A New Method for Increasing the Efficiency of a PCP

[rsterne]

A while back on my .30 cal Grizzly HPA I tried a new system on the hammer spring.... It rides on a guide that is adjustable in length and comes to a positive stop just before the front of it hits the inside of the hammer.... The idea is to use a long, light hammer spring, which is easier to cock, and preload it on the guide.... This increases the initial cocking force, but reduces the final cocking force, making the gun smoother and lighter to cock.... What I thought I achieved was also a reduction or elimination of hammer bounce, which reduced wasted air and increased the efficiency.... I used the same design in my new Monocoque PCP, but of course it is yet untested.... So I decided I would take a gun I was familiar with and build one of these guide stops to see if I could document any increase in efficiency.... BOY, DID I !!! ....

The gun is a 2560.... ie a 2260 converted to a PCP, with a .25 cal LW barrel on it.... The valve has been hogged out, and the gun breathes extremely well.... The standard setup is a 1.75" x 0.040" wire spring on a Challenger Power Adjuster (RVA).... I use this gun for testing because it has a small reservoir so I don't have to shoot a lot to test changes, but it is big enough at 65 cc to not lose power in this .25 cal setup which is tuned for about 50 FPE with 25.4 gr. JSB Kings.... Here is a plot of the Velocity vs Preload with the gun tethered to a regulator with a 1900 psi output (actually 1880 according to the gauge I used for these tests)....



As you can see, this is pretty much the perfect spring for the gun, and the knee of the curve, where the gun starts gaining efficiency without losing much velocity, is at 4-5 turns out from coil bind.... I then chose a setting of 5 turns out and shot a string with the gun tethered but the SCUBA tank turned off.... The hoses were full, one going into the regulator and the other out, along with a gauge, so the total volume works out to 90 cc.... I got 5 shots starting at 938 fps, peaking at 950, and ending at 904 fps, and the pressure at the end was 1120 psi, so the gun used 760 psi of air, and got an efficiency of 0.85 FPE/CI.... While this in not great, don't forget that this a 50 FPE string from 1880 psi down to 1120.... I couldn't hear any hammer bounce on the first shot, but as the pressure dropped I heard more and more of that telltale B-RR-AAA-PPPP indicating it was wasting air.... I then installed the new preloaded setup, which uses a 2200 hammer spring with 0.80" of preload, which works out to over 5 lbs.... However, the maximum force to cock the gun is only 9.5 lbs. instead of over 10.6 lbs. with the original setup, which had a preload of less than 1.5 lbs.... Here are the two setups tested....



You can see the guide, made from a 3" long piece of 3/16" drill rod, threaded 10-32 on both ends.... The front nut is turned down to fit inside the hammer, and at the rear are two nuts locked against each other to adjust the preload.... The 3/8" hex head bolt is drilled through for the guide, and allows you to position the front of the guide just clear of the inside of the hammer.... It is longer than needed, and if shortened 1/4", the guide could be shortened the same amount, making it more compact.... Here are photos of the gun, first uncocked, and then cocked, so that you can see what is going on....






The small O-ring just acts as a cushion when the guide crashes to a halt.... The hammer then carries on, coasting on its own to open the valve to the appropriate lift and dwell.... The magic occurs on the return trip.... Since the spring is not touching the hammer while the hammer is touching the valve stem, as the valve closes it does not store any energy in the spring.... When the valve closes, the hammer has to move a bit further before it touches the spring guide, but instead of having a spring with little or no preload, which is easy to compress, it hits the end of the guide, which can't move unless you push on it with over 5 lbs. of force.... so it doesn't budge.... the hammer rattles to a stop between the valve stem and the spring guide, and doesn't have enough energy to open the valve for a second time.... No hammer bounce, so no wasted air, and instead of 5 shots.... I got 7, and used less air doing it.... While the shots got louder as the pressure dropped, as expected, NONE of the shots, even the last one at the lowest pressure, had that telltale burp of wasteful hammer bounce.... Here are the two shot strings....



