Polymer composites are used as facing materials in hydrodynamic bearings for their low friction and compliant properties, which play an important role during machinery operation. In journal bearings, this low friction property can be of significant importance during start and stop cycles when insufficient oil is available to fully separate the surfaces in relative motion. Current work is aimed at determining a suitable material for use in hydrodynamic journal bearings for applications in hydroelectric power plants. This study investigates friction and wear encountered during the transition from the stationary state to operational speed (acceleration) during initial start-up. This is examined for virgin poly-tetra-fluoro-ethylene (PTFE) together with a series of commercially available PTFE-based composites and a babbitt material in boundary/mixed lubrication conditions. Tests are performed using standard laboratory block-on-ring test apparatus with a VG32 mineral oil.

The use of environmentally adapted lubricants (EALs) is a subject of growing interest to industry as legislation increasingly demands the replacement of mineral oil lubricants. Vegetable-based fluids are widely seen as providing lubricants from a renewable source, as well as meeting demands for improved biodegradability. However, at present, utilization of such fluids is limited due to their rapid oxidation. EALs produced from other base stocks (i.e. synthetic esters) have been shown to provide performance benefi ts in hydrodynamic thrust bearings. In the present study, a hydrodynamic journal bearing test rig has been employed to compare the performance of three EALs (a VG32 saturated ester, rapeseed base fluid and a propylene glycoldioleate) relative to three mineral turbine oils (ISOVG32, ISOVG46 and ISOVG68) in the hydrodynamic regime. Results are given in terms of temperature, power loss and minimum fi lm thickness. The impact of oil viscosity index is also discussed.

Polymer composites are employed as facing materials in hydrodynamic bearings for their low friction and "compliant" properties which play an important role during machinery operation. In journal bearings, this low friction property can be of significant importance during start and stop cycles when insufficient oil is available to fully separate the surfaces in relative motion. Current work is aimed at determining a suitable material for use in hydrodynamic journal bearings. This study investigates friction and wear encountered during the transition from the stationary state to operational speed (acceleration) during initial start-up. This is examined for virgin PTFE together with a series of commercially available PTFE-based composites and a babbitt material in boundary / mixed lubrication conditions. Tests are performed using a standard laboratory block-on-ring test apparatus with a VG32 mineral oil.

The use of polymer composites as facing materials in hydrodynamic thrust bearings gives performance benefits in terms of temperature distribution and friction losses. A study was carried out to investigate the break-away friction encountered during the transition phase (acceleration) under initial start-up from stationary state to full operational speed. This was examined for a series of commercially available PTFE-based composites. The lubricant used was a mineral oil. Best performance in terms of relative friction and wear was found for a carbon filled PTFE composite material. This is an abstract of a paper presented at the 16th International Colloquium Tribology Lubricants Materials and Lubrication Engineering (Stuttgart/Ostfildern, Germany 1/15-17/2008).

The use of Environmentally Adapted Lubricants (EALs) is a subject of growing interest to industry as legislation increasingly demands the replacement of mineral oil lubricants. Vegetable based fluids are widely seen as providing lubricants from a renewable source as well as meeting demands for improved biodegradability. However, at present, utilisation of such fluids is limited due to their rapid oxidation. EALs produced from other base stocks (i.e. synthetic esters) have been shown to provide performance benefits in hydrodynamic thrust bearings. In the present study, a hydrodynamic journal bearing test rig has been employed to compare the performance of three EALs (a VG32 saturated ester, rapeseed base fluid and a Propylene Glycol Dioleate) relative to three mineral turbine oils (ISOVG32, ISOVG46 and ISOVG68) in the hydrodynamic regime. Results are given in terms of temperature, power loss and minimum film thickness. The impact of oil VI is also discussed.

Environmentally adapted lubricants (EALs) are becoming of increasing interest to the industry as legislation demands the replacement of mineral oil lubricants. However, little or nothing is known about the infl uence of ageing on the performance of these fluids in mixed and boundary lubricated tribological contacts. In this study, samples of three fully formulated lubricating oils, both in their 'unaged' form and 'aged' by means of an oxidation process in the laboratory, were utilized in a detailed study whereby variations in relative lubrication performance were examined. Standard laboratory pin-on-disc equipment was utilized in order to allow comparisons to be made and the definition of conclusions for a sliding tin bronze on steel contact. This simulated the interaction between an oil-lubricated bronze bushing and steel shaft as found in the vane adjustment mechanism of a Kaplan turbine. X-ray photoelectron spectroscopy analysis was employed to assess the effect on surface layer structural chemistry. Results showed, for the specific application being investigated, that EALs continue to function well in terms of their friction and wear performance even at high acid numbers

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.

Boundary lubricated journal bearings are found in various applications involving oscillatory sliding conditions. Environmental adaptation of hydraulic systems includes the introduction of synthetic esters. These new environmentally adapted lubricants (EALs) have shown very good boundary lubrication performance but also condition sensitivity. This study examines an oil lubricated bronze pin on hardened steel configuration in a reciprocating friction and wear test machine. Three synthetic esters were tested with a 1 mm stroke length. Results were compared with those for a mineral oil. The tribological performance with synthetic ester lubricant can, under certain conditions, be very good. SEM-EDS and XRD surface sensitive studies indicate the formation of a soft, copper enriched outer contact layer. The layer's nature and contact mechanisms clearly affect the performance of the different lubricants. (17 refs.)

Lubricated journal bearings operating at moderate temperatures under conditions of slow intermittent or oscillating motion are important components in many items of machinery. In this application, choosing a suitable lubricant is critical to prolonging the lifetime of the component. The choice of lubricant is often dependent on the material pairing between the bearing and shaft. In parallel with growing concern about the environment, the development of environmentally adapted lubricants (EALs), such as synthetic esters, is gradually gaining pace. Studies have shown the ability of synthetic esters to reduce friction and wear under boundary lubricated conditions, especially with low alloyed bronzes on hardened steel. This study deals with the investigation of three different shaft materials, i.e. hardened, DLC-coated or normal steel, for use with a tin-bronze journal bearing lubricated with a synthetic ester. Experiments using the journal bearing show encouraging friction and wear results with the synthetic ester lubricant in comparison to tests with a mineral oil under the same operating conditions. (16 refs.)