16 Inch Four-Point Contact 398x628.8x80mm Ball Slewing Ring Bearing - Outside Gear16 Inch Four Point Contact 398x628. 8x80 mm Ball Slewing Ring Bearing with Outside Gear Four Point Contact Ball Slewing Bearing with Outside Gear A slewing bearing ring is a rotational rolling element bearing that typically supports a heavy but slow turning or slow oscillating load, often a horizontal platform such as a conventional crane, or the wind facing platform of a horizontal axis windmill. (To 'slew' means to turn without change of place.)