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Mastering the Monitoring of Low-Speed Bearings

Machinery that operates at speeds below 600 rpm falls under the category of low-speed machines. These machines are typically large and possess high rotating inertias, making them crucial components of the production line. Although these machines are less prone to breakdowns, they are considered critical, and their failure can result in enormous production losses, significant downtime, and substantial replacement costs. Historically, there has been limited interest in the condition monitoring of these machines due to their infrequent failures.

The parts of these machines that necessitate condition monitoring are primarily the bearings and gears in motion. This article will cover modern and innovative techniques for monitoring the condition of low-speed machinery, with a particular emphasis on monitoring the condition of rolling element bearings.

Monitoring low-speed bearings present unique challenges. In the case of high-speed bearings, vibration analysis, thermography, and wear debris analysis are standard tools used in predictive maintenance (PdM) programs. However, when it comes to low-speed bearings, these conventional technologies are not effective until the speed is less than 250 rpm. Early bearing failure is a persistent issue in low-speed applications, and the solution lies in using ultrasound.

Ultrasound is an effective solution for monitoring slow-speed bearings, and the process is simpler than one might expect. High-end ultrasound instruments possess a broad sensitivity range and frequency tuning, which enables the acoustic quality of the bearing to be heard, even at lower speeds. However, in extremely slow-speed applications (typically below 25 rpm), there may be little or no ultrasonic noise generated by the bearing. Therefore, it is crucial to analyze the recorded ultrasound sound file using spectrum analysis software, focusing on the time waveform for any anomalies. The presence of "crackling" or "popping" sounds indicates the occurrence of a deformity. For bearing speeds above 25 rpm, a baseline decibel level can be established, and the associated decibel level readings can be trended over time.

Ultrasound devices primarily function by converting high-frequency sound into audible sound through heterodyning. An operator who understands the fundamentals of bearing friction can differentiate between a healthy bearing producing a steady and quiet signal and a faulty bearing causing an intermittent or repetitive ringing or crackling sound. However, listening alone is insufficient. Reliable measurements are necessary to establish a robust PdM program; otherwise, the instrument is no more useful than a stethoscope. For instance, the UE Systems Ultraprobe 15000 enables the user to listen to sound quality and compare baseline information before saving the recording for upload to DMS software. Alarm levels can be set, and data can be analyzed to determine the bearing's condition.

In summary, when monitoring slow-speed bearings, it is essential to rely on sound quality and pattern. Using an ultrasonic instrument with sound recording capabilities such as the Ultraprobe 15000 or OnTrak system is recommended to facilitate data analysis. These tools can effectively manage the lifespan of your bearings and significantly reduce the number of bearing failures caused by improper lubrication. Once the sound is recorded, it can be analyzed using sound spectrum analysis software. Maintenance professionals can then load the file into the software and analyze it, providing valuable insights into when a bearing needs lubrication or replacement if a failure is likely to occur.

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