Engine bearing failure: detection improved with new technology

Flying Safety, August, 2002 by Andy Rodwell

The modern jet engine operates at the very limits of mechanical technology. The speeds and temperatures of the components are pushed to maximum performance at minimum weight, while being able to deliver that performance with reliability that will maintain safety and mission capability. The main engine bearings that support the high speed rotating shafts of the engine operate in a particularly challenging environment, needing to support the loads of the internal engine speeds and pressures in addition to larger maneuver and gyroscope loads generated by the aircraft. These engine loads, as much as 15,000 pounds on each bearing, are supported on the minute contact areas between the balls or rollers and the bearing races they run against while riding an oil film just microns thick.

The unavoidable result of the repeated stress cycling of the bearing's surface metal created by the passing balls or rollers is metal fatigue which, in highly loaded bearings, can ultimately lead to the surface of the bearing breaking off flakes of material, normally referred to as "spalling." If the bearing continues to run after this spalling has initiated, then the rough contact surfaces will ultimately result in breakup of the bearing and seizure of the engine.

Most jet engine bearings operate at loads that will result in fatigue failure of some of the bearings, and the detection of the spalling debris in the oil system is essential to prevent the effects of full failure. In single-engine aircraft this becomes critical, as undetected failures typically result in engine failure with high potential for loss of the aircraft.

Although a joint oil analysis program (JOAP), which focuses on the number of tiny wear particles present in the oil, has been successful in detecting bearing failure on certain engine types, it is typically nonsuccessful with detecting spalling failure debris, because this type of debris is too large to be detected by JOAP. This means that this critical detection of bearing debris has been exclusively dependent on a visual inspection of the engine master chip detector. Unfortunately, some bearings located deep within the engine have a long and obstructed path to get the debris from the bearing to the chip detector, with the result that very little debris may be available at the chip detector to announce the impending failure. This requires extremely tight limits on chip detector debris, with a particle as small as 20 thousandths of an inch indicating potential failure. These small particles are required to be evaluated on a chip detector of the same color, while covered in oil, in less than ideal conditions t hat sometimes exist on the flightline. An additional problem with these tight debris limits is that routine maintenance can introduce many particles of the same size from sources outside the engine which are visually impossible to differentiate from the bearing debris, often resulting in unnecessary maintenance engine runs or engine removal and teardown.

Recently the USAF began implementing a new technology solution to the bearing failure detection problem in the form of an automated debris detection and classification system that is intended to improve both safety and maintenance by the early and correct diagnosis of bearing failure. Currently, the system is being installed to monitor all GE F110 engines in the F-16, while other potential applications are under consideration.

The system, which goes by the JetSCAN[TM] trade name, employs an SEM/EDX system, standing for Scanning Electron Microscope with Energy Dispersive X-ray. As this name suggests, the system is essentially a highly accurate electron microscope. The EDX part of the system is used to detect the material composition of the particle it is examining. This type of detection solution was selected as it was already available, had been in use with the U.K. Royal Air Force and required no modification to the existing fleet of engines.

While this is a complex piece of equipment, it has been developed and "ruggedized" to tolerate the normal conditions where it might be installed and to be movable to deployed locations. Just as importantly, it has been developed for ease of use with minimal training requirements. At the flightline nothing is changed except that following visual inspection of the chip detector, the chip detector is bagged and submitted for JetSCAN[TM] analysis and a clean chip detector is installed that has been returned from the analysis location. The visual inspection is still required and remains critically important, as the analysis may not always be conducted prior to the next flight.

To conduct the analysis, the chip detector is de-greased in solvent and any debris removed by pressing it onto an adhesive tab. Up to 24 separate engine samples can then be loaded into the machine and the corresponding engine details entered into the computer. The analysis begins with the operator stepping through the automated calibration, following which the system will perform the analysis with no further input or oversight. The system examines each sample for any particles, then each particle is measured and the elemental composition by the EDX sensor. The composition of each particle can then be compared to known alloys used in the engine bearings, gears and other oil system components and compared against programmed limits for each material. This allows small amounts of important materials to be detected in amongst less important or foreign debris, and limits are set accordingly to prevent unnecessary maintenance. On conclusion of the analysis, which typically takes around one minute per sample, a repor t is generated for each engine, and if debris is detected that exceeds the limits, a maintenance warning is generated for which direction is provided by the technical manuals, depending on the type of material detected.