As the machines we are building increase in size we faced the problem that ballscrews of lengths over 1 meter will give trouble in the long run. Inbalance and vibrations are quite common. Using rack and pinion drives is the solution. The only problem is the backlash. We have developed a module that takes out the backlash between the rack and the gear. Ofcourse you will still have a litle backlash in the wormgearbox, in practice it works.. See for yourself on youtube, this machine has these modules on the X and Y axis http://www.youtube.com/watch?v=ByEZ2jyjPV8