Lightening the ram/piston?

[john]

Paul68

I believe you are still missing the point that the spring is going to transmit the same amount of energy to both pistons, regardless of their respective masses. The pellet is going to absorb the same amount of that energy, namely most of it, regardless of which piston provides that amount of energy. In other words, both pistons are going to deliver equal amounts of their total energy to the pellet. The heavy piston is going to have more energy left over which will cause it to rebound further and/or harder. Vice versa for the light piston which will have less energy left over and not rebound as much .

We are of course assuming both pistons have the same physical dimensions, namely the same length and, hence, the same cocking stroke length and, hence, the same stored energy in the compressed spring.

This cracked me up today.
Question: What is worse, ignorance or apathy?
Answer: I don't know and I don't care. 

[Scotchmo]

john,

You got the point. But I would have reworded your response slightly:

The appropriate pellet is going to absorb the same amount of that energy, namely most of it, regardless of which piston provides that amount of energy.

And, in a good design, neither piston will rebound a substantial amount.

[Paul68]

Paul68

I believe you are still missing the point that the spring is going to transmit the same amount of energy to both pistons, regardless of their respective masses. The pellet is going to absorb the same amount of that energy, namely most of it, regardless of which piston provides that amount of energy. In other words, both pistons are going to deliver equal amounts of their total energy to the pellet. The heavy piston is going to have more energy left over which will cause it to rebound further and/or harder. Vice versa for the light piston which will have less energy left over and not rebound as much .

We are of course assuming both pistons have the same physical dimensions, namely the same length and, hence, the same cocking stroke length and, hence, the same stored energy in the compressed spring.

This cracked me up today.
Question: What is worse, ignorance or apathy?
Answer: I don't know and I don't care. 

Naa, I get that entirely. I already explained the problem is that it is not only the force of the spring working on the piston that decides the end force acting on the air, which in turn applies force to the pellet. It is mass x velocity = momentum. Greater momentum = greater force.

Lets try this a different way..

You have a sealed tube with a piston and seal in it on the ground. The piston is positioned at the top of the tube and protrudes out of it.

Drop a 5 lb brick on the end of the piston from a specific height and it will push the piston a certain distance into the tube and compress the air within it.

Now do the same thing with a 10 lb brick, and the brick would force the piston in farther, and compress the air within the tube more.

Why? The force acting on the piston is exactly the same, but for some reason the heavier brick will produce higher compression. It is because although the force acting on the bricks is the same, the heavier brick has more mass and thus more momentum, so it will be harder to stop it.



Something else to consider.

If you have two pistons, and one is heavier than the other, but both have the same momentum, which is going to stop first when it encounters the resistance of air compression? Obviously the lighter piston because although initial momentum is the same, greater mass still has greater resistance to changes in its current state. However, in order for the lighter piston to have the same momentum, the spring would have to move it faster ie applying more kinetic energy. So, that would also mean that the same amount of intial kinetic energy does not produce an equal end force provided by pistons of different weights.



And again, the mass of the pellet is also being ignored. A heavier pellet will resist movement more than a lighter one. Hence a heavier piston is more effective at moving heavier pellets. And the pellet does not just "absorb" the energy. It is acclerated, which in turn imparts momentum to it , which we measure in FPE. FOOT POUNDS OF ENERGY. Increase the force applied to the pellet, you increase velocity and increase momentum, and thus gain FPE. Reduce the force applied to the pellet, you reduce FPE. Keeping pellet weights the same, a lighter piston typically results in lower pellet velocities and lower FPE and vice versa.

You can alter pellet weight to gain efficiency and increase FPE with a light piston, but only to a point, beyond which the lighter piston just wont produce the necessary force to achieve the same pellet energies as a heavier piston without adding a lot more intial kinetic energy.

[Scotchmo]

Paul68,

Things would be easier if we were on the metric system. Or we could use the proper imperial unit of mass - slugs.

I think that you are confusing pounds mass (lbm) with pounds force (lbf).

The gas spring generates the same force regardless of the mass of the piston that it is propelling.

A falling brick is accelerated by gravity. In which case the force on the 10 lb. brick is twice as much as the force on the 5 lb. brick.

The force of gravity is dependent on the mass of the object. The force of a gas spring is not.

You said that we measure momentum in FPE. That makes about as much sense as saying that we measure distance in MPH. Sorry for the sarcasm. I should have just said that it makes no sense.

You stated that "you increase velocity and increase momentum and thus gain FPE."

However, you must realize that when you increase the piston mass, you will actually have a reduced piston velocity and still have greater piston momentum. That heavier piston has more momentum, but the energy is unchanged.

