Sliding bearings for hydropower applications novel materials, surface texture and EALs
Abstract: Hydropower, the utilisation of water as a means of driving machinery, has been used for many centuries. Today, the great power of water is being harnessed to generate as much as 20% of worldwide electricity. The requirement for reliable and efficient electricity generation needs to be matched by environmental awareness on the part of the industry. Work carried out during the course of my doctoral studies at Luleå University of Technology has been aimed at addressing these demands by investigating means of improving sliding bearing performance through the use of environmentally adapted lubricants and the employment of novel materials and surface texturing to reduce friction. The term "sliding bearing" refers to a type of bearing where two surfaces (usually the stationary bearing and a moving shaft) slide relative to one another with load distributed directly across the interface. In hydrodynamic bearings, a lubricant layer is built up in the contact region such that the two surfaces are completely separated. Examples of these are hydrodynamic journal and thrust bearings. These types of bearing are major components in many large machines, including hydroelectric turbines. Their safe operation relies on the maintaining of sufficient oil film thickness. As long as this condition is met, the bearing could continue to do its job indefinitely. This is particularly important during transient operating conditions (i.e. rapid changes in load or speed) when oil films can momentarily become extremely thin. Such conditions have been examined for a tilting-pad thrust bearing to assess impact on operation. More and more onerous demands are being placed on such bearings and their associated methods of lubrication, e.g. increased frequency of start/stop of hydroelectric turbines. This means that new solutions are required to maximise their operational performance. One such solution attracting current attention is the employment of surface texturing. The impact of surface texturing on the performance of a tilting-pad thrust bearing has been studied in a series of tests in comparison with a plain Babbitt surface. These show a reduction in friction with the textured bearing. Film thickness is seen to be greater suggesting improved load-carrying capacity. Another possible option for improving operational performance is the use of new lubricants. Environmentally Adapted Lubricants (EALs) are produced from simple hydrocarbons meaning that the final product tends to consist of only a few different types of molecule. This also means that an EAL can be carefully tailored to a specific function. In several experimental investigations using such fluids in both thrust and journal bearings, it has been shown that minimum film thicknesses can be maintained using an EAL with lower viscosity grade than that of the mineral oil currently used. This has the added impact of reducing bearing power (friction) losses and lowering temperatures, all beneficial to bearing operation. Thicker film has been found for an EAL of identical viscosity grade to the mineral oil. Unlike mineral oils, which have been employed as lubricants for decades, little data is available for the long-term performance of EALs. This is of interest as their use has only recently begun to catch on in the power generation industry. A study of aged EALs in a tin bronze-steel contact, typical of what may be found in hydroelectric turbines, has shown continued satisfactory performance even at high acid number (high oxidation level), beyond the point where an oil would normally be changed out. The introduction of new materials provides a means of solving certain problems associated with machine operation and reliability. The use of PTFE as a bearing facing material is well established but little has been published about its performance in this application. A series of experiments have been performed to investigate the impact of employing such a facing on a tilting-pad thrust bearing. A thicker minimum film is found at the outlet edge of the pads in comparison with a babbit (white metal) faced bearing. Temperatures within the pad bulk material are seen to be lower as a result of the insulating effects of the polymer, thereby reducing the impact of thermal crowning. An investigation has also been performed with a range of PTFE composite materials to assess their suitability as a replacement for babbitt facing material in hydrodynamic journal bearings. The polymer composites have been shown to introduce clear benefits in comparison with pure PTFE and babbitt in terms of reduced wear and break-away friction at start-up.
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