The two curves on the graph are the average of two strings each, where individual shots only varied by a few fps.... You can see that the starting velocity at 1880 psi is the same, but look at how fast the pressure drops with the original setup, compared to the new one with the preloaded guide that stops before the hammer hits the valve.... Instead of using 760 psi for 5 shots, the gun now uses only 620 psi for 7 shots, starting at the same 1880 psi and ending at 1260.... This gives an efficiency of 1.48 FPE/CI.... which is absolutely stunning for a 50 FPE gun at these pressures.... I NEVER expected to see such a huge difference.... It sure shows how much air a PCP can waste if the hammer is bouncing enough to hear it....

I shot a 10 shot string with the gun tethered at 1880 psi, and the ES was just over 1%, about 11 fps.... That is pretty typical.... One thing I noticed is how fussy the preloaded setup is to tune.... The amount of preload is the "coarse" adjustment, you have to get enough so that the average spring force, with the longer, weaker spring, is a bit more than the standard setup.... I found that the best setup for the clearance between the end of the guide and the inside of the hammer, in this case, was very close, about 0.030".... This may have to do with the rather short hammer stroke on a Disco, about 0.67" on this gun (and that is longer than stock).... It turns out that each flat on the adjusting bolt (about 0.010") is about 10 fps of velocity.... Coil bind with this spring was about 985 fps.... One thing I have done on the Monocoque PCP, which I think is a good idea, is to drill the inside of the hammer large enough that the spring and the end of the guide cannot touch the sides of the hole.... so that there is no chance of extra drag on the hammer.... It does, after all, have to open the valve while coasting....

This setup worked so well that I plan to retrofit all my PCPs eventually.... It does take a bit of extra length to accomdate the hammer spring, because you need a longer, weaker spring.... but not excessively so.... How can I pass up the opportunity to use something that, in this case, improved the efficiency by 74%.... We do need to come up with a good name for it, though....

Bob

[rsterne]

I tried this idea on a regulated PCP today, and it went very well.... The gun was my QB79 Ninja, regulated at 1200 psi, that shoots 25 FPE with 15.9 gr. JSB Exacts.... I used two 1.75" x 0.040" springs on a 7/32" guide, giving a total spring length of 3.4" and a rate of about 7 lb/in.... I tried three different amounts of preload, 0.70", 0.80", and 0.90", and adjusted the position of the end of the guide from 1 turn of compression against the hammer to 3 turns of gap (24 TPI), so the total range of adjustment in position was 0.167".... Here is what happened....



You can see that the maximum velocity this gun can reach is ~850 fps with these pellets.... As I increased the preload, the velocity at any given gap between the end of the guide and hammer increased, as expected.... For instance, with 2 turns of gap (0.084"), the velocity with 0.70" of preload in the spring was 760 fps, with 0.80" of preload it was 790 fps, and with 0.90" of preload it was 820 fps.... With the guide just touching the inside of the hammer, the velocities were 836, 840, and 845 fps.... and with the guide pushing on the hammer (actual preload on the hammer at rest), the velocity was within 2 fps with all preloads, about 847 fps....

The interesting part of the graph above, is the efficiency curves (the dotted lines).... With the guide pushing on the hammer, the efficiency was horrible, about 0.55 FPE/CI.... I think what is happening is that the large amount of preload is slowing the closing of the valve, and causing what amounts to a dump shot.... Once the guide is clear of the hammer, however, the efficiency is greatly improved.... With it just touching, it is 1.15 FPE/CI, which is about the same as I used to get on the knee of the curve with the original setup.... As soon as there is a gap, and it doesn't need to be much, just 1 turn (0.042"), the efficiency is great, between 1.27 - 1.32 FPE/CI at all preloads.... The efficiency at any given velocity is slightly better with more preload, which to get that same velocity requires more gap between the hammer and the guide....

