From looking at the videos, the shop sabre pneumatic cylinder looks like one of the two I have and that is 32mm bore, but it does look like a larger bore, I'm guessing 40mm as a standard size unless it uses Imperial units. You can see from this video when they hook up the pneumatics. I am curious about the air system, reservoir size, regulator, flow rate requirement for the compressor that's always dumping air....etc. Router Bob, come to the forum and answer some questions about Shopsabre? LOL, who knows, he may be a member here. Also curious why they have hoses hooked up to the top and bottom of the cylinder.

https://www.youtube.com/watch?v=IHeanOy43e4

Lower pressure needs a larger piston area to provide the same force, the volume of the piston effects the stroke, and could effect the needed reservoir size. When using cylinder with the rod in tension, the area is the bore area minus the area of the rod, in compression, you just use the bore area for the calculations.

Smaller bore cylinders need higher pressures to get the required force, but can also use smaller reservoirs, and that is essentially what a traditional gas strut is. Higher pressure, small bore, small reservoir that is small enough that it is built into the spring. Dampening that is built in, and non-adjustability, and higher force differences at top and bottom of stroke, are the problems there, although some can let air out to make it adjustable in one direction. I would have considered gas struts if the dampening wasn't there, you can buy some without dampening but I think that is lots of $$$. Also I am thinking if the spindle I have doesn't work out for me, I may use a lighter one, and this way I can adjust the force for the new weight and fine tune the pressure for the best balance.

Traditionally, these pneumatic systems are used to do other things. The example Chris posted, I believe this is used to be able to help lift up different objects of different weights, so there is a level of complexity there that doesn't necessarily need to exist in counterbalancing something of fixed weight, although it was interesting to look at, and I appreciated the post!

Chris, you were talking about adding dampening on purpose? To my knowledge, dampening is bad for a CNC application, but you know what, a little bit of it might not be? The easiest way I can think of to be able to have adjustable dampening is simply to add a simple screw based flow control valve(s) to the pneumatic cylinder(s). Making the exit hole for the unpressurized side of the cylinder restricted with a much smaller opening will dampen the system. Although doing this will make it fight against your ballscrew on fast moves. There may be some dynamic responses that I am not considering which is why I'm saying I don't know if a little bit of dampening is better.

Bob, the sample you posted was an example of a rod in compression, what I'm doing, but I'm already thinking it's not the best, that system will be continually dumping air with each stroke and will need a compressor that is constantly running to make it work, which may be fine, if that's what you want.

My thoughts are that you should design your system based on the force you need to counterbalance and the reservoir size that gives you an acceptable force difference at top and bottom of stroke. 10-20 lbs difference, is it important, I don't think so....you can always add a regulator and turned on compressor into the system later if it leaks or doesn't work to your expectations? It's not like sizing a gantry tube where you are stuck with it unless you want to make another. The regulator is small and could be mounted in a number of places....so why not try a closed system first?

If you could tune the compressor only to come on below your optimum mid stroke pressure value and the regulator to only dump air at a value higher than the calculated extremity, then you might have a nice "closed - open" system that only takes care of leaks and has a safety release, so that the compressor isn't constantly running and air isn't being constantly dumped.

A few questions I still don't know the answers to that could change everything.....are these cylinders prone to leaking...does using a lower pressure make them less prone to leaking....?

So how much weight do you want to counterbalance?

Let's do an example.......

Let's say you want to counterbalance 120lbs with a stroke of 300mm and a single gas spring (rod in tension), and you want to use a 2 gallon air reservoir.

Option 1 is a 32mm bore with a 10mm rod

Option 2 is a 40mm bore with a 10mm rod

Option 3 is a 63mm bore with a 20mm rod

^^^^^^^^^^^^^^^^^^
Area1 is (3.14 x 0.63^2) - (3.14 x 0.197^2) = 1.12 in^2

Pressure 1 is 120 lbs / 1.12 = 107 PSI

Cylinder volume 1 is 11.81 x 1.12 = 13.2 in^3

2 gallons is 462 inches cubed

P1V1 = P2V2, so assumed balanced at bottom stroke (in reality you might want to do mid stroke). The 2 here refers to the top of the stroke, not example 2.

107 x 462 = P2 x (462 + 13.2), P2 = 104 PSI

104 x 1.12 = 116.5 lbs

So there is a top to bottom force difference of 120 - 116.5 = 3.5 lbs

^^^^^^^^^^^^^^^^^^^

Area 2 is 1.83 in^2

Pressure 2 is 65.5 PSI

cylinder volume 2 is 11.81 x 1.83 = 21.61

65.5 x 462 = P2 x (462 + 21.61), P2 = 62.6 PSI

62.6 x 1.83 = 114.5 lbs

So the force difference between top and bottom stroke is 120 - 114.5 = 5.5 lbs

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Area 3 is 4.34 in^2

Pressure 3 is 27.6 PSI

Cylinder volume 2 is 11.81 x 4.34 = 51.25

27.6 x 462 = (462 + 51.25) x P2, P2 = 24.84 PSI

So the force difference is 12.2 lbs.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

So really you see that as you lower the pressure and increase the bore, the force difference between top and bottom stroke becomes greater, however, with a 2 gallon reservoir, it still doesn't matter that much. Smaller reservoirs magnify this effect.

On beer 4 now guys, may have made a math error, check it out for yourself, may have another look later and subsequent edit.....

My thoughts for you Bob, size the thing and build it, add a regulator and compressor system later on if you need to.