A slewing ring or slewing ring is a rotary rolling bearing, usually bearing a heavy but slowly rotating or slowly swinging load, usually a horizontal platform, such as a conventional crane, swing winch, or windward crane.The platform of a horizontal axis windmill.Rotary bearings have a thinner cross section than other rolling bearings and are usually one meter or more in diameter.Lifts Slewing Bearing on the falkirk wheel is 4 meters in diameter and is mounted on a 3.5 meter axle.Slewing bearings usually use two rows of rolling elements.