Spring-Power paradox: Cutting the spring to increase power?

[Rijo Joseph]

Really appreciate the time and effort you spare for shooting...

[bennedose]

This is a very strange paradox.
Only reason could be the bouncing of piston, that affects smooth acceleration of pellet due fall in pressure .

Basu

Using a 1000 fps camera I have managed to document piston bounce in an old air pistol I have.

In this gun, the inner part of a concentric barrel springs forward to compress air between the barrels. The fully cocked position is with the muzzle tip pushed in fully and after shooting the muzzle is fully extended.

In the animated gif below the barrel springs forward almost fully and then stops for a moment - then it actually moves back a notch while the pellet emerges as a fuzzy blurred line. Then the barrel extends fully to its relaxed/extended position. Weird stuff.

pistolgif.gif

[brihacharan]

This is a very strange paradox.
Only reason could be the bouncing of piston, that affects smooth acceleration of pellet due fall in pressure .

Basu

Using a 1000 fps camera I have managed to document piston bounce in an old air pistol I have.

In this gun, the inner part of a concentric barrel springs forward to compress air between the barrels. The fully cocked position is with the muzzle tip pushed in fully and after shooting the muzzle is fully extended.

In the animated gif below the barrel springs forward almost fully and then stops for a moment - then it actually moves back a notch while the pellet emerges as a fuzzy blurred line. Then the barrel extends fully to its relaxed/extended position. Weird stuff.

pistolgif.gif

Hi bennedose,
> It certainly seems "Weird" since the barrel I presume is not of a "Floating" kind
> After cocking when the barrel is locked in position how can it move forward?????
> On second thoughts it could be the entire gun that moves forward & not the barrel per say - what do you think?
> Re: Basu's contention about the "Piston Bounce" - Unless the fitment of the seal is not perfect, it cannot happen at least technically....
> Again technically the gap between the OD of piston & ID of receiver should ideally be less than "1mm"....
> Acknowledged "tuners" around the world solve this anticipated problem by adding on a 'thrust bearing' at the end of the piston so that the piston does not bounce when travelling forward...
> Nevertheless a very interesting experiment & observation on your part
Briha

[bennedose]

Briha, this air pistol is a weird one - an outdated "concentric barrel" design in which the barrel itself is moved backwads to cock/lock it. A concentric cylindrical chamber outside the barrel holds the air and the compression is achieved by the barrel itself shooting forwards, and a small air transfer port is there near the breech, just aft of the pellet. The pellet can only be loaded after full cocking and unscrewing a breech seal cum pin. The pellet is loaded and the breech seal replaced and the pin pushes the pellet some distance (1 inch) down the barrel so that the pellet ends up sitting just in front of the ar transfer port between outer air chamber and inner barrel. I found a design photo on the web which I will upload later.

I examined and compared four of the stills in my video above making measurements using a graphic program. It may not be piston bounce. It seems that the barrel shoots out to its maximum extent and then retracts a little bit - but what surprised me was that the pellet does not emerge as the barrel is movimg forward. Th pellet takes its own time to emerge after the barrel has extended to its fullest extent. At best - this seems to be a highly inefficient and leaky (and inaccurate) mechanism.

[brihacharan]

Hi bennedose,
> Very interesting mechanism...
> Wonder what was the rationale' behind this kind of design... there must be something to it...
> See if you can mail this (your clip + write-up) to Mr. BB Pelletier & ask for his comments... should make some interesting reading & knowledge gain
Briha

[mercury]

I would think that all pistons , even in finely tuned air rifles will bounce...or maybe for want of a better word rebound. what I mean is that as the pressure increases in the chamber and exerts itself on the pellet , the piston does does come to a stop ( end of stroke) and then moves back or pushed back , maybe a fraction and then moves back to finally stop. the pellet is on its way on the first "stop" and exits the barrel at the second stop as the last of the compressed..or recompressed air... is expanded.

piston bounce is , as rightly pointed out.... when a light spring / piston is used in combination with a heavy pellet , the pressure within the chamber is greater which causes the piston to move back further than normal and not much movement of the pellet. one of the reasons why many air gunners feel that heavy pellets can damage springs / seals etc.

its all about getting the perfect balance of the internal components and pellet weight.

