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Hybrid bearings

Hybrid bearings
Hybrid bearings
Hybrid bearings have rings made of bearing steel and rolling elements made of bearing grade silicon nitride (Si3N4). Because the silicon nitride ceramic material is such an excellent electrical insulator, hybrid bearings can be used to effectively insulate the housing from the shaft in both AC and DC motors as well as in generators.

In addition to being an excellent insulator, hybrid bearings have higher speed capabilities and provide longer bearing service life under the same operating conditions than samesized bearings with steel rolling elements. Hybrid bearings also perform extremely well under vibrating or oscillating conditions. Often, it is not necessary to preload the bearing or apply a special grease under these conditions.

Assortment

The assortment of SKF hybrid bearings comprises popular sizes for electric motors and generators. It includes:
  • single row deep groove ball bearings
    • basic design
    • sealed design
    • XL hybrid design
  • single row cylindrical roller bearings
Hybrid bearings provided here constitute the basic SKF assortment and are only part of the total assortment. Other hybrid bearings include:
  • Hybrid super-precision bearings (→ Super-precision bearings)
    • hybrid super-precision angular contact ball bearings
    • hybrid super-precision cylindrical roller bearings
    • hybrid super-precision angular contact thrust ball bearings, single and double direction
  • Hybrid angular contact ball bearings
  • Hybrid stainless steel deep groove ball bearings
  • Bearing units incorporating hybrid bearings
For availability and detailed information, contact the SKF application engineering service.
Factors that influence SKF hybrid bearing performance
Bearing performance is not only determined by load or speed ratings. There are a number of other factors that contribute to bearing performance. Main factors that enhance SKF hybrid bearing performance, when compared to same-sized bearings with steel rolling elements, include:
  • Insulating properties
    As a non-conductive material, silicon nitride protects the rings from electric current damage and, therefore, can extend bearing service life in applications like AC and DC motors and generators, where there are electric currents that could damage the bearing.
  • Lower density
    The density of a bearing grade silicon nitride rolling element is 60% lower than a same-sized rolling element made of bearing steel. Lower weight means lower inertia – and that translates into superior behaviour during rapid starts and stops, as well as higher speed capabilities.
  • Lower friction
    The lower density of a silicon nitride rolling element, combined with its low coefficient of friction, significantly reduces bearing temperature at high speeds. Cooler running extends the service life of both the bearing and the lubricant.
  • High hardness and high modulus of elasticity
    The high degree of hardness of a silicon nitride rolling element means high wear-resistance, increased bearing stiffness and longer bearing service life in contaminated environments.
  • Resists false brinelling
    If a stationary bearing is subjected to vibrations, there is a risk that “false brinelling” occurs. False brinelling is the formation of shallow depressions in the raceways that eventually lead to spalling and premature bearing failure. In cases where steel rolling elements were replaced by ceramic rolling elements, the bearings were found to be significantly less susceptible to false brinelling.
    Hybrid bearings lubricated with SKF wide temperature grease (WT) were found to sustain less false brinelling damage than hybrid bearings lubricated with other types of greases.
  • Reduced risk of smearing between silicon nitride and steel surfaces
    Even under inadequate lubrication conditions, there is reduced risk of smearing between silicon nitride and steel surfaces. This enables hybrid bearings to operate much longer in applications where there are high speeds and rapid accelerations, or applications where there is an insufficient hydrodynamic film to support undisturbed bearing operation (i.e. κ < 1, → Selecting bearing size). For conditions where κ < 1, it is common to apply κ = 1 for hybrid bearings for bearing life calculations. Hybrid bearings may perform well when lubricated with ultra-thin film forming media, such as refrigerants, typically used in compressor applications or fuel pumps, enabling oil-free designs.
  • Runs faster, lasts longer
    Combine the lower density of silicon nitride with its lower coefficient of friction, high hardness and the fact that silicon nitride does not smear the raceways under poor lubrication conditions, and the result is a bearing that runs faster and longer even under the most difficult operating conditions.
  • Low coefficient of thermal expansion
    A silicon nitride rolling element has a lower coefficient of thermal expansion than a same-sized rolling element made of bearing steel. This means less sensitivity to temperature gradients within the bearing and more accurate preload/clearance control.
    When designing bearing arrangements for very low temperatures, a hybrid bearing with greater internal clearance than standard may need to be selected. In these cases, contact the SKF application engineering service.
  • Speed capability
    Typically, hybrid bearings have a higher speed capability than same-sized bearings with steel rolling elements, however, in some cases, the cage execution may limit the attainable speed.
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