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RVTMaverick · Member

Hey Bob,

Awesome info. and I thank you...
Hey before you do any milling (IF it's not too late?) Could I see a picture of your Air Rifle Scope set up?

Also, I am asking this because I do not understand... Why would this matter?
I'm thinking I may have to reduce the hammer spring preload a bit to reduce the air consumption
I under line the thing I wondering about and that is..... WHY does\would doing any kind of adjustment to the "hammer spring" have any kind of affect on AIR useage?

And again Beautiful Rifle!

Peace Jeff

rsterne · Moderator

The hammer spring on a PCP governs how hard the hammer hits the valve when you pull the trigger.... That in turn governs how far/fast the valve opens.... which controls how much air escapes to propel the pellet....

Most PCPs are tuned so that the first shots are a bit below the maximum velocity.... the velocity increase.... and then starts to drop again.... This is called the "shot string" or "bell curve".... It occurs because at high pressure the valve is resisting being opened all the way by the air pressure, and the pellet receives a very short, sharp pulse of air.... As the pressure drops, the valve opens longer, and the pellet gets more air, but at a lower pressure.... Eventually, the valve can't open any more, and as the pressure drops, so does the velocity....

When you increase the hammer spring hit, the velocity tends to increase (more air released).... but in addition, the pressure of the maximum velocity part of the curve moves upwards.... Once you have the first shot being the fastest (as is the case for my DAQ).... the gun is operating outside of its "normal" parameters.... and tends to waste a LOT of air in doing so.... Taken to the extreme, the valve will still be open after the pellet has left the barrel, simply wasting air.... Decreasing the hammer strike MAY lose a bit of velocity, but the air usuage drops faster than the velocity does.... I say MAY, because if the valve is being held open until after the pellet has left the barrel it can't be accelerated any more.... and the wasted air is being expelled, doing no work.... I have a feeling my DAQ is operating in that mode (blasting air out the barrel after the pellet has left).... It is the only explanation for the extremely high air usage.... IMO....

Bob

RVTMaverick · Member

Hey Bob, good morning,

Thank You very much for the detailed explanation sir, AWESOME!

Hey Bob, You taught me something NEW today,.... does this mean I get to go Home with PAY!? Smile

Peace Jeff

rsterne · Moderator

Only if I do as well!.... Mr. Green

Bob

rsterne · Moderator

I got out to the camp today and got the DAQ sighted in at 50 yards.... that will put me about 1" high at 25 yards.... Before sighting it in, I tried the first couple of shots across the Chrony starting at 3200 psi and it ran 855 and 808 with the ending pressure about 2400 psi.... This compares to 836 fps at 3000 so the gun is still gaining speed at 3200 psi.... Definitely gonna have to back down the hammer spring preload a bit.... it's still using 400 psi per shot, even at 3200 psi.... Shocked

I tried a shot at 2700 psi at two pieces of fir 2x6 stacked together at 50 yards.... The muzzle velocity would have been about 795 fps, and the remaining velocity about 765 fps (155 FPE).... It blew through the first one and imbedded in the second, stopping about 1/2" from going through.... ie 2.5" of penetration at 50 yards.... I'm pretty sure a maximum power shot would penetrate both pieces of wood at point blank range.... Here are the pics....

The first pic shows the two pieces stacked and the bullet in place.... The black line indicates where the wood was crumbled and fell apart when I cut the piece of wood in half, ie the damage zone.... The bullet curved over in the second piece of wood, rotating almost 45 degrees from the entry line.... The second photo shows the exit hole from the first piece of wood, and the bullet after I removed it from the second piece of wood (above it).... You can see where it stopped, about 1/2" from going through.... Crazy Power.... Mr. Green

Bob

rsterne · Moderator

OK, so here's what I found out when I pulled the DAQ hammer out today.... First, some dimensions....

Hammer
Weight: 177 gr. + cocking lever (15 gr.) = 192 grams total
OD: 0.995"
Length: 2.36"
Bore for spring: 0.516"
Depth of bore: 2.10"
Stroke: 1.62"

There is a flat milled along the bottom of the hammer for the sear to slide on.... The sear catches on the square front end of the hammer (which is hardened) when cocked.... There is a milled notch for the sear to catch on to keep the hammer from sitting on the end of the valve stem, providing a "safety catch" to allow filling the gun without fully cocking it.... The aft end of that notch is showing wear from hitting the sear already, in about 100 shots.... Shocked

Spring
OD: 0.505" +/- 0.002"
Wire: 0.062"
Coils: 25
Free length: 5.44"
Preload: 1.20" (uncocked)
Cocked length: 2.62"
Compression (cocked): 2.82"

