A slewing bearing , also called a slewing ring bearing or slewing ring, is a large-diameter rolling-element bearing designed to support combined loads while allowing one machine component to rotate relative to another. Unlike conventional bearings, slewing bearings can accommodate relatively large axial and radial loads as well as overturning moments within a compact structural arrangement.
They are widely used in excavators, cranes, wind turbines, construction machinery, industrial robots, rotary tables, and other equipment that requires controlled rotational movement.
A typical slewing bearing consists of several key components:
Inner ring– The inner ring contains the raceway and is connected to one part of the machine.
Outer ring– The outer ring provides another raceway and is mounted to the stationary or rotating structure, depending on the application.
Rolling elements– Balls or cylindrical rollers transfer loads between the raceways while reducing friction.
Cage or spacer– It keeps the rolling elements properly separated and helps maintain their distribution around the raceway.
Seals– Sealing elements help protect the raceways and rolling elements from dust, water, and other contaminants while retaining lubricant.
Depending on the design, a slewing bearing may also include an integrated gearon the inner or outer ring. The gear can engage with a pinion to transmit driving torque and rotate the bearing assembly.
The internal geometry varies according to the bearing type. Ball slewing bearings generally use point contact between the balls and raceways, while roller slewing bearings provide line contact. This difference affects load capacity, stiffness, friction, and suitability for different operating conditions.
The basic operating principle is similar to that of other rolling bearings: rolling elements move between precision-machined raceways, allowing one ring to rotate relative to the other with reduced friction.
However, slewing bearings are specifically designed to handle combined loading.
For example, an excavator slewing bearing may simultaneously experience:
Axial loadcaused by the weight of the upper structure
Radial loadgenerated during operation
Overturning momentcaused by the position of the boom, arm, and payload
These loads are transmitted from one ring to the other through the balls or rollers and their raceways.
The bearing's internal contact geometry determines how these loads are distributed. In a four-point contact ball slewing bearing, for example, the raceway geometry allows the bearing to accommodate axial and radial loads as well as moment loads through the contact between the balls and raceways.
For applications requiring higher stiffness or load capacity, roller-based designs may be used. Their line contact can provide greater load-carrying capability under appropriate operating conditions.
The rotation itself can be driven externally by a hydraulic motor, electric motor, or other transmission system. When the slewing bearing has an integrated gear, a pinion engages with the gear teeth and transfers torque to rotate the connected structure.
The performance and service life of a slewing bearing depend on several factors, including:
Applied axial and radial loads
Overturning moment
Rotation speed and operating cycle
Raceway and rolling-element design
Lubrication
Mounting structure and bolt preload
Alignment and deformation of the supporting structure
Contamination and sealing conditions
Therefore, selecting a slewing bearing should not be based only on bore diameter or external dimensions. The actual load spectrum, mounting conditions, operating environment, and required service life should also be considered.
In the next part, we can look at the main types of slewing bearings , including four-point contact ball bearings, crossed roller bearings, and three-row roller bearings, and compare their structural characteristics and typical applications.