Bennedose...I would think you hit the sweet spot with the right coil numbers. also sent you a PM.

[brihacharan]

[quote="mercury"]I would think piston bounce is , as rightly pointed out.... when a light spring / piston is used in combination with a heavy pellet , the pressure within the chamber is greater which causes the piston to move back further than normal and not much movement of the pellet. one of the reasons why many air gunners feel that heavy pellets can damage springs / seals etc.

It's all about getting the perfect balance of the internal components and pellet weight. Bennedose...I would think you hit the sweet spot with the right coil numbers.


Briha

[Basu]

G.V Cardew,G. M. Cardew and E R Elsom said,
here I quote "using standard light piston, it can be seen that the velocity
is approximately constant after initial acceleration, until it nears the end of the cylinder, when it slows down abruptly and stops for an instant at about 1/10inches away from the cylinder end.From this position it ' bounces' back to a point nearly 1/2 inch away from the cylinder end , it then returns and comes to rest against the top of the cylinder".

I am of the opinion that there is a fall of pressure during the time when piston bounces back and that hampers the smooth acceleration of pellet.
Harder the spring more is the bounce.But heavier piston will carry more MV , so the chance of bouncing is less, thus pellet acceleration is smoother.
Benne , as Briha suggested , you may refer this matter to some international Gurus to get their view point.


Basu

[brihacharan]

G.V Cardew,G. M. Cardew and E R Elsom said,
Benne , as Briha suggested , you may refer this matter to some international Gurus to get their view point.
Basu

Air Rifle Recoil

When the piston reaches the end of its travel, it stops suddenly. When that happens, it imparts a hammer blow to the air-gun, sending it in the same direction the piston was traveling. This is the second recoil, and it’s much more noticeable. At this point in time, the pellet is probably between three and six inches down the barrel and the entire gun’s moving.

The movement is in several forms. First, there’s high-speed vibration running through all the parts of the gun. You can’t see this vibration, even on a high-speed camera, but you can feel it. This is the buzz that you feel from some guns, and it can be so sharp that it actually hurts to hold the stock against your cheek.

Next, there’s a lower-speed vibration that’s both larger and much slower. If you had a high-speed camera, you could actually see the various parts of the rifle moving. The pellet is still inside the barrel when this happens.

Finally, there’s the recoil in both directions. Both are visible on a high-speed camera; and the forward movement, assuming we’re talking about a conventional spring-piston setup, is by far the largest. The gun starts moving forward before the pellet leaves the muzzle, but completes the movement after the pellet has gone.

SPRING-PISTON GUNS & ACCURACY

Simply stated, break-barrel spring guns are the most difficult to control. They may be just as accurate as under-lever and side-lever air rifles, but they’re almost always more sensitive to the movement of the gun when it fires. That’s not to say that side-levers and under-levers are not sensitive; but in comparison to break-barrels, they’re less sensitive.

Let’s stay with break-barrels for now. The ones with the longest piston stroke have the longest period of time for movement. That includes the high-speed vibration, the low-speed vibration and the recoil in both directions. As a rule, long-stroke spring-piston guns are the most sensitive to hold, and long-stroke break-barrels are the most sensitive of all.

Then there’s the weight of the piston to consider. A heavy piston causes more rearward recoil when it begins moving and more forward recoil when it comes to a stop. You tend to find heavier pistons in guns with more power.

Put this all together, and you know that a break-barrel spring-piston rifle that has a long piston stroke and high power will probably be the most sensitive air-gun, as far as hold goes. It may be potentially very accurate; yet also be so sensitive that unless the hold technique is perfect, it’ll spray pellets everywhere.
(Extract from BB Pelletier’s article)

PS:
Now one can safely conclude that the whole thing is nothing but “Basic Physics” that concerns:

1.Mass (Piston Wt.)
2.Velocity (Speed of the piston in motion)
3.Distance travelled by the piston
4.Friction
5.Mechanical tolerances between moving parts
6.Potential & Kinetic energy in the spring

Surprisingly nowhere in all my research on the above did I come across any standard formula that specifies the method to arrive at the right piston length & weight suitable for air rifles that ensure a smooth, vibration free & accurate performance. May be it’s a well-kept secret!!!!

