[Max Lobovsky]

If you would be ok with using a cylinder with significantly the force that you actually need, you could wrap the chain part way around the sprocket with one end connected to the piston and the other end connected to a spring with a fixed mount. You would waste a lot of force (half the force?) tensioning the spring.

If you only need 140 degrees of motion, a better solution is probably to attach the cylinder to a spool of some sort with a cable. If i'm not mistaken, the maximum amount of force the cylinder produces will be applied through the whole 140 degrees as opposed to using a fixed linkage (like a cam).