Dual ballscrews are for situations where the carriage could bind, e.g., CNC router where the gantry is three pieces: 2 sides + bridge.
If the router had a rigid gantry with four pieces - 2 sides + bridge + lower bridge - only one screw would be necessary. In fact, two screws would be detrimental. You'd add unnecessary expense, unnecessary friction, and unnecessary overall complication.
You always want the largest and/or highest precision screw that'll fit your application design-wise and budget-wise. Two crappy screws or two small screws will be less accurate than one large, accurate screw. Large screws are also easier to machine.
Backlash should be eliminated by a preloaded 2nd nut and properly preloaded thrust bearings. If you still get backlash, somethings wrong, and I guarantee you that the fix is cheaper than buying an entire 2nd ballscrew assembly.
If multiple ballscrews were actually practical, preloaded nuts, multi-circuit nuts, multi-start screws, etc would not exist. All those contraptions were devised to solve motion problems with a single screw.  |