Abstract

In this talk, Dr. Jonny Hansen discusses the fundamentals of tribology in electric vehicles (EVs). Special attention is given to the lubrication and electric breakdown mechanisms that may occur in bearings and gears of heavy-duty EVs.

More specifically, it will be shown that when subjected to an electric field, surface initiated problems are possible even under conditions previously considered safe, i.e., in the full film elasto-hydrodynamic lubrication (EHL) regime. Because of this, and when considering tribological contacts for EV’s, it is crucial to be able to estimate the transition between the EHL and mixed lubrication (ML) regimes to a good degree of accuracy.

Traditionally, Tallian’s lambda ratio, i.e. the nominal film thickness over the composite surface RMS level (Λ=hm/Sq), has often been employed for this purpose, and for assessing rolling contact fatigue (RCF) life. However, although some research has proven the model beneficial with regard to the latter, it cannot be used to accurately assess the transition between EHL and ML [1-5]. In fact, and as will be shown, it may falsely suggest contact interference, boundary lubrication (BL), despite a fully developed EHL film has formed –  a very risky outcome in the design of tribological contacts for EV’s.

The presentation will proceed by revealing the main deficiencies of the Λ-ratio that stems from the nature of surface roughness and micro elasto-hydrodynamics. With this in mind, and inspired by Tallian, a new recently published film parameter [4], Λ*, is put forward as a more reliable tool to estimate the transition between EHL and ML. Subsequently, it is shown that the new film parameter can be used to assess under which lubrication conditions electrically induced surface damages may occur, and when the lubricant film is sufficiently thick to protect surfaces by electrical insulation.

With knowledge of the fundamental film formation and breakdown mechanisms, the presentation expands to cover the lubrication conditions of EV drivetrains in the context of the proposed film parameter, and is finally concluded by putting forward prospects pertinent to EV lubrication research.

References

The presentation was given for TribNet.org and a recording can be found on the following webpage (see below):

https://network.tribonet.org/webinar/fundamentals-of-tribology-in-electric-vehicles-by-dr-jonny-hansen/

Please find attached the presentation from the talk by Dr. Jonny Hansen.

Key references (all Open Access) leading to this work include:

1. Hansen J, Björling M, Larsson R. Mapping of the lubrication regimes in rough surface EHL contacts. Tribol Int 2018;131:637–51. https://doi.org/10.1016/j.triboint.2018.11.015.
2. Hansen J, Björling M, Larsson R. Topography transformations due to running-in of rolling-sliding non-conformal contacts. Tribol Int 2020;144:106126. https://doi.org/10.1016/j.triboint.2019.106126.
3. Hansen J, Björling M, Larsson R. Lubricant film formation in rough surface non‑conformal conjunctions subjected to GPa pressures and high slide‑to‑roll ratios. Sci Rep 2020;10:1–16. https://doi.org/10.1038/s41598-020-77434-y.
4. Hansen J, Björling M, Larsson R. A New Film Parameter for Rough Surface EHL Contacts with Anisotropic and Isotropic Structures. Tribol Lett 2021;69:1–17.
5. Hansen J. Elasto-hydrodynamic film formation in heavily loaded rolling-sliding contacts (Doctoral dissertation). Luleå University of Technology, 2021.