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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

A New Method for Increasing the Efficiency of a PCP - the SSG

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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....

Follow through the thread and you will see this device is now called the Stopping Spring Guide or SSG for short....

Bob

Motorhead:
AZ ( Alan Z )tunes the Brit Theobens very similar in that NO preload is used having hammer do the coast to impact thing.

Great idea bob given the room to have the mechanics of it not interfere with your grip hand.

* Now, would not having the rod holding spring be better yet if LIGHTER ?
would seem the weight of the rod being accelerated along with hammer slows up the lock time some.
also being its weight contributes nothing to the hammer energy upon poppet ... lighter is better, no ?

Also if i understand this correctly ....
The jam nuts hold head of spring rod off the hammer at impact yet bottoms out against bolt used for preload.
So ... any changes done to preload ALSO require resetting the rods depth ?

oldpro:
 we did something similar with the FLEX it has 1/2 of free play on the hammer spring with no pretension and runs on twin guides. Prevents all hammer bounce and any spring bind or deflection. But this has to be built into the gun design because you need a lot of hammer throw for this to work or you end up having very stiff hard cocking effort. Good work there Bob.   

K.O.:
"
We do need to come up with a good name for it, though....
"

Since I have a Norwegian last name and it controls the Hammers Strike...

I'm all in for calling it...

Thor

 ;)

rsterne:
I have been using short heavy springs with negative preload for quite a while, the idea started here in North America a few years ago by Lloyd Sikes.... It can reduce hammer bounce to be sure, but the price you pay is greater cocking force required at the end of the cocking stroke.... For whatever hammer energy and momentum you need it is the AVERAGE spring force and the hammer travel that generate it.... Don't confuse how this setup works with a short heavy spring with no preload.... they are VERY different.... Cock one and then the other, and you will see....

I agree with you that the weight of the spring guide must be accelerated by the spring, but does nothing to the valve.... So, the lighter you can make the guide, as a percentage of hammer weight, the better.... If you are dealing with a regulated PCP, and tune it to be efficient, you likely have little hammer bounce anyway.... However, most unregulated PCPs tend to have at least some bounce at the low pressure end of the string, and some poorly tuned ones even at the high pressure end.... I have never been a fan of any HDD that uses friction to slow the hammer, because as well intentioned as they may be, most seem to hinder the hammer on the strike as well as the rebound.... When you start playing around with Big Bores, hammer bounce is something that for the most part hasn't even been addressed....

Some guns may not benefit from this idea.... but others obviously will.... It's just one more tool in the toolbox, to use or throw away, as you see fit....

Bob

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