[EricH]

Originally Posted by yash101

Maybe fill it up with pressurized air (like a piston out) to make it longer, and have a spring retract it. Maybe that will work for light/medium loads

Except that if you don't do that with a COTS pneumatic cylinder, you're in violation of the rules (specifically R76). I don't see the ban on non-COTS pneumatic items going away anytime within the next decade, if not century. On top of the large volume of air you'd need for any reasonable extension length, and the fun of fitting at least one cylinder that long inside the arm, which make for a very nasty engineering challenge between them...