DISCLAIMER;

First of all, an apology. Steve and I started exchanging more and more information and I got carried away on a tangent. He was kind enough to "call me back on track". Perhaps I am too much of a scientist and love bare truth too much. So, I need to rely on good friends to make me see the forest, as I am too entangled in the trees; the delay is mine and mine alone.

What follows are my own personal observations, conclusions, and hopes of further work.
I take full responsibility for what I say or write. If someone demonstrates me that I am wrong, as I have done in the past, I will stand corrected.
Nobody's "Purfect".

Let's re-cap: the FOCUS of this series of articles was the Harmonics Tuner (HT); a relatively simple device that allows the shooter to tune a gun to the best possible performance of a given pellet in the gun, WITHOUT altering the MV.
​
The idea was to compare the performance of the HT under two completely different powerplants to see if there was ANY measureable improvement from both, as different as they are.
As you can see from the results, there is. Smaller or larger, but there is a measureable improvement.
This is what I feel is most original.
Typical airgun tuning is done through cutting coils, change of springs/guides, change of pellets, and adding or removing washers that go from full sized spacers to paper thin power adjusters. BUT that means altering the trajectory (because MV changes) so that accuracy can be obtained.
The HT, in a way and within reasonable limits, allows us to "have our cake and eat it too"; because we can select an MV (obviously within the stability range of the pellet we want to use, and assuming that the barrel does not deeply dislike the pellet), and THEN tune to optimal accuracy and precision (depending on what we want to do with the airgun) to the best possible results within that system.

I have also received "questions" about the validity of using 5 shot groups as "data points" being that sometimes variations between groups are as large without any change, as they are when performing small changes. As limited as our tests were, we are confident that the results are reproducible in any individual's airgun, as long as the proper procedures are followed. 
​The RESULTS for individual guns may be different (as is normal with most aspects in airgunning), but everyone can find an "optimum spot" for harmonics tuning.
​That is part of the essence of the beast.

So, having clarified all that, let's get going

We might make some tweaks still, as this is a pre-production model.
​We are working on making the HPM available as an after market part for ALL 54's (as usual, in good DIANA tradition, it will be fully compatible with all previous editions of the 54, though the T01's may need a little gunsmithing).
I am told that the stock will be available in several colors and that also more traditional Beech (and perhaps some Walnut) stocks will be made available. It all depends on the wholesalers, so if you are interested, write to your favourite wholesaler to request some attention to the point of your interest.

Steve and I have been exchanging a lot of interesting information, and I have also been receiving results from the Harmonics Expert, such as this:

So, I will be writing another entry, this one dedicated to a more detailed and better analysis of what TRULY happens with the spring-piston shot cycle, what determines the efficiency, what are common misconceptions about it, and analyzing more in depth the "phases" of the shot cycle.

Just to give you an advance taste:
The curve above belongs to an ASP 20
You can clearly see that the first acceleration spike (red oval) is where the piston reaches the end of the air compression run, and that a few msecs later (green oval) there is an even greater "slam" BUT, the human senses cannot detect it.
You can also see the 70 g's force vibrations developing during the time of the pellet in the barrel (-20 to +50 g's for 7 msecs, though the dwell time of the pellet in the barrel is shorter).

It takes relatively sophisticated instruments to detect and measure them, not only in magnitude, but also in time. The chart also shows a good, long decaying vibration pattern (blue square).
And all this from a gas spring gun that has been reported as a "smooth shooter" and excellent at bench rest.
¡Surprise!

Anyway, it will be fun to actually make the real behaviour of the shot cycle evident for different airguns. And to see what works and what does not work, independent of what shooters "feel".

Conclusions:

The HT is a device that has been tested and proven in different power plant designs and has proven its value.

​DIANA will include it in the next " edition" of D54 rifles.

There are still other technologies that await research; as long as there are people that are fascinated by the spring-piston airgun, whether it is a steel spring, or a gas ram, there will be things to discover and improve.
And THAT is also part of the magic of airgunning.

Again, I need to thank Steve for his excellent participation and impeccable work. It has been a great pleasure working with him on this.
And also thanks to our readers because without dedicated airgunners willing to read a LONG, FOUR part, entry our words would be like the sound of a tree that falls in the middle of an empty forest . . .

Keep well and shoot straight!






​HM

N.E.
For those of us that remember B&W TV, "B" Westerns, and were raised on a visual diet of clear-cut good and evil as white hats and black hats, the name 'Dragnet' will always recall Det. Sgt. Joe Friday.
And badge # 714 will always mean dedication to the truth and unswerving loyalty to the Law.
Among its most famous phrases:
"All we want are the facts, Ma'am"
In honor of such a simple, unswerving, and devoted to service attitude, we decided to title this entry.
​
Take it away, Steve!
​

​As promised, I wanted to provide you with more in depth detail and comparisons between the two power plants that were used in the recent Harmonic Tuner testing.  

