Condition monitoring of spherical bearings

Pitch control links on helicopters are a safety critical component that require regular maintenance checks to ensure the wear on the composite liner within the two embedded spherical bearings is not above a specified limit. These are expensive and time-consuming maintenance checks, that have the potential to be replaced with a condition monitoring method instead. A review of potential methods for monitoring this liner was conducted and three techniques, Acoustic Emission (AE), ultrasound and capacitance were chosen to be investigated in this thesis. AE was investigated on a bespoke test bench (Rig Q) that was redesigned and improved prior to testing. The results from this test bench were analysed and various data analysis techniques have been applied to the collected AE data. Success was found using techniques such as bandpass filters around the resonant frequency of the Nano30 AE sensor. Furthermore, the techniques were then applied to a preliminary dataset from full bearing tests. Ultrasound was investigated with a limited degree of success. The data collected was found to correlate only to the steel that the liner was bonded onto. However, methods of overcoming this initial failure have been outlined and explained for further investigation. The temperature effect on the speed of sound in the steel backing has been determined to aid future research. Capacitance was investigated through the means of static testing to understand the influence of temperature and pressure on the dielectric value of the liner. Having found pressure to have a major influence and temperature to have a minor influence, dynamic testing was completed. The dynamic testing results followed the expected trend although the capacitance values were typically slightly higher. Overall, the three techniques all show potential in varying ways. Both AE and capacitance have been proven to be able to monitor the wear of the composite liner. Continued testing of all three techniques is recommended until the influence of real-world issues such as contamination are understood