Real-time electrostatic charge monitoring of the wear surfaces and debris generated by sliding bearing steel contacts

Abstract

An electrostatic sensor was originally developed for condition monitoring of the gas path of jet engines and turbines.  It was used to detect changes in the level of debris present in the gas path, which can be related to component deterioration such as blade rubbing, combustion chamber erosion and poor combustion performance.  Preliminary studies have been carried out in the University of Southampton to apply this technology to monitor oil-lubricated contacts.  Precursor charge events have been successfully detected prior to the onset of significant wear and the charging mechanisms involved are thought to be tribocharging, triboemissions, surface charge variation and debris generation.  This thesis presents research conducted to establish the contributions of surface charging and debris generation to the overall charge detected during wear.

During the monitoring of lubricated sliding wear of bearing steel, the occurrence and location of precursor signals prior to catastrophic failure were detected using improved signal processing that utilises a three-dimensional colour map which relates the exact magnitude, duration and position of charge features on the wear surface.  The polarity of the signals was predominantly positive for tests with a mineral base oil and both positive and negative for a fully synthetic oil.  The sources of the precursor signals have been confirmed to be surface charging due to the formation of white layers and the generation and entrainment of wear debris.  Electrostatic sensing was also found to be sensitive to very small changes in wear rate, not detectable by the measurement of volume loss.  Further tests were carried out to study the effects of surface charge and debris generation and entrainment separately.  The electrostatic monitoring system was found to be sensitive to small changes in the surface charge due to contact potential difference (CPD) effects arising from subtle differences in surface work functions.  It is also sensitive to the transition between mild and severe oxidational wear under unlubricated steel sliding conditions by detecting the dynamic behaviour of CPD change related to the dynamics of wear.  Charge features related exclusively to the pressure of wear debris have been identified during tests carried out on a modified pin-on-disc rig with base oil lubrication.

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