Machine Design

Rack-and-pinion gears keep cage and rollers centered

Precision rail-guide systems using cross rollers are commonly found in X-Y positioning applications where speed and accuracy are essential. When the machines are put to nonstop use, however, they suffer a common problem the cage and roller assembly move toward one end of the guide's travel. This movement, dubbed "cage creep," is particularly prevalent in cross-roller systems subjected to high speeds and accelerations, uneven loads, and vertical orientations. As the cage moves farther from its centered position, system friction increases at the stroke limits. The cage can also hit the end stops and make the rollers skid. In the end, the machine must be shut down to reposition the cage or it must be equipped with a larger, more costly motor with the power to reset the cage.

To stem cage creep, engineers at SKF Motion Technologies, Bethlehem, Pa., have developed a rack-and-pinion system which can be used as a drop-in design for most cross-roller systems. It has a rack gear for each rail and a pinion gear on the cage. The pinion gear engages the teeth on the rack and controls the cage's position, keeping it centered.

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