The Art of Monitoring Low-Speed Bearings


Low-speed machinery is classified as machines with operating speeds less than 600 rpm. These are known to be the most critical items in the production line and are generally large with high rotating inertias. In the past, there was little interest in condition monitoring of these machines as they have less tendency to break down. However, if a failure does occur, the downtime and replacement costs can be huge, which can lead to massive production loss.

Using Ultrasound to Inspect and Monitor Slow Speed Bearings

The moving components of these machines that require condition monitoring are mainly bearings and gears. In this article, we’ll discuss the new and advanced tools to facilitate condition monitoring of low-speed machinery, with the main focus on rolling element bearing condition monitoring.

Challenges of Low-Speed Bearing Monitoring

For high-speed bearings, different technologies are commonly a part of a PdM program: vibration, thermography, and wear debris analysis. Low-speed bearing monitoring is a different story. These common technologies are ineffective until it is too late regarding speeds less than 250 rpm. In slow-speed applications, early bearing failure remains a notorious problem – unless ultrasound is used.

Why Ultrasound?

Slow speed bearing monitoring with ultrasound isn’t as difficult as you might think. Because most high-end ultrasound instruments have a wide sensitivity range and frequency tuning, you can listen to the acoustic quality of the bearing, especially at slower speeds.

In extreme slow-speed bearing applications (usually less than 25rpm), the bearing will produce little to no ultrasonic noise. In that case, it is important to not only listen to the sound of the bearing but more importantly to analyse the recorded ultrasound sound file in a spectrum analysis software, focusing on the time waveform to see if there are any anomalies present. If “crackling” or “popping” sounds are present, then there is some indication of a deformity occurring. In bearing speeds above 25rpm, it is possible to set a baseline decibel level and trend the associated decibel level readings over time.

How Does It Work?

The primary function of an ultrasound device is to turn high frequency into audible sound. This is called heterodyning. An operator who knows the basics of bearing friction can distinguish a healthy bearing, producing a quiet steady signal from a defective bearing, causing an intermittent or specific repetitive ringing or crackling sound. However, listening is not enough. Reliable measurements are required to build a solid PdM program. Otherwise, your instrument is no more than a stethoscope.

The UE Systems Ultraprobe 15000, for example, allows you to listen to sound quality and compare baseline information before it saves the recording to be uploaded to DMS software. From there, you can set alarm levels and analyze data to determine a bearing’s condition.

Bottom Line

For slow-speed bearings, it’s crucial to rely on sound quality and pattern. To facilitate data analysis, it’s recommended to use an ultrasonic instrument with sound recording capabilities, like the Ultraprobe 15000 or the OnTrak SmartLube system, which can manage the life of your bearings and significantly reduce the number of bearing failures caused by improper lubrication. After the sound has been recorded, it is then analysed on sound spectrum analysis software. Then, maintenance professionals can record the sound produced by a slow-speed bearing, load the file in the software, and analyze it. This software provides valuable insights into when a bearing needs to be lubricated or replaced if failure is imminent.


UE Systems