The chances are you haven’t reflected on the bearing that maintains a crane’s vertical position while lifting or the mechanism that enables a hospital scanner to rotate accurately. That’s the whole point; if a mechanism functions flawlessly, it goes unnoticed. Slewing rings are a prime example.
What are they?
A Slewing ring is a special type of bearing. Because of their large diameters, reckless and axial moments can be combined and handled simultaneously. They are installed between two parts and enable controlled rotation; most slewing rings can rotate through a full 360 degrees. Designed with large scale applications in mind, Slewing rings are used in applications that require a combination of large slewing diameters, load carrying capacities and rotational accuracy.
The different Types
Four-point contact ball bearings is one of the most frequently used. A single row of balls makes contact at four different points within the raceway, enabling them to handle reversing loads, in addition to being useful in applications with space limitations. These are used in light-duty cranes, medical equipment, and turntables.
Cross-roller bearings consist of cylindrical rollers that are arranged in alternating directions perpendicular to one another. The unique structural design of cross-roller bearings provides improved rigidity and a significant load added capacity in compact environments. This design is used in robotics, and in precision rotary tables.
Three-row roller bearings operate a little differently. Independent axial and radial systems are designed in separate rows, and as such, they are capable of handling a combination of load’s that are significantly higher. This design is used in offshore cranes, large excavators, and tunnel boring machines.
Applications
A considerable portion of the demand comes from the construction and materials handling industries. Slewing rings enable safe rotation under load of tower cranes, mobile cranes, and reach stackers. These bearings are involved in hazardous applications where a failure can lead to catastrophic consequences.
The marine and offshore applications present a more challenging environment due to the presence of salt water, shocked loading, temperature cycling and structural requirements. Typical applications include heave compensation systems and platform cranes.
The wind energy sector is also a significant market. Slewing rings of large diameter are used to track the wind and control the pitch of wind turbine blades. These bearings are designed to have a long service life to accommodate the infrequent access for maintenance. The loading on the bearings is also cyclical and the use of these bearings in low maintenance applications also impacts their design.
Industrial automation and defense applications use cross roller bearings where more load capacity is less important than positional accuracy. This also applies to the aerospace applications where systems employed for turrets of weapon systems and radar antennas require high reliability and consistent rotation to function the bearings.
Having the Exact Specification
There is never one answer for an entire load profile and operating speed. Space availability, environment, and accessibility for maintenance all influence what type of bearing is most appropriate. For example, a four-point contact bearing designed for a light duty turntable is totally unsuitable for an offshore crane, and vice versa.
Regarding the drive connection, slewing bearings are designed with or without integral gear teeth, externally, internally, or are ungeared. This is an additional layer of decision-making on top of bearing type, which is easily overlooked too late in the project.
It is also worth understanding slewing rings because the typical load rates in a catalogue are for a calculated duty cycle, unlike most bearing manufacturers, and slewing rings are designed for a duty cycle, not a single worst case load. If you are starting from scratch, it is best to consult with a specialist prior to finalising the design, as the time and effort required to correct later issues is often substantial.