Swept Volume in Spring Piston Air Rifles
Posted: Mon Aug 29, 2011 3:53 pm
Talk to any air gun geek and after the usual spiel about springs, guides, seals and the lubes used the topic takes a U turn and winds up with the quiz – hey what kind of Swept Volume are you getting? Now this is a real poser, because it isn’t easy to answer, as it concerns the very basic principle on which air guns are designed.
A spring piston air rifle is designed to propel a pellet via the rapid expansion of a fixed volume of air. All parts of a finely tuned air rifle interact with one another producing a consistent energy release during every shot cycle, unless of course something is wrong with the mechanism.
For a given design, the only real fixed part is the diameter of the compression tube. The volume of air available for compression is a function of the diameter of the tube (area) and the length of the stroke. Generally a larger swept volume would lead to greater air mass and energy output, but because of interaction between the various parts, there will be an optimum swept volume for each rifle.
But, where does the energy come from? Conservation of energy dictates that the maximum available energy must come from the energy stored in the compressed spring. Theoretically, if everything was perfect and you had a frictionless mechanism without leaks, you could get all the energy from the spring into the pellet, but of course in reality there will be losses. So, there are three main areas to be considered, in order to optimize the power output:
1.Increasing the energy stored in the spring -
The energy a spring stores is E=0.5FL, where F is the spring load and L is the amount the spring compresses. A longer spring and cocking stroke can compress more and give more power than a shorter one; also a stiffer spring produces more force for a given compression and will store more energy.
2.Reducing Friction -
Friction will consume some of the energy when the spring is released, and every contact surface is involved. This is where materials and lubes come into play.
3.Optimizing the swept volume -
This allows the spring energy to be transferred trough compression/expansion of the air mass into kinetic energy in the pellet.
4.Other considerations include –
•Size of transfer port
•Friction of pellet in the barrel
•Pressure losses through breach seals
All the above 3 factors play a role in robbing energy from the pellet.
The mass of the piston also plays a role, since the piston energy is 0.5MV - where M is the piston mass and V is its speed. Empirical evidence shows that there is an optimum piston mass for a given system. Please note that there is no mechanical link between the piston and the air column, so in reality the spring energy must become piston kinetic energy to compresses the mass of air.
If there is oil or grease in the system, the heat generated by the compression of air can ignite it generating more energy and increasing power. If this happens in the air chamber it is called detonation and can wreck the piston and spring and even blow up the compression tube. If it happens in the barrel it's called dieseling and can produce inconsistent energy, reducing accuracy. Some high power spring air rifles supposedly rely on these fuel burning techniques to achieve their rated power.
So in the end, you have a complex system, with the spring being at the heart of it. When a springer is "tuned" you see a variety of springs, seals, lubes being used to optimize the energy developed and transferred to minimize the banging and twanging of the internal components.
Hence Velocity, Power and Accuracy in air rifles are the result of conceptualizing a need by the application, of ‘basic physics’ and the principles of ‘mechanical engineering’. This entire activity begins on the drawing board and with attributes such as aesthetics, ergonomics and design being added in the process by manufacturers to create product differentiation.
Now the very mention the most aspired ‘catchword’ in Air Gunning, “Tuning” and everyone wants to jump into the bandwagon! Now spare a thought – have you come across any owner of foreign made air guns such as Diana, Weihrauch, Gamo, BSA, Walther to name a few asking for their guns to be tuned? None or very rare isn’t it – why? Simply because these air guns are conceptualized and designed by engineers, who work by the proven principles of physics and mechanical engineering. In the process they also seek the help of industrial designers who contribute in providing the aesthetics and ergonomics of the end product.
Unfortunately Indian manufacturers of air guns, woefully lack this approach, they either simply make a very bad copy or disregard these vital inputs and throw quality to the winds. Moreover when demand outstrips supply, anything made - sells. The net result is that every Indian made air gun is a potential candidate for “Tuning”.
Go ahead and tune your air rifle by all means – but remember that the kit consisting of spring, seal, adaptor, rear & front guides are suitable for your air rifle, because these have to work in synch with the receiver (tube) of your air rifle. Remember that their combined functions are reciprocal, the reason being that the ‘ID’ of the receiver (tube) is constant as is the diameter of the transfer port.
If you are attempting to re-size the piston, its’ another ball game altogether since altering the size of the rod with the sear is a tricky job. A bad job and you will wind up with ‘auto fire’ problems when the air rifle is cocked or the gun may not cock at all.
A well tuned air rifle could improve its’ smoothness – reduce vibration and even contribute an increase in velocity & power. But don’t be obsessed with power and if you do, you will compromise its’ accuracy.
