that, I had to include the weight of the SSG guide (22 grams) during acceleration, to get the velocity, and then subtract it to get the energy and momentum of the hammers.... Here are the results that produced identical (963 fps) velocity and FPE in the pellet, which is what matters, of course....

104 gr. Hammer
Velocity 11.54 fps
Energy 0.471 ft.lb.
Momentum 0.082 ft.lb/sec
Lock time 10.1 mSec
Cocking Force 14.4 lbs.

51 gr. Hammer
Velocity 17.83 fps
Energy 0.554 ft.lb.
Momentum 0.062 ft.lb/sec
Lock Time 7.1 mSec
Cocking Force 16.6 lbs.

These results are skewed somewhat by the admittedly heavy SSG assembly in this gun.... as it represents a much higher percentage of the weight of the light hammer than the heavy one.... However, in this particular application, that's what I got.... There is no question that this lightweight hammer will work in this gun, with either valve, at 1900 psi with the 34 gr. JSBs, it would just be a matter of tuning it for the velocity you want.... There is also no question that it requires about a 15% increase in cocking force to get the same performance.... I expected an increase, but it's nice to actually put some numbers to it.... Bear in mind, this is a pretty powerful .25 cal, delivering 70 FPE at only 1900 psi....

Although I did not try it at 2900 psi, the results I could expect were pretty obvious.... I was unable to max. the velocity out with the 34 gr. JSBs at that pressure with the heavy hammer and 0.40" of preload on the spring.... I think I could just get there with 0.60" of preload at zero gap, which would put the cocking force at about 19 lbs.... With the light hammer, I'm pretty sure I couldn't come close to maxing the velocity using an SSG, I would have to go to a preloaded spring.... Using heavier bullets at 2900 psi it would be even harder to flow enough air to max. them out, even with the heavy hammer.... Even the Cothran valve had to have the SSG dialed down to zero gap with the heavy hammer, to get to maximum velocity with my 51 gr. BBTs....

My conclusion is that the MDS hammers certainly have their place, and can do wonderful things at moderate power and pressures.... However, when you are trying to extract maximum FPE, particularly with heavy bullets and at high pressure.... a heavy hammer helps you get the hammer strike required without using a spring that make the gun too hard to cock.... The Cothran Powerhouse valve, with its ability to open with a much lighter hammer strike, can certainly make use of lighter hammers, however.... I have done no efficiency testing with either hammer or valve, so can draw no conclusions regarding that at this time....

Bob

Rdsail · « **Reply #44 on:** January 18, 2017, 03:23:47 PM »

Very impressive. Do you know what kind of efficiency you are getting?

Back when I built my .25 mrod I was able to do 52g slug at 995fps which was about 115 fpe (if I remember correctly). That was with a modified WAR valve. But it was an air hog so I ended up turning it down to 85-90fpe for 20-25 good shots with a 500cc bottle. The efficiency was never great but that was not the point

The power peak for me was around 2500-2400 psi

I did end up using a 18-20# spring with a lighter hammer. The biggest issue I found was hammer bounce which I end up fixing thanks to travis's help. The hammer was acting as a piston and creating a large air cushion which increase the hammer bounce significantly. I also ended up putting an oring around the poppet stem to create a seal with the valve housing.

I should have kept it and tried all the new goodies as SSGs

The AirGun Guild

A friendly place to share one's interest in all things airguns.

News: