I’ve had to do maintenance work on sliding cross-roller bearing stages before, and I realized first-hand that if you don’t do a good job of centering the cage to the midspan of the rails, the cage will slam into the hard stops prematurely. Even if you install them perfectly, if for whatever reason the rollers slip and cause the cage to move when it isn’t supposed to (usually during high-frequency moves), the cage can wander away from its original position with potentially disastrous results – this is known as cage creep. The video below from Nippon bearing shows the problem and their solution, the Studroller (TM) – at around 10 seconds, you can see that the cage tries to move beyond the base rail. It would hit a hard stop at that point (or worse, the rollers would fall out)!
Double rack and pinion, Schneeberger
This solution uses two racks and a floating gear in the center to kinematically link the motion of the cage to the motion stage without relying on friction.