Damage inside of a rolling element bearing begins with sub-surface fatigue and results in a detectable sound signal that occurs in the ultrasonic frequency range. Ultrasound can be used to detect these Stage 1 bearing defects which would not be visible looking at typical vibration spectrum. If you listen to the sound that is being produced from a bearing with a Stage 1 defect, it would sound relatively smooth and sound similar to a bearing that was in need of lubrication. The difference between the two is that after adding lubricant to the bearing with the Stage 1 defect, the amplitude of the sound would not decrease, whereas with a bearing requiring lubrication the sound level should reduce and return to baseline. As the wear progresses and the sub-surface cracks become…