I mentioned in that write-up that after recrowning the barrel on my 56TH, Hector completely assembled my barreled action with some of his own components and returned to me for  testing.  It arrived with one of Hector's Short Stroked HMO powerplants, a tuned T06 trigger and the most beautifully detailed Diana 54 stocks I have ever seen! He went all out on this set up provided to me for testing.

We have already seen the rifle in action during the Harmonic Tuner testing.  Now lets turn our attention to the powerplants,  beginning with the Short Stroke HMO set-up.

28mm x 76mm HMO Short Stroke

The set-up as provided to me included everything you see pictured below;

Compression Tube with Reinforced Breech Seal
HMO piston
Main Spring with dual guides
Cocking Handle with modified fulcrum and linkage extension

It was set up for my requested energy level (11.2 to 11.4 fpe range)

​The HMO piston is a work of art with an o-ring main seal, nylon top cap, and a rear guide that is nicely machined to ride solidly inside the action, an improvement to the flared out piston skirt on factory pistons. 

You can also see the dual spring guide set-up.  This includes a conventional guide that inserts into the main spring, and an outer guide that surrounds the spring.

​Looking at the Transfer Port you can see the seal has a center reinforcement collar measuring approximately 3mm ID.

​The piston weighs in at about 317 grams.

​Inside the outer spring guide are the preload spacers, one of which appears to be a softer compound anti-bounce washer.  It is the blue washer in the picture below.

​I mentioned the "Modified Fulcrum" on the cocking handle.  Here is a close-up showing the re-positioned cocking linkage pin.

​The linkage rod end is solidly supported with nylon washers on either side for a smooth cocking stroke.

​To account for the relocated pivot point, the linkage rod has a threaded extension added.  In this picture you can see a factory linkage rod (bottom), alongside of the modified cocking linkage (top)

​It is evident that Hector has done his homework on this set-up.  It is precise, consistent and was easily tuned on target using the harmonic Tuner.  The cocking stroke is extremely light and will have you questioning whether or not your are truly making over 11fpe.

After all testing was complete, I purchased this set-up from Hector.  It is well made, durable and easy to tune on target.  I was able to successfully tune to sub 3/16" c-t-c 5-shot groups across two series of tests at 32 yards.
​
AGT 22mm x 96MM Skirtless

This set-up has been covered in much greater detail in a couple posts I have made on GTA and AGN, but I will give a quick overview;

This set-up consists of a factory Compression Tube sleeved down to 22mm.  The transfer port retains the factory 4.1mm ID.

The piston uses the factory piston rod, and has a dual seal piston affixed on the end.  The dual seals consist of a parachute seal and an o-ring main seal, and help reduce the effects of temperature changes on your velocities.  The piston also has two supporting guide bearings, with a special top hat that also functions as a third support bearing.

This is an exceptionally smooth powerplant and was producing groups as small as 1/4" c-t-c at 49 yards.  It is also very well made, solid and durable.  Adjusting preload can be done without a compressor.  I am able to compress the trigger assembly into place with one hand and insert the trigger block pins with the other hand.

​This piston weighs in at 149.5 grams

​In addition to the mechanical and dimensional comparisons, I also set up a test to measure the cocking effort for each set-up.  

This was accomplished by taping a Protractor above the Cocking Handle, centered directly over the pivot point.  The rubber grip cap was removed from the cocking handle and a slotted wooden block was inserted inside the end of the handle.  The block was supported by rubber sheeting, and had a groove in the center.  I then looped a wire tie around the handle, centered in the groove.  The rifle was supported firmly in a padded support and measurements were taken using a digital scale. 

The weight measurements were taken at specific locations in the cocking stroke, with the max reading being recorded just before the piston rod contacted the trigger mechanism.  Due to the differences in cocking stroke length across the power plants, the max readings were not the same as you can see in the chart below.

The pull point was centered right where you would place your hand when cocking the rifle and scale was kept inline with the direction of pull.

Here is the set-up for testing:

With the barrel crown issue now resolved, we moved to the next step in this project which was the HT (Harmonics Tuner) installation and testing.  

With the rifle set up, Hector conducted his own series of tests just to verify what setting it would prefer on target.  Once complete, he reached out to me to see if I would be willing to continue with this testing.  Hector said that he would like to keep the rifle set up just the way he had it for my first phase of testing.  After discussing the terms of this testing I agreed without any hesitation.

A few days later the rifle arrived.  For this testing you would think that the equipment would consist of an old beat up stock and parts that were laying around in Hector's shop.  After all, it would be packaged and shipped to my shop, plus be assembled and adjusted throughout the testing, and finally disassembled packaged and shipped back to Hector.  

