Whenever the topic turns to the internal ballistics of spring-piston airguns, it's never long before the term "piston bounce" will bounce onto the scene. But how much sense does it make in the first place? After all, the only thing that opposes the motion of the piston is the air trapped between it and the end of the compression chamber -- merely air and only air.
And air is soft and squishy. Right? Why should anything violent be expected to happen?
Well, normally yes. But when air is violently compressed, not so much. And the way it's compressed in the course of a springer's firing cycle makes the stuff the piston hits at the end of its stroke less like the "soft and squishy" air we're all used to, and more like a solid brick wall. The plot below, of a 25mm diameter piston moving down a 100mm stroke pushed by a constant 100lb mainspring force, illustrates what's happening.
When the piston begins to move, the air in front of it is uncompressed (blue curve = psig) and offers little resistance to its acceleration (the very epitome of "soft and squishy"!), allowing the piston's kinetic energy (red curve = ftlb) to accumulate virtually unopposed. It's not until 80% (80mm) of the stroke has been completed and the piston has gathered its maximum kinetic energy (~21ftlb) that pressure rises high enough to cancel mainspring force and begin to slow it down.
But by now the piston is travelling like a bat-out-of-you-know-where (~16m/s = 16,000mm/s), so it continues to fly forward, causing pressure to soar to 1000s of psi and only finally stopping with only a few mm (and 10s of microseconds) of stroke remaining and the air in front of it become that "brick wall" mentioned earlier. Short version...
WHAM!
Notice that the underlying reason for this phenomenon is the properties, not of a particular piston, compression chamber, or mainspring, but of air itself.
Which, fundamentally, is why all springer pistons bounce: Because they're all filled with air and therefore all have that "brick wall."
When I first examined the test target. I surprised to see a relatively-heavy for-bore 10.34gr pellet was being utilized. I could somewhat rationalize the use-of a relatively-heavy pellet for accuracy testing. But-for chronographing the 80K ... that's another matter all-together.
Accordingly, after reading one-of Hector J Medina entries in his Connecticut Custom Airguns blog. I think I may-have some-understanding as-to “why” Airguns of Arizona would do-so.
Scope was mounted and gun was tested (of all things with Baracuda 10.6 grs.pellets) at 20 yards. Why that choice, I really don't know. Odd choice anyway and not representative of the gun's performance potential. The ONLY reason I can think of is to protect the gun by preventing the dieseling that COULD occur using lighter pellets.
This set-me to ponder-about ... what are the peak-pressure differences between a light-for-bore pellet versus a heavier-for-bore projectile. This would-be the key for explaining why a heavier-pellet may-be less-prone to dieseling.
Moreover, "piston-bounce" being the spring-and-piston air rifle dynamic that induces "forward recoil." And, considering that "forward recoil" is the single-most destructive element effecting mounted optics. The question-is ... whether is it heavier or lighter pellets for-bore that generate the most "forward recoil?"
After some-thought, I'm thinking it's lighter pellets that produce the most "forward recoil." As-that, lighter-pellets provide less-cushion for the advancing piston.
I believe that heavy for caliber pellets cause more piston bounce and that the old axiom that springers like mid-weight pellets while pcps and CO2 airguns like heavies is generally true.
Heavy pellets fractionally delay pellet release which affects the design ratio - (traditionally 9 or 10: 1) - of swept volume to minimum necessary air cushion to keep the piston head from slamming into the front wall. The slower than designed release creates a larger air cushion (pellet hasn't started travelling down the down the barrel increasing volume) which reverses piston movement sooner inducing more piston bounce or multiple bounces or both. The same effect can be created by an oversprung rifle which compresses the air more quickly prior to pellet release. This is the reason why a more powerful (than spec or tolerances allow) spring generates harsh recoil.
Very light for caliber pellets can have the opposite effect. Such pellets are released sooner increasing volume which may allow the piston to slam into the front wall.
Thoughtfully engineered springers are designed to work well - within with a range of normal pellet weight. A diet of pellets at either extreme can induce harsh firing behavior, inaccuracy, and, eventually, damage the rifle.
Jmo, based on long discussions with airgun designers and many years of personal experience. And I have the burned and bashed piston heads that show the results of my misguided experimentation!
