Linear bearings reduce the friction of straight movements. But here the sliders are put to another clever use: They adjust to and absorb the forces resulting from misalignment.
Say one rail of a 10-ft.-long table is assembled several millimeters lower than the one opposite. With most bearings, any misalignment becomes additional load. The mounting surface is rigid. Bearings are rigid. There is no freedom; the bearings are pulled in opposite directions at once. What happens? In most cases, the bearings bind.
On the other hand, adjustable sliders assemble into the system at an angle to conform to the table's imperfect geometry while still taking their share of radial load or moment. Similarly, if one rail of a table is skewed laterally a few degrees, adjustable sliders can be mounted at an angle to allow linear strokes without binding or increased friction. These bearings come in pairs of hardened C-shaped channels: The first has flat races and allows lateral freedom while the second holds the bearings on one side, allowing a rotational freedom on the other. When used together, this system allows up to ±2Þ of rotational and 4 mm of lateral freedom. This means that the rails can be mounted to nonmachined surfaces out of parallel by up to 4Þ.