Ultrasound or vibration?
There is no easy answer to this question, but one thing is for sure: if a maintenance team wants to reach excellence, both technologies should be used. Ultrasound will provide the earliest warning of failure and is also very easy to use, since it relies on simply trending decibel levels. Vibration analysis is extremely complete and will give maintenance professionals a deep overview of the issue and the root cause of such issue. Almost as if ultrasound is the doctor who detects the problem, and vibration is the health specialist that will diagnose it properly. We will now talk about a few situations where, in general, ultrasound can be used instead of vibration analysis.
Slow Speed Bearings
Slow speed bearings are difficult to monitor. Since they rotate very slowly, it’s difficult for vibration sensors to pick up significant changes in vibration. Even with an ultrasound instrument it may be difficult to pick up failures if we rely only on decibel levels, since in extreme slow speed bearing applications (usually less than 25rpm), the bearing will produce little to no ultrasonic noise. However, high-end ultrasonic devices will allow for sound recording: by recording the sound of the bearing and checking it in a spectrum analysis software, we can easily find peaks in the sound spectrum amplitude which indicate a fault in the bearing.
Lubrication
Because ultrasound is based on the friction levels, it is perfectly adequate for bearing lubrication. Is the bearing lacking lubrication? Then the friction levels will increase, and we can hear that through the ultrasonic instrument and see it in the dB levels. If we start lubricating the bearing, most likely we will see a decrease in the sound intensity and the dB levels. Did the bearing receive too much lubricant? Then again, friction levels will increase, and we will know that using the ultrasonic instrument. Thus, ultrasound is perfect to avoid under- and over-lubrication issues.