the non-covalent bonds between the molecules of the thickener are broken, which shortens the fiber lengths and irreversibly degrades them physically. This causes the grease to soften and change its rheological properties. (Meijer, et al., 2023). This study examines the effect of total physical thickener failure in a grease in a real gear application. It shows that the irreversible shearing of the thickener cannot be simulated using established model tests for grease. Based on this, a modified model test is developed that replicates the effect. The modified model test developed is used as a method for formulating an adapted thickener system of the same type. The suitability of the modified model test for early-stage development is validated in a real gearbox application. Physical thickener degradation in planetary gearboxes The investigation of the degradation effect is based on the results of (Ochs, et al., 2024), which showed, among other things, that a defined gear test after just 300 hours of operation allows a significant differentiation of the condition of fundamentally suitable greases and that the Science and Research 50 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0019 Introduction In industrial gear systems, the lubrication system can be designed as a grease-lubricated system for cost-driven applications or to reduce the risk of leakage. Greases for gears generally consist of one or more base oils, a thickener, and various additives. Metal soap-based thickener systems are widely used in industrial applications and impart the viscoelastic properties of gear grease through their fibrous network. The subgroup of complex soaps, which are composed of a base, a fatty acid, and a non-fatty acid, are characterized by their high performance. (Kuhn, 2017). This study examines greases based on complex soap, which are primarily intended to provide lifetime lubrication in planetary gears in precision applications. The lubricity and service life of a grease are limited by physical and chemical degradation mechanisms caused by shear stress, pressure, high variations in operating conditions, and temperatures in the gear. Chemical degradation includes additive degradation and oxidation reactions at elevated temperatures, which are accompanied by bleeding and evaporation of the base oil. Physical degradation is primarily caused by the shear stress and shear rate exerted, which leads to increased oil separation and destruction of the thickener structure. Both types of degradation are irreversible processes (Rezasoltani, et al., 2016). At moderate operating temperatures (40 °C -70 °C), physical degradation is one of the most dominant damage processes that shortens the remaining service life of the grease (Rezasoltani, et al., 2016). The energy introduced into the system by shear stress dissipates into heat, and Design and Implementation of a Model Test for Reproducing Thickener Degradation in Grease-Lubricated Gearboxes and Its Role in Grease Formulation Development Katrin Alt, Markus Wöppermann, Frank Plenert, Jürgen Liebrecht* Presented at GfT Conference 2025 The irreversible structural degradation of thickeners in lubricating greases under gearbox operating conditions cannot currently be replicated using conventional model test methods. To overcome this limitation, a modified model test has been developed that enables the reproduction of this failure mechanism. This modified model test is used as a screening test for the formulation of alternative lubricating greases, and their suitability is validated in gearbox tests. Keywords Lubricating grease, Thickener degradation, Gearbox, Model test, Shear stress, Grease formulation Abstract * M.Sc. Katrin Alt Dr.-Ing. Markus Wöppermann SEW-EURODRIVE GmbH & Co KG Ernst-Blickle-Straße 42, 76646 Bruchsal Dipl.-Ing. (FH) Frank Plenert Dr.-Ing. Jürgen Liebrecht FUCHS LUBRICANTS GERMANY GmbH Friesenheimer Str. 19, 67661 68169 Mannheim condition of the grease correlates with the wear condition of the gear components. A flank-critical operating point with a high transmission ratio is tested in a planetary gear. The tested operating point causes surface pressures of over 1 GPa on the tooth flank, and the narrow spaces in the planetary bearing lead to high shear stress on the grease. After the gear test, Grease A, as designated in this paper, showed complete physical degradation of the thickener, as shown in Figure 1. Table 1 shows the relevant characteristics of Grease A. Transferability to model tests With the aim of reproducing the physical thickener degradation from Figure 1 of Grease A at the model level, various tests are performed on Grease A. According to the state of the art, established laboratory and model test methods already exist. The tested methods and the optical and haptic condition of Grease A after each test are shown in Table 2. Figure 2 shows examples of the grease condition described in Table 2 after the FZG, FE8 test, and the oxidation roll tester. Grease A has become significantly discolored and softer. The thickener structure is intact, and reversible behavior of the consistency can be observed upon cooling. The tests performed in Table 2 do not cause the damage pattern of total physical thickener degradation of Grease A in the gearbox shown in Figure 1. Science and Research 51 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0019 Figure 1: Condition of Grease A after planetary gear testing: Total physical degradation from (Ochs, et al., 2024). Figure 2: Grease condition Grease A according to FZG (left), FE8 (center), oxidation roll tester (right). Name NLGI Base oil viscosity Base oil type Thickener Additive Grease A 2 100 mm 2 / s Synthetic oil Calcium sulfonate complex A1 Table 1: General characteristics of Grease A. Test Grease condition Test Grease condition FE9 Roling bearing test DIN 51821 A/ 1500/ 6000-120 Softer, oily -consistent Work stability DIN ISO 2137 Softer, oily -consistent FZG FLP-TF-0520 A/ 2,8/ 50 Softer, oily -consistent VKA Welding load DIN 51350-4 Softer, oily -consistent FE8 DIN 51819-2 C-75-80 Softer, oily -consistent Oxidation Roll Tester ASTM D 1831 Softer, oily -consistent Shell-roller test 72h/ 100°C Pw nach ASTM D 1831 Softer, oily -consistent Klein´s shear Mikro PU / mikro PW60 [0,1mm] Softer, oily -consistent Work penetration DIN ISO 2137 Softer, oily -consistent Table 2: Performed model tests and their results with Grease A. temperature during the test is kept constant at T = 100 °C. The test duration is 20 million revolutions. Adapted grease formulation and validation The modified model test is used as a method for the predevelopment of an adapted grease formulation. In order to minimize the influence of the