Load expectations of up to 8MPa on the thrust bearings in these motors are similar to oil-lubricated systems even though the motor is cooled by water rather than oil. This requires thrust bearings that are not only resistant to corrosion and swelling, but can also run under the boundary lubrication conditions associated with high loads in water. Material compatibility between the stationary bearing and the rotating collar is one element of a successful system. The traditional solution for this has been stainless steel pads running against a collar with a graphite insert or ferrobestos pads running against a hard interface. Ferrobestos, however, is now illegal due to its asbestos content.
Over the past five to 10 years Waukesha has introduced solid polymer thrust bearings initially as a problem solving design and now as a full product line. Solid polymer components are water-resistant and can adopt an optimum shape to maximise the load capability of the bearing. Trials on components reached loads of between 15 and 25MPa, depending on speed and size of the bearing, before loss of the water film occurred. A further advantage of this material is that it embeds any dirt such as rust particles from within the closed system. The embedding of dirt gives a greater wear resistance than systems using graphitic materials when run against a hardened collar. This is because the dirt particles are adsorbed into the pad rather than becoming trapped in the interface where they can machine the relatively soft graphite.
Solid polymer components can be supplied as traditional tilt-pad items allowing sizes of bearings to be tailored to suit a particular application. This is beneficial for large units and those with low quantities as they can benefit from the mass production techniques of producing the pads. There are other applications, however, that are small and high volume and use low viscosity fluids, such as water, freon and hexane, or are subject to high temperatures up to 250°C (482°F). These can benefit from the same material by forming it into a one-piece ring of connected pads. This combines the simplicity of a fixed profile design with the performance of a tilt-pad arrangement. Applications suitable for this include exhaust-driven turbochargers, high speed motors and oil and process lubricated turbines.
In summary it can be said that the solid polymer material offers the high load capability and low friction of graphite/stainless steel systems combined with the wear resistance of ferrobestos. Such components have now been running in industrial environments for almost four years and have proved a robust solution.
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