I don't think it is a matter of heavier for bore as it is for the size of the power plant. Too light a pellet does not seem to have enough inertia to dampen the piston and this causes greater bounce and seal damage.
The extent of piston bounce is not just determined by weight, but also by start pressure, and start pressure trumps weight.
A high start pressure pellet (RWS 8.3gn Superdome and 6.9gn Hobby, for instance) will cause greater bounce than a low start pressure pellet of equivalent weight, such as a JSB 8.4gn and 7.3gn (the latter marketed in the UK as Falcon Accuracy Plus).
An example of the Hobby and Falcon Accuracy Plus in a Mk.3 TX200 with an 85mm stroke and standard soft spring.
@jim-in-uk - I agree that piston bounce is directly related to start pressure, but thought the question related to pellet weight. Therefore the response was to explain the effect of weight on start pressure. Absolutely concur in your conclusion that bore fit is the primary bounce bugger.
Hey Man - No problem. We are all (mostly) friends here trying to figure out and share airgun info. Steve runs a good forum and everyones' questions and advice is appreciated. It is one of the things that makes the Yellow special.
A long-time ago, I recalled reading some cautionary advice against using heavy-for-bore pellet in a spring-&-piston air rifle. I accidently came-across the information by accident while searching for an aftermarket-trigger for my F4.
Although heavy pellets such as the Kodiaks will not damage Co2, PCP, and Pumper type airguns, they can and will sometimes cause severe damage to the main spring in Springer Guns. They can cause damage and spring fatigue beginning with just a few shots and when disassembled, the damage caused by heavy pellets and detonation is easily detected and identifiable. The spring failure usually is not the fault of the spring.
For the longevity of your spring gun: My suggestions are for .177 caliber gun that themaximum pellet weight should be 9 grains. My suggestions are for .22 caliber gun that themaximum pellet weight should be 15 grains. Use heavy pellets at your own risk and expense.
A long-time ago, I recalled reading some cautionary advice against using heavy-for-bore pellet in a spring-&-piston air rifle. I accidently came-across the information by accident while searching for an aftermarket-trigger for my F4.
Although heavy pellets such as the Kodiaks will not damage Co2, PCP, and Pumper type airguns, they can and will sometimes cause severe damage to the main spring in Springer Guns. They can cause damage and spring fatigue beginning with just a few shots and when disassembled, the damage caused by heavy pellets and detonation is easily detected and identifiable. The spring failure usually is not the fault of the spring.
For the longevity of your spring gun: My suggestions are for .177 caliber gun that themaximum pellet weight should be 9 grains. My suggestions are for .22 caliber gun that themaximum pellet weight should be 15 grains. Use heavy pellets at your own risk and expense.
The weight of a pellet has less effect on detonation than its start pressure.
This shows the recoil of a .177" Anschutz 335 deliberately set up to diesel, and to detonate. The recoil is a reasonable guide to what the piston is doing, and in the detonation it is clearly being accelerated way back up the cylinder.
If you look at the area of piston bounce, you can see that the diesel starts the pellet moving earlier than normal in the compression stroke, lowering cylinder pressure and allowing the piston to travel further but, that the high start pressure pellet sticks in the breech and raises cylinder pressure at any given point in the compression stroke compared to the pellet that's on the move, elevating air energy (temperature) and pressure sufficient to cause a detonation of lubricant vapour. A pellet with a lower start pressure will not allow air temperature to rise sufficiently to cause a detonation, regardless of its weight.
A high start pressure pellet alone is not sufficient to cause detonation; it also needs lubricant vapour at saturation point, and a pretty beefy spring capable of pushing the piston far enough to elevate air temperature, and it needs all three.
The rapid re-compression of the spring is not in itself the cause of breakage, that's a question of the timing of the expansion of the very rear of the spring and the compression, starting at the front and moving back during piston bounce. Where the two waves meet, the wire is being twisted in opposite directions - result, snap.
Spring breakages are commonly (not always) around an inch from the rear, and can result from cumulative mild wave displacement front collisions, or a single very harsh collision caused by detonation.
THANKS for making the difference between dieseling and detonation!
LOL!
😉
It's relatively easy to ascertain if a spring broke through fatigue (whether the gun habitually dieseled or not), and detontation.
