Polymeric Materials for Bearing Applications Tribological Studies in Lubricated Conditions

Sammanfattning: One of the most important demands in hydropower and other industries today stems from the emphasis on preserving the environment. Usage of mineral and synthetic oils in sliding bearings of hydropower plants raises concerns about the environmental impact of these lubricants in case of spillage into downstream water. These concerns have led to attempts in using water as lubricant and the concept of oil-free systems in hydropower generation. Realization of this concept however poses many challenges including the choice of bearing materials and this necessitated research pertaining to the tribological behaviour of polymers in lubricated conditions. In this work, several studies have been carried out in order to obtain an insight into polymers’ tribological performance and associated mechanisms in the presence of lubricants. Extensive tribological studies on several unfilled polymers in water lubricated contacts demonstrated the superior wear resistance of UHMWPE. It was also found that the frictional behaviour of the unfilled polymers was influenced by their water contact angle and relative energy difference with regard to water. Studies on the effect of counter surface roughness characteristics on tribological behaviour of polymeric materials revealed their significant influence on friction and wear behaviour of polymer-metal contacts. Using a small scale journal bearing configuration, it was found that dynamic friction of polymer bearings can be significantly reduced using a shaft of reduced surface roughness. However, depending on the bearing material, this can adversely affect the material’s breakaway frictional response and thereby increase the torque required for machine start-up; a critical issue in applications such as pumped storage hydropower plants. Further studies using a pin on plate configuration revealed that variations in surface roughness characteristics of the metallic counter surfaces can significantly alter the wear resistance of the polymeric materials which cannot modify the topography of the counter surfaces in a tribological contact. Investigations regarding the influence of incorporation of various micro/nano carbon based fillers/reinforcements in the polymer matrix revealed their great potential for enhancing the friction and wear behaviour of the polymer composites. However these effects have been found to be strongly influenced by presence/absence of other reinforcements in the polymer matrix and/or by alterations in the operating conditions. Application of polymers as bearing lining materials also exhibits the potential of enhancing the performance of oil lubricated bearings. Therefore a part of this work was aimed at investigating the tribological characteristics of several polytetrafluoroethylene based materials at the onset of sliding (breakaway) at various pressures and temperatures. The results of this study showed dramatic reduction in breakaway friction using PTFE based materials in comparison to the conventional Babbitt (white metal) bearing lining material utilized in oil-lubricated sliding bearings of hydropower plants.