That is exactly like our arm too. Except we typically refer to it as a wrist and shoulder instead of elbow and shoulder. We ended up using a CIM-U-LATOR gearbox tied to a GEM 500 gearbox to get enough torque for our arm. When we started it in testing we imediatly started breaking chain left and right. It was crazy, but then we found a really nice system for hooking turnbuckles into the chain directly by using master links. This fixed the connection points in our chain, and it hasn't broken since. Here is a video of everything working:
http://www.youtube.com/watch?v=sturFfjv_bU