In the first case, the broken ends will exhibit a progressive crystallization (grainy structure when seeing with a magnifying glass) that usually starts from the outside edge, and progresses inwards. On the second case, the snap will be a like broken branch.
A long-time ago, I recalled reading some cautionary advice against using heavy-for-bore pellet in a spring-&-piston air rifle. I accidently came-across the information by accident while searching for an aftermarket-trigger for my F4.
Actually, Jim, it's been my observation that springers are very prone to energetically detonate when fired with zero-weight pellets -- i.e., no pellet at all.
I would be grateful if you didn't inquire into the circumstances of said "experiments." ?
A long-time ago, I recalled reading some cautionary advice against using heavy-for-bore pellet in a spring-&-piston air rifle. I accidently came-across the information by accident while searching for an aftermarket-trigger for my F4.
Actually, Jim, it's been my observation that springers are very prone to energetically detonate when fired with zero-weight pellets -- i.e., no pellet at all.
I would be grateful if you didn't inquire into the circumstances of said "experiments." ?
Been there, done that myself, many times, Steve. I give you the HW95 in .22" flavour.
A long-time ago, I recalled reading some cautionary advice against using heavy-for-bore pellet in a spring-&-piston air rifle. I accidently came-across the information by accident while searching for an aftermarket-trigger for my F4.
Actually, Jim, it's been my observation that springers are very prone to energetically detonate when fired with zero-weight pellets -- i.e., no pellet at all.
I would be grateful if you didn't inquire into the circumstances of said "experiments." ?
Been there, done that myself, many times, Steve. I give you the HW95 in .22" flavour.
The blue line is from a microphone..
Scratch that. The blue trace is not a microphone, but a 400g accelerometer which records 3.3 mV per g, making the pressure of the very thin layer of air (<1% of available piston travel) that separates the piston seal and cylinder end wall 4,726psi.
Whenever the topic turns to the internal ballistics of spring-piston airguns, it's never long before the term "piston bounce" will bounce onto the scene. But how much sense does it make in the first place? After all, the only thing that opposes the motion of the piston is the air trapped between it and the end of the compression chamber -- merely air and only air.
And air is soft and squishy. Right? Why should anything violent be expected to happen?
Well, normally yes. But when air is violently compressed, not so much. And the way it's compressed in the course of a springer's firing cycle makes the stuff the piston hits at the end of its stroke less like the "soft and squishy" air we're all used to, and more like a solid brick wall. The plot below, of a 25mm diameter piston moving down a 100mm stroke pushed by a constant 100lb mainspring force, illustrates what's happening.
When the piston begins to move, the air in front of it is uncompressed (blue curve = psig) and offers little resistance to its acceleration (the very epitome of "soft and squishy"!), allowing the piston's kinetic energy (red curve = ftlb) to accumulate virtually unopposed. It's not until 80% (80mm) of the stroke has been completed and the piston has gathered its maximum kinetic energy (~21ftlb) that pressure rises high enough to cancel mainspring force and begin to slow it down.
But by now the piston is travelling like a bat-out-of-you-know-where (~16m/s = 16,000mm/s), so it continues to fly forward, causing pressure to soar to 1000s of psi and only finally stopping with only a few mm (and 10s of microseconds) of stroke remaining and the air in front of it become that "brick wall" mentioned earlier. Short version...
WHAM!
Notice that the underlying reason for this phenomenon is the properties, not of a particular piston, compression chamber, or mainspring, but of air itself.
Which, fundamentally, is why all springer pistons bounce: Because they're all filled with air and therefore all have that "brick wall."
Steve,
I never thought of a springer as producing such high pressures as 4000 psi. I always had the idea that the slower buildup of pressure meant that the pellet would start moving earlier and pressure would never build up to such and extent. The end of the receiver tube becomes quite a high pressure vessel.
I never thought of a springer as producing such high pressures as 4000 psi. I always had the idea that the slower buildup of pressure meant that the pellet would start moving earlier and pressure would never build up to such and extent. The end of the receiver tube becomes quite a high pressure vessel.
David Enoch
Yup. Springers generate pressures at least as high as found in most any PCP, if only for a millisecond or two.