[shorty]

Something to read.

http://www.google.com/url?sa=t&rct=j&q=piston%20airgun%20drawings&source=web&cd=11&ved=0CGwQFjAK&url=http%3A%2F%2Fhome2.fvcc.edu%2F~dhicketh%2FDiffEqns%2Fspring07projects%2FStephenCompton%2FSpringAirModel.pdf&ei=pMitUdnMM4jS9ATf6YCQBQ&usg=AFQjCNGadG9oAywk8aUGePWRYBkchyahnQ

[Paul68]

Paul68,

Things would be easier if we were on the metric system. Or we could use the proper imperial unit of mass - slugs.

I think that you are confusing pounds mass (lbm) with pounds force (lbf).

The gas spring generates the same force regardless of the mass of the piston that it is propelling.

A falling brick is accelerated by gravity. In which case the force on the 10 lb. brick is twice as much as the force on the 5 lb. brick.

The force of gravity is dependent on the mass of the object. The force of a gas spring is not.

You said that we measure momentum in FPE. That makes about as much sense as saying that we measure distance in MPH. Sorry for the sarcasm. I should have just said that it makes no sense.

You stated that "you increase velocity and increase momentum and thus gain FPE."

However, you must realize that when you increase the piston mass, you will actually have a reduced piston velocity and still have greater piston momentum. That heavier piston has more momentum, but the energy is unchanged.

When I refered to momentum and FPE with the pellet, I was trying to demonstrate that a faster moving pellet of the same weight has higher energy because that is more well known among air gunners.

I understand what you are saying about gravity. But I'll concede that the force of gravity is greater. It doesn't matter anyway.

If the force of gravity isn't a good way to demonstrate the force produced by momentum, then try hitting a nail with a basic household hammer, then use a small sledge hammer and swing it with the same force and hit the same nail. The heavier hammer will drive the nail into the wood farther.

I also understand about gas springs versus coil springs. Thats why I understand that the constant force applied by the gas spring can affect the end result. BUT, it doesnt negate the effect of momentum, only masks it to an extent because the piston is being accelerated at a constant rate rather than the variable rate supplied by a spring. Momentum still applies. You are still accelerating a greater mass.

Since you are a number cruncher, why not calculate the amount of force needed to compress a given volume of air, say the amount in a popular rifle we know the spring specs for, then calculate the force supplied by the spring only? Would that not provide something useful?
 

[Bullit]

I think Paul's post #68 give a good summary of why the extra weight produces higher pressure, given the timing of the "pellet launch".  By introducing more time in breech/barrel with a heavier pellet, the heavier piston's momentum will yield higher pressures.  It's momentum that aids the piston in "driving home", against the building pressures in the sealed system.

[Paul68]

I think Paul's post #68 give a good summary of why the extra weight produces higher pressure, given the timing of the "pellet launch".  By introducing more time in breech/barrel with a heavier pellet, the heavier piston's momentum will yield higher pressures.  It's momentum that aids the piston in "driving home", against the building pressures in the sealed system.

That pretty succinctly sums up what I've been trying to say in an extremely long winded way

[Scotchmo]

The link that shorty gives shows the most comprehensive spring gun model that you will find.

I read it when I was doing my spring gun testing last year.

The math involved is much more thorough than used in the Cardew books.

No where is momentum even mentioned. Momentum is a symptom of the process. Energy is the cause. Adding mass to a closed process can indeed result in more momentum. But it does not increase the energy available.

You can add a heavier flywheel to a car engine. You have increased the angular momentum of the engine. It does not increase the power output. Using heavier pistons in the engine will not increase the power either.

I'll say this:

I can cut the piston weight in half and guarantee that I can maintain the same FPE out of any gas spring rifle. It will require using a lighter pellet and sometimes a less restrictive transfer port.

[Bullit]

I thought we were talking "apples to apples" with the same spring, and the same pellet, and the same bore/stroke?   The only variable being the piston mass.    Staying within those parameters,,, it's a slam dunk.

[Scotchmo]

I thought we were talking "apples to apples" with the same spring, and the same pellet, and the same bore/stroke?   The only variable being the piston mass.    Staying within those parameters,,, it's a slam dunk.

“same spring, and the same pellet, and the same bore/stroke”

The only thing that I changed was the piston mass.

Here are the actual results:

7.9oz piston-ram – yields 4.7fpe of muzzle energy
6.6oz piston-ram – yields 8.4fpe of muzzle energy
4.4oz piston-ram – yields 9.5fpe of muzzle energy

What do you mean by “slam dunk”?

