ENDURANCE TESTING OF DAMAGED MAIN ROTOR UPPER THRUST BEARINGS

Two tapered roller thrust bearings of the type installed in the upper main rotor hub of the AH-64A “Apache” Advanced Attack Helicopter were tested after spalls were intentionally induced in their inner races. This bearing, paired with a lighter tapered roller bearing in the lower hub, sustains the rotor thrust and moment in flight. After the initial spalls were induced, the damaged bearings were tested back to back under a spectrum of thrust and moment levels representing the flight loads. The test was run for almost 1000 hours, with inspections conducted every 300 hours. The progress of the damage was photographed and recorded. As the damage grew, the bearing remained capable of rotating at operating speed while continuing to carry the full spectrum of loads. The test was eventually terminated at 998.8 hours when the test machine’s hydraulic drive stalled due to the increase in friction as the race and roller surfaces deteriorated. The results of the test show that this bearing is able to rotate while carrying full load long after the onset of easily detectable damage. The fail-safety of the installation is therefore demonstrated. In a follow-up test, two used bearings were obtained from the field and tested in the same rig under the same loads for 1200 hours each. Neither sustained any detectable damage. This paper describes the initial spalling and its method of infliction, shows the progress of the race and roller damage and the severity of this damage at the termination of the test, and draws some conclusions with respect to retirement of rolling element bearings on condition or at a safe life.