24th International Colloquium Tribology - January 2024 187 Enhancing Understanding of Grease-Retention and Lubrication-Mechanisms of Oscillating Sliding Contacts with Long Stroke Lengths Andreas Keller 1* , Markus Grebe 1* , 1 Hochschule Mannheim/ Kompetenzzentrum Tribologie, Mannheim, Deutschland * Corresponding author: andreas.keller@hs-mannheim.de 1. Introduction Modern automotive tribosystems face strict demands for performance, cost-effectiveness, and energy efficiency. Many of these systems, like small worm gear drives, linear guides and actuators use lifetime lubrication with grease to enable compact, low-maintenance designs. However, as friction partners displace grease from the contact area, particularly in systems with long displacement distances, the risk of dry running and subsequent system failure is a constant problem. The lubrication’s success hinges on the ability of displaced grease to return to the contact region, a complex process influenced by grease’s rheological properties and chemical composition. Practice shows that current standardized tribological and rheological testing methods do not permit a sufficient prediction of the suitability of lubricants for lifetime-lubricated linear plain bearings and lead screw drives. 2. Current State of Tribological Testing of Greases There is already a great amount of knowledge and experience available to evaluate the suitability of greases for lifelong lubrication in rolling bearings. Common testing methods are described in DIN51819 ( FE8) and DIN51821 ( FE9 ) . Most literature search results show a focus on rolling bearing lubrication and related grease research areas, including investigations into thickener structural degradation, chemical advancements in additives and thickeners. These include, for example, the structural degradation of the thickener [1] [2], the thickener influence on lubrication state [3] and the impact of thickener type and base oil viscosity on lubricant film height [4]. However, research on tribosystems involving grease movement over larger distances under oscillating sliding friction is still limited. For instance, when grease adheres to a lead screw nut and is displaced to the ends of the spindle, relying solely on the flow of released oil for lubrication upon the nut’s return is inadequate (see Figure 1). In the absence of mechanisms that facilitate the “return transport” of the grease, the stickiness of the lubricating grease becomes crucial. It helps retain lubricant at the friction point or minimizes the amount of displaced lubricant. Currently, there is no standardized test-method for these kinds of load-collectives and lubrication conditions. The standard test for grease classification in tribology is the 4 ball method, described in ASTM D2596 and ASTM D2266 and their counterpart DIN variants (DIN 51 350/ 4 and / 5). Obviously, these test-methods are very far away from any real-life application and cannot sufficiently recreate those conditions, as contact pressures and speeds are too high and grease/ oil can easily reflow from the sides. Figure 1 Schematic depiction of grease displacement in lead screw nut 3. Current State of Rheological Testing of Greases Lubricating greases are primarily characterized by their NL- GI-Class, which describes their consistency properties. The NLGI class is determined using the ASTM D217 “cone penetration of lubricating grease” test at 25°C. Other common rheological characterization methods use: • Rotational viscometry/ rheometry with a cone-and-plate or parallel-plate setup, which measures viscosity as a function of shear rate, offering insights into shear resistance, flow properties, and linear viscoelastic properties like storage and loss moduli as well as other factors like thixotropy and creep behavior • Thermal Analysis (Differential Scanning Calorimetry - DSC), identifying phase transitions like melting or softening across temperature ranges. These methods provide crucial data on how greases behave under various conditions and might offer a better understanding of grease mobility compared to standard NLGI classification alone. Additional major grease characteristics might be oil release and stickiness/ tackiness. If grease is pushed out of a friction point by the movement of the friction partners, then the oil release capacity is one of the factors that determine how much lubricant remains in the tribological zone. The oil release capacity is usually determined according to DIN 51817. As mentioned before, part of the grease might return due to tackiness when the friction partner resumes the original position. Achante et al [5] were the first to investigate the tackiness of greases in a systematic manner following the tackiness tests for pressure sensitive adhesives in ASTM D2979. In the case of tackifier-free greases, they were able to demonstrate that the thickener itself has a decisive influence on the 188 24th International Colloquium Tribology - January 2024 Enhancing Understanding of Grease-Retention and Lubrication-Mechanisms of Oscillating Sliding Contacts with Long Stroke Lengths stickiness of greases. A correlation of stickiness and friction coefficient showed a weak correlation for sliding friction (plate-cylinder geometry) and a strong correlation for rolling friction (plate-ball geometry) was also shown by E.P. Georgiou et. al. [6]. 