This post is a grossly over-simplified explanation of some mechanical hammer behaviors and the variables that are often able to be manipulated.
What is the hammer in a PCP? It is what carries kinetic energy and momentum to deliver a blow to the poppet within a PCP's valve, which upon impact of the valve stem, decompresses the poppet from the seat, and with the remaining energy, produces what is called Lift, and Dwell. The hammer lock time is the time the hammer takes to strike the valve stem once you pull the trigger. The less compressible the poppet material, the more energy left over to produce Lift/Dwell, which is why peek is a popular material among tuners over other thermoplastics, as peek has a high young modulus while still being soft enough to create a seal.
What are the primary factors that determine the available kinetic energy?
Shown below is a relatively stock looking .25 cal Marauder. Lock time is roughly 6.2 ms, force to cock the gun is 10.8 lbs.
Now, what happens if we simply reduce the Travel from .75" to .5", using the adjustments allowed on the gun by increasing the distance the striker protrudes from the hammer...
The velocity is reduced as well as lock time from 6.2 to 5.4 ms, but at a great expense of reduced momentum and kinetic energy transferred from hammer to valve stem, which will result in much less lift and dwell being produced, if the hammer can even knock the valve off its seat. What would be needed to restore the momentum and energy transferred? More spring and/or hammer weight, but adjustment within the gun only allows spring, so you'd have to crank down on the preload a LOT to restore power back to the original settings, or fabricate a new hammer.
Shown here, is an increase of .6" of preload which restores the energy needed to operate the valve, which knocked down lock time even more, down to 4.1 ms, however, now the gun is much more difficult to cock, at 13.6 lbs. For a bolt action that is a bit much for most users, however for a side lever gun, no big deal.
What if we want to change hammer weight, and leave the stock travel alone? Lets reduce the hammer from 78grams to 50 grams.
Now we've lost quite a bit of momentum, which will result in less dwell, so we will have to hit the valve harder with more spring be it preload or a higher rated spring, producing more lift, to restore the necessary dwell we once had, which will result in, again, a harder to cock gun. This simple calculator won't tell you the exact amount, but below is an example of what probably would restore the gun to original power...
Shown is simply an increase from 8 lb spring to 10 lb spring...thus the gun is harder to cock than stock (from 10.8 lbs up to 13.5 lbs), however, the lock time is now 4.4ms and we exchanged momentum for more kinetic energy, which is favorable for mitigating hammer bounce.
As you add lift to a valve (primarily determined by KE/FPE), you add dwell as it takes the poppet longer to return to the valve seat, however, if you were to add dwell with a heavier hammer, you won't add lift. Once lift exceeds roughly 1/4 of the valve throats porting equivalent (after considering the cross sectional surface area of the stem within it), your valve's mass air flow rate does not increase, this is commonly referred to as the valves flow curtain, in effect, this means once the curtains height is exceeded, you're creating more dwell, and not adding more flow rate. An example would be a .23" ported gun, once lift exceeds roughly .0575", the flow rate does not increase, only the duration of max flow rate the valve can achieve. Heavier hammers allow more dwell by remaining in contact with the stem for a longer period due to conservation of momentum. Momentum decays much slower than Kinetic Energy, akin to comparing stopping a train going 15 mph versus stopping a drag car going 100 mph, which is why heavier hammers are more prone to hammer bounce, as their momentum conservation is greater than that of a lighter hammer, while kinetic energy decays rapidly in inelastic conditions, which is what we experience in our PCPs, and is more favorable for managing hammer bounce, as well as producing quicker hammer lock times.
One may ask, why doesn't hammer weight greatly effect kinetic energy by itself? The answer to that is, the kinetic energy is stored in the hammer springs tension once compressed, adding weight to the hammer results in less hammer velocity, resulting in near equal energy output upon decompression of the spring, given optimal conditions.
