Integrated health monitoring is beneficial but due to reliability, weight, size, wiring and other constraints, the incorporation of instrumentation onto aircraft propulsion systems is limited. Conventional wired sensing systems are not always feasible due to size, weight constraints, and issues associated with cable routing. This paper presents an integrated and selfpowered wireless system for high temperature (above 125 • C) environments powered by a thermoelectric generator for bearing condition monitoring. Thermoelectric generator with internal oil cooling chamber is proposed to achieve higher energy output for small temperature gradient recorded in the jet engine in comparison with other thermoelectric generators with heat sinks. The experimental results demonstrate that, under a simulated engine environment, the thermoelectric generator can provide sufficient energy for a wireless sensing system to collect environmental data every 46 s, and transmit every 260 s, during the critical takeoff phase of flight and part of cruise.

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White etching cracks (WECs) have been found to form in rolling element bearings as early as 6-24 months into operation, especially in large wind turbine gearboxes [1] . [2,3] . [4] . [3] . To investigate the feasibility of WEC detection using the electrostatic (ES) sensing technique, an ES wear site sensor was installed on a bearing test-rig where WECs had been created under the influence of electrical load. The ES responses were compared with those from an acoustic emission (AE) sensor that had been shown to detect WEC failures in a previous study The physical findings related to WEC failures in the bearings and basic analysis of the sensor signals have been reported in a parallel paper by Zuercher et al

The reliability of rolling element bearings has been substantially undermined by the presence of parasitic and stray currents. Electrical discharges can occur between the raceway and the rolling elements and it has been previously shown that these discharges at relatively high current density levels can result in fluting and corrugation damages. Recent publications have shown that for a bearing operating at specific mechanical conditions (load, temperature, speed, and slip), electrical discharges at low current densities (<1 mA/mm2) may substantially reduce bearing life due to the formation of white etching cracks (WECs) in bearing components, often in junction with lubricants. To date, limited studies have been conducted to understand the electrical discharges at relatively low current densities (<1 mA/mm2), partially due to the lack of robust techniques for in-situ quantification of discharges. This study, using voltage measurement and electrostatic sensors, investigates discharges in an oil-lubricated steel-steel rolling contact on a TE74 twin-roller machine under a wide range of electrical and mechanical conditions. The results show that the discharges events between the rollers are influenced by temperature, load, and speed due to changes in the lubricant film thickness and contact area, and the sensors are effective in detecting, characterizing and quantifying the discharges. Hence, these sensors can be effectively used to study the influence of discharges on WEC formation.

In bearing applications, the presence of stray and parasitic currents in combination with lubricants has been studied for almost a century and has been found to cause fluting and corrugation damages under high current densities. However, recent research has suggested that at low current densities (<1 mA/mm2) under specific operating conditions, electrical discharges can substantially reduce bearing life due to the formation of white etching cracks (WECs). To date, limited studies have investigated the critical operating and electrical conditions for WEC formation and demonstrated effective fault detection techniques. This study uses a novel monitoring technique known as the electrostatic sensing technique to detect, monitor and characterise electrical discharges in an oil-lubricated steel–steel rolling contact on a TE74 twin-roller machine. The findings demonstrate that WECs can be formed under the influence of electrical discharges in less than 50 h, and the electrostatic sensors are effective for the early detection of critical electrical discharges related to WEC-induced failures.