A New Method of Calculating the Attainable Life and Reliability in Aerospace Bearings

Abstract

The aviation industry made significant progress improving reliability, efficiency and performance throughout the last decades. Especially aircraft engines and helicopter transmission systems contributed significantly to these improvements. The kerosene consumption decreased by 70 % and the CO2 emissions due to air transport decreased by 30 % per passenger kilometer within the last 20 years. Simultaneously, the flight safety was increased with aircraft engine in-flight-shut-downs as low as 1 ppm and „unscheduled engine removals” as low as 4 ppm. Flight safety is equal to the reliability of the systems in service. Failure of these systems directly leads to exposure of human life. Among the most critical aviation systems are aircraft engines including the rolling element bearings which support the rotors. A serious damage to the aircraft engine main shaft bearings during flight requires shout-down of the engine to avoid a further damage escalation subsequently leading to engine fire. Today, it is a requirement for aircraft to operate with one engine shut down. However, each in-flight-engine-shut-down typically is connected with flight diversion or abort and immediate landing. Inflight-shut-downs translate into increased risk for passengers and crew and substantial on cost. Therefore, rolling element bearings for aircraft engines are developed – similar to other aircraft engine components – targeting a reliability of nearly 100 % over an operation time of more than 10 000 hours prior to overhaul. To achieve this requirement despite the extreme operating conditions such as high speed and temperatures occurring in gas turbines, special high-performance materials are used for the rolling bearing components which are partially integrated in surrounding engine parts like shafts and housings. These special conditions - deviating from conventional industrial rolling element bearing applications - are currently not sufficiently considered in the standardized method of calculating the bearing life per ISO 281. A new method of calculating the attainable life of rolling elements bearing in aerospace applications is presented. This method considers the special aerospace conditions and materials and thus enables a higher reliability of the theoretical analysis and life prediction.