Developments in Hydro Generator Thrust Bearings

The use of PTFE coatings to replace whitemetal in hydro generator thrust bearing design offers a number of advantages, which have been demonstrated by recent applications at both new and refurbishment schemes. This concept, and experience to date, are discussed here.

Thrust bearings for hydro generators cover a wide range of designs. However, the working surface of the bearing is usually coated with a layer of whitemetal (or babbitt), a tin- or lead-based alloy. This material has operated satisfactorily for many years, but is subject to rapid failure if taken beyond its operational limits. Sudden unexected transients, misalignment, and poor geometry of the  machine, as well as failure of high pressure oil injection systems for example can all lead to failure. If this condition is caught early by good monitoring then usually a light wipe of the whitemetal surface is the result. However, this can very often be a precursor to a more dramatic type of failure in which melting and removal of the whitemetal results in catashtopic failure of the bearing.

A recent development in hydrodynamic bearing design is to replace the whitemetal with PTFE (polytetraflouroethylene). This material, which is reowned for its low frictional properties and chemical inertness, allows for operation at much higher duties without the type of risk associated with whitemetal.

A section through a typical PTFE-faced thrust pad is shown ih the photo below. Attachment of the PTFE is by extrusion into a compressed copper wire mat. This composite is then soldered onto the steel substrate of the pad. In this way a mechanical bond is made between the PTFE and the copper wire, while a met­allurgical bond is made between the copper wire and the steel. The function of the copper wire is not only to allow bonding of a chemically inert plastic which is difficult to bond using adhesives, but also to provide a flexible interface to allow for expansion of the PTFE during operation. This latter feature is important, as the temperature differentials between the PTFE and supporting steel of the pad, coupled with the high coefficient of thermal expansion of the PTFE, give rise to differential large expansions in the plane of the pad.