According to international experts the following are the optimum velocities that give air rifles both power and accuracy –
* 0.177 cal > 900 / 950 fps
* 0.22 cal > 750 / 800 fps
So whether you are target shooting, plinking or hunting, the whole idea is to enjoy the sport.
Happy shooting!!!
Cheers
Briha
A spring piston air rifle is designed to propel a pellet via the rapid expansion of a fixed volume of air. All parts of a finely tuned air rifle interact with one another producing a consistent energy release during every shot cycle, unless of course something is wrong with the mechanism.
For a given design, the only real fixed part is the diameter of the compression tube. The volume of air available for compression is a function of the diameter of the tube (area) and the length of the stroke. Generally a larger swept volume would lead to greater air mass and energy output, but because of interaction between the various parts, there will be an optimum swept volume for each rifle.
But, where does the energy come from? Conservation of energy dictates that the maximum available energy must come from the energy stored in the compressed spring. Theoretically, if everything was perfect and you had a frictionless mechanism without leaks, you could get all the energy from the spring into the pellet, but of course in reality there will be losses. So, there are three main areas to be considered, in order to optimize the power output:
1.Increasing the energy stored in the spring -
The energy a spring stores is E=0.5FL, where F is the spring load and L is the amount the spring compresses. A longer spring and cocking stroke can compress more and give more power than a shorter one; also a stiffer spring produces more force for a given compression and will store more energy.
2.Reducing Friction -
Friction will consume some of the energy when the spring is released, and every contact surface is involved. This is where materials and lubes come into play.
3.Optimizing the swept volume -
This allows the spring energy to be transferred trough compression/expansion of the air mass into kinetic energy in the pellet.
4.Other considerations include –
•Size of transfer port
•Friction of pellet in the barrel
•Pressure losses through breach seals
All the above 3 factors play a role in robbing energy from the pellet.
The mass of the piston also plays a role, since the piston energy is 0.5MV - where M is the piston mass and V is its speed. Empirical evidence shows that there is an optimum piston mass for a given system. Please note that there is no mechanical link between the piston and the air column, so in reality the spring energy must become piston kinetic energy to compresses the mass of air.
If there is oil or grease in the system, the heat generated by the compression of air can ignite it generating more energy and increasing power. If this happens in the air chamber it is called detonation and can wreck the piston and spring and even blow up the compression tube. If it happens in the barrel it's called dieseling and can produce inconsistent energy, reducing accuracy. Some high power spring air rifles supposedly rely on these fuel burning techniques to achieve their rated power.
So in the end, you have a complex system, with the spring being at the heart of it. When a springer is "tuned" you see a variety of springs, seals, lubes being used to optimize the energy developed and transferred to minimize the banging and twanging of the internal components.
Hence Velocity, Power and Accuracy in air rifles are the result of conceptualizing a need by the application, of ‘basic physics’ and the principles of ‘mechanical engineering’. This entire activity begins on the drawing board and with attributes such as aesthetics, ergonomics and design being added in the process by manufacturers to create product differentiation.
Now the very mention the most aspired ‘catchword’ in Air Gunning, “Tuning” and everyone wants to jump into the bandwagon! Now spare a thought – have you come across any owner of foreign made air guns such as Diana, Weihrauch, Gamo, BSA, Walther to name a few asking for their guns to be tuned? None or very rare isn’t it – why? Simply because these air guns are conceptualized and designed by engineers, who work by the proven principles of physics and mechanical engineering. In the process they also seek the help of industrial designers who contribute in providing the aesthetics and ergonomics of the end product.
Unfortunately Indian manufacturers of air guns, woefully lack this approach, they either simply make a very bad copy or disregard these vital inputs and throw quality to the winds. Moreover when demand outstrips supply, anything made - sells. The net result is that every Indian made air gun is a potential candidate for “Tuning”.
Go ahead and tune your air rifle by all means – but remember that the kit consisting of spring, seal, adaptor, rear & front guides are suitable for your air rifle, because these have to work in synch with the receiver (tube) of your air rifle. Remember that their combined functions are reciprocal, the reason being that the ‘ID’ of the receiver (tube) is constant as is the diameter of the transfer port.
If you are attempting to re-size the piston, its’ another ball game altogether since altering the size of the rod with the sear is a tricky job. A bad job and you will wind up with ‘auto fire’ problems when the air rifle is cocked or the gun may not cock at all.
A well tuned air rifle could improve its’ smoothness – reduce vibration and even contribute an increase in velocity & power. But don’t be obsessed with power and if you do, you will compromise its’ accuracy.
According to international experts the following are the optimum velocities that give air rifles both power and accuracy –
* 0.177 cal > 900 / 950 fps
* 0.22 cal > 750 / 800 fps
So whether you are target shooting, plinking or hunting, the whole idea is to enjoy the sport.
Happy shooting!!!
Cheers
Briha
