If they backdrive, is there a braking system besides that of the holding torque of the stepper? Given that ball screws are used (I'm guessing, but pretty sure) in high-end CNC machines, wouldn't they need some way to ensure precise braking points? If the workpiece is of considerable weight, isn't it possible that the inertia at some point could considerably over power the stepper's holding torque? I know they probably verify distances with encoders, but even then the error would have to be rare, if just to make sure the machine runs fairly smoothly. So ball screws = needs brakes, too?
I'm just running things in my head, thinking about making a walking robot using lead screws. The way I see it, you could make a fairly realistic, albeit slow-moving, walking robot using lead screws to control the motion of the various joints. I'm presuming a kind of robot/walking that would be balanced at any given time during the cycle, so kinetic balance is not an issue (hence speed of the robot is not an issue).
The way the ball screws came up was I was kind of anticipating a more evolved, efficient version further down the line. But if ballscrews don't have the holding resistance of lead screws, then that's a new issue I'd have to factor in farther along, too.
You know what's weird? I have a model I made out of Lego to demonstrate my thoughts on slow-moving, but balanced, walking robots. I guess I'll try to get a video of that on Youtube this weekend. But the thing is, the model I made looks very similar (to my mind, anyway) to the first version of Asimo from back in 1986. That, as far as I'm aware, is pure coincidence.
https://world.honda.com/ASIMO/history/e0.html
In my opinion, my Lego model has a significant improvement over the initial Asimo design. As counter-intuitive as it might seem, I think it's a big mistake to overlook the importance of keeping a considerable portion of the mass of the robot
above the legs. This higher mass oscilates in a pendulus motion to counter what would otherwise be a lateral shift in the centre of gravity of the robot as it moves from leg to leg.