I ended up using the 0.90" preload and 1/2 turn of gap (just 0.020") between the end of the guide and the inside of the hammer.... This gave me 843 fps (25.1 FPE) at an efficiency of 1.27 FPE/CI, which is better than I had with the original setup.... The ES, at 10 fps over a 20 shot string, is a bit better, and the SD is definitely better than before, with by far the majority of shots being within about 4-5 fps, with only the occasional high or low shot.... possibly due to differences in individual pellet weight or fit.... All in all, I am very pleased with the results.... It would appear that this setup does, indeed eliminate hammer bounce.... I think the efficiency graph, being so flat once there is a gap, proves that point....


Bob

[rsterne]

I pulled apart my 2560 today and swapped it over to the new style one with the internal stop.... Here are the old and new parts....



I am now using a 10-32 machine screw with a piece of 7/32" OD tubing slid onto it for the guide.... Then you slide on the spring and an aluminum collar that hits the end of the gap adjusting bolt when cocking the gun and compresses the spring.... The stop nut is turned from a piece of 5/16" rod, drilled and tapped 10-32 and tightened against the end of the brass tube.... I start out a bit longer than I need and keep shortening the tube until I can get the gun shooting on the plateau with no gap.... Once I get the right length, I install the stop nut with a drop of blue Loctite, tightened against the end of the tube.... The stop nut has a slot in the end for this purpose.... The gap adjusting nut is drilled just over 5/16" because the stop nut has to slide through it.... The end cap is tapped 7/16"-20NF for the adjusting bolt, and then counterbored just over 7/16" to miss the cocking collar.... At the bottom of the photo is the original SSG I made.... You can see how much shorter the new guide is.... Below is the new SSG assembly, ready for installation....



When the gun is uncocked, the stop nut is recessed about 1/2" inside the gap adjusting bolt, so you can't see it.... Compare this with the one above, where the stop nut and bumper O-ring (no longer required) are outside the gap adjusting bolt.... Below is what the gun now looks like when cocked....



This installation is much cleaner, and there is no way you can pinch your fingers like could happen with the old one.... However, you still have a cocking indicator because the stop nut (the part with the slot) is visible when cocked.... With no gap I had about 985 fps, which I could adjust down to whatever I wanted.... At 1-1/2 turns out the velocity was just over 900 fps and the gun was very quiet.... I ended up about 1 turn out, with a average velocity of 960 fps, the same as I had before.... I might have lost a couple of shots compared to the first SSG, but I have still gained about 40% more shots per fill than without the SSG.... and I no longer have to worry about pinching my fingers and I can wrap my thumb over the wrist of the stock again because is it so much more compact.... The efficiency is around 1.40 FPE/CI....

Bob

[rsterne]

We have dubbed this device the "Stopped Spring Guide" or SSG for short....

Bob

[rsterne]

I was curious what I could get for efficiency using the SSG.... so I took my 2260 HPA, which is regulated at 1600 psi, and started dialing back the power and recording the pressure drop for 20 or 30 shots, using 18.1 gr. JSBs.... Here is what I came up with....



This is the first time I have had an efficiency greater than 2.0 FPE/CI.... At just under 12 FPE, I got 2.12 FPE/CI, which is 7.7 barcc/FPE.... That would give me 250 shots per fill on the 13 CI tank.... Adjusted for 23 FPE, the efficiency would be 1.7 FPE/CI and 100 shots per fill.... The way the gun is normally tuned, at 37 FPE, I can actually shoot down to 1400 psi without velocity drop, and I get 60 shots (6 mags.)....

Bob

[rsterne]

Yesterday and today I worked on fitting an SSG to my QB Ninja.... The gun is regulated at 1200 psi, and is modified to cock-on-open.... The best previous tune delivered 81 shots at 850 fps from a 13 CI tank filled to 3000 psi and refilled at 1100 psi.... For the preloaded spring for my "Stopped Spring Guide" modification I used two 1.75" x 0.040" wire springs on a single 7/32" guide, with 10-32 nuts both ends (the front one turned to fit inside the hammer).... The adjusting bolt is a 3/8" - 24 NF bolt drilled for the guide, and it is carried in a short piece of aluminum which is bolted inside the QB tube.... Here is the previous cock-on-open RVA, and the new SSG setup, positioned relative to each other the way they sit inside the tube....



