Authors
Mutsumi Setoi, Assistant Manager, NSK-Warner K.K.
Hideaki Takabayashi, Manager, NSK-Warner K.K.
Kenji Maruo, Deputy HQ Head, NSK-Warner K.K.
Shinji Okada, Director, NSK-Warner K.K.
1. The electrification challenge: bearing electrical erosion and EMC noise
As electric vehicles, hybrid electric vehicles, and industrial machinery adopt higher-output, higher-voltage motors and inverter drives, electrical potential differences can develop along the motor shaft. If the voltage across a bearing exceeds the dielectric strength of the lubricant film, electrical discharge can occur within the bearing. Repeated discharge can create microscopic damage that develops into ridge marks and wear on the raceway.
Shaft voltage and high-frequency current can also contribute to electromagnetic noise. An effective grounding solution should therefore provide a controlled low-impedance path that bypasses the bearing, supporting both protection against electrical erosion and improved electromagnetic compatibility. However, space around the bearing is limited, and oil films at sliding interfaces make it difficult to maintain stable conductivity under lubricated, high-speed operating conditions.
2. Product structure and installation
The Electrical Bypass Plate (EByP) consists of a thin diaphragm spring and a proprietary conductive paper. The outer-diameter side is fixed between the bearing and housing, while spring force presses the conductive paper on the inner-diameter side against the end face of the bearing inner ring. This arrangement creates a current path from the bearing inner ring through the EByP to the bearing outer ring or housing.
The spring is 0.15 mm thick, minimizing the additional axial space required. The spring force maintains contact pressure, while the bent geometry follows axial displacement. Slits in the plate preserve the lubricant supply path to the bearing. The geometry can be adapted to the bearing size and surrounding layout.
3. Conductive mechanism of the porous paper
In an oil-lubricated sliding contact, an oil film tends to increase contact resistance. The EByP uses the porous structure of the conductive paper together with broad surface contact. Oil at the interface can move into the pores, helping to exclude or thin the oil film and maintain conductive contact points. Compared with point or line contact, the larger contact area supports a stable current path even when lubricant flow, viscosity, or rotational speed changes.
The plate uses a stainless-steel diaphragm spring as the structural support and a proprietary porous conductive paper as the functional contact material. The metallic spring provides elastic contact force and axial compliance, while the paper provides a compliant, oil-compatible conductive interface. Separating the mechanical and electrical functions allows the spring geometry and paper microstructure to be optimized independently for installation space, contact stability, conductivity, and durability. The segmented paper arrangement also supports local conformity at the rotating contact interface without blocking the oil path to the bearing.
The conductive paper applies NSK-Warner’s experience in wet friction materials to a new electrical function. Material formulation, microstructure control, manufacturing technology, and CAE are used to balance strength, oil resistance, flexibility, surface characteristics, pore structure, conductivity, and durability.
4. Evaluation method
A rotating test rig was constructed with insulating components to suppress unintended leakage current. Axial and radial loads were applied to the bearing while drivetrain oil was supplied, and impedance was measured with an LCR meter. Representative conditions were:
- Rotational speed: 0 to 30,000 rpm; durability conditions were set according to product size.
- Load: 1,000 N axial and 2,000 N radial under the representative condition.
- Lubrication: drivetrain oil at 60 °C and 150 cc/min; the effects of oil flow, temperature, and oil type were also evaluated.
- Impedance measurement: 30 mA at 10 kHz using a HIOKI IM3536.
- Durability: 18,000 rpm for 300 hours with a 6206-size bearing. Additional operation at 25,000 rpm for 130 hours and 30,000 rpm for 7 hours was also confirmed.
5. Conductivity and durability
Under oil lubrication, representative impedance values were 5 Ω at standstill, 30 Ω at 3,000 rpm, and 10 Ω at 18,000 rpm. During continuous operation at 18,000 rpm for 300 hours, impedance remained stable at approximately 10 Ω, with no degradation in conductivity over the test duration.
After the initial loading, the conductive paper thickness stabilized. No progressive thickness reduction or measurable wear was observed during the subsequent durability test, and no measurable wear was found on the bearing side. Surface discoloration resulted from adsorption of oil and additives rather than material decomposition, and no effect on conductivity was identified.
Application feasibility was evaluated over lubricant flow rates of 50 to 600 cc/min, temperatures equivalent to −30 to 120 °C, and with ATF, EV transmission fluid, and gear oil. Rotational testing up to 30,000 rpm also confirmed applicability across a broad range of high-speed lubrication conditions.
6. Suppression of bearing voltage and electrical erosion
In a component-level comparison without the EByP, repeated dielectric breakdown occurred in the bearing and 4 μm of raceway wear was measured after testing. With the EByP installed, no dielectric breakdown was observed and measured raceway wear was 0 μm.
In a customer unit-level evaluation using a high-output motor, the EByP prevented dielectric breakdown in the bearing and demonstrated an effective grounding path that reduced shaft voltage under the evaluated condition. These results show that the EByP forms a low-impedance path around the bearing and can suppress bearing voltage, discharge current, and electrical-erosion damage. By controlling high-frequency current flow, the same grounding concept can also contribute to EMC noise reduction.
7. Application features
- Thin design: 0.15 mm spring thickness minimizes additional axial space.
- Simple installation: the plate is positioned between the bearing and housing, with the outer-diameter side fixed in place.
- Axial compliance: spring force and the bent geometry maintain contact during axial displacement.
- Lubrication compatibility: slits preserve the oil supply path to the bearing.
- Design scalability: prototypes are available for representative motor-support bearing sizes, and application can be studied according to the surrounding layout.
8. Conclusion: Electrical Bypass Plate (EByP)
The Electrical Bypass Plate combines oil-film exclusion and thinning by porous conductive paper with stable surface contact generated by a diaphragm spring. Its thin structure is suitable for installation around existing bearings, and it maintains low impedance in oil-lubricated, high-speed environments. Component durability and unit-level evaluations confirmed stable conductivity, limited wear, and suppression of bearing dielectric breakdown and electrical-erosion damage. The EByP is therefore a practical grounding solution for EV and HEV drivetrains where bearing protection, compact packaging, high-speed oil lubrication, and EMC performance must be considered together.
