23rd International Colloquium Tribology - January 2022 109 Influence of mechanical, thermal, oxidative and catalytic processes on the thickener structure and thus on the service life of rolling bearings Markus Grebe Competence Center for Tribology, Mannheim University of Applied Sciences Corresponding author: m.grebe@hs-mannheim.de Michael Ruland Competence Center for Tribology, Mannheim University of Applied Scien 1. Introduction Constant further developments in application technology with the aim of higher economic efficiency and power density place ever greater demands on mechanical components and construction elements and thus also on the lubricating greases used. This is particularly true in the area of roller bearings, in which lubricating greases are often used with high mechanical stress and in wide temperature ranges. A current example is the rolling bearings in the assemblies of hybrid vehicles, which are subjected to extreme thermal and mechanical loads due to engine downsizing, high speeds and the radiant heat from the combustion engine. Investigations at the Competence Center for Tribology Mannheim (KTM) show that the grease service life for roller bearing lubrication, even at high temperatures, does not only depend on classic oil aging [Greb18]. In numerous roller bearing tests and by means of rheological measurements it could be shown that the loss of the lubricating effect is often a consequence of the change in the thickener structure [Greb19]. Mechanical, thermal, oxidative and catalytic processes play a decisive role here. As part of the lecture, the current status of a DGMK project will be presented, in which the significance of these individual influencing factors on the change in the thickener structure and thus on the bearing life is determined through targeted individual investigations and detailed tests. The calculation of the service life of grease-lubricated rolling bearings is based on the fatigue strength of the bearings in accordance with ISO 281. However, under critical operating conditions, such as high temperatures, failure due to a lack of lubrication occurs even before this fatigue limit is reached. Since many roller bearing systems are lubricated for life in practice, the service life of the bearing depends on the service life of the grease and not on the fatigue life of the bearing. The service life of the grease, however, is not a clearly determinable characteristic value that can easily be determined by a standard test. For this reason, various laboratory aging processes (RapidOxy, TGA, DSC), laboratory tests (Shell roller) and endurance runs in roller bearing test rigs (e.g. FE8-test, FE9-test, R0F-test) are usually used in development in order to obtain information about the expected service life of a component. These tests are very time-consuming and costly, so that the industry is constantly looking for new, innovative and meaningful screening tools [Greb2021]. In the DGMK project 788 “Screening test method for lubricating greases” it was shown that the loss of the lubricating effect of a grease is strongly influenced by the thickener degradation. Recent publications by other scientists confirm this hypothesis and reinforce the need for research [e.g. Dorn16, Kuhn17, Yuxi18, Zhou18]. In the case of soap thickened greases, this effect has been shown to occur well before the actual base oil aging and is accordingly limiting service life. The catalytic effect of the cage material apparently has a different and previously unknown effect on this effect than on classic oil aging. The current running research project aims to clarify which influencing factors ultimately play the decisive role and how the effect can be positively influenced. In technical applications, a wide variety of causes can lead to irreversible damage to the lubricants. A thermo-oxidative degradation depends strongly on the ambient conditions. In non-encapsulated systems, the oxidation of the lubricant leads to the formation of acids, polymers, condensates and deposits. This process depends on the temperature, the presence of catalysts such as metal surfaces or wear debris, the oxygen supply and the aging products produced. Furthermore, coking effects can occur due to the aging of the lubricating grease and thus the degradation and polymerization of the lubricating grease, but 110 23rd International Colloquium Tribology - January 2022 Influence of mechanical, thermal, oxidative and catalytic processes on the thickener structure and thus on the service life of rolling bearings also due to insufficient lubrication, especially at elevated temperatures and high speeds [Klein98; Brau15]. In