Speed vs power

[LordKronus]

are 3/4 inch or 1 1/2 inch pistons better for launching the ball in terms of speed to power ratio. i would really appreciate solid numbers that i could present to the group.

[sethw]

Both the speed and power you get out of a piston is directly related to the bore size. A larger bore will give you more power and more speed. This is because the compressed air is impacting upon a larger surface area within the larger pistons. The extending force of a piston in pounds is equal to the pressure being sent into it (in pounds per square inch) multiplied by the surface area of the disk inside the cylinder (the radius of this is half the bore size). There is a chart listing the force output of the allowed bore sizes in the FIRST pneumatics manual.
The speed at which different size cylinders will fire is more difficult to calculate, as it depends on the flow rate of air into the cylinder. This will depend on the valve you are using, whether you are using flow controls, the inner diameter of your tubing, and the arrangement of your tanks. All other things being equal, though, a larger bore cylinder will fire more quickly than a smaller bore cylinder of the same stroke. The longer the stroke, the slower the extension, typically speaking.
The easiest way to demonstrate all of this to your team would be to set up a small pneumatics system on a piece of plywood or something, and fire two cylinders of differing bore sizes if you have some already. Hope this helps.

[EricH]

Originally Posted by sethw

Both the speed and power you get out of a piston is directly related to the bore size. A larger bore will give you more power and more speed. This is because the compressed air is impacting upon a larger surface area within the larger pistons. The extending force of a piston in pounds is equal to the pressure being sent into it (in pounds per square inch) multiplied by the surface area of the disk inside the cylinder (the radius of this is half the bore size). There is a chart listing the force output of the allowed bore sizes in the FIRST pneumatics manual.
The speed at which different size cylinders will fire is more difficult to calculate, as it depends on the flow rate of air into the cylinder. This will depend on the valve you are using, whether you are using flow controls, the inner diameter of your tubing, and the arrangement of your tanks. All other things being equal, though, a larger bore cylinder will fire more quickly than a smaller bore cylinder of the same stroke. The longer the stroke, the slower the extension, typically speaking.
The easiest way to demonstrate all of this to your team would be to set up a small pneumatics system on a piece of plywood or something, and fire two cylinders of differing bore sizes if you have some already. Hope this helps.

This doesn't make sense. A larger volume of air is required for a larger bore cylinder, so it should fill slower and as such fire slower at the same pressure. In my experience, a smaller bore cylinder always fires faster than a larger one.

[Frank Neuperger]

Agreed on filling the larger volume taking longer. If there are restrictions in the path between the storage tank and the cylinder, you may get far less projectile height than the calculated potential energy of the compressed air. The issue is the restriction of the plumbing. The flow rate may prevent you from acheiving the launch velocity (and height) you expected. Even though a 1 inch cylinder will do less work than a 2 inch for the same static air pressure, The 1 inch ram may have a faster terminal velocity after accelerating a certain mass.

There is likely be a curve that peaks in terms of optimal bore size for given air pressure pressure, stroke length and mass to be accelerated.

Think of the extreemes... 12 inch diameter bore for 12 inch stroke would have way too much volume and fill very slow. Alternately a 0.1 inch bore would not provide enough force. Somewhere in between is the Goldilocks bore that is "just right" for the flow restrictions in your plumbing and the mass you are trying to launch.

[sethw]

It will certainly depend on the flow rate into the cylinder, but in my experience the additional force provided by the larger bore cause the piston to accelerate more rapidly, and hence extend in a shorter time. However, the systems I was working with when I made those observations were a good bit larger than those we're working with in FIRST (especially the tubing). If the flow rates are low enough here to counteract the greater force of the cylinder, I stand corrected. I haven't tried it yet with these components.

[EricH]

Originally Posted by sethw

It will certainly depend on the flow rate into the cylinder, but in my experience the additional force provided by the larger bore cause the piston to accelerate more rapidly, and hence extend in a shorter time. However, the systems I was working with when I made those observations were a good bit larger than those we're working with in FIRST (especially the tubing). If the flow rates are low enough here to counteract the greater force of the cylinder, I stand corrected. I haven't tried it yet with these components.

I still don't get what you are trying to say. A large bore cylinder has more volume to fill, so at a constant flow rate, it will take longer to fire. Operating pressure here is 60PSI.

