Establishment of lubrication diagnostic technique with ultrasonic method

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An ultrasonic technique is applied for detection of abnormalities in a ball bearing in its life test. Ultrasonic wave pulses are entered into a bearing housing from a piezoelectric vibrator attached to the outside of the housing. Those pulses are partially reflected from the interface between the housing and the outer ring of the bearing. Because of the different acoustic impedance between the solid-to-solid contact and the contact with an intervening gas or liquid layer, the intensity of the echo depends on the real contact area formed at the interface, and it can be used as a measure of the local contact pressure. When determined in the loading zone ofthe bearing which is running in a normal condition, the intensity of the filtered echo pulses produces smooth, nearly sinusoidal peaks corresponding to passages of the balls. However, invasion of a solid particle like wear debris to the rolling surface between a ball and either of the raceways causes a sudden rise of the local contact pressure, while a flaw formed on their rolling surface causes its sudden drop. The experimental results show these "events" are detected by the ultrasonic technique, and, further, a simple sum of the intensity or of the time duration of those events over a certain period starts to increase when the cumulative number of revolution reaches, typically, 80% of the total bearing life. This indicates the possibility of the early detection of abnormalities in ball bearings which lead to fatigue failure due to surface-originated cracks.