addition to thermo-oxidative aging, a lubricating grease is also subject to high mechanical-dynamic stress in use. The importance of the individual influencing factors was clearly shown in the recent DGMK project 788. The focus of this follow-up project will be on changing the thickener structure as a result of mechanical, thermal, oxidative and catalytic stress. The chemical and structural changes are detected and examined using the most modern analytics and microscopy: • Mechanical stress: FE9, Walk apparatus according to Klein (Figure 1) • Structure of the thickener system: scanning electron microscopy, partly “cryo-SEM” • Rheology: Yield stress, flow curves, elastic and plastic modulus • Aging: RapidOxy, TGA, DSC, oven aging [Dorn19] • Analytics: FTIR, HPLC-MS, GC-MS, PDSC, ICP. Figure 1: Shear tester acc. Klein This project is based on an empirical approach. That means, numerous laboratory tests on aging depending on temperature, ambient medium and catalytic elements will be carried out and evaluated. The focus is on the so-called RapidOxy test, as this is becoming increasingly important in industry (ASTM D8206, DIN 51808) [Matz2021]. The main part of the tribological tests will be application-oriented roller bearing tests on the multi-station roller bearing test bench MPWP based on the FE9 test. Different parameters and bearing types are tested. These tests are supplemented by high-speed tests in order to meet the special requirements of e-mobility. The aim is to gain knowledge for the first time as to how the thickener structure changes during aging and mechanical stress, how this can be proven and what influence this has on the lubrication and thus on the performance and service life of the rolling bearing. 2. First test results Figure 2 shows the different run times in the FE9-test for all 8 model tests. Figure 2: Run times of the different model greases in the FE9 test (blue mineral oil based, red: PAO) With the help of the apparatus acc. Klein, greases can be sheared under defined conditions. The grease is pumped in a circle, which works like a gear pump. The change in the rheological parameters (yield point & loss modulus) is then determined using a rheometer measurement. Figure 3 shows the derease of the shear stress of the modell greases after 24h and 72 h. Figure 3: Decrease in shear stress at 3 1 / s after 24h or 72h in the shear test acc. Klein 3. Conclusion As part of the project, the influence of individual effects on the change of the thickener structure of a lubricating grease is examined. The procedure and the first detailed results were presented here. Further test results will be shown during the lecture. 23rd International Colloquium Tribology - January 2022 111 Influence of mechanical, thermal, oxidative and catalytic processes on the thickener structure and thus on the service life of rolling bearings References [Klei98] Kleinlein, E.: Einsatz von Wälzlagern bei extremen Betriebs- und Umgebungsbedingungen, expert Verlag, Renningen, ISBN 3-8169-1608-2, 1998. [Brau15] www.braun-waelzlager.de. [Dorn16] Dornhöfer, G.; Ermittlung der Schmierfettgebrauchsdauer mit zeitraffender Prüfmethode und Übertragbarkeit; Tagungsband GfT-Jahrestagung; 2016. [Kuhn17] E. Kuhn: Modellierung zum Schmierfettverschleiß im stationären Reibungsprozess; Tagungsband GfT-Jahrestagung; 2017. [Yuxi18] Yuxin Zhou, Rob Bosman & Piet M. Lugt (2018) A Model for Shear: Degradation of Lithium Soap Grease at Ambient Temperature, Tribology Transactions, 61: 1, 61-70. [Zhou18] Yuxin Zhou, Rob Bosman, Piet M. Lugt (2018) A Model for Shear Degradation of Lithium Soap Grease, Trib. Trans. 61: 1, 61-70, DOI: 10.1080/ 10402004.2016. 1272730. [DGMK788] Abschlussbericht des Forschungsvorhabens 788; IGF-Vorhaben Nr. 18615 N: 12/ 2018. [Greb18] M. Grebe; C. Müller; J. Molter; S. Hiesinger: Einflussfaktoren auf die Schmierfettgebrauchsdauer im Wälzlager, 59.Tribologie-Fachtagung 2018, Gesellschaft für Tribologie e. V. (GfT), ISBN 978-3- 9817451-3-9. [Greb19] M. Grebe, J. Molter; T. Isik: Einflussfaktoren auf die Schmierfettgebrauchsdauer; 60. Tribologie-Fachtagung, Gesellschaft für Tribologie e. V. (GfT), Tagungsband 2019; S. 418 - 427; ISBN 978-3-9817451- 4-6. [Matz21] Matzke M., Beyer-Faiss S., Grebe M., Höger O.: Thermo-oxidative grease service life evaluation - laboratory study with the catalytically-accelerated method using the RapidOxy; Digitaler Tagungsband 62. Tribologie-Fachtagung, Gesellschaft für Tribologie e. V. (GfT), 2021. [Greb2021] Grebe, M: Tribometrie - Anwendungsnahe tribologische Prüftechnik als Mittel zur erfolgreichen Produktentwicklung; Expert- Verlag, ISBN 978-3-8169-3521-6 (print), 2021.