Re: pic: 488 Arm Prototype
 

Again, I was able to redesign some parts of the mechanism so that a considerably shorter stroke piston is able to accomplish the same task. However, your general assessment is correct.

Two tetras at the end of a 60" arm produce approximately 100 ft./lbs. of torque. To lift that, you'd want a piston acting as far away from the pivot point as possible to minimize the force needed, but as that point of actuation moves away from the pivot, the length of stroke required to get the desired radial motion (~120*) increases. A 24" stroke piston is the bare minimum required as shown, as that allows us to act on a point approximately 12" from the pivot. 2" bore pistons are rated at 188 lbs. of force extended, but, that's assuming 60 PSI. It would operate at something less, but I wouldn't feel comfortable relying on its operation over and over without making the compressor do some serious work.

I shortened the piston stroke needed by moving the point of actuation a bit nearer to the pivot. This means, of course, that we'll need to be closer to 60 PSI to operate correctly, however, it also means that we need much less volume of air, as well; 50% less, in fact. Coupled with weight saved, it seems like a fair trade.

Re: pic: 488 Arm Prototype
 

Correct me if I'm wrong, but the weight neccesary for a given torque and range of movement is constant. Saving weight on the cylinder is possible, but you seem to be arguing that by moving the mounting point and changing the length of the cylinder, you managed to reduce the air requirement.