Abstract
Nowadays, hydropower plants are forced to have more frequent power control and the self-lubricated bearings used in the applications are one of the most critical components affected by the continuously changing operating conditions. In this study, microstructure and composition of two commercially available bearing materials (Orkot TXM Marine and Thordon ThorPlas) used in hydropower turbines were studied. In addition, the influence of sliding speed and applied pressure on the friction and wear behavior of the materials was investigated systematically for dry sliding conditions. The bearing materials were characterized using X-ray microtomography, Nuclear Magnetic Resonance (NMR) spectroscopy and Inductively Coupled Plasma–Sector Field Mass Spectrometry (ICP-SFMS) techniques. Friction and wear tests were carried out with a polymer pin sliding against a stainless steel (SS2333) plate with a linear reciprocating motion. Test conditions were: room temperature, 9–28 MPa pressure and 10–40 mm/s sliding speed ranges. Surface analysis of the polymer pins and the wear tracks were performed by optical profilometry, Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) techniques. Test results show that, for both materials, the coefficient of friction (COF) is decreasing at higher pressures. Surface analysis reveals higher concentrations of solid lubricants in the transfer layers formed at higher pressures, explaining the decrease in COF. Furthermore, the specific wear rate coefficients are increasing at higher sliding speeds, especially at lower pressures. Results of this study demonstrate that, under dry sliding conditions, changes in sliding speed and pressure have a significant influence on the tribological behavior of these bearing materials.
Highlights
The use of renewable energy has increased dramatically over the last few years due to increased environmental awareness, technological developments as well as tougher legislation and politicalLubricants 2018, 6, 39; doi:10.3390/lubricants6020039 www.mdpi.com/journal/lubricantsLubricants 2018, 6, 39 decisions resulting in higher costs of energy production from fossil sources
Results suggest that ThorPlas is a mixture of a thermoplastic aromatic polymer containing PTFE and silicon-based inclusions as solid lubricants
The X-ray microtomography reveals the presence of larger pores in Orkot as well as smaller voids in the areas surrounding fibers, indicating imperfect adhesion between the resin matrix and the structure fibers
Summary
The use of renewable energy has increased dramatically over the last few years due to increased environmental awareness, technological developments as well as tougher legislation and politicalLubricants 2018, 6, 39; doi:10.3390/lubricants6020039 www.mdpi.com/journal/lubricantsLubricants 2018, 6, 39 decisions resulting in higher costs of energy production from fossil sources. The self-lubricated bearings used to control the turbine blades and the guide vanes are one of the most critical components affected. These boundary lubricated bearings often experience extreme working conditions as they operate at high pressures, low sliding speeds and small oscillatory movements. In spite of the severity of the issue and additional maintenance and/or replacement costs involved, only a limited number of studies dealing with wear and friction behavior of commercially available self-lubricating polymer composite bearing materials used for hydropower applications has been reported [2,3,4,5,6]. Jones et al [2] presented an extensive work, studying coefficients of friction and wear rates in both wet and dry conditions and proposed a rating system for several bearing materials
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