Much to my surprise when I received the package, I found an exceptionally well packed rifle in a padded hard case, with precisely cut out areas for each item.

Included were a Vortex 6-24x50 Diamondback Tactical, a custom parallax adjustment wheel, an accurized ZR mount, and the most beautifully detailed D54 stock I have ever seen.  Wrapped over the stock was a padded leather cheek piece...very nice!  

​Inside the back it looks like a nylon collar machined for an exact flush fit to the end of the muzzle with zero o-rings added

​Adjustments are made by adding or removing o-rings.  Adjustment range is between 0 and 10 o-rings.  Simply loosen the 3 set screws, slide the HT off, add/remove o-rings, reinstall HT and lightly snug the set screws.

​Here is a view from the front of the HT.  I think this picture was taken with 9 o-rings installed.

Now let's talk about the arrows:
​While I was doing this chart I saw a pattern to both tests that I did not notice before.  Both tests have a sort of rolling pattern to them where the groups gradually increase, then decrease in size, and then start over again.  Look attentively at the arrows in the chart above.

Ignoring the normal group to group variation that you would expect to see when shooting consecutive groups without making any changes, you can definitely see these patterns.

It looks like the 28mm set-up has a more pronounced climb from smallest to largest group sizes, and the 22mm has a less pronounced climb.  This was reflected in my article and supported by the data.  However, in the 0-10 o-ring range of testing, the 22mm had three cycles of this pattern, while the 28mm had only two.  In my opinion this supports the theory of increased frequencies being produced by the 22 mm's kits. 

The 28mm generates higher harmonics (as shown in the greater spreads from smallest to largest group sizes), but lower frequency (only 2 complete cycles of small to large groups in this testing)

The 22mm on the other hand made three complete cycles in this testing, indicating higher frequency.  The peaks and valleys (group size differences) were lower so the harmonics were not as pronounced, but it occurred more frequently.

So, looking at this chart, I would bet that the targeted ranges for o-ring settings should be similar if you target the valleys (smallest groupings).

For example, on the 28mm the best settings could be either 0 o-rings or 9 O-rings (at this exact set-up, velocity, pellet, etc).  A slight change to the set-up would require a new test, but results should be similar with 2 specific settings being revealed in the range of 0-10 O-rings.

On the 22mm set-up, you should find 3 valleys across the range of 0-10 O-rings due to the increased frequency of the harmonics.

This is VERY interesting!!  I am theorizing that tighter groups in both tests would relate to the barrel being tuned to match an end point of muzzle movement with the release of the pellet.

​Testing summary;

28mm x 76mm HMO Short Stroke

The HT was very effective in tuning the 28mm x 76mm short stroked powerplant. You could see dramatic changes in group size with even the slightest change in HT settings.  The size difference between the largest and smallest group was .512".  
You could also clearly see when you were in the sweet spot ranges for the tuning (1-4 o-rings, and 8-10 o-rings).  This powerplant did not like the range of 5-7 o-rings.  This would be in direct relation to the barrel harmonics when using this particular powerplant, and at this particular power setting.

AGT 22mm x 96mm Skirtless

While I could see slight changes across the adjustment range of the HT, the overall effects of the adjustments were less.  There is no doubt that a sweet spot could be found for dialing in for the smallest group, but the size difference between the largest and smallest group was only .312".  

Further experimentation will be needed on the 22mm set-up.  Now that the crown issue has been addressed, I will be following up with group testing using both the HT and the factory muzzle weight.  This particular powerplant may very well prefer the added heft of the factory muzzle weight.  Previous testing revealed .25" groupings at 49 yards on this set-up using the factory muzzle weight.

I hope you found this HT Testing interesting and helpful.  

Up next will be an article that focuses on the two powerplants and some of the differences between them.


Steve

A Guest Blog by Stever Herr

PROLOGUE by Hector Medina (Editor)

As humanity moves along in its "progress", it is more and more clear that the only thing that is constant in the UNIVERSE, is: change.

The modern world has brought a number of inconveniences (like worldwide running Flu's), a number of disasters (like Global Warming), and a lot of veritable Tragedies (The continuous State of War since the beginning of the 19th Century).
BUT, it has also brought serious marvels; some of them, undoubtedly, double edged swords that can cut either way.
Among those marvels is the Internet and the level of communication that we can have now with people we have not ever met face to face. And, yet, the commonality of passions, the willingness to suspend (or lower) the "offense" level, and the usage of a "lingua franca" (English), allows us to work; share knowledge, ideas and experiences; and grow together with people that are hundreds of miles away, or thousands, it really doesn't matter.

And that enriches our life. Yes we can cuss the internet for the amount of garbage that we receive on the EMail everyday, but we also have to be grateful for the genuine and wonderful communication opportunities it affords us.

