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

[rsterne]

Al, I don't have a near-stock 22XX around here, and there was little point doing it to my 1740 or 1750, as they already get over 100 shots per cartridge (at just under 500 fps).... I would not be at all surprised if on a stock 2240 it might double the shot count with basically no loss in power.... I'm REALLY looking forward to your results....

Bob

[I_like_Irons]

Here is a non-protruding method that I put  into my .25 Armada.  The stock design prevents anything from protruding out the end cap. 



It may still be holding the spring a little bit long as there is almost no hammer play between the valve and the pre compressed spring.

The spring is from a stock Gen 1 .177 Marauder.  The hammer stroke is now at max.  I lost a little bit of velocity with this lighter spring, but I now get 30 uniform shots on a 165 BAR (2395 psi)  fill.  I'm going to turn the velocity up soon.  I am also making another slightly improved version using the spring that I took out of

The velocity for the JSB Kings (25.4 gn.) started at 709 ft/s, peaked at 721, and fell back to 708 at 124 BAR (1800 psi).  The average velocity was 712 with a sample standard deviation of 3.6.  This appears to be an efficiency of about 1.6 ft-lbs E/Cu in.  (If I found the right calculator). 

The parts for the cable stopping spring guide are two  cut down .38 special cases that started to have cracks on the mouth, a bit of 50 pound test Dacron fishing line---four strands, a bit of glue, and a piece of plated brass for the stop in the end cap. 

The caps were drilled out and notched, and the line threaded through and secured with a plug and glue.  The hammer side has a larger hole to accommodate the adjustment screw.  The line wraps around the case on this side as well. 

The improved version has 17/64th holes drilled in each case head, notched and the line is secured with JB weld.  It is untested as of yet. 

[BigTinBoat]

, a bit of 50 pound test Dacron fishing line---four strands, a bit of glue, and a piece of plated brass for the stop in the end cap. 

FYI - Dacron line will stretch. Not sure how it will affect the design.

[Gippeto]

Little update;

Took some tinkering to get this working sort of right. Initial shot count was no better than stock. Seemed like too much dwell due, I suspected to too much striker mass. Tried it with a hipac and could only manage a decreasing string from 2000psi. Sooo...put the striker on a diet.



Still too much hit. :roll:  Changed to a spring that I thought was WAY too light and got an increasing string with the hipac. Cocking effort is pretty much the same as stock.

Also made the spanner wrench...WAY easier to make adjustments now.



So back to the 12g carts...not really "tuned", just enough preload to pierce the carts and about a .030" gap. Quieter than before and no cloud of co2....and no braapp. :wink:  Zeros are a read error on the chrony.



Pretty pleased with these as initial results, will do more tweaking yet....which is the fun part.

Should perhaps note the shots were not stretched out...load, aim, shoot, write it down, repeat.

Al

[I_like_Irons]

, a bit of 50 pound test Dacron fishing line---four strands, a bit of glue, and a piece of plated brass for the stop in the end cap. 

FYI - Dacron line will stretch. Not sure how it will affect the design.

True, but if it didn't it could not absorb any energy.   Even  the steel rods in the devices have to stretch in order to absorb energy.   It appears that the  Dacron line has a 2% elongation at break.  This is .04 for a 2" long string.  At 50 pound test, it means that each strand will absorb 2 in-lbs at break.

A spring that has about 10 inch pounds of energy would break the four strands were it not for the significant mass of the hammer.  The actual energy the strands have to absorb is going to be a lot less than bringing them to break.  The stretch is also going to be significantly less than .04" 

However, this may be part of why I don't think I currently have enough of a gap.  The efficiency is not bad, though, with what I have.   I'm going to play next with the original spring (shorter) and make a flush hammer head. 

Oh, the other nice thing about the Dacron line is that it has no appreciable creep.  That is it does not get longer over time while under stain. 

[Rallyshark]

, a bit of 50 pound test Dacron fishing line---four strands, a bit of glue, and a piece of plated brass for the stop in the end cap. 

FYI - Dacron line will stretch. Not sure how it will affect the design.

True, but if it didn't it could not absorb any energy.   Even  the steel rods in the devices have to stretch in order to absorb energy.   It appears that the  Dacron line has a 2% elongation at break.  This is .04 for a 2" long string.  At 50 pound test, it means that each strand will absorb 2 in-lbs at break.

A spring that has about 10 inch pounds of energy would break the four strands were it not for the significant mass of the hammer.  The actual energy the strands have to absorb is going to be a lot less than bringing them to break.  The stretch is also going to be significantly less than .04" 

However, this may be part of why I don't think I currently have enough of a gap.  The efficiency is not bad, though, with what I have.   I'm going to play next with the original spring (shorter) and make a flush hammer head. 

Oh, the other nice thing about the Dacron line is that it has no appreciable creep.  That is it does not get longer over time while under stain.

I wonder how spider wire would work in this application?

[ngozdex]

any suggestion about how many gap bettween hammer and valve stem
maybe some one here already got some data abaout some various gap and the efect

[Gerard]

I'm not sure how much 'data' can be presented for this measurement, as there are so many variables depending on each person's other modifications, their desired power level, etc. But generally speaking I think it's safe to say that the smaller the gap the better when the right spring strength and preload are in place for a given tune, as this makes the most use of the spring's energy. Too large a gap and striker impact will be less, resulting in lower power. Of course this can be used to fine tune the velocity... but again, getting into too many variables, and adjusting preload for power then closing the gap to quite small seems a better approach.

[BigTinBoat]

 Even  the steel rods in the devices have to stretch in order to absorb energy. 