Now, if you are referring to the tubing, yes, larger tubing will fire faster. Operating pressure here is 60PSI.

[Frank Neuperger]

Eric, the optimal bore diameter for max velocity at the end of stroke depends on the degree of restriction (and some other factors)and in the FIRST KOP, the restriction of the approved valve is significant when the task is launching the ball some 8 feet high. .

If the bore dia is excessive (think 12 inch dia or more), as you narrow the bore, clearly your end of stroke velocity and thus the height of projectile will improve.

At the other extreme, say your piston bore is the diameter of the smallest restriction in the air path to the cylinder. Because of the small bore size, it will have insufficient force (given your 60psi pressure limit) to accelerate the ball very well... perhaps not even be able to lift it. Widening the bore from this extremely narrow scenario will improve acceleration.

As you can see both of the extremes I describe are very poor performance. IF you charted it, there is a peak for performance ( maximum end of stroke velocity) in between these two extremes of bore size. To calculate the perfect bore size is fairly complex but it can be done if you have a really good handle on flow calcs. It may be easier to experiment with standard sizes you have on hand. 0.75, 1, 1.5, 2

Frank

[MrForbes]

Originally Posted by EricH

In my experience, a smaller bore cylinder always fires faster than a larger one.

My guess is that you really have to know what the load is on the cylinder before you can compare...if the cylinders don't have to do any work, then sure, the small one will fire more quickly. If they both have to push the short end of a long lever that has to lift a 10 pound ball on the other end, then the 3/4" bore cylinder with 60 psi applied to it will just sit there, while the larger bore cylinder might actually make something happen, slow though it may be.

As sometimes happens, there is not enough information here to give a good answer to the original question

[ALIBI]

Wild guess. The air flow is restricted by the regulator, tubing and valves. Think about heart bypass surgery and multiple cylinders. To conserve air we limited air consumption by using multiple pressure regulators for those applications that did not require 60psi in the past. You can use up to 4 Clippard Receivers. Could you get max airflow by having 4 cylinders on your launching mechanism? Fill 4 Clippards in parallel from the compressor. Have a regulator-valve-cylinder off from each clippard and then fire all 4 valves simulatainiously? It would then be up to you to determine stroke and bore that works for your application. I have only looked at the Typical Pneumatic Circut and I do not know if more than one Norgren Regulator at 60psi is permitted (we used Monnier regulars downstream of the Norgren). R88 seems to say that this is possible. This solution would do a lot of damage to your weight budget, but it seems plausable. Could you end up in a place where the valves are fighting eachother to fire? This should be easy enough to prototype and find out.

[AdamC]

Everybody else before me is correct. we're operating now with 2 1.5" cylinders, but there isn't much luck right now. Pneumatics operating with FIRST regulation parts and pressures are not strong enough to launch a trackball BY THEMSELVES! You can create a mechanical linkage powered by a cylinder, but just punching the cylinder will not work.
The main issue is the hoses we are required to use. The flow-rate through the hoses is not fast enough to fill the cylinder fast enough to "punch" it. You will apply a very powerful force to the trackball with a beefy cylinder, but it will not impart a powerful enough impulse against the ball.
If you want to crunch the numbers, the formula is: Area*Pressure = Force Output. To calculate area, you simply find the area of a the cylinder head (pi*.5bore^2).
You can crunch the numbers further with some kinematics equations to find the resulting velocity. If you choose to pursue the idea (which I highly recommend you don't), invest in a cylinder that will provide you with about 25% to 50% more force than you technically require to launch the ball the desired height.
I will GREATLY envy you if you manage to pull the pneumatic launcher off, and I'll want to see some pictures.

[LordKronus]

Originally Posted by squirrel

My guess is that you really have to know what the load is on the cylinder before you can compare...if the cylinders don't have to do any work, then sure, the small one will fire more quickly. If they both have to push the short end of a long lever that has to lift a 10 pound ball on the other end, then the 3/4" bore cylinder with 60 psi applied to it will just sit there, while the larger bore cylinder might actually make something happen, slow though it may be.

As sometimes happens, there is not enough information here to give a good answer to the original question

Sorry i meant to say is which is better for launching the ball, the 3/4 inch or 1 1/2 inch pistons.