This FOUR PART SAGA is the result of one of those opportunities. A veritable "meeting of minds" that resulted in some fantastic experimentation and some solid science being made.

And so, without further ado, I yield the floor to the Steve Herr, the Gentleman from Pennsylvania, noting that it was a privilege and a pleasure to work with such a fine mind in these experiments.

​A 56TH Gone Awry
About 7 months ago I purchased a Diana 56TH in .177.  This was my first Diana with the sliding action design and I was anxious to give it a try.  After much reading and correspondence with Hector Medina, I tore into the rifle, going over everything from top to bottom.  The sled mechanism was polished up, T06 trigger disassembled, polished, relubed and set to a mere 8.2 oz pull.  The powerplant was replaced with an AGT 22mm skirtless design from Tony Leach, and the sled tension was tuned for best groups on target.
While I was very pleased with the results, I was seeing some fliers that concerned me.  It was shooting extremely well, don't get me wrong....but the fliers weren't going away.

After much work to try and resolve this issue, I turned my attention to the factory crown.  The factory crown had been machined at a slight angle, see attached picture.  When looking at the crown you could see that it was deeper and wider in the area of 1:00 O'Çlock to about 7:00 O'Çlock. You could also see that the grooves were blocked off along the edge in the area hi-lighted in yellow with only 5 of the 8 grooves being visible. 

​I finally decided to hand lap to see if I could improve on the crown, and help with the issue of occasional fliers. Using a brass hand lapping tool from Brownells, I went to work.  After the hand lapping, I was now able to see all 8 lands.  However, the crown was still angled.  If there is a lesson to be learned here, hand lapping will not correct a crown that is not square to the bore.  In fact, it may actually make the angle worse.
Here is the crown after Hand Lapping.

​After hand lapping I returned to the target to see if there were any improvements.  At 32 yards it was quite apparent that not only was the problem unresolved, but it had actually gotten worse with groups beginning to scatter.  As the title of this article indicates, this rifle was going HAYWIRE!!

After many hours of experimentation to try and resolve, I reached out to Hector to see if he knew of anyone who could re-crown a barrel square to the bore.  I was pleasantly surprised to find out that this was a service that he performs in most of his builds, and that he would be willing to look into my barrel in detail. 

We also discussed the heavy factory muzzle weight, and the tendency they may have in contributing to fliers.  He suggested some options for replacing the factory muzzle weight.  The options offered were a Harmonic Reducer or a more simple design called a Harmonic Tuner.  I said that I was open to his suggestions and would go with whatever he recommended.

A short time later my rifle was completely stripped down and the bare barreled action along with the barrel shroud was on it's way to Hector.

After evaluating the rifle, Hector began the process of re-crowning.  Here are a few pictures showing the step-by-step process.    Hector continued to machine the surface until the entire internal edge was the same depth and square with the bore.    If you look closely you can see how the crown was squared up nicely.  ​

On the final step, Hector added a taper around the outer edge, the end result being a nice target crown.

Hector's next step was to verify that the barrel would shoot.  In order to do this, Hector utilized one of his own powerplants, tuned to the desired level that I would be shooting the rifle (11.4 fpe), and using the same pellets that this rifle seems to prefer...JSB Exact 8.44 (4.53).  He also installed one of his stocks and a scope for the testing.
At 35 yards in a seated position (not benched), Hector confirmed that the rifle would offer up nice uniform groups rather than the scattered groups I had been seeing previously.

It was a HUGE relief to hear that Hector was able to save the barrel on my 56, and seeing the more uniform groups returning on target was extremely encouraging!
A job well done!!!
After the recrowning, Hector moved to the next part of the project, that being a suitable replacement for the heavy factory muzzle weight.  I will cover that in another write-up.

​
Steve

There has been a lot of interest in the 430L lately.
Probably as a result of the fantastic price that it's being offered at ($150 for a high quality gun is a great deal in anybody's book).
We have been working with the little rifle for some time now, but before we can go into "making it all it can be" we need to explain a little about how the GEOMETRY of a spring affects its characteristics.

To be quite frank, I owe a debt of gratitude to a young friend that plainly and to my face told me:
"You launched a bunch of information on materials when people do not really understand how springs even work"

So, yes, my fault. I should have posted this entry first and THEN the entry on Specialty steels now available for springs (which already have two reactions from spring/gun manufacturers). I do apologize for that and should have done things more in order.

So, Recapping:
Last entry about springs, we talked about the materials and how the different inclusions of metals as additives, or as alloying agents (not the same thing), change the behaviour of the material itself, it's capacity to store energy (as opposed to deforming permanently with it), and the spring's resistance to fatigue.

