23rd International Colloquium Tribology - January 2022 357 High pressure, high shear viscometry - Lubricant characterization for highly loaded contacts Lukas Mebus Institute for machine elements and systems engineering, RWTH Aachen University, Aachen, Germany Corresponding author: Lukas.mebus@imse.rwth-aachen.de Georg Jacobs Institute for machine elements and systems engineering, RWTH Aachen University, Aachen, Germany Clément Larriere TotalEnergies, OneTech, Centre de Recherches de Solaize, France Arnaud Riss TotalEnergies, OneTech, Centre de Recherches de Solaize, France Jonathan Raisin TotalEnergies, OneTech, Centre de Recherches de Solaize, France Florian König Institute for machine elements and systems engineering, RWTH Aachen University, Aachen, Germany 1. Introduction Lubricants play a major role in reducing friction and preventing wear inside various applications. In general, these phenomena are responsible for yearly losses equalling up to 5% of the gross domestic product [1]. Tribological optimization has been identified as one possibility to reduce friction and wear of tribological systems, with a potential avoidance of up to 40% of these losses over the next years [2]. Whilst significant attention has been given to the optimization of lubricants for common applications like internal combustion engines, upcoming applications have to be considered as well. The challenge for both common and upcoming applications remains the same: Reduction of friction of moving surfaces without the increase of wear by using lubricating oils. They, on the one hand, separate the contact bodies from each other and, on the other hand, ensure minimum frictional losses in the tribological system. For this a lubricant is chosen based on the lubricant’s properties, mainly the viscosity, and its dependency on the applied strains inside the contact. The viscosity is measured in so called viscometers, which can be divided by concept into falling body, capillary, rotational or quartz viscometers. However, these viscometers are currently not able to recreate the simultaneous acting lubricant strains of pressures up to 3 GPa, shear rates up to 10 6 s -1 and temperatures up to 150°C, which are commonly observed for example in gearboxes and roller bearings in E-Mobility drive trains [3,4]. Hence, to this date, additional component and system level test are necessary for a sophisticated lubricant evaluation and selection. To solve this problem, a new viscometer has been developed at MSE, which is presented in this work. It is capable of measuring the viscosity under simultaneous application of pressure, shear and temperature leading for better understanding of lubricants and an improved application oriented lubricant selection. 2. Material and method 2.1 High pressure high shear viscometer The new viscometer concept for high-pressure and highshear developed at MSE is shown in Figure 2-1. This viscometer uses a rotational viscometer concept with a rotating inner cylinder, a so-called Searle-type viscometer. The viscometer is composed of the electrical drive, viscometer stator and rotor, velocity sensor, temperature sensor and torque sensor. In between viscometer stator and rotor, the measurement gap forms with a defined gap width. For shear rate adjustment the rotational speed of the motor or the gap width can be adjusted. This setup is submerged in a lubricant specimen and inserted into the pressure chamber during the measurement. For proof of concept a prototype viscometer was built and tested inside an existing high-pressure test rig capable of pressures up to 800 MPa. Based on the positive results of the prototype a new High-Pressure-High-Shear (HPHS)-vis- 358 23rd International Colloquium Tribology - January 2022 High pressure, high shear viscometry - Lubricant characterization for highly loaded contacts cometer using the same concept was developed. With this, measurements under conditions similar to the effective lubricant strains inside tribological contacts can be conducted. Figure 2-1: Schematic presentation of viscometer con-cept 2.2 Measurement procedure For measurements, the pressure vessel is heated below the desired measurement temperature to enable the compensation of viscous heating due to fluid friction during the measurement. Afterwards, the viscometer is inserted into the vessel and is allowed to heat up to the pressure vessel temperature. As soon as the temperature is equalized, a pressure is applied and the electrical motor is started. During measurement, the inner cylinder is rotating shearing the pressurized lubricant inside the measurement gap leading to a measurable torque at the torque sensor. In combination with the temperature measurement at the measurement gap and the velocity measurement of the viscometer rotor the necessary data for viscosity determination is generated. 