When using ultrasound for slow-speed systems, there are some things to consider. The first is our alarm levels. Slow-speed bearings will have a relatively low level of friction, which will give us a low level of ultrasonic signature. So we may have low decibel values, and we wouldn’t set the alarm level the same as we would for a 3000 rpm motor. We might use time waveform analysis to look for impacts—changes in the deviation of the friction.

For VFD applications, we might widen the bands of the alarm levels due to the variable speed, which also has an effect on the friction levels.

A common question when starting is “How do I know if the baseline is a good baseline?” This is a valid question. If you take what you think is a good baseline, and you come back a few weeks later and the value has gone up, indicating that something is not right, and it goes up the next time too, the baseline may not be good. It will always increase if it is not a good baseline. If there is something wrong, you will always see that sound increasing.

Sometimes we may be able to listen to a bearing and lubricate while we do that—condition-based lubrication using ultrasound because, again, we’re listening to the friction level. As we apply grease, hopefully we are reducing the level of friction and therefore the level of ultrasound, giving us a more controlled lubrication program so we avoid under and over lubrication.

For extreme slow-speed bearings (1 rpm to 20 rpm), it may be impossible to trend due to the lack of high-frequency sound. You may get zero decibels, but you can still hear it on your headset and do a sound recording. It’s such a low level of friction that you don’t quite register it, but there may still be a fault. This is where we use time waveform analysis because we can see what’s going on from the sound recording we get. Remote continuous monitoring may help with that too, depending on criticality.

An ultrasound instrument with the ability to frequency tune can be recommended for slow-speed bearings. We might want to lower the frequency to improve the sensitivity of the technology. A lot of fixed-frequency ultrasound devices are set to 38 or 40 kHz. When listening to slow-speed bearings, 20–25 kHz is recommended. We can pick up more sound that way.