In this entry, we will explore a little about the science of "resistance of materials"  and, hopefully, we will help our kind readers understand the concepts and the relationships between single characteristics/variables that make or break (pun intended) a spring in an airgun. By the end of the entry, we hope that the readers will have a fairly good understanding of what "stress" and "strain" are, what geometrical aspects of a spring define those two quantities, and how the numerical values attained by those quantities relate to the "fit for purpose" quality of the material being used for an airgun spring.

Elastic/Inelastic.- The first thing we need to understand is that ALL materials are partly elastic and partly inelastic.
By elastic we mean that: IF subject to a force, and retained in place by some resisting mechanism (otherwise the object would simply move), THEN the material will deform in a proportional relation to the force applied.
By inelastic, we mean that the material, IF subject to a force and retained in place, will deform in a manner that is NOT proportional to the force applied.

Elastic limit / Yield Limit.- We have all seen relatively stiff and hard materials (like structural steel), bent beyond any recognizable shape, whether in a car crash, an explosion, or a building collapse due to some human error (the New Orleans Hard Rock Cafe Hotel being the most recent that comes to mind), but this tells us that even a metal that is DESIGNED to be stiff and resilient, turns from elastic to inelastic when the force becomes too much to bear. Materials Science aims to understand WHY and HOW, so that materials can be improved over time.

Stress and Strain.-
Stress is what we, airgunners, understand in HPA (High Pressure Air), as "Pressure". When applied to "solids" it cannot distribute evenly all across the material, as it does in gases or liquids, but it is the same concept: A FORCE (vector) applied over a surface, its units are pressure units.
Strain is the deformation of the material when subject to a stress (note that we now do not need to establish that there is retaining mechanism because the actual presence of an INTERNAL "pressure" in the material actually tells us that the material is being restricted and is accumulating that pressure inside the material components). It does not have units, as it is a proportion of whatever dimension the material had before being stressed to the dimension of the material when subjected to said stress.

Example: Let's think of a rubber band. Simple office/household/workshop/sports device. It can be the rubber bands obtained from old inner tubes you used when you were a kid to build your slingshot, or it can be the rubber band that holds the deeds, wills, contracts, and other important documents in your house fireproof safe.
We know that, if you apply a force to extend it, while retaining the other end, the band's LENGTH grows. We may also have noticed that the band's width and thickness were REDUCED by the extension of the length.
We surely will have noticed that if you pulled too hard on your slingshot, sometimes the subsequent shots came out "weak", or that if there are too many documents in the sheaf, then the rubber band seems to not hold them as well.
It means we have exceeded the elastic limit of the rubber band. That is, the point where stress and strain stop being proportional.
Using our new words, we can say that the stress induced in the rubber band by our pulling exceeded the allowable strain (yield point) for the rubber used in the band.
Savy slingshot builders will know from experience that there are many ways to build a better slingshot, but probably the easiest is to use surgical tubing instead of rubber bands made of cut inner tubes.
By improving on the material used, we built a better product.

Interesting so far, but, if you stop to think: ¿Can you compress a rubber band?
Hmmmm.- not really so, ¿how come we can compress steel springs, but not rubber bands?

Of course the answer lies in the geometry, but also in the way the molecules/atoms of materials stick to each other.

Some materials form crystals, some create amorphous structures. A Crystal is just a material where the molecules of the material are arranged in a geometric pattern, and therefore they resist pretty well any force that tries to tear them apart (operative word here is TEAR).
An amorphous material is a material where there is no geometric pattern, but still the molecules bind to each other by electromagnetic forces and therefore resist forces in a less "stiff' way, but they do resist deformations.

That sounds logical? Hope so, it still does not answer why we can compress steel springs, but not rubber bands.

Are there any examples of pure crystalline materials? Almost. Jewels are a good example, they are not perfect because even the most perfect diamond will have inclusions and flaws. It is THESE that the experienced cutter uses to split a large chunk into smaller pieces that can then be sold as the engagement ring stones that start most couples in the long road to family bliss.

IF the rough diamonds were perfect, there would be no way of cutting one in an orderly fashion and the huge gem would most likely explode if someone tried to cut it. It would have to be eroded/ground away little by little and a huge "rock" would be the result, like the British Crown Jewels.

Is there a perfectly amorphous material? Play Doh comes to mind. It does have SOME spring to it, but very little, so that little hands can create things out of it.

Between these two extremes, everything else in the world finds a place.

Now, ¿why steel?

We use steel because it is a fairly abundant mineral (Iron/Fe), therefore cheap. And we have been doing research and development on it for about 3,200 years.

HEY! Someone will say: We still do not know why we can compress steel springs and not rubber bands!

Yes, I know, we're getting to that, Patience and Perseverance, my young Padawan.