The spring appears to be hand made.... The coils, particularly at the ends, are not perfectly spaced, and the ends of the spring are not bent down (closed) to contact the next coil, merely ground off on an angle.... There is a shouldered plastic spring guide that fits into the end cap which is drilled 0.516" to locate the spring.... The knurled end cap is threaded into the end of the main tube, and prevented from rotating by the rear breech screw.... To be frank, the quality of the machining inside the tube where the hammer slides is quite rough.... Of everything I have seen thus far, this bothers me the most.... Because of the design of the end cap, I think the easiest way to make a velocity (preload) adjuster would be to use a relatively small (1/4" or less) diameter fine thread screw and counterbore the spring guide for the end of it.... The only thing that bothers me about that is that once the shoulder on the spring guide moves very far, the spring will no longer be contained by the hole in the end cap.... In addition, there is no simple way to REDUCE the preload as the guide is already bottomed in the hole in the end cap.... Since the whole point here is to reduce the preload, I decided to look at modifying the spring....

I could have simply cut off a coil or two on the spring, but the uneveness of the coils bothered me.... This usually indicates that he spring has never been "set".... ie collapsed completely to coil bound and then allowed to spring back.... When you set a spring, the coils rebound to an even spacing, at least even enough that you can't see it with the naked eye.... I slid the spring over a piece of 3/8" threaded rod with a nut and washer on each end and proceeded to collapse it.... I first wound the nuts in until the spring was at its "cocked" length (2.62").... The coils were pretty much all over the place for spacing.... I wound the nuts in another 4 turns (1/4") and let the spring rebound and measured it's free length.... It had just begun to collapse, the free length had shortened by about 1/32".... I repeated the operation, going 8 turns beyond the cocked length.... The spring was beginning to look better, with more even spacing between the coils as the ones that had the bigger space collapsed first.... The spring rebounded to 5.25", so it had collapsed about 0.2".... It looked MUCH better, the spacing appearing much more even.... I checked the OD, and it had only increased about 0.001".... Encouraged, I went another 4 turns, so that I had now compressed the spring 3/4" more than it's cocked length.... The distance between the coils was now becoming much more even, and the free length was now down about 0.35".... I went another 4 turns, and the spring was now looking very nice, straighter, very even coil spacing, and nearly coil bound.... When I released it and checked the length, I found that it had collapsed a full 1/2", with the free length now measuring 4.94".... It had taken a full inch of compression past the original cocked length to achieve this result.... The spring coil spacing was now very even, and the spring was also straighter than when I started, indicating that every coil was now more symmetrical....

I checked the spring rates using a web based calculator, and in both cases, it says the spring rate is 9.9 lb/in.... so now I know that "setting" a spring doesn't change the spring rate, only the free length.... Here is how the spring compares to what I started with....

Preload: 0.72" (instead of 1.22") Force = 7.1 lbs (instead of 11.9) - a reduction of 40%
Compression (cocked): 2.32" (instead of 2.82") Force = 23.0 lbs (instead of 27.9) - a reduction of 18%

Lloyd shared his hammer spring energy calculator with me, and it gives the following results:

Stock spring
Hammer Velocity = 70.4 fps
Energy = 1.63 in.lb.
Momentum = 0.72 in.lb./sec.

Set spring
Hammer Velocity = 61.0 fps
Energy = 1.23 in.lb.
Momentum = 0.62 in.lb./sec.

If I cut 3 coils from stock
Hammer Velocity = 61.9 fps
Energy = 1.27 in.lb.
Momentum = 0.63 in.lb./sec.

What do these numbers mean?.... I dunno, but I'm sure I will begin to understand them as I get more data.... I do know that setting this spring produced similar results to cutting off 3 coils in terms of the effect it should have on the hammer strike.... I don't have a backstop that I can fire the DAQ into inside as of yet.... but I decided that one test I could do was to see how much air pressure it dumped with the set spring installed but no bullet.... It now dumps 300 psi per shot.... I assume that it would dump less with a bullet in place.... and I don't have a comparison for how much pressure it would have lost with a dry fire when stock.... but my gut feeling is that I'm going to be close to what I want.... I asked my wife if she could hear me shooting in our detached garage, and she said no, which is a good sign.... That means that if I have a backstop strong enough I can at least do some Chrony readings without having to go out to our camp.... That's a good thing, because it snowed today.... so my days at the camp are likely over for the year.... The plan is to make a temporary backstop from some timbers and find out what the velocity is and the air consumption.... If it turns out I need to detune the gun more, then I'll shorten the spring a coil and try again.... If it turns out that I've already gone too far, then I'll either make a new spring guide, a spacer for the end of the spring inside the hammer, or an RVA....

Stay tuned.... Mr. Green

Bob

SMP · Silver Status Member

Jumpin Jimminies

That's a BIG A$$ spring Shocked

Sounds like your havin fun Bob! Ya know going a little too far and chefing up a pretty RVA has a lot of merits. Think double cut acme, with a pitch rate large enough to go from minimum to maximum in less than 1 revolution.