Can you explain this? Are you saying the guide rod "stretches" on each shot to absorb the energy from the spring? If so then how are you keeping the gap the same? Wouldn't the gap (between spring and hammer) be getting smaller on each consecutive shot?

[Gerard]

Over time some steels may creep slightly, but I think what was meant was that there's energy being absorbed via elastic elongation of the guide rod, which then returns to its normal length. Very, very slight elongation. Like an incredibly stiff extension spring.

[I_like_Irons]

 Even  the steel rods in the devices have to stretch in order to absorb energy. 

Can you explain this? Are you saying the guide rod "stretches" on each shot to absorb the energy from the spring? If so then how are you keeping the gap the same? Wouldn't the gap (between spring and hammer) be getting smaller on each consecutive shot?

All non brittle materials will stretch or compress depending on the load direction.  It may not be much in the case of a steel rod (think in terms of ten thousandths of an inch).  But it will spring back to its original length for a given load.  It is not a permanent lengthening until you exceed the yield point.

The energy the rod or cable absorbs is the equal to the force times the distance.  If the distance of the stretch or compression is very small, the force is quite large.  This is why you can break cast iron with a hammer, but it will hold a large static load without breaking. 

Cast iron has very little give, which makes it brittle.  If you take something very resilient like a rubber band  there is a lot of give, and it can absorb more energy with a lower average force. 

Steel, string, and all materials act a bit like a rubber band, but on a scale that you cannot see with your eyes. 

[rsterne]

I think he means the steel guide rod is stretching elastically (which is true), but it will be well within the yield strength of steel.... Many of the SSG designs use an O-ring to cushion the blow on the stop nut on the rod (which is likely hundreds of "G" without it)....

Spectra (spider wire) has appreciable creep (elongation with load over time).... This is mostly due to its slipperiness, the strands in the line don't stick on each other, allowing the braided line to get longer and thinner as the gaps in it move.... (We used to use Spectra for the sheets on our R/C Model Yachts, which is why I know that)....

The amount of "gap", in theory, can be as small as possible, particularly on the original design where the guide stops and only the hammer keeps going.... I notice a complete elimination of hammer bounce in just 0.020" of gap most of the time.... Less than that, and you are probably in the grey area where the O-ring is compressing and the head of the poppet is too.... With some of the designs, it seems that the gap needs to be a bit larger to complete the transition between preload (bounce) and fully debounced, perhaps as much as 0.040-0.060".... In addition, Travis ran into a bit of a weird characteristic with the bstaley style SSG, where the guide passes through the hammer and it is the end of the guide that opens the valve.... He found that as he increased the gap from zero, the velocity actually increased for a bit before declining again.... We think what is happening there is that since the guide is stationary, back away from the stem, and is picked up by the moving hammer before it hits the valve, it has to have a short distance to accelerate.... and hence the velocity peak at 0.040"-0.050" of gap (on the one Travis was working on, anyway)....

All of my SSG installations are on guns where I know where the plateau is (ie what is the maximum velocity the gun can shoot), so I basically keep increasing the preload until I get to that (or very close) with no gap.... I then increase the gap to drop the velocity onto the knee of the curve, which does two things simultaneously.... It makes sure that the valve is closing well before the pellet reaches half way down the barrel (which is about where the plateau starts).... and the SSG eliminates the hammer bounce.... Tuned this way, the efficiency, and the report, are like an on-off switch.... LOUD AND INEFFICIENT changes to QUIET AND EFFICIENT within about a half turn on the gap adjuster.... BRRAAAPPP to SNAP!....

Bob

[SynRodSteve]

This is a fascinating thread.  Pardon my ignorance but can you explain what is meant by 'knee of the curve'?  Also, how do you measure the gap. 

Steve

[real7]

Anyone that has done Bob's "original" SSG on a Marauder, did you remove the stroke screw in the hammer? If you did, what did you replace it with?

[BigTinBoat]

Spectra (spider wire) has appreciable creep (elongation with load over time)....

This is what I meant by "stretch", now Irons was saying that the "steel" rods "stretch too", but they really don't (in the same sense of the word) unless the pressure placed on them is beyond their "yield" strength, right?

So using the "steel" the gap will stay the same, but using the "dacron" (or spectra) the gap will get smaller (with use).......this is what I was providing the FYI for, is this not correct?

[mooseslayer]

Steve...if you are using the design that has the 3/8 bolt screwing into the endcap, when you turn the bolt  in, the oring will move away from the endcap when the spring guide hits the hammer. If you mark one of the flats of the bolt you could measure the distance from the endcap and then turn the bolt out (to start creating a gap) 1 flat or half a turn and measure the distance between the endcap and bolt and adjust accordingly.

[SynRodSteve]

Makes sense.  Thanks,.

[Gerard]

  Pardon my ignorance but can you explain what is meant by 'knee of the curve'?

Bob has written extensively on the subject, you can find references to the 'knee' in just about all his posts on airgun tuning. It's that part of the measured shot string where velocity peaks. Just after that peak is just 'below the knee', and that's the most efficient part of a given tune in terms of air use for a given power setting. Where that knee sits for a given airgun will vary depending on other elements of the tune. So it's up to you to do chrony testing and chart your results and determine where your knee is for that setup, then in applying that to the SSG one has a number to look for on the chrony while also getting the gap just right.

[sawtoothscream]

really getting up to 50 shots in a .25 m rod with this mod?    Need to get a parts list and install one of these when I tune up in the spring, sounds amazing

[SynRodSteve]

Thanks Gerard.  I was assuming it was something like that.