So, Iron, in its relatively wild state, is an amorphous material (Iron Oxide for the most part). and even when reduced in a kiln and rendered into ingots, the amount of carbon inclusions present as a result of the smelting process will make the "pig iron" fairly brittle. ¿Is it brittle because it is crystalline, or because there are lots of impurities and inclusions? Actually both.

PURE iron would be a crystal (actually there are two types of iron crystals, both cubes), albeit unstable ones (but that would be getting into Quantum Physics and that is beyond the scope of this entry), and so it would be brittle. It is also brittle because the casting process creates stresses along lines of non-homogeneity (impurities and inclusions), and because SOME forms of steel in the iron would actually act like a "wedge" in the material and make the stress fractures grow  rapidly and uncontrollably.
Assuming a very high degree of purity in the iron, then it would bend easily. Not much stronger, stiffer, nor resilient than the best Bronzes.

So, we refine and change the pig iron into steel by including and diffusing the carbon content (reduction process of the oxide, and then purposely oxidizing the excess carbon by blowing air into the molten metal), and then increase the content of other metals, and non-metals, to increase other characteristics (hardness; stiffness; resistance to fatigue, to high or low temperatures, to oxidation, etc); from Molybdenum and Vanadium, to Copper, Silicon, Manganese, Chromium, Nickel, and Cobalt.

Now, the elements above mentioned, whether added as alloying agents, or as additives, diffuse themselves into the crystalline lattices that iron forms. And when temperature is changed dramatically (quickly) either in a quenching, tempering, drawing or other thermal process, then further compounds form: Carbides and Silicates being the prevalent ones.  More important than the compounds is how the crystalline structure in the steel changes with heat treatments.

When we said that Iron creates two types of crystals, we were referring to a Body-Centered Cube (2 Iron atoms) that is called Ferrite, and Face-Centered Cube (4 Iron atoms), called Austenite.
When the Austenite cools down abruptly, it shears the cubes into two tetrahedrons, where the carbon is diffused into a much more compact space, and therefore the crystal itself exhibits very high "strength". Problem is that it also become brittle. These tetrahedrons structures are called Martensites and they are the ones most responsible for the hardness in steel.
Since the crystalline structure change is forced by thermal processes, it is mostly, and controllably, reversible. So, for every quenching, there must be a tempering and a drawing, in order to create a material that is "balanced" between hardness, stiffness and resistance to breakage.

Now that we have explained how the physical properties of steel change due to the change in the crystalline structures, we can say it:

We CANNOT destroy any material by "pure compression" . We can compress materials but compression by itself does not induce neither stress nor strain.

If you remember, when we stretched the rubber band, we observed the thickness and the width of the band to decrease. THIS is what WE call "compression", but in reality is the RESULT of a stretching.
The PROPORTION between the elongation in the direction of the stress that is being applied and the contraction in the perpendicular direction is the Poisson Ratio for that material.

In the same fashion, if we were to have a perfect cube of steel that we put it in a press and then apply force on it, it will fail not because we are compressing the material, but because the material that we are compressing is placing a STRETCHING force sideways into the surrounding, unsupported, material, and that is what tears the material apart (¿remember I told you that the operative word was TEAR?).
​
If I am not making too much sense, consider the same case, but in a hydraulic environment, where we apply a force to the fluid, and the fluid itself applies evenly and in all directions, a pressure to the steel cube.
¿Do you think it will fail?
.
.
.
Nope!

Let's conduct a "thought experiment":
Think of a material as a collection of "leaves", formally called lattices, and then let's think about how important is the directional connection of the internal structures of the material.
Imagine a sheaf of paper. While it is packed and in the wrapper, it can be incredibly strong and behave almost a like a solid brick.  You can stand it upright and you can stack a bunch of them one on top of the other.
BUT, if we remove the wrapper and try to stand a single one on its own, it will collapse.
While on the wrapper, we cannot bend a whole ream, but out of the wrapper, the whole ream can be bent and fanned, and "aired", as we used to do with the feedstock of old copying machines.

IF we apply enough force to a wrapped ream of paper, it will BREAK the wrapper, if we apply a little force, and then place it on a table it will return to is prismatic shape out of its own weight.
In one case we have exceeded the yield strength of the material, in the other we have not.

Important here is to stress that, when out of the wrapper, if we bend the ream one way, there will be one end that becomes "longer", if we bend it the other way, then it's the other end the one that becomes "longer".
In the middle, there will be one sheet that keeps its apparent length, and we will call that the "Neutral Fiber"

So, back to steel airgun springs: ¿What on earth are we doing when we compress the spring?