2.3 Evaluation Based on the results of the viscometer the dynamic viscosity of the specimen is calculated with the generalized Newtonian fluid model . For calculation, the measured torque is converted into the applied shear stress based on the geometry of the measurement gap. To calculate the correct viscosity only the torque produced inside the measurement gap has to be considered. Therefore, a Computational Fluid Dynamics (CFD-) simulation is used to obtain a correction factor to reduce the measured torque to the torque generated inside the measurement gap. This factor is comparable to the correction factor applied by known viscometer standards e.g. DIN EN ISO 3219-2 [5]. For this CFD model, a 120° wedge of the viscometer including parts of the housing were modelled and simulated. This wedge is chosen due to possible influences of the torque sensor on the measurement results and the torque sensor symmetry. The chosen wedge and the simulated fluid domain are shown in Figure 2 2. In addition to this the temperature influence of the sensor is compensated by measurement-based correction factor for each temperature. This approach’s advantage is a reduced amount of calibration measurements to determine the correction factor of the viscometer. Figure 2 2: CFD model generation from viscometer to fluid domain: viscometer (left), stator (center), surface of the fluid domain without solid-body structure (right) 3. Results The prototype viscometer, described in chapter 2.1-2.2 and evaluated via the methods described in chapter 2.3, is used as a reference for the HPHS Viscometer measurements. To ensure the validity of the results a comparison between results conducted in the prototype and a falling body viscometer is used, which is shown in figure 2-3. The measurements presented were conducted at a desired temperature of 60°C and shear rates of at different pressures levels between ambient pressure and 200 MPa. A mineral group III base oil with Newtonian behaviour was used for validation. The measurement data was compared with the results obtained in a high-pressure falling body viscometer at 60°C at various pressures. The prototype results show a good accordance to the trend of the falling body viscometer measurements, shown in Figure 2-3. In addition to this, the trend of the prototype measurements corresponds to the trend of the falling body viscometer accurately with an error over the complete pressure range < 2%. Comparison of deviations of the individual measurements shows a higher deviation (up to 10%) of the prototype measurements compared to the falling body viscometer measurements (up to 4.5%). This can be explained by the dynamic of the rotational viscometer compared to the gravity driven falling body viscometer. 23rd International Colloquium Tribology - January 2022 359 High pressure, high shear viscometry - Lubricant characterization for highly loaded contacts Figure 2 3: Measurement results of group III base oil 4. Summary and conclusion A performance evaluation of lubricants is necessary for selecting a suitable lubricant. The evaluation should be performed under realistic load conditions. For this purpose, a new rotational viscometer concept for lubricant characterization was developed. The viscometer concept has been prototyped and a new HPHS-viscometer was developed. For evaluation of the measurement results a new evaluation approach has been developed, based on CFD simulation and measurement results. The shown prototype results of the measured lubricant show good accordance compared to the measurements conducted with falling body viscometers, validating the prototype results. These measurements also show, that for this concept a further reduction of the deviation of individual measurements to < 5% is desirable for better measurement quality. This improvement in combination with the increased measurement range of the HPHS-viscometer leads to further understanding and quantitative description of lubricants under realistic operational conditions. 5. Acknowledgement The authors would like to thank TotalEnergies for the funding and support of this project. References [1] Czichos, H. , Habig, K-H.: Tribologie-Handbuch 4th edn., Springer Vieweg, Wiesbaden (2015). [2] Holmberg, K., Erdemir, A.: Influence of tribology on global energy consumption, costs and emissions. Friction 5(3): 263-284 (2017) [3] Morhard, B., Schweigert, D., Mileti, M. et al.: Efficient lubrication of a high-speed electromechanical powertrain with holistic thermal management. Forsch Ingenieurwes 85, 443-456 (2021). https: / / doi.org/ 10.1007/ s10010-020-00423-0 [4] Schröter, J., Jacobs, G., Straßburger, F.: Schnelldrehende Elektrische Antriebe für Bau- und Landmaschinen. Antriebssysteme 2015 : Elektrik, Mechanik, Fluidtechnik in der Anwendung ; Aachen, 11. und 12. November 2015 / VDI Produkt- und Prozessgestaltung, Seiten/ Artikel-Nr: 27-37. DOI: 10.18154/ RWTH-2015-06857 [5] DIN EN ISO 3219-2: 2021-08, Rheologie - Teil 2: Allgemeine Grundlagen der Rotations- und Oszillationsrheometrie (ISO 3219-2: 2021)