Machine Design

Rolling-Element Linear-Motion Bearings

Linear-motion bearings deal with the problem of providing low-friction translational (rather than rotational) motion. Sliding-surface bearings are stiffer and deflect less under load than rolling-element types, and sliding-surface bearings allow greater load capacity per unit of surface area. But they also exhibit appreciable stick-slip, or a sudden surge of "breaking loose" when motion begins. Stick-slip results from a large difference between static and kinetic friction. Hydrostatic bearings do not have stick-slip, but they are complex and expensive.

Rolling-element linear bearings are simple and inexpensive and do not exhibit significant stick-slip because their static and kinetic friction is nearly the same. (Coefficients range from about 0.002 to 0.005.) These bearings, therefore, are preferred for precision machinery. They require less power to put a machine in motion. This factor is especially important in servo control where cost increases greatly as power requirements increase.

Linear bearings are of two types. Flat-way bearings are designed to bear against flat surfaces. Shaft bearings are designed to ride along a round shaft.

TAGS: Bearings
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