An ultrasonic technique was applied for the measurement of bearing load of deep groove ball bearing operating under low rotational speed without the influence of temperature rise and vibration. Ultrasonic wave pulse entered from an angle probe attached to the outside of housing is partially reflected from the interface between housing and outer ring of the bearing. Reflection echo can be used for a measurement of the bearing load, because the intensity of echo depends on the local pressure

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... ed to the ball load. In this study, ultrasonic transmittance decided with the reflection echo height was employed as an index for the measurement of bearing load, and measurement was carried out with using the relationship between load and transmittance calibrated under extremely low rotational speed. As a result, it became clear that required calibration curve was obtained by the measurement of transmittance for two normal loads having a difference moderately, even though a direction of load was changed. The potential of in-situ observation of bearing load using ultrasonic technique was confirmed, since the bearing load, its direction and vertical / horizontal component (which were estimated from echo height and calibration curve) agreed with the actual value of them which were applied to the bearing.