4. Working hypothesis, research approach and conclusion Our working hypothesis is that the limited service life of linear plain bearings and spindle drives can be explained by the depletion of grease from the contact zone. When lifetime-lubricated systems are assembled, sufficient grease is available in the contact zone at the beginning. Due to the component-specific movement of the friction partners, the grease is displaced over long distances from the actual tribological zone. If it does not return to the loaded contact after stressing, it can no longer provide its function as a lubricant reservoir. This is presumably the case if a lubricating grease has a low stickiness and therefore does not adhere to the moving mating body, or if a lubricating grease “stiffens” to quickly and therefore remains in the edge zone after displacement. Without additional supply of grease, then the contact point will become depleted of lubricant over time. From this working hypothesis, it can be concluded that, on the one hand, the stickiness, viscoelasticity and oil release properties of lubricating greases must be characterized in a suitable manner and, on the other hand, both the time dependence of the lubricating grease distribution and friction and wear in test configurations with translational sliding at low frequencies with long strokes and must be determined. The correlation of all results should therfore allow the formulation of a robust and meaningful screening test strategy to derive the suitability of greases for lifetime lubricated linear sliding systems. We propose the following composition of rheology and tribometry: Regarding rheology, greases should be characterized by determining viscosity curves following DIN 51810-1 and viscoelasticity (yield point, shear modulus, loss modulus) according to DIN 51810-2. In addition, stickiness and oil release capacity are to be determined. There is currently no standardized test method for determining the tackiness of lubricating greases. Here, it makes sense to develop and evaluate a rheological measurement method based on the known publications as described by [5] [6]. There is a standard for the evaluation of the oil release capacity; however, strictly speaking, DIN 51817 only applies to oil separation under static conditions (storage conditions) and not tribological induced stress. Therefore, more practical approaches might be necessary. Regarding tribometry the distribution of lubricating greases is to be recorded optically in oscillating sliding friction tribometers capable of long stroke oscillation to provoke “pushout” of grease and subsequent starvation. In a first step this can be done with a simple category 5 model test setup which would allow easy correlation of qualitative and quantitative grease distribution and friction and wear measurements (see figure 2). Figure 2 Depiction of pin on disc testing with continuous optical documentation of grease distribution In a second step, results from the tribometric model testing and advanced rheology have to be confirmed by Cat. IV component testing for example by testing application relevant trapezoidal screw drive assemblys or linear plain bearings in a suitable test setup. Using this testing methodology together with a specifically defined and produced set of model greases should yield further understanding on the behavior of greases in long stroke sliding contacts and their dependency on rheological characteristics. Ultimately this should improve the ability of lubricant producers to tailor their grease specifically for these kinds of applications and industrial end users to design their systems towards longer lifetime and/ or higher load capacity. References [1] M. A. Delgado, C. Valencia, M. C. Sanchez, J. M. Franco,* and C. Gallegos: Influence of Soap Concentration and Oil Viscosity on the Rheology and Microstructure of Lubricating Greases; Ind. Eng. Chem. Res. 2006, 45, 1902-1910. [2] Grebe, M; Ruland, M: .Influence of mechanical, thermal, oxidative and catalytic processes on the thickener structure and thus on the service life of rolling bearings; Lubricants 2022, 10(5), 77; https: / / doi.org/ 10.3390/ lubricants10050077; [3] P. M. Cann; Grease Degradation in a Bearing Simulation Device; Tribol. Int., 39, 1698 - 1706 (2006) [4] B. Vengudusamy; C. Enekes; R. Spallek: On the film forming and friction behaviour of greases in rolling/ sliding contacts; Tribology International, Volume 129, pages 323-337, 2019 [5] S. Achanta; M. Jungk; D. Drees: Characterization of cohesion, adhesion, and tackiness of lubricating greases using approach-retraction experiments; Tribology International; 44 (10), 1127 - 1133, 2011. [6] E.P. Georgiou, D. Drees, M. De Bilde, M. Feltman, M. Anderson; Grease Adhesion and Tackiness: Do They Influence friction? ; NLGI Spokesman, 2020, 84(4), 20 - 25.