In the uncocked position, you will notice that the end of the guide is basically flush with the end of the tube, but of course it sticks out when cocked.... I cured that later, as you will see below.... There are two screws to mount the aluminum block in the tube, a short one on the bottom and a long one on top, that goes through the breech and holds it down.... Since I didn't need the back part of the bolt that normally moves the pin for the cocking block, I cut it off flush with the back of the setscrew that holds the bolt handle, and the hole for the top screw for the SSG is immediately behind the bolt when fully back.... You can see the relative positions of the parts in the photo below....



In order to cover the back of the breech, I made a special bolt from a short metric flat-head screw by turning the head to 1/4" OD and thinning it slightly.... and then milled a recess in the breech cap to allow it to sit below flush, as below....



The bottom SSG mounting screw interfered with the trigger, so I used a low profile SHCS and counterbored a recess in the trigger as shown to miss it....



Since when cocked the guide and nuts stick out the back of the tube, and I needed something to mount the back of the trigger into, I made a cover from aluminum as shown below.... It is tapped for a 10-32 screw to attach the back of the trigger through the original hole in the back of the tube.... It does double duty to prevent the gap adjusting bolt from rotating as well.... The lip that sticks forward inside the tube has a flat milled on the top that just clears the flat on the bolt, so that once inserted the bolt can no longer rotate.... You do the adjustment with a socket wrench and then slide the cap in and bolt the trigger in place and the adjuster can no longer rotate....



When the gun is reassembled, the only change in appearance is the extended aluminum cap on the back of the tube, and the head of the upper screw on top of the breech under the scope.... It doesn't interfere with my scope rings, although something to consider if yours were in a different place....



During final tuning today I found a few more fps, and the gun now averages 851 fps (25.6 FPE) with the SSG one turn out from touching the hammer (0.042").... The ES is narrower than before (it was 12), only 8 fps over the first 86 shots of a 90 shot string down to 1100 psi.... Shot 91 was 838 fps.... This SSG gave me a gain of 9 shots over the previous arrangement (11%), increasing the efficiency to a pretty stunning 1.36 FPE/CI.... Here is the shot string....



I am now a firm believer in the SSG system as having the potential to increase the efficiency of a PCP, even a regulated one that was already pretty good.... If you can hear the telltale BUURRRPPPP of hammer bounce in you gun, and getting rid of it has been a problem for you, considering this modification could be the way to go....

Bob

[rsterne]

For some time I have been wondering how effective the SSG would be on a CO2 gun like the Crosman 2260.... I had the opportunity to try that out today.... The testing started out with the gun configured exactly as a stock 2260.... Stock valve, hammer and spring, stock transfer and barrel ports, and a 24" barrel, although the gun did have a Crosman steel breech on it.... I loaded a brand new 12 gr. CO2 cartridge, fired the gun twice to pierce the cartridge, and then shot a string using 18.1 gr. JSB Heavy pellets until the velocity dropped to 300 fps, at which point I stopped.... I fired one shot every 30 seconds, and the temperature was 68*F.... I then repeated the testing using a much lightened hammer that was 50% of the weight of a stock one.... In both cases the stock rear cap was in place, so the preload on the 2260 spring was completely stock.... I then fitted an SSG, using the same spring, with as much preload I could use and still cock the gun with no gap between the end of the guide and the hammer.... I adjusted the gap to 0.020", installed a new cartridge, fired two shots to pierce it, and then fired another string.... The results are shown in the graph below....