Savy airgunners and specially those tinkerers that turn their own spring guides know that springs will increase in Outside Diameter (D) when compressed.
If you think about it, a length of metal can only increase its length if it is being STRETCHED. So, when we compress a steel spring, in reality we are stretching each and every coil.
This stretching is what induces the stress and the strain, and it is the reason why we must pay special attention to the SURFACE of the spring. Any imperfection or point of stress concentration will start a micro-fracture that, in the end, will break the spring.
Reason why good quality springs are shot peened and / or covered with some material that prevents the formation of micro-cracks.
Usually, when we receive a spring from a manufacturer, it is longer than expected, unless we specify that we want our springs to  be shipped "set". In this case, the spring manufacturer will compress the spring to solid and then ship it to us. By compressing to solid, the yield / elastic limit is reached and the spring is reduced to the shortest possible length. No further compression cycles can exceed the yield limit of the new conditions (length and diameter), and therefore all further usage will happen within the elastic limit of the material in question.

Can we make springs out of other materials? Yes, and we do. Beryllium bronze is used in explosion proof applications, copper is used in some other -mainly electrical- applications, even aluminum can be used but it's very limited; hopefully, once we master the creation of crystalline aluminum ("Transparent Aluminum" of Start-Trek fame), we will have a fantastic material, specially for scope building. That, though, is several years into the future.

One last thought experiment: If you drop a steel ball bearing into an anvil, it will re-bound. But, it will not rebound to the same height it was dropped from. Even in vacuum.
And if we measured the temperature of the steel ball, we would see that after many re-bounds the temperature has gone up slightly. ¿What is happening?
What is happening is that even the hardest of steels that can absorb huge forces and not yield, still cannot return 100% of the received energy.
SOME energy is lost in the INTERNAL FRICTION of lattices and molecules that bend and suffer recoverable internal displacements.
The SUM of all these INTERNAL energy losses is called Hysteresis

Now, putting it all together:
A material can receive a stress, accumulate it as a strain, AND RETURN IT as a force, only when the elastic yield limit has not been reached.
It will not return ALL the energy, some portion of it is inevitably lost to hysteresis.
The Elastic Yield Limit will be determined by the structure of the material as much as by the chemical and crystalline arrangement that is controlled by post material manufacture heat treatments.

So, now that we understand how springs work; how materials work, and fail; and how we need to look at the limits of the material to select a "fit for purpose" material for our springs, we can go into the GEOMETRY.

Let's dive off the deep end and go directly into an equation:

Frightening, ¿Isn't it?

And yet, this equation says nothing more scientific than if I would assert: "The moon is a huge, spherical, rock that goes around the earth in ever increasing ellipses governed by Newtonian physics"

Both, the equation and my assessment, are true and consistent with the human experience of thousands of years. Both give humans a good frame of reference to study the object of interest, whether it is the moon, or the coil spring. Both can yield good insights into what the effects of changes can be. 
And yet there is a degree of empiricism into both, they are not "pure science".

The reason is simple:  since the formation of the moon from the proto-earth to the present, there have been many phenomena that have taken place that we can neither prove, nor disprove. So we can only establish theories of HOW the moon came to be, WHY it is going away, and WHAT happened in the past that cannot happen in the future because the distance to the moon is only increasing.

In the same way, the complication of a coiled wire that changes apparent cross-section with each millimeter  of compression, that is subject to microscopic heterogeneity and that can have vastly different characteristics, while at the same time having vastly different histories, is very hard to pin-point.
BUT
The experience of the last 162 years (since the machine to economically make coil springs was invented), has shown that it is "Close enough".

And, so, let's take element by element in the equation into analysis and consideration:

"k"

k is the constant of the spring. In other words, how much force it can store for a given length of compression. You can think of it as lbs(force)/in, or Newtons/m, or kgs/m, or oz/in, or . . .
In the end, you will have a rifle with a power stroke of length "X" and you will want a spring that can develop "Y" force, so that the piston can be accelerated and pushed into the compression chamber that will make the air inside go through the transfer port and push in its turn the pellet out of the barrel, ¿no?

So, given an ARCHITECTURE of a gun (power stroke, bore, swept volume, piston's mass, and transfer port length) we can estimate how much force we will need, and therefore, what characteristics of the spring we need to achieve.

OR, assuming an average efficiency of the basic airgun architecture, we can also start from desired energy yield at the muzzle and go back all the way to the force required and therefore the "k" we need to achieve within the constraints of both the gun and the spring material.

So, let's simplify, clarify, and amplify (SC&A):
"k" just means how strong the spring will be, given a length of compression, it comes from Hooke's  Law that states that  'F=kX' where F stands in for Force, k is itself, and X is the power stroke.

"G" 

Is the "Shear Modulus". It means how rigid a material is, how much stress is required  to create the necessary  amount of strain that will tear the material apart.

It CAN be calculated using the Elasticity/Young's Modulus (the tangent of the angle of the line that describes the stress-strain behaviour of a material in the linear -elastic- zone; also called the slope of the curve in this first section -before the yield point-) and the Poisson Ratio (the relation between a stress applied in a direction with the strain created in a perpendicular direction).

