347 23rd International Colloquium Tribology - January 2022 347 Laboratory-based reproduction of shear-degraded greases by use of a grease worker Christoph Schneidhofer AC2T research GmbH, Wiener Neustadt, Austria Corresponding author: Christoph.Schneidhofer@ac2t.at Michael Schandl AC2T research GmbH, Wiener Neustadt, Austria Nicole Dörr AC2T research GmbH, Wiener Neustadt, Austria Thomas Macheiner Siemens Mobility Austria GmbH, Graz, Austria Lukas Fritzer Siemens Mobility Austria GmbH, Graz, Austria 1. Introduction Grease is a system component, which is essential for the functionality and reliability of rolling bearings. Thermal, oxidative, and mechanical stress during oper-ation are major reasons for loss of lubrication perfor-mance of a grease. Thus, knowing its long-term per-formance is crucial. In typical bearing applications, chemical grease degradation is mostly induced by high operating temperature, whereas mechanically induced grease degradation is caused by the applied forces (shear stress) and velocities (shear rate). The breakdown of the thickener structure results in increased oil bleed-ing and consequently in reduced service life of rolling bearings. [1, 2] To properly correlate grease degradation with grease performance, e.g. lubricity, aged greases with defined condition and in sufficient amounts are required. There-fore, a laboratory-based method of grease degradation using a modified grease worker was developed. This method of artificial alteration aimed at significantly accelerated mechanical degradation compared to the field. In this work, greases sampled from axle box bearings on wheelsets for railway application were characterized as basis for the laboratory simulation of grease degrada-tion. Artificially stressed greases obtained after 500 000 and 1 000 000 cycles in the modified grease work-er were compared to the respective greases from the field by qualitative analyses of the thickener structure, among others. Shear stability of three greases is discussed. 2. Experimental setup 2.1 Fresh and used greases Used greases were collected according to EN 12082 [3] from four axle box bearings from a railway application with a milage of around 1.6 Mio. km. For the laboratory investigations, three commercially available greases were used. Table 1 lists the grease properties extracted from the datasheets. 348 23rd International Colloquium Tribology - January 2022 Laboratory-based reproduction of shear-degraded greases by use of a grease worker Table 1: Overview of greases Properties Grease A Grease B Grease C Base oil Mineral oil Mineral oil Synthetic Thickener Lithium Lithium Lithium complex NLGI consistency 2.5 2.5 2 Base oil viscosity 40° C 100 40 100 [cSt] 100° C 11 8 14.5 Dropping point [° C] 180 >180 274 2.2 Analysing grease condition Following analyses were performed to investigate the greases deteriorated in the field and in the laboratory: • Cone penetration using one-quarter scale cone equipment according to ASTM D1403. • Bleeding tendency according to an in-house method to evaluate the amount of oil bleeding at 80 °C after 6 h. • Infrared spectra obtained by a Fourier trans-form infrared (FTIR) spectrometer equipped with an attenuated total reflection (ATR) unit. • Thickener structure by scanning electron mi-croscopy (SEM). Figure 1: Trend of consistency (unworked cone penetration) (TOP) and bleeding (BOTTOM) of used greases depending on the position in the bearing 2.3 Modified grease worker for artificial alteration The greases were artificially altered in the grease worker, as described in ASTM D217-10, for 500 000 and 1 000 000 cycles with a speed of around 1 double-stroke per second. The perforated plate was modified, where the diameter of the holes was reduced to increase the shear stress. Afterwards, the grease conditions were determined and compared to those of the used greases. 3. Results and discussion Figure 1 shows the analyses of used greases from the field. The sampling positions (A, B, C1, C2, D) are corresponding to the positions defined in EN 12082 [3]. The consistency (unworked cone penetration) shows a typical W-shape when drawn over sampling positions. Grease samples from the raceway zones (C1, C2) get stiffer, caused by increased bleeding due to progressed shear degradation. Greases from A and B get softer due to mechanical degradation of the thicken-er. Grease samples from position D showed similarities to the fresh grease condition. The same trend can be observed for the bleeding tendency. 23rd International Colloquium Tribology - January 2022 349 Laboratory-based reproduction of shear-degraded greases by use of a grease worker Figure 2: Trend of consistency (TOP) and bleeding (BOTTOM) of artificially stressed greases in the modified grease worker The results from mechanical degradation in the grease worker are shown in Figure 2. Generally, the higher the number of strokes the higher the consistency (cone penetration) and, therefore, the softer the grease. How-ever, a certain saturation effect (flattened curves) can be detected. Grease A and B showed very similar trends during alteration, both based on a Li thickener. Grease C based on a Li-complex thickener with synthetic base oil exhibited a higher shear stability in comparison to grease A and B. Also, the bleeding tendency of grease A and B are similarly characterized by a slight increase. By contrast, grease C shows a slight decrease in artifi-cial alteration. Analyses of the thickener structure by SEM clearly illustrate a breakup of the thickener fibres in the used and artificially altered greases in comparison to the fresh grease (see Figure 3). The artificially altered grease after 1 Mio. cycles can be well correlated to the structure of the used grease sample from position A. Furthermore, the consistency of these samples is very similar. The modified grease worker is a useful tool to simulate shear degradation of greases and benchmark the shear stability of greases. Its application reproduces shear degraded greases in a rapid manner for further investi-gation of the influence of shear degradation on the functionality of greases in field operation. Figure 3: Qualitative optical comparison of thickener structures; a) fresh grease B; b) used grease sampled from position A c) artificially altered grease after 1 Mio. cycles 4. Conclusion Analyses of greases sampled from axle box bearings for railway application revealed mechanical stress as main grease degradation mechanism. A laboratory method developed to simulate shear degradation using a modi- 350 23rd International Colloquium Tribology - January 2022 Laboratory-based reproduction of shear-degraded greases by use of a grease worker fied grease worker provided greases that were compared to respective greases from the field. A good correlation between mechanically induced grease degradation in the laboratory and in the real application was confirmed. Therefore, laboratory-generated greases allow the accel-eration of investigation of grease performance. References [1] P. M. Cann, M. N. Webster, J. P. Donner, V. Wikstrom and P. Lugt, “Grease Degradation in R0F Bearing Tests,” Tribology Transactions, no. 50: 2, pp. 187-197, 2007. [2] A. Rezasoltani and M. M. Khonsari, “On Monitoring Physical and Chemical Degradation and Life Estimation Models for Lubricating Greases,” Lubricants, vol. 4, no. 34, 2016. [3] EN 12082: Railway applications - Axleboxes - Performance testing, CEN, 1 September 2017.