Aus Wissenschaft und Forschung / TAE-Plenarvorträge 40 Tribologie + Schmierungstechnik · 67. Jahrgang · 4/ 2020 1 Introduction Over the years, operating conditions of rolling bearings and gears have evolved towards higher severity. Trends like: downsizing, higher speeds, higher temperatures and poorer lubrication conditions have increased stresses at the contacting surfaces. Tribology of roll-ing/ sliding contacts has become more important than simple Hertzian fatigue in life predictions. This trend has the tendency to further increase with the acceleration of electrification in vehicles. Electrification brings new challenges for rolling bearings and gears. High acceleration, high speeds, low viscosity and new chemistry in lubricants. Besides potential corrosion and electrical damage on the raceways and teeth flanks. Therefore, engineers must be prepared to develop and apply modelling techniques that can help them to predict the surface life of a lubricated contact in electrical applications. This paper presents modelling techniques applied to rolling bearings and gears to predict the surface effects on the life of these machine components. Including tribological phenomena like surface distress, seizure, wear and spall propagation. 2 Surface Distress Surface distress also called micropitting or grey staining is a surface failure mode that is caused by poor lubrication conditions in heavily loaded contacts. It is the result of the competition between surface fatigue at asperity level and mild wear [1]. An overall model to capture this phenomenon has been presented earlier [1] and its flow chart is summarised in Figure 1. First, representations of two contacting surface topographies are measured and entered in the model. They together with the operating conditions and rheology are utilized by the mixedlubrication model [2] to find instantaneous pressure, deformations and stresses in the near-surface area as the topographies move into the contact. The stress history of the accumulated load cycles is compared with a fatigue damage criterion, if the threshold is exceeded a pit will be opened. Then a local Archard wear model [3] is applied to modify the topography. The new topography is solved again until the number of load cycles intended is achieved. Since the local load constantly changes, a damage accumulation criterion is needed to add the damages in time. At the end of the simulation the final modified topography including micropitting is predicted. Tribology Modelling of Lubricated Contacts for Electrification Guillermo E. Morales-Espejel* * Guillermo E. Morales-Espejel SKF Research and Technology Development, Houten, The Netherlands LaMCoS, INSA-Lyon, Lyon, France Corresponding author: guillermo.morales@skf.com Figure 1: Flow chart for the surface distress model Aus Wissenschaft und Forschung / TAE-Plenarvorträge 41 Tribologie + Schmierungstechnik · 67. Jahrgang · 4/ 2020 3 Seizure High-speed rolling bearings of electrical motors lubricated with grease, are likely to work under kinematic starvation conditions. There, the risk of high adhesive wear (or seizure) due to very high temperatures inside the contact and collapse of film thickness is very real [4]. A seizure risk calculation procedure can be developed by considering a simple starvation model in rolling bearings [5] to calculate the real lubrication conditions, knowing the internal sliding in the contacts a simple analytical expression for the flash temperature can be used [6]. In [7] it is shown that a temperature of around 250 °C in experiments seems to be a safe thresh-old limit for potential seizure, as shown by Figure 2, corresponding to a simulation and experiment for a hybrid ball bearing 7218. 4 Wear Wear can be an important failure mode when corrosion, abrasive particles or chemical attach is present in bearings and gears due to, e.g. new lubricants more chemically compatible with electrical components or water condensation due to high changes in temperature. Wear alone can potentially take very long time to become a failure mode in may components. However, in heavily loaded contacts with high sliding (high speeds) abrasive/ adhesive or chemical wear (corrosion) can modify the contact profile substantially to accelerate stress concentrations in certain contact points and therefore fatigue [7]. 5 Spall Propagation Micro or large spall propagations is an interesting topic, since the risk of actual surface failure is related to how fast spalls propagate. This is an emerging topic of study which is taking more relevance with electrical contacts, especially when some electrical damage has already occur in bearings or gears. The surface distress model described earlier can be a good starting point to model this phenomenon. For large spalls, the pressure cushioning effect caused by the lubricant film is less important and the model can be simplified by considering only dry contact [8]. 6 Conclusion In the current paper several models for surface failure modes in heavily-loaded lubricated contacts have been described, with special emphasis on potential triggering mechanisms related to electrification of vehicles. Certainly this new industrial trend is likely to produce higher stress on the loaded surfaces of rolling bearings and gears and will bring new challenges to engineers, tribologists and modellers. References [1] Morales-Espejel, G.E., Brizmer, V., “Micropitting Modelling in Rolling-Sliding Contacts: Application to Rolling Bearings”, Trib. Trans., vol. 54, 2011, 625-643. [2] Morales-Espejel, G.E., Wemekamp, A.W., Félix-Quiñonez, A., “Micro-Geometry Effects on the Sliding Friction Transition in Elastohydrodynamic Lubrication”, Proc IMechE, Part J, J Eng. Trib., vol. 224, 2010, 621-637. [3] Morales-Espejel, G.E., Brizmer, V., Piras, E., “Roughness Evolution in Mixed Lubrication Condition due to Mild Wear”, Proc IMechE, Part J, J Eng. Trib., vol. 229(11), 2015, 1330-1346. [4] Czichos, H., Kirschke, K. “Investigation into Film Failure (Transition Point) of Lubricated Concentrated Contacts”. Wear, vol. 22, 1972, 321-336. [5] Chiu, Y.P., “An Analysis and Prediction of Lubricant Film Starvation in Rolling Contact Systems”. ASLE Trans vol. 17, 1973, 22-35. [6] Tian, X., Kennedy F.E., “Maximum and Average Flash Temperatures in Sliding Contacts”. ASME Trans J Tribol, vol. 116, 1994, 167-174. [7] Morales-Espejel, G.E., Gabelli, A., “Rolling Bearing Seizure and Sliding Effects on Fatigue Life”, Proc IMechE, Part J, J Eng. Trib., vol. 233(2), 2018, 339-354. [8] Morales-Espejel, G.E., Engelen, P., van Nijen, G., “Propagation of Large Spalls in Rolling Bearings”, Tribology Online, vol. 14(5), 2019, in press. Figure 2: Simulation (lines) and experiment (circle) for average flash temperature estimation inside the heaviest loaded contact of a hybrid ball bearing 7218 at the first signs of seizure