Evaluation of an ultrasonic oil film measurement technique for PTFE faced thrust pad bearings

There is a growing trend in the replacement of the babbit facing in thrust pad bearings with a composite PTFE surface layer. The PTFE faced bearings have been shown to allow a greater specific pressure, reduce thermal crowning, and in some cases negate the need for an oil-lift (jacking) system. These designs of bearing require new methods for the measurement of oil film thickness both to assist in their development and for plant condition monitoring. In this work, an ultrasonic method of oil film measurement is evaluated for this purpose. An ultrasonic transducer is mounted on the back face of the thrust pad. Pulses are generated and transmitted through the pad material, bonding interlayer, and PTFE surface layer. The proportion of the wave that reflects back from the oil film layer is determined. This is then related to the oil film thickness using a series of calibration experiments and a spring stiffness model. In practice the reflected signal is difficult to distinguish, in the time domain, from other internal reflections from the pad. Signals are compared with reflections when no oil film is present and processing is carried out in the frequency domain. Experiments have been performed on a full size PTFE-faced thrust pad destined for a hydroelectric power station turbine. The instrumented pad was installed in a test facility and subjected to a range of loading conditions both with and without oil lift. Whilst there were some problems with the robustness of the experimental procedure, oil films were successfully measured and used to study the effect of the oil lift system on film formation. Copyright © 2004 by STLE.