Excessive shaft wear due to radial shaft seals in lubricated environment. Part I: Analysis and mechanisms

Within the last years, the amount of fluorelastomer (FKM) seals are increasing dramatically in industrial applications such as gearboxes due to its outstanding characteristics and superior performance. In terms of thermal stability resistance and compatibility towards lubricants FKM has wider application ranges compared to other sealing elastomers. However combined with low-viscosity, synthetic lubricants i.e. polyalphaolefin (PAO) or polyalkylene glycols (PG) macroscopic wear grooves were reported on hardened shafts. This can, but does not necessarily lead to leakage and the breakdown of a sealing system, which may cause serious damage to the environment and the machine, where the functionality of the seal is often crucial for the whole system. So far a clear explanation does not exist, just a few theories or assumptions, for this behavior can be found in the literature. Driven by that fact this article presents a detailed wear analysis of a critical combination for shaft wear in a system of fluorelastomer seal, polyalphaolefin lubricant and case hardened and twist-free ground shafts of AISI 5115 (1.7131,16MnCr5). The wear behavior of state-of-the-art radial shaft seals out of fluorelastomer seals was investigated. This wear phenomenon was reproduced using endurance tests on a multi-shaft test rig. Typical characteristic parameters and values of the sealing system performance were analyzed and correlated to shaft wear. For further understanding of this system, selected samples were analyzed with different recent methods of surface and thin film analysis. The results indicate that there is a clear relation between seal and shaft wear. The occurrence of shaft wear is also caused and influenced by a complex combination of lubricant-elastomer interaction. Based on this findings, a wear hypothesis is derived.