What are you planning on using the rifle for?

rsterne · Moderator

Well I couldn't wait to try it out.... I got a 12" log about 16" long to use as a backstop and tried a few shots with the hammer spring I modded yesterday.... At 3000 psi, the first shot was 856 fps.... 20 fps MORE than stock.... I did a shot string starting at 3300 psi (my hand slipped, really it did!) and got the following....

882 fps (206 FPE)
858
824
797

There was 2100 psi left, so the gun was still using 300 psi per shot.... Way better than stock, but the hammer spring was still too strong.... By now I'm convinced that somehow Dennis used the wrong hammer spring in my gun, so I pulled the gun apart and modded the spring some more.... This time I closed the end two coils to that they sit nice and flat.... I then fully set the spring, completely coil bound.... and then sanded the end two coils flat.... The spring is now only 4.50" long (neary an inch shorter than stock) and has 23 active coils instead of 25.... The spring rate is therefore higher, at 10.8 lb./in.... I ran the numbers through Lloyds hammer spring calculator and got the following....

Set spring, 23 active coils
Hammer Velocity = 53.6 fps
Energy = 0.95 in.lb.
Momentum = 0.55 in.lb./sec.

By now I'm wondering if I went too far.... I reassembled the gun and took a dry shot at 2500 psi (that was in the gun) and it was obviously working (not valve locked) so I filled it to 3000 and fired one over the Chrony.... 835 fps, the same as it was stock.... Now was the time to shoot some serious strings....

I filled to 3200 psi, and shot 6 shots before it fell below 800 fps.... WOW! that's 6 shots faster than the stock rifle's second shot.... Shocked

.... I shot several strings, a few from 3200 and then gradually less as the tank pressure dropped, stopping at a 2900 psi fill.... In every case, the string used 950 psi for 5 shots or 1150 psi for 6 shots.... less than 200 psi per shot.... That is less than HALF the air the gun was using stock.... Mr. Green

The graph above also shows the stock performance.... Starting at 3000 psi I was getting only 3 shots, with the second shot under 790 fps.... With the modded hammer spring I'm getting 6 shots from a 3200 psi fill (5 shots from 3000) with the same maximum velocity and the last shot is over 790 fps.... Here is the summary....

Stock
3 shots averaging 787 fps (163 FPE) with the velocity dropping like a stone between shots
400 psi per shot (0.39 FPE/CI) - and the gun was VERY LOUD as a result

Now
5 shots averaging 825 fps (180 FPE) between 811 and 833 fps
<200 psi per shot (0.88 FPE/CI) - over twice the efficiency with the gun MUCH quieter
6th shot at 792 fps is faster than the second shot from the stock setup

Is the 3200 psi safe?.... Well, Dennis has an article about the Exile on his website that talks about using 3200 psi, so I would presume if he felt that was unsafe he would have either not used it, or put in a disclaimer.... I'm not going to worry about it.... Had I used a 3200 psi fill on the stock gun (I actually did try one string), the shots would have been 855, 808, 760.... That gives an average of 808 fps (172 FPE).... While the first shot is stronger than the gun shoots now, the average over those three shots is less than I get over 6 shots now.... I think that's a darn good tradeoff, so the gun is going to stay the way it is for a while.... There remains one unexplained anomaly, however.... With the intermediate spring setup.... I got a shot at 882 fps (206 FPE) at 3300 psi.... and 855 fps at 3000 psi.... That is significantly stronger than the gun shot stock and I am at a loss to explain it.... unless the gun is simply wearing in a bit....

Bob

SMP · Silver Status Member

Is it possible that when you set the spring there was less hysteresis and therefore a cleaner transfer of force
edit: In the sense that the valve was recieving several little smacks as opposed to a clean strike

rsterne · Moderator

I suppose anything is possible, it could also be a change in the lubricant on the hammer and spring.... I had to wipe off the (rather thin) oil that was there originally and used some moly paste on the spring ends, a tiny amount (mostly wiped off) on the spring exterior, and a couple drops of "Moly E" (a mix of moly and motor oil) which I use as a general purpose lubricant on assembly for any moving parts....

Bob

SMP · Silver Status Member

Bloody nice string btw....I bet your pretty thrilled with the improvement

rsterne · Moderator

How can you not be.... same peak power, very small fps range, way more average FPE, and twice as many shots on less air.... Mr. Green

Bob

SMP · Silver Status Member

I'd be willing to bet the accuracy has gone up as well now that the boolit is not getting a kick in the butt by the excess air blast.

rsterne · Moderator

That may require a wait until next spring to get into the camp again.... I have target boards set up there at 15, 35, 50, 75, and 100 yards, and can probably stretch it out to 125.... Ya gotta love Crown land.... Mr. Green

Bob

RVTMaverick · Member

WOW Bob! IF I may Sir, Your NEW Call SIGN>>>====> "Mr WIZARD" Smile

As this is, to put it mildly, some Super Magical Work Brother, Awesome! Cool

Peace Jeff