Proper lubrication is fundamental for bearing health. Applying too much or too little lubricant will drastically reduce the life of bearings and, even worse, cause production halts and unplanned downtime. Using ultrasound instruments to precisely lubricate bearings is an established, well-known and reliable method to assure proper lubrication. This method also allows maintenance teams to have a first level of bearing health assessment, as we will see in these examples.
Ultrasound-assisted lubrication & bearing monitoring: How does it work?
By using ultrasound, we measure bearing friction levels in real time, translated into decibel (dB) levels. It enables lubrication problems and requirements to be detected at an early stage, well before the bearings are damaged.
By using bearing friction as a guide, this enables bearings that require lubrication to be precisely lubricated with the right amount of lubricant, avoiding over- and under-lubrication. As the friction level is measured in real time, during lubrication, we can stop lubricating as soon as the friction level has dropped to its reference value, or baseline.
By establishing a bearing baseline in dB and comparing the baseline with the current reading, we can have a good idea on the bearings’ health. Generally speaking, we will consider the dB reading against the baseline as the following:
- +8dB above baseline indicates lack of lubrication
- +12dB above baseline indicates microscopic damage
- +16dB above baseline indicates visible damage to the bearing
- +35dB above baseline indicates critical failure
This is an easy process to implement and follow, but of course, there are always nuances. For example, how can we assess if a bearing simply needs lubrication or is also damaged?
Let’s look at the case study below.
Assessing Bearings’ Condition at a Steel Plant
The act of lubricating a bearing with the assistance of an ultrasound instrument will gives us some clues about its health, but there are a couple more steps we can take to more accurately assess its condition:
- Evaluate sound quality: Ultrasound instruments allow you to listen to what the bearing sounds like. A uniform smooth sound normally indicates no issues, while a rough sound where crackling or popping sounds may be present will indicate bearing damage.
- Check sound spectrum: Some ultrasound instruments allow you to record the sound from the bearing and further analyse it on a sound spectrum software. By looking at the time wave form we can have a better idea of what’s going on with the bearing by checking sound wave amplitude and eventual peaks.
- Take regular readings: taking regular dB readings, especially when you suspect the bearing might be damaged, is good practice. The more readings you take the more data you have to work with.
This is a good example of all these steps in use: on a steel plant, we carried on an inspection on 2 bearings from a combustion fan. They were called East bearing and West bearing.
West Bearing:
To determine a baseline for this bearing, we have lubricated it while watching the dB levels and the sound recording. While lubricating, we could observe how the dB levels dropped to and stayed consistent at 60dB. We can also observe below, in the sound recording, what happens as we apply lubricant – the amplitude drops as the lubricant gets in the bearing, thus lowering the friction levels.
If this bearing is in good condition, the 60 dB we established as baseline will remain for some time. However, if the bearing presents issues beyond a lack of lubrication, the dB levels will go up and the sound quality will worsen in the short term.
Thus, on the following day, we took another reading and sound recording of the West Bearing. The reading showed 69 dB: that’s 9db above baseline, in just one day.
We decided to lubricate the bearing once again, and immediately noticed an improvement in sound quality as well as a drop in the dB levels, to 64 dB.
We waited 8 minutes and took another reading and sound recording and we could observe that the 69dB reading was back, as well the previous sound quality we got before this second round of lubrication.
Thus, we conclude that lubricating the bearing is not bringing it back to baseline and is not enough for it to keep a uniform and smooth sound quality. This bearing needs to be regularly monitored, as it is possible it has already some microscopic damage.
East Bearing:
We used the same method to evaluate the East bearing of this combustion fan. After a first round of lubrication, we got to 60db, which corresponds to our baseline. The screenshot from the sound file clearly shows the drop in amplitude when lubricant is applied.
Again, we took the same measurement and sound recording one day after, only to realize the bearing went way above baseline, by displaying a reading of 72dB. Sound quality showed some signs of concern, with pops and sputtering. After a 2nd round of lubrication, we were able to get the bearing to 67db and noted a quick sound quality improvement but retaining some tones of damage.
Just like with the West bearing, we waited 8 minutes before taking another reading and observed that the 72dB and the prior lubrication sound quality had returned.
Conclusion
Both bearings show clear signs of damage, even though still on an early stage (microscopic damage). Normally, healthy bearings will respond well to lubricating and will stay longer on their dB baseline, while presenting a smooth and uniform sound.
We can see how useful ultrasound technology is when assessing what’s going on with a bearing, and how easy it is to do so. Even though ultrasound may not be able to indicate the root cause of the issue, it provides an effective, easy and quick way to understand which bearings need more attention.
For this combustion fan, regular monitoring is advised, as well as including other predictive maintenance technologies to determine the root cause of the additional friction occurring with the bearings. While we cannot make a prognosis as to the underlying issue, we are able to observe that there is an issue causing strain on the bearings that will eventually lead to damage and/or failure.
This article is published in Motion+Drives December 2024 issue on page 101-103