[Paul68]

I thought we were talking "apples to apples" with the same spring, and the same pellet, and the same bore/stroke?   The only variable being the piston mass.    Staying within those parameters,,, it's a slam dunk.

“same spring, and the same pellet, and the same bore/stroke”

The only thing that I changed was the piston mass.

Here are the actual results:

7.9oz piston-ram – yields 4.7fpe of muzzle energy
6.6oz piston-ram – yields 8.4fpe of muzzle energy
4.4oz piston-ram – yields 9.5fpe of muzzle energy

What do you mean by “slam dunk”?

That's a bit misleading don't you think?

You also have results which show a heavier piston producing more FPE. What you have here looks like what is expected with too light a pellet with a heavy piston. That would fit with what I've been saying because the light pellet doesn't provide enough resistance. We both agree, balance is important as well.

[shorty]

Well gentleman, I think that's it. This post can go on and on. I know what I did and I know it's works for me. Do whatever you want because there's pro's and con's to any modification.

Scotchmo, thank you for your input and info. Your an asset to GTA. I hope we can work on a project one day. Paul68, your still cool in my book too.

[Bryan Heimann]

light piston likes lighter pellets, heavy piston likes heavier ones.  Long stroke= slow stroke, heavy piston= even slower stroke.  Long stroke lighter piston mid weight pellet with strong enough spring (and swept volume) is going to give you the optimum power.  Chris at C.A.P. pretty much proved that.  With a long stroke, you either gotta have enough weight and friction in the bore to keep that pellet from budging until the perfect time every time, or a light enough piston and strong enough ram to close the distance before the middle weight pellet moves.

[Paul68]

I think I'll join Shorty in bowing out now. This thread to be honest is getting redundant and I'd rather let some bigger brains pick it up if they want to continue it. Thanks for all the great discussion, particularly Scotchmo.

[Privateer]

All the math stuff makes my head hurt.
I still say if it was a proven fact a lighter or heavier piston made a big difference?
There would a billion people out there ready to take your money.
I still can't find one selling such a product.

Now I have a couple tubes of special Snake Oil derived from a little known serpent only found in Central America that will do the job.
It also cures cancers of the skin, removes ticks, repels elephants, makes hair grow on bald heads, and stops all computer viruses!

Both Einstein and Steven Hawkings were proven wrong in thier math. Not by ACTUALLY doing what they proved by math.
But just doing marks on a chalkboard.
I see the same thing here. It's all marks on a chalkboard and only one person offers numbers which are not backed up with further proven results.
To prove those results means one must duplicate what he did and verify every step.

All I see is fuzzy theory only applied to AG's.

[Scotchmo]

I think I'll join Shorty in bowing out now. This thread to be honest is getting redundant and I'd rather let some bigger brains pick it up if they want to continue it. Thanks for all the great discussion, particularly Scotchmo.

Thanks Paul68 and shorty.

I did have to correct one thing.

Paul68, you said:

"...You also have results which show a heavier piston producing more FPE...."

If you look back at my results, you will see that the heavier piston never achieved as high an FPE as the light piston. The heavy pellets performed best with the heavy piston but I never got quite as much energy as I did with the light pellets and light piston. I'm not saying that a heavy piston can't produce just as much FPE when everything is balanced correctly.

At least we agree that it is all about balancing the variables.

Some day I plan on getting back to experimenting. The Scotchmo Express #2 has been sitting in the shop half done for the last 7 months. I have been working on my "Big (airgun) Year". I have been competing in as many Grand Prix field target matches as I can. I plan on doing every one west of the Mississippi, from Texas to Oregon until the Nationals in Texas in October.

I have also been dedicating time and effort to getting airgun competitions started at our local range. I found a great club with available range time for matches. We have already had two matches. I just started a web site to help promote the smallbore club as well as the new airgun shooting. It is a great shooting spot. I invite anyone within a reasonable distance to attend one of our monthly matches.

www.slsba.org

Scott Hull

[robtmc]

As much as I have enjoyed y'all thrashing this out,  I think all has been covered adequately.  Big thanks for the links to documents for further reading. 

With that:

[Laser-Fist]

Trail xl1100
Piston and seal weight: 414 grams
Nitro ram: 166 grams, weight appears to be evenly distributed, balance point near middle

[Paul68]

Trail xl1100
Piston and seal weight: 414 grams
Nitro ram: 166 grams, weight appears to be evenly distributed, balance point near middle

@Laser
When we talk about balance, we are talking about the relationship between spring, piston weight, air volume, and pellet weight and how it affects the shot cycle and transfer of energy.