What is Hammer Gap? Commonly referred to as free flight, it is a distance where the hammer is traveling without the assistance of compression from the spring, this in essence, basically kills two birds with one stone. The distance that the hammer has to travel is the gap distance twice prior to returning to the valve stem, so a .03" gap forces the hammer to travel .06" prior to secondary strikes. This distance allows time for the poppet to return to its seat and fully compress, making it more difficult to re-open, as well as a distance that assists in the decay of both momentum and kinetic energy that is transferred by the closing force of a valve, that's right, the valve closes often so violently, that it sends the hammer flying back, and without gap or a short stiff spring or the like in place, what happens is re-compression of the hammer spring, which in many conditions, produces enough of both momentum and kinetic energy to strike the valve again with enough power, before the poppet returns to the seat, producing a secondary blast of air that doesn't contribute to projectile velocity. Only caveat to hammer gap is, the more you add, the more you have to compensate the 'free flight' with added spring pre-load, or hammer travel. Nominal gap is generally 1mm-1.2mm or .04"-.047", however bigger bores with heavier springs likely benefit from a bit more.
Attached is a basic spreadsheet that will calculate all of the above data. It may be useful to someone so, I thought I'd share a stand-alone of these calculations, available as XLSX.
Keep in mind this is a simple spread sheet meant to represent the relationship between hammer travel, hammer spring pre-load, hammer weight. No losses are calculated in the sheet currently, such as gravity, friction, angle of shot, and air resistance.
*edit* modified the spreadsheet thanks to @caliusoptimus for pointing out a flaw in a calculation, spreadsheet now uses both Average force * distance method and a Potential energy method provided by caliusoptimus.
-Matt
View attachment 497196
View attachment Hammer_Data_Calculations_V3.xlsx
What is the hammer in a PCP? It is what carries kinetic energy and momentum to deliver a blow to the poppet within a PCP's valve, which upon impact of the valve stem, decompresses the poppet from the seat, and with the remaining energy, produces what is called Lift, and Dwell. The hammer lock time is the time the hammer takes to strike the valve stem once you pull the trigger. The less compressible the poppet material, the more energy left over to produce Lift/Dwell, which is why peek is a popular material among tuners over other thermoplastics, as peek has a high young modulus while still being soft enough to create a seal.
What are the primary factors that determine the available kinetic energy?
- Spring Rating
- Total spring tension, includes pre-load and cocking distance to engage the sear under spring tension.
- (Hammer weight is notable mention as without any hammer weight you create none, however adding/removing nominal weight does not change KE)
- (Hammer Gap aka free flight notable mention, however when nominal the effect is minimal)
- Hammer weight
- Hammer travel (distance hammer travels between sear and valve stem)
- Spring rating
- Spring Pre-load
- Hammer travel
- Spring rating
- Total Spring tension
- Hammer weight
- (notable mention, Hammer Gap aka free flight, however minimal when set at optimal distance)
Shown below is a relatively stock looking .25 cal Marauder. Lock time is roughly 6.2 ms, force to cock the gun is 10.8 lbs.
Now, what happens if we simply reduce the Travel from .75" to .5", using the adjustments allowed on the gun by increasing the distance the striker protrudes from the hammer...
The velocity is reduced as well as lock time from 6.2 to 5.4 ms, but at a great expense of reduced momentum and kinetic energy transferred from hammer to valve stem, which will result in much less lift and dwell being produced, if the hammer can even knock the valve off its seat. What would be needed to restore the momentum and energy transferred? More spring and/or hammer weight, but adjustment within the gun only allows spring, so you'd have to crank down on the preload a LOT to restore power back to the original settings, or fabricate a new hammer.
Shown here, is an increase of .6" of preload which restores the energy needed to operate the valve, which knocked down lock time even more, down to 4.1 ms, however, now the gun is much more difficult to cock, at 13.6 lbs. For a bolt action that is a bit much for most users, however for a side lever gun, no big deal.
