A slewing bearing is a piece of hardware used to facilitate circular movement, usually of a large device such as a crane. The term "slew" means to turn without changing location, so a slewing bearing is one that will not move out of place but will instead facilitate movement while in one position. These bearings are usually quite large and are used for heavy-duty applications; they are therefore usually made from heavy-duty metals such as steel, though other materials including aluminum and titanium can be used for their construction as well.
The slewing bearing is not mounted on a shaft, but is instead bolted to a flat surface. The gears allow one part of the bearing to move while the other stays in place, thereby facilitating movement. The path of the movement can vary; sometimes the slewing bearing is used to facilitate round movement, while in other cases an oscillating movement can be performed. The type of movement depends on the configuration of the slewing bearing as well as the location of the gears. The gears allow a platform to be driven in a certain motion, and the location of the gears will depend on the design of the device.
The profile of the slewing bearing is usually quite thin, allowing the distance between platforms to be very narrow. Ball bearings are present within the bearing race to ensure movement is smooth and constant; these ball bearings may need to be lubricated or greased periodically to prevent premature breakdown or abnormal shaping of the balls. Most devices that use these types of bearings are quite large and slow-moving, but constant movement and high temperatures can alter the shape and function of the bearings over time, thereby necessitating maintenance or repair.
Heavy machinery devices often use a slewing bearing between the vehicle's base and the upper part of the vehicle, which is sometimes known as a house. An excavator, for example, features a base and a house that rotate independently from each other. This is accomplished by using a slewing bearing between the two parts of the vehicle; the outer ring of the bearing will be bolted to the base of the machine, while the inner ring will be bolted to the house. A drive gear may be located on the outside or inside of the ring to allow a drive unit to turn the house as necessary.