The presence of electric or magnetic fields can significantly alter the lubrication process in various types of bearings used as support elements in machines, as well as the flow properties of the lubricants within these bearings. While innovative applications are being developed to leverage these properties for enhanced lubrication, improper use of electric or magnetic fields can adversely affect the lubrication process. The rapid adoption of electric vehicles (EVs) introduces new lubrication challenges due to the presence of electric currents. Therefore, it is crucial for machine designers, engineers, and technical personnel in maintenance and repair to have a comprehensive understanding of lubrication systems exposed to electric and magnetic fields and influenced by charged particles.
The electromagnetic field consists of two components: electric and magnetic fields. The magnetic field is the region where moving and electrically charged particles are influenced by force, created by the electrons in atoms rotating around the nucleus and on their own axis. Although the magnetic field itself is invisible and not easily felt, its effects can be observed or sensed. All substances possess magnetic fields of varying strengths. If a system’s magnetic field is not properly isolated, it can adversely impact the operation of other systems. For instance, when a mobile phone is used near a television or computer, static noises may be heard, and screen distortions may occur. There are various types of electric motors, including direct current (DC), alternating current (AC), synchronous, asynchronous, servo, and stepper motors. In electric motors, the rotor shaft is typically supported by rolling element bearings or cylindrical radial sliding bearings known as bushings. The lubrication function in these bearings is usually performed using grease or solid lubricants.
All motors exhibit some level of shaft voltage. When this voltage exceeds a certain threshold, it indicates potential failure. In electric motor bearings, electric currents in the rolling elements and raceways can sometimes cause spot welding. Stray magnetic fields within electric motors can generate high electric currents that pass through the motors, potentially damaging the bearings. To mitigate these issues, it is recommended to use grounding brushes in electric motors.
There is limited research in the literature on the effects of magnetic fields on friction in rolling bearings used in electric motors. However, with the rapid adoption of electric vehicles (EVs), this area is gaining attention due to the presence of electric currents. Lubricants in EVs need to have higher electrical insulation to prevent arcing, as they come into direct contact with the e-motor and other electrical components. The operating conditions of EVs are harsh, often involving high temperatures, increased oxidation, and particle abrasion. To withstand these conditions, lubricants must maintain stable dielectric properties throughout. Additionally, lubricants in EVs are in close contact with various materials, which can lead to issues such as component breakage, swelling, and cracking. Many of these components are made of copper due to its high electrical conductivity, making excellent copper compatibility crucial for the lubricants. There is an optimal operating temperature range where lubricants are most efficient and durable for the electric motor and other power electronic components. These lubricants must provide high heat dissipation at temperatures up to 180°C. The higher torque in electric vehicles can also lead to wear problems that are unprecedented in internal combustion engine vehicles.
The main component of all lubricants is the base oil (BO). Almost all lubricants begin as BO, with various additives incorporated over time to enhance performance and/or improve energy efficiency. While BOs and their viscosity are considered crucial for cooling performance, additives play a critical role in the electrical conductivity of EVs. It is important to note, however, that additives can also have a minor impact on cooling performance.
References for further reading:
Kadıoğlu M., Durak E., Study of the tribological properties of rolling element bearings under the effect of magnetic field. Industrial Lubrication And Tribology, 71(10), , 1200-1205. Doi: 10.1108/ILT-02-2019-0044, (2019), (SCI-Expanded), Daban Z. ve Durak E., “Manyetik alana maruz toz metalurjisiyle (T/M) üretilmiş bronz yatakların sürtünme özelliklerinin incelenmesi”, Politeknik Dergisi, 23(1): 137-149, (2020).Manyetik Alana Maruz Kalan T/M Esaslı Yatakların Tribolojik Özellikleri, Tunay, R. F., Durak, E., TMMOB Makina Mühendisleri Odası Konya Şubesi, VI. Makina Tasarım ve İmalat Teknolojileri Kongresi, 22-23 Ekim 2011, Chen Y, Jha S, Raut A, Zhang W and Liang H (2020) Performance Characteristics of Lubricants in Electric and Hybrid Vehicles: A Review of Current and Future Needs, Front. Mech. Eng. 6:571464. doi: 10.3389/fmech.2020.571464 Wang X., Wang Q.J., Ren N., England R., Lubrication subjected to effects of electric and magnetic fields: recent research progress and a generalized MEMT-field Reynolds equation Frontiers in Mechanical Engineering, 9, 2024, https://www.frontiersin.org/journals/mechanical-engineering/articles/10.3389/fmech.2023.1334814, DOI=10.3389/fmech.2023.1334814E.