Perhaps precision engineering with an eye on minimal tolerances in static & all moving parts play an important role. Now all said & done there arises, questions on the right quality (shape, size, weight, metallurgy) of pellets to be confronted with!!!!

Briha

[bennedose]

G.V Cardew,G. M. Cardew and E R Elsom said,
here I quote "using standard light piston, it can be seen that the velocity
is approximately constant after initial acceleration, until it nears the end of the cylinder, when it slows down abruptly and stops for an instant at about 1/10inches away from the cylinder end.From this position it ' bounces' back to a point nearly 1/2 inch away from the cylinder end , it then returns and comes to rest against the top of the cylinder".

Thanks to Mercury I got the book and despite saying that I wouldn't start reading it until I finish reading the book I am curently reading, I could not resist this beautfully written and informative book.

Interestingly Cardew and Cardew say that the stoppage 1/10 inch from the end just as the pellet begins to move cannot be prevented. They tried to stop that by using various mechanical systems but the systems simply broke apart from the stresses and were unable to prevent this inevitable stoppage of piston just before hitting the end of its stroke.

The other point that Cardew say is that a certain amout of combustion of lubricant or "something" is inevitable and adds 20% or so to the power. That combustion could not be stopped by removing all the oil. But it was stopped by removing all oxygen and replacing with nitrogen.

[mercury]

when i used the term bounce / rebound , i did not mean to imply that the piston bounces / rebounds off the end of the chamber....like for example a ball rebounds off a wall...NO.

bounce is an inherient part of an air gun cycle and all air guns have them. the extreme high pressure in front of the piston will slow it down and bring it to a stop on a thin film of air , bounce/ rebound of this , moving back those fractions of an inch then move forward to finally stop at the end of chamber. whether this bounce in any way aids in recompressing air or just energy loss on the spring/ piston...dont know!

Basu , as to bounce leading to a loss of pressure affecting the pellet velocity...given that the pellet is accelerating down the barrel and the bounce time is in nano seconds...honestly i do not know. in a badly set up gun where there is pronounced bounce..again i do not know , though theoratically it does sound logical.

"..Harder the spring more is the bounce.But heavier piston will carry more MV , so the chance of bouncing is less, thus pellet acceleration is smoother..". do not forget there is also the weight of tthe pellet and the length of the stroke to be taken into consideration to come to a satisfactory conclusion.

can piston bounce be eliminated.....i do believe that there have been experiments using custom made pistons with 0 rings ; but there seems to a hung jury on the verdict.

Briha , you do bring up a good point on the length/ weight of pistons. how is that calculated?? yes end of the day we are left with the pellet quality to oompenstate manufacturing standards.

Bennedose....enjoy maadi.

[brihacharan]

Can piston bounce be eliminated.....i do believe that there have been experiments using custom made pistons with 0 rings ; but there seems to a hung jury on the verdict

Hi mercury,
> As international experts opine Piston Bounce can only be minimized... as Piston Bounce is endemic in ARs...
> This is achieved (as you have mentioned) by incorporating polymer thrust bearing in the fore & aft of the piston...
> These bearings almost touch the ID of the receiver and being made of polymers reduces friction to a palpable extent...
> There are plenty of "IFs & BUTs" as far as ARs are concerned...the question is "will the twain ever meet"????
"Enjoy Maadi" - Good Sign off
Briha

[bennedose]

can piston bounce be eliminated.....i do believe that there have been experiments using custom made pistons with 0 rings ; but there seems to a hung jury on the verdict.

Bennedose....enjoy maadi.

LOL thanks. According to Cardew et al , piston bounce cannot be eliminated. When they tried to eliminate that using a system to prevent backward movement of the piston - the rings/bearings used in the mechanism simply shattered.