Two air cylinders acting against each other

[djc9273]

I'm trying to design a system in which one air cylinder (single piston) can reach 4 repeatable positions. Multi-piston cylinders won't work because I need fine adjustment in the position of each of the four stages. The four positions are at 0 inch, 1 inch, 2 inch and 3 inch.

My idea was to have one cylinder as the main driving cylinder (let's call this cylinder A with a stroke of 3 inches and bore of 25mm), and two cylinders (theoretically coaxial with Cylinder A, but acting in a the opposite direction) with a stroke of 1 and 2 inches respectively (and a bore of 32mm). Let's call these cylinders B (2 inch stroke) and C (1 inch stroke).

My idea is to use the cylinders B and C as "hard stops" to obtain an adjustable position that Cylinder A can extend to (through the use of a through piston rod with a fine adjustment screw on the back)

There will be four positions, and in order to reach position 1 & 4, Cylinder A would be extended or retracted as usual. To reach position 2, Cylinder B would be extended and cylinder A would be Extended. This way the rod of cylinder A would be driving against the rod of cylinder B (which has a larger bore, same pressure). And likewise for position 3.

My concern is how "hard" these hard stops would be in this situation. With the same pressure throughout the system, the force in Cylinders B & C would always overcome Cylinder A, but will there be any drift in the position once the system "settles" at positions 2 & 3?

 

[Mech_Engineer]

I think rather than using opposing air pistons you're better off using pop-up stops for the main piston using solenoids. Depending on where you want it to stop, actuate the bump stop for the position you want. As you say, opposing air pistons could have some position variation if there are pressure equalization issues, and they will have some settling time as well.