[MrForbes]

We've made two arm robots, each with a small motor and a "spring" to support most of the weight. Neither arm was fully balanced, but they did work ok.

Gas springs do not really have a constant force, the force is higher when the spring is compressed all the way. You can play with the geometry of the mounting points, to try to get a more linear spring force out of it, but it will never be perfect.

And surgical tubing also has this problem, even more than the gas spring. The fun part is trying to get the geometry right so that it works the way you want, ie. naturally holds the arm retracted in the "stowed" position, then provides more help as the arm raises up. We haven't been able to do it yet, but one of these years we'll actually design this part of the robot.

The 330 approach of using a lot more motor power, seems to be quite successful. I'd listen to Eric.