24th International Colloquium Tribology - January 2024 135 Analysis of Biodegradable Lubricants for Radial Shaft Seals Under Critical Conditions Stefanie Haupt 1* , Dr. Florian Johannes Heiligtag 1 , Maria Frackowiak 1 , Tanja Püler 1 , Danijela Grad 1 , Dirk Fabry 1 1 KLÜBER LUBRICATION MÜNCHEN GmbH & Co. KG/ GPI&GBT, Munich, Germany * Corresponding author: Stefanie.Haupt@klueber.com 1. Introduction Dynamic compatibility tests for radial shaft sealings are becoming increasingly important for matching elastomer and lubricant to the corresponding application. The tests are often time and cost intensive but are more informative than a static test and the correlation with the application is higher. The selected base oil in combination with the additive have a big influence on the performance of the sealing. Lubricant development becomes even more challenging, when users want an innovative bio-based and environmentally acceptable lubricant (EALs), which must meet the following requirements: biodegradability, renewability of the raw materials, low toxicity and non-bioaccumulation. [2] In this work, the tribological performance of elastomers is investigated when lubricated with an EAL under various technical operating conditions. The latter were adjusted iteratively to examine the limits of the respective tribo-system. The measured results and tribological characterization are the basis for the lubricant development that expands the limits of the elastomer sealing performance. 2. Methodology 2.1 Test principle To investigate the dynamical elastomer compatibility, Klüber Lubrication uses a self-developed Dynamical Elastomer Screening test rig (DES). The testing principle is comparable to a ring disk tribometer. [1] The elastomer samples were produced from FKM plates. The elastomer sample is pressed onto the counter surface. The pressure force on the counter surface remains constant for the entire test time. The counterpart material is an austenitic steel compliant to DIN 3761. The test rig also includes an oil reservoir. The oil reservoir can be heated and controlled with a cryostat to a defined oil sump temperature, which is measured during the test. The friction force, the elastomer wear and the temperature in the proximity of the sealing edge are measured. The elastomer wear is determined using a displacement sensor. In case of leakage, the leakage behaviour is observed from the time of first leakage. 2.2 Iterative procedure to determine critical conditions An oil lubricant containing synthetic biodegradable ester with a high renewable content and a special additive package (ISO VG 320) is used for the test series with a biodegradability of over 60-% according to OECD-301-F. The tests were performed at different normal line loads, operational times, sump temperatures (60-°C, 100-°C and 120-°C) and rotational speeds. Each sump temperature variation is measured at least three times. The following Figures 1 and-2 show the temperature behaviour near the sealing edge and the associated friction coefficient, respectively for three test runs and for all considered oil sump temperatures at a normal line load of ~0.85-N/ mm and a circumferential speed of ~1.7-m/ s. The running-in behaviour of approximately 18-hours is not shown. Figure 1: Temperature near the sealing edge at different sump temperatures Figure 2: Friction coefficient at different sump temperatures The sealing edge temperatures are subject to higher fluctuation and higher spread at an oil sump temperature of 60-°C (orange to red lines) compared to 100 °C and 120-°C. After the run-in, it proceeds more stably at an oil sump temperature of 100-°C (green) than by 120-°C (gray to black). The friction coefficient at 60-°C is also higher than at higher oil sump temperatures. At 100-°C, the friction coefficient is on a lower level and is more stable than at 120-°C, where the measurements show a bigger spread. 136 24th International Colloquium Tribology - January 2024 Analysis of Biodegradable Lubricants for Radial Shaft Seals Under Critical Conditions Figure 3: Elastomer wear at different sump temperatures The continuous wear measurement shows a significant increase after the run-in time (Figure 3). At sump temperatures of 100-°C and 120-°C respectively, the wear measurement curves are stable after the run-in. At first glance, the shaft run-in and the elastomer track width do not show any correlations. The visual inspection is also unremarkable. The optical appearance of the counterparts tested at 100-°C show deposits. To further understand the different performances at different temperatures, the samples (elastomer and metal counterparts) were subsequently examined by Secondary Electron Microscope (SEM) with EDX. The samples which were tested at 100-°C clearly show a tribo film and on all samples a transfer film can be observed (data not shown). 2.3 Increase of the loading conditions Using the same parts as before except for the seals from FKM material, the operating conditions are iteratively intensified until thermal damage occurred (Figure 4). Figure 4: Thermal damaged sealing edge with orig. formulation The excessive frictional heat at the sealing edge changes the elastomer boundary layer such that hardening occurs. This hardened layer reduces the elastic compliance and weakens the hydrodynamic sealing mechanism. Furthermore, the contact surface becomes brittle. [3] The sealing edge can tear and partial breakouts from the sealing edge cause a greater spread in the coefficient of friction. Based on these results, the lubricant composition was iteratively adjusted and the new formulations are tested under the same conditions. A formulation was found that increases the thermal load capacity of the sealing system (Figure 5). This formulation also shows a significantly lower friction coefficient than the original formulation and was biodegradable according to OECD-301-F. Figure 5: Sealing edge left: with new formulation; right: combination with ester oil and synthetic hydrocarbon oil 3. Conclusions Environmentally acceptable lubricants in elastomer sealings were tested under dynamic conditions. Critical operating conditions have been identified on an elastomer screening test rig for a biodegradable lubricant. At a certain oil sump temperature, a tribo layer formed on the metal and the elastomer sealing edge as well. The performance limits of the lubricant have been tested iteratively until thermal damage occurred. The additive package in the lubricant has a significant impact on the thermal load capacity of elastomer sealing systems at the considered severe operating conditions. References [1] Burkhart et. al., Development and Optimization of a Tribometer for Radial Shaft Seals, Journal of Tribology Vol 143 (4), 2021, DOI: 10.1115/ 1.4049597. [2] United states Environmental Protection Agency, VES- SEL GENERAL PERMIT FOR DISCHARGES INCI- DENTAL TO THE NORMAL OPERATION OF VES- SELS (VGP), 2018. [3] Bauer, Federvorgespannte-Elastomer-Radial-Wellendichtungen: Grundlagen der Tribologie & Dichtungstechnik, Funktion und Schadensanalyse, Taschenbuch, 2021.