Browse Publications Technical Papers 2002-01-2951

Reliable Bearing Wear Detection System for On-Condition Maintenance of Electric Generators 2002-01-2951

Demand on the reliability of Electric Generators for Aerospace applications is assuming more importance everyday with the advent of “Fly-by-Wire” and “More-Electric-Aircraft” concepts. With today's high-powered avionics and sophisticated control systems, airline operators expect better performance and would no longer accept weak links in the system that need frequent maintenance.
One of the weakest points in an electric generator is its reliance on rolling element bearings, which are subject to unpredictable and frequent failures. Huge redundancy and frequent maintenance ensure uninterrupted supply of electricity in an aircraft. While a replacement of rolling bearing by air bearing or magnetic bearing, is still the subject of research and development, many attempts have been made to improve upon the unpredictable nature of bearing failure, one of which is to render the bearing “on-condition” by predicting its imminent failure well in advance, so that a maintenance action can be initiated, instead of premature and regular replacements.
Most promising of these methods is a patented system of Auxiliary Bearing, where a lighter bearing placed adjacent to the main bearing, takes the load temporarily when the main bearing fails or in danger of failing. Although this is a very novel system in principle, practical application proved to be difficult as unreliable detection of imminent failure resulted in “false alarm” and “catastrophic failure” due to non-detection when complacency dictated replacement only on warning signal.
A new and reliable method of bearing wear detection was needed to make the Auxiliary Bearing system work as intended. Thus a new development evolved where the rotation of the auxiliary bearing is counted by a proximity detector (magnetic induction) and the signal is digitally processed and compared with pre-determined threshold that indicate a load transfer to the auxiliary bearing. The lighter and inexpensive auxiliary bearing is mounted on the same shaft as the main bearing and a small radial gap is maintained between the outer race of the auxiliary bearing and its housing. As the main bearing wears down, the auxiliary bearing comes in contact with the housing and starts sharing the load with the main bearing. The outer race and the inner race of the auxiliary bearing were initially forced to a synchronous rotation by a face magnet attached to them. As the auxiliary bearing starts taking load, the torque overcomes the weak magnet and the outer race slows down and ultimately comes to a halt. A magnetic sensor passing over a coaxial groove in the outer race detects rotation of the outer race. As the outer race slows down, the rev. counter signals the electronic circuit, which processes the quality of the signal and issues a warning to the cockpit under specific qualifying conditions.
Since there is possibility of insulation debris or particles of sand being lodged in the gap between the bearing and the housing, the outer race may slow down or stop. However, due to high-speed rotation and the presence of the coaxial groove, it is likely that the particle will be propelled out of the groove almost instantly. Signal delays and resets are built into the electronic control to ensure against false signal, which also enable signal calibration for applications in different environments. It is expected that the on-condition maintenance will become a reliable method thus saving the airlines from unexpected failures and premature replacements of bearings.
The invention was submitted for and granted a US Patent.


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