There are some interesting things to note.... In stock form, even though I was only shooting every 30 seconds to limit cooling, the velocity declined in a near linear fashion for 30 shots, and then dropped rapidly, with a total of 37 shots above 300 fps.... I calculated the FPE of every shot and added them up, getting a total of 341 FPE from a 12 gr. cartridge.... With the lighter hammer, the initial velocity was lower, but within 15 shots it leveled off, staying within a very narrow ES for another 30 shots before dropping, with a total of 53 shots above 300 fps.... The total FPE from that cartridge was 457 FPE, which is pretty good, and a 34% improvement over stock.... With the light hammer and the SSG, the velocity stabilized quite quickly, and I got over 60 shots before it started to decline.... and the initial decline was not as dramatic.... and I got a total of 78 shots above 30 fps.... over twice as many as stock.... The total FPE from a cartridge with the SSG fitted was 627 FPE, breaking my own previous record of 608 FPE with an 18" barreled 1750.... That represents 84% more energy from a 12 gr. than I got stock....

I am extremely pleased with the huge increase in FPE per 12 gr. CO2 obtained by using the SSG.... This was, additionally, the first attempt using an SSG on a CO2 gun for me, and breaking the 600 FPE mark without tuning was pretty impressive.... I look forward to trying some different settings to see if I can get even more energy from a 12 gr. in the future.... but right off the bat, we know it works great on CO2....

Bob

[rsterne]

Today I installed an SSG in my 2260 reversed tank HPA.... It previously got 43 shots with JSB 18.1 gr., from 3000 psi down to 1400 averaging 958 fps (37 FPE).... It had a 1.75" x 0.040" wire hammer spring (14 lb/in) and took about 11 lbs. of effort to cock.... This is the gun I'm talking about....



I made a Stopped Spring Guide to carry a 2.00" long 0.040" wire spring (12 lb/in) with about 0.45" of preload (~ 5.5 lbs), and played around with the gap to get the same velocity I had previously.... The gap ended up about 0.040", and the cocking effort was about 12.5 lbs.... The ideal spring would have been slightly longer with a 10 lb/in. rate, but I didn't have one, and as it turned out it doesn't matter.... Even though the maximum cocking force is slightly greater, because it starts out at 5 lbs. instead of less than 2 lbs., it feels smoother and you don't have that sudden increase in force right at the end, that sometimes causes you to not quite cock the gun, which with the MRod magazine causes a double feed.... In all the shots fired during testing (over 200) I never had a mis-feed.... Here is what the old RVA and the new SSG look like....



The old RVA used a floating spring guide that was captive between the spring and the hammer and extended through the adjusting bolt as a cocking indicator.... The 10-32 nut on the back of the new SSG is against a shoulder on the 7/32" guide, so I didn't need a 2nd nut to lock it, I just snugged it up with Loctite.... so the total length when cocked is nearly the same.... During testing springs I noticed that the pressure guage wasn't dropping as fast as before, so I was pretty anxious to check the shot count and efficiency.... Using the same pressure range as before (3000 psi down to 1400) I got 60 shots (instead of 43) averaging 966 fps (8 fps faster) with an ES of 10 over the string (12 previously).... The shot count, and the efficiency, increased 40%, to 1.49 FPE/CI (from 1.07).... I was absolutely SHOCKED and delighted by this result.... There is no question that the SSG can make a huge difference, particularly in a gun where you are pushing the power....

Bob

[rsterne]

Here is a general arrangement for the Stopped Spring Guide, for those who may be having trouble seeing how it is built....



The black rod is the spring guide, I make it from the appropriate size drill rod, and the spring seat on the right from a nut, threaded on, peened in place with green Loctite and then machined to shape and the same OD as the spring.... It sits in the end of the hammer, with a small gap.... The green part is the adjusting nut to set the gap between the end of the guide and the inside of the hammer, and acts as a velocity adjuster.... I make it from a fine thread bolt, and you will need to prevent it from self-adjusting, either with a locknut, or a plastic plug pushed against the side with a setscrew, as a brake.... It is threaded into the purple part, which is the rear end cap for the tube, tapped to accept the bolt.... The blue is a 90D O-ring that fits the guide.... The red is a nut, threaded onto the end of the spring guide, and can either be a NyLoc, two nuts locked together, or tightened against a shoulder on the guide (if you don't need it adjustable).... It sets the preload on the spring....