Now, BOTH the elastic modulus and the yield point, as well as the Poisson ratio are determined experimentally for specific materials, again, this is empiricism. So, there are tables that tell us what are TYPICAL Shear Modulus values for different kinds of steels and other materials just for the sake of comparison:

As you can see, the range of "G" for economically viable materials is pretty narrow: between 76 (GPa) for "Nickel Steel" (basically stainless), and 79 for "Structural Steel".

I am including other materials just for the sake of comparison, and if we go to serious spring makers' websites, some of them list the characteristics of the materials they use.
Still, a pretty much uniform value for G is around 79.3 GPa for ALL steels that are of viable use, whether they are labelled as "valve" or "specialty" steels.

​So, SC&A: G is, for all intents and purposes, an empirically derived value that for airgun use should be taken as the constant 79.3 GPa/11.5 Million PSI

"d", "D", and "na".-

"d"  Is the diameter of the wire used. Using a fairly standard diagram:

"D" is the mean diameter of the spring, or the diameter between the neutral fibers of the steel used in making the spring, which just means that we need to subtract one wire diameter from the Outer Diameter.
A little like calculating a center to center group size: You measure the two farthest edges of the two extreme shots and deduct a caliber. Same process, same principle, same arithmetic.

"na"  is the number of active coils, which for most purposes (except Vortex's PG-3), would mean the number of coils that are ACTIVE, therefore the two end coils do NOT count.

SO;  By now we have almost everything we need to start playing with the numbers and understanding how one variable affects the possible outcomes.

As usual, since the advent of computers and the internet, there are online calculators that do the number crunching, this is one of them:
https://www.acxesspring.com/spring-calculator.html
and it is fairly easy to use.

Let's try an example:

We take the 640 T spring from ARH's website:
https://www.airrifleheadquarters.com/catalog/item/251488/8128905.htm

Just because it is new and it provides an interesting contrast with most other springs offered. It is a WIDE spring : 0.640" ID, with a relatively thin wire: 0.120", 35 coils and an OAL of 13.25" (unset).

The 0.640" ID would mean a 0.880" OD and a "D" of 0.760" or 19.3 mm's

So, what would the calculator predict?

And so, we have the force that the spring can store: 20.564 #/in with those design parameters.

But, what would happen if we increase the Wire diameter (for the purpose of this numerical simulation, we will change ONE thing at a time.

Let's go to 0.130" wire:

We've gone from a k of 20.564 to a k of 29.47, this is a HUGE increment for a mere 0.010" in diameter of the wire.
But then, remember the equation? In the equation, the d is raised to the 4th power, so a 9% increase in wire diameter would imply over 40% increase in k.

What would happen if we had REDUCED the mean diameter of the spring?

Let's go back to our 0.120" wire and reduce the OD to 0.774" that would imply a D of 0.654:

Now we have a k of 32.272 #/in.
Why that value? because that value gives an ID of 0.534" that is a tight fit into the OEM DIANA spring guides.
What would the equation have predicted?
A reduction of 14% gets raised to the power of 3 in the divisor, so the net effect is an increase of 57%
For a base case of 20.564, a raise of 57% would mean 32.27 #/in, so we now have a feel of how changing one aspect affect the outcome.

The last variable we can play with is the number of active coils, but that is an inverse relation, so it is not too complicated to imagine what effect reducing, or increasing the number of active coils will have in the result.

Ideally, we should choose the biggest OD that will yield the power we are looking for.
We can increase the d, within limits, and every gun will achieve a "plateau" where more force will not increase muzzle energy because the swept volume has been completely used.

The biggest possible OD will yield the least stress in the spring, therefore, this will guarantee the longest and most stable life possible.

For most calculations, the springs we use in airguns are doomed to failure because calculations cannot take into account the effect of a guided spring (fore and aft). SOME calculators do, but you need to pay for them and it is not in the intent of this blog to make you pay for information.

Most spring makers would balk at the duty cycle we submit our springs to. But then they are concerned mostly with industrial applications where millions of operations are desired before you need to shut down the plant for maintenance. Maintenance, for some of us, is part of the hobby.

In the next blog entry, we will deal with what can be done to the DIANA 430L, short of changing the cocking lever arrangement, because that will be looked into in another blog entry that will deal with efficiency of powerplants.

Hope you have enjoyed this trip into the rarefied world of the airgun designer.

I apologize for the length of the entry and thank you all for staying with the thread of ideas. It is a complicated aspect to explain and I hope I have not confused everyone even more.

;-)

Keep well, shoot straight, and if I don't get the time to write the entry for the 430L before the holidays come tumbling down on us, like the walls of Jericho, I wish you all, the best of peaceful, contented, filled with family and friends, holidays!






​HM