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Performance of Different Types of Insulated Bearings

Traction motor bearings are critical components in urban rail transit vehicles, directly impacting operational safety. Common failure modes include electrical erosion, cage fracture, and spalling of the rings or rolling elements; notably, electrical erosion is the most frequent cause of failure. Extensive research—both domestically and internationally—has been conducted on electrical erosion in bearings, covering topics such as the mechanisms of shaft current generation and the severity of erosion damage. To mitigate this, insulated bearings are frequently incorporated into electric motor designs. The primary types of insulated bearings include those with aluminum oxide (Al₂O₃) coatings, polyphenylene sulfide (PPS) resin coatings, and hybrid ceramic bearings. In China’s urban rail transit sector, aluminum oxide-coated bearings remain the most widely used, with limited application of PPS resin-coated bearings. Hybrid ceramic bearings, however, have not yet been adopted, primarily due to cost considerations and a lack of operational experience.

 

Given that the insulation strategies for aluminum oxide-coated and PPS resin-coated bearings differ significantly in terms of coating materials, thickness, and the design parameters of the bearing rings, this paper presents a comparative study of their insulation performance, weather resistance, and the mechanical properties of the bearing rings.

 

Comparison of Coating Materials and Bearing Parameters

This study focuses on 6215 and NU216 insulated bearings—the types most commonly used in metro traction motors. An insulating layer was applied to the outer rings of these bearings: an Al₂O₃+TiO₂ coating (with a density of 3.8–4.0 g/cm³ and a thickness of 0.1 mm) and a PPS resin coating (with a density of 1.3–1.9 g/cm³ and a thickness of 0.7 mm).

The aluminum oxide coating exhibits higher insulation strength—approximately 1.5 times that of the PPS resin coating—and superior resistance to voltage breakdown. Conversely, the dielectric constant of the aluminum oxide coating is roughly three times that of the PPS resin coating, indicating that the PPS resin coating possesses better dielectric and polarization properties. The 6215 insulated bearing has dimensions of Φ130 mm × Φ75 mm × 25 mm, with dynamic and static load ratings of 68.9 kN and 49 kN, respectively, and a limiting speed of 6,700 r/min. The NU216 insulated bearing has dimensions of Φ140 mm × Φ80 mm × 26 mm, with dynamic and static load ratings of 166 kN and 160 kN, respectively, and a limiting speed of 5,600 r/min. While both types of insulated bearings possess comparable load-carrying capacities and limiting speeds, the PPS resin coating has a thickness of 0.7 mm, which is significantly greater than that of the aluminum oxide coating.

 

Conclusions

1) At room temperature, bearings with a PPS resin coating exhibit superior performance—in terms of DC withstand voltage, high-frequency leakage current, electrical characteristics in hot and humid environments, and weather resistance—compared to bearings with an aluminum oxide coating.

 

2) Bearings with a PPS resin coating demonstrate better power-frequency withstand voltage capabilities than those with an aluminum oxide coating; furthermore, the power-frequency withstand voltage values ​​for the PPS resin-coated bearings are more stable, whereas those for the aluminum oxide-coated bearings exhibit greater fluctuation.

 

3) Bearings with a PPS resin coating are more sensitive to temperature changes; their insulation resistance decreases more rapidly with rising temperature than that of the aluminum oxide-coated bearings. However, at 150 °C, the insulation resistance of both types of coated bearings becomes comparable.

 

4)Under functional random vibration and shock conditions, the von Mises stress on the outer ring raceway surfaces of both the 6215 and NU216 bearings—featuring either a 0.1 mm-thick aluminum oxide coating or a 0.7 mm-thick PPS resin coating—remains below the maximum allowable contact stress. Consequently, the mechanical properties of the rings for both types of insulated bearings meet the operational requirements for use in metro traction motors.