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A slewing bearing is a critical machine component that carries significant axial, radial and tilting moment loads while allowing relative rotary motion between machine parts. Its performance directly affects the stability and reliability of the whole equipment. Choosing the right material and the right bearing type for each operating condition is therefore essential to long, dependable service.

1. Material Types for Slewing Bearings

1.1 Bearing Steel

Rolling elements of a slewing bearing are generally made from through-hardened high-carbon chromium bearing steel, such as GCr15 or GCr15SiMn. The rings, in contrast, use surface-hardened steel. When no special requirement is given, 50Mn is the usual choice; for specific applications, other surface-hardenable grades such as 42CrMo or 5CrMnMo can be selected according to the host machine’s operating conditions.

Bearing steel offers high hardness, high strength and good wear resistance. GCr15, for example, has a high carbon content; after quenching and tempering it reaches approximately HRC 60–65. Slewing bearings in bearing steel suit light-to-medium duty applications in general industrial equipment, such as small cranes and material handling machines. In these conditions they resist radial and axial loads effectively while maintaining low wear over long service, preserving the precision and service life of the equipment.

1.2 Carburizing Steel

Carburizing steel is used when both high surface hardness and a tough core are required. In grades such as 20CrMnTi, the carbon content is relatively low; after carburizing, the surface forms a high-carbon hardened layer with significantly increased hardness, while the core retains good toughness. This makes carburized slewing bearings perform well under impact loading — for example in the traveling mechanisms of construction machinery that operate on rough terrain with frequent shock and vibration. The hardened surface resists wear, while the tough core absorbs impact energy, preventing sudden fracture from overload and improving equipment reliability.

1.3 Quenched and Tempered (QT) Steel

QT steels such as 42CrMo QT and 50Mn QT are quenched and tempered (quenching followed by high-temperature tempering) to obtain balanced mechanical properties — strength and toughness in good proportion. They suit medium-load applications with high dimensional stability requirements, such as the joints of industrial robots, large cranes and material handling equipment. In industrial robots, the slewing bearing must control rotation angle and position precisely; the well-balanced properties of QT steel keep deformation small under load, preserving motion accuracy, while still withstanding cyclic loads over long continuous operation.

2. Selection by Operating Condition

2.1 Low Speed, Heavy Load

In low-speed, heavy-load equipment such as mining machinery and large port cranes, the slewing bearing is subjected to large axial, radial and tilting moment loads. Priority should be given to high-strength carburizing or QT steels, combined with larger rolling element diameter and a greater number of rolling elements to raise load capacity. In large ball mills, for example, carburized steel with larger-diameter steel balls and additional rows spreads the load, reduces contact stress between the balls and the raceway, and lowers the risk of wear and fatigue failure — keeping the equipment stable and reliable under prolonged, high-load operation.

2.2 High Speed, Light Load

For high-speed, relatively light-load applications such as the rotary table of a precision CNC machine tool, accuracy and rotational speed matter most. Bearing steel is preferred, together with special cage designs — phenolic laminate fabric tube cages or engineering plastic cages — to reduce friction and centrifugal effects at high speed. This combination meets the low-friction, low-heat requirements of high-speed rotation, supports rapid positioning accuracy and stable high-speed operation, and improves the machining efficiency and precision of CNC machine tools.

2.3 Severely Corrosive Environments

In chemical equipment and marine engineering applications, slewing bearings face severe corrosion. Materials with good corrosion resistance — such as stainless steel — are required. In the yaw and pitch slewing bearings of offshore wind turbines, which are exposed to sea spray and salt mist for years, 316 stainless steel resists chloride attack, protecting the raceway, rolling elements and seals from corrosion damage, extending service life and keeping the turbine operating reliably in harsh marine conditions.

2.4 High-Temperature Conditions

In metallurgical continuous casting equipment and heat treatment furnaces, slewing bearings absorb heat from radiation and conduction. High-temperature-resistant materials with dedicated lubrication and sealing systems are needed. For example, high-temperature bearing steel such as Cr4Mo4V, combined with high-temperature grease and metal seals or special high-temperature rubber seals, prevents lubricant loss at high temperature and keeps contaminants out. This keeps the bearing operating normally in high-temperature environments, supports continuous production and reduces downtime and maintenance cost.

Conclusion

Slewing bearing materials vary, and each grade brings its own performance advantages. In practice, selection must consider the actual operating conditions — load magnitude, rotational speed, corrosion level and temperature — and match the material and bearing type accordingly. Done correctly, the slewing bearing operates stably, efficiently and reliably in every condition, protecting the whole machine system, extending equipment life, raising productivity and lowering maintenance cost.