thickener type on the functionality of the grease in the gearbox, the thickener type of Grease A is kept unchanged. A fully synthetic base oil is used, the base oil viscosity is increased to 150 cst, and the thickener structure, including the additive package, is developed further. Table 3 shows the basic characteristics of Grease B compared to Grease A. Figure 4 shows Grease A and B before and after the test on the modified VKA from Figure 3. Both greases have Science and Research 52 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0019 Modified model testing Since it is not possible to replicate the degradation process of Grease A in the gearbox using the model tests carried out in Table 2, a modified VKA (four-ball tester) was developed. A parameter study was used to specify the test setup and operating conditions that reliably replicate the degradation process of Grease A. The setup is typically used to investigate the shear stability of oils according to CEC L45-A-99 and is shown in Figure 3. The test setup based on a VKA is extended by a temperature-controlled roller bearing holder with a cover. A tapered roller bearing from the 32008 series is used as the test bearing. The test bearing is fully filled with grease and operated at a constant speed of n = 1500 min -1 . The load is 10 kN (corresponding to a pressure on the outer ring of approx. p = 1.5 GPa). The Figure 3: Modified four-ball-tester setup. Figure 4: Condition of Grease A (left) and Grease B (right) before and after testing in the modified VKA. Name NLGI Base oil viscosity Base oil type Thickener Additive Grease A 2 100 mm 2 / s Semi-synthetic oil Calcium sulfonate complex A1 Grease B 1 150 mm 2 / s Synthetic oil Calcium sulfonate complex A2 Table 3: General characteristics of Grease A and Grease B. changed not only in color but also in terms of their texture. It should be noted that the change in Grease A (left) is irreversible. It is clear to see that the thickener has been destroyed by the shear in the tapered roller bearing. In comparison, Grease B shows an expected discoloration and a normal softening, which is reversible upon cooling. To validate the suitability of the modified VKA as a new model test for assessing the grease condition in the gearbox and to validate the thickener stability of the newly developed Grease B, Grease B is tested in the same gearbox test as Grease A from (Ochs, et al., 2024). After a running time of 300 hours, the gearbox is disassembled and the grease condition and gearbox components are analyzed. Figure 5 shows the grease condition in the planetary gearbox after the test run. After the test, the grease is clearly discolored and softened but still has an oily consistency. Compared to the condition of Grease A after the same gear test in Figure 1, the thickener is intact and the softening is reversible. Discussion During gear tests according to (Ochs, et al., 2024), Grease A and B exhibit moderate operating temperatures (60 °C -70 °C). This leads to the assumption that high shear stresses are the most dominant factor reducing the remaining service life of the greases and that the chemical degradation mechanisms can be considered negligible. Furthermore, the temperatures and temperature curves do not indicate any abnormal wear in the narrow spaces between the planetary bearings, so that the flow behavior of Grease B (and the degraded Grease A) appears to be sufficient for the conditions tested. Due to the unremarkable wear condition of the bearing components after the gear tests with Grease A, it can be assumed that the effect of thickener degradation is compensated by the associated high flow behavior of the resulting grease mixture and is not yet critical for the operating time of 300 hours. However, during further operation, it can be assumed that the inhomogeneous grease mixture will have lost a significant amount of its thermal conductivity and will cause increased temperature and friction. On the other hand, the degraded thickener residues may increase the oxidation reactions in the base oil/ grease mixture, so that the improved Grease B with an intact thickener structure can be attributed a longer residual service life. In order to simulate the effect of total physical thickener degradation of Grease A in the gearbox at the model level, it becomes apparent that both the mechanical load and the available grease reservoir and its oil supply to the tribological contact are relevant. Modification of the established model tests with regard to test setup, test equipment, and operating conditions is essential for effect replication. The high shear stresses and the modified oil supply from the grease reservoir to the tribological contacts in the modified VKA cause the thickener structure of Grease A to fail, as in the gearbox test, but not the improved Grease B. Conclusion It is shown that the physical degradation of a grease in a real gearbox application could not previously be reproduced in established model tests. Simulating the effect at the model level is essential in order to optimize the thickener structure for mechanical stability during the early stages of grease development. The model test developed in this work on a modified VKA allows the total physical thickener degradation to be isolated and reproduced for the time being. A new grease with an adapted thickener system proves to be mechanically stable on the modified VKA. The stability of the new thickener system and thus the suitability of the modified model test for early development was validated in a real gearbox application. The modified VKA is proposed as a new method for the specific investigation of the mechanical stability of the thickener structure of grease. References (1) Kuhn Erik Zur Tribologie der Schmierfette [Buch]. - Renningen : expert verlag, 2017. - Bde. Eine energetische Betrachtungsweise des Reibungs- und Verschleißprozesses. (2) Meijer Robert Jan, Osara Jude A. und Lugt Piet M. On the Required Energy to Break Down the Thickener Structure of Lubrication Greases [Journal] / / Tribology Transactions. - [s.l.] : Taylor & Francis, 2023. (3) Ochs Georg [et al.] Experimentelle Untersuchung der Übertragbarkeit von Fettkenngrößen auf fettgeschmierte Getriebe [Journal] / / Tribologie und Schmierungstechnik. - [s.l.] : Narr Francke Attempto Verlag GmbH + Co. KG, 2024. (4) Rezasoltani Asghar und Khonsari M.M On Monitoring Physical and Chemical Degradation and Life Estimation Models for Lubricating Greases [Journal] / / Lubricants. - 2016. Science and Research 53 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0019 Figure 5: Condition of Grease B after planetary gear testing under operating conditions (Ochs, et al., 2024).