You use a longer, lighter spring than normal, with significant preload, perhaps 1/3 of the cocking force.... You want enough preload that with the guide just touching the hammer the gun will shoot the highest velocity you want (or is capable of with the porting and pressure you are using).... It is therefore a good idea to know what the gun can do before you start playing with springs.... When properly set up, with the correct spring and preload, you will be able to adjust the gun over a wide range of velocity by adjusting the gap, up to the point where the end of the guide hits the hammer.... If you reduce the gap to zero or tighter (actual preload on the hammer), the gun will waste a LOT of air for very little increase in velocity, because the preload will prevent the valve from closing quickly, blowing air out the barrel after the pellet has left.... The muzzle report will be a dead giveaway if you do that, it gets MUCH louder if there is no gap....

Bob

[rsterne]

For those of you trying to wrap your brain around what is happening to the efficiency here, this drawing may help.... It is "generic", the numbers are ficticious (but representative), the purpose of the graph is to give you a visual comparision of how the various types of hammer spring setup affect the efficiency curves.... and then hopefully explain why they are the shape they are....



With a conventional, preloaded hammer spring, the spring is in contact with the hammer all the time, even when it is opening the valve.... It actually helps the hammer open the valve, and slows the poppet on the return trip.... In a stock Disco, for example there is about 1/4" of preload and the valve only opens 1/16-1/8".... As you increase the preload, the hammer spring holds the valve open longer and longer, and eventually you get to the point where the valve is still open after the pellet leaves the barrel, and the velocity no longer increases.... This is shown by the plateau in the velocity curve on the above graph.... As you continue to add preload, you continue to waste more and more air however.... The efficiency curve is typically a shallow "S" curve as shown....

If you use a shorter, stiffer spring, if it is adjusted for a gap between the spring and the hammer (let's say zero preload occurs at -3 Turns out from coil bind), from there and out, the gap means that the hammer spring is no longer pushing on the valve stem directly, it has "launched" the hammer, which then does all the work.... It is proven that this setup generally has less air-wasting hammer bounce, and better efficiency, providing there actually IS a gap.... If you increase the preload to where there is no gap, you can waste even more air than with a weaker spring, because the higher force can retard the valve closing even more.... However, for the way you would normally tune a gun, on the "knee" of the velocity curve, you tend to get better efficiency.... This setup, however, for a given average spring force, will be harder to cock, particularly at the end of the cocking stroke.... and it is still possible to get some hammer bounce, because the spring, when the hammer first strikes it, has by definition zero preload, and will give easily, storing some energy to be returned as hammer bounce....

With the SSG, when properly adjusted to have a gap between the end of the spring guide and the hammer when uncocked, the spring again does not push on the hammer while it is in contact with the valve, the hammer does all the work.... If you adjust the gap down to nothing, and then start adding preload, then there is a very large spring force against the hammer while it is opening the valve, which holds the valve open, wasting air.... The point where the gap reaches zero would be at -3 Turns out on the graph.... The velocity at which this occurs (the knee of the velocity curve) will depend on the amount of preload on the spring, and the average spring force available to launch the hammer.... You might gain a few fps by adding preload, but the amount of air wasted will NOT be worth it.... On the other hand, once there is a gap, the efficiency is very good, because the large amount of preload prevents the hammer, when it is thrown back by the valve on closing, from storing energy in the hammer spring which is then returned, causing air-wasting hammer bounce.... The gap acts very much like an "of-off" switch for the efficiency.... With a conventional setup, there is a slope to the efficiency, increasing as you decrease preload.... while with the SSG, it is pretty much either "efficient" (with a gap), or "inefficient" (without one)....

I hope this helps you understand why I'm so excited by this development....

Bob