What if we want to change hammer weight, and leave the stock travel alone? Lets reduce the hammer from 78grams to 50 grams.
Now we've lost quite a bit of momentum, which will result in less dwell, so we will have to hit the valve harder with more spring be it preload or a higher rated spring, producing more lift, to restore the necessary dwell we once had, which will result in, again, a harder to cock gun. This simple calculator won't tell you the exact amount, but below is an example of what probably would restore the gun to original power...
Shown is simply an increase from 8 lb spring to 10 lb spring...thus the gun is harder to cock than stock (from 10.8 lbs up to 13.5 lbs), however, the lock time is now 4.4ms and we exchanged momentum for more kinetic energy, which is favorable for mitigating hammer bounce.
As you add lift to a valve (primarily determined by KE/FPE), you add dwell as it takes the poppet longer to return to the valve seat, however, if you were to add dwell with a heavier hammer, you won't add lift. Once lift exceeds roughly 1/4 of the valve throats porting equivalent (after considering the cross sectional surface area of the stem within it), your valve's mass air flow rate does not increase, this is commonly referred to as the valves flow curtain, in effect, this means once the curtains height is exceeded, you're creating more dwell, and not adding more flow rate. An example would be a .23" ported gun, once lift exceeds roughly .0575", the flow rate does not increase, only the duration of max flow rate the valve can achieve. Heavier hammers allow more dwell by remaining in contact with the stem for a longer period due to conservation of momentum. Momentum decays much slower than Kinetic Energy, akin to comparing stopping a train going 15 mph versus stopping a drag car going 100 mph, which is why heavier hammers are more prone to hammer bounce, as their momentum conservation is greater than that of a lighter hammer, while kinetic energy decays rapidly in inelastic conditions, which is what we experience in our PCPs, and is more favorable for managing hammer bounce, as well as producing quicker hammer lock times.
One may ask, why doesn't hammer weight greatly effect kinetic energy by itself? The answer to that is, the kinetic energy is stored in the hammer springs tension once compressed, adding weight to the hammer results in less hammer velocity, resulting in near equal energy output upon decompression of the spring, given optimal conditions.
What is Hammer Gap? Commonly referred to as free flight, it is a distance where the hammer is traveling without the assistance of compression from the spring, this in essence, basically kills two birds with one stone. The distance that the hammer has to travel is the gap distance twice prior to returning to the valve stem, so a .03" gap forces the hammer to travel .06" prior to secondary strikes. This distance allows time for the poppet to return to its seat and fully compress, making it more difficult to re-open, as well as a distance that assists in the decay of both momentum and kinetic energy that is transferred by the closing force of a valve, that's right, the valve closes often so violently, that it sends the hammer flying back, and without gap or a short stiff spring or the like in place, what happens is re-compression of the hammer spring, which in many conditions, produces enough of both momentum and kinetic energy to strike the valve again with enough power, before the poppet returns to the seat, producing a secondary blast of air that doesn't contribute to projectile velocity. Only caveat to hammer gap is, the more you add, the more you have to compensate the 'free flight' with added spring pre-load, or hammer travel. Nominal gap is generally 1mm-1.2mm or .04"-.047", however bigger bores with heavier springs likely benefit from a bit more.
Attached is a basic spreadsheet that will calculate all of the above data. It may be useful to someone so, I thought I'd share a stand-alone of these calculations, available as XLSX.
Keep in mind this is a simple spread sheet meant to represent the relationship between hammer travel, hammer spring pre-load, hammer weight. No losses are calculated in the sheet currently, such as gravity, friction, angle of shot, and air resistance.
*edit* modified the spreadsheet thanks to @caliusoptimus for pointing out a flaw in a calculation, spreadsheet now uses both Average force * distance method and a Potential energy method provided by caliusoptimus.
-Matt
View attachment 497196
View attachment Hammer_Data_Calculations_V3.xlsx
Last edited:
