Even with some big motors and drivers, I suspect that I will have some speed limits on a 1/2" (12.5 mm) pitch screw. That's with a direct drive setup. I can live with the speed I'll (hopefully) get. I would not be happy with what the calculations suggest I'd get on a 1/4" pitch screw.
Going to gears or belts to drive the screws would be one way to get around that limit. A 2:1 to 4:1 ratio would be fairly easy to do either way. Backlash is the first thing you would have to take care of. Doing that without adding complexity to the design would be a challenge.
As it is, "good" thread rods are not a lot more / may be less than the cost of a good (fat) smooth rod. Drive nuts do cost money, so do bearings for large diameter smooth rod.
The next issue I see is the binding up of the thread rod to the smooth rod. On long drive distances, having the end of the rod just flop around isn't very pretty. Confining it brings up the possibility of a pinch point somewhere in the travel.
Going to gears or belts to drive the screws would be one way to get around that limit. A 2:1 to 4:1 ratio would be fairly easy to do either way. Backlash is the first thing you would have to take care of. Doing that without adding complexity to the design would be a challenge.
As it is, "good" thread rods are not a lot more / may be less than the cost of a good (fat) smooth rod. Drive nuts do cost money, so do bearings for large diameter smooth rod.
The next issue I see is the binding up of the thread rod to the smooth rod. On long drive distances, having the end of the rod just flop around isn't very pretty. Confining it brings up the possibility of a pinch point somewhere in the travel.