Vibration analysis has long been the instrument of choice to use for bearings and other rotating equipment. More commonly, ultrasound is being used in conjunction with vibration analysis to help technicians confirm the condition of mechanical assets.

Ultrasound vs. Vibration Analysis for Low-Speed Bearings

Because of the versatility of ultrasound, if a facility does not have a robust vibration analysis program in place, ultrasound can be implemented to detect early-stage bearing failures, as well as other issues. If the vibration analysis is performed by an outside service provider on a quarterly or monthly basis, ultrasound can be used during the interim. This will help the facility to know the condition of the more critical assets prior to the service provider entering the facility; therefore, the service provider’s time can be used more efficiently because the plant knows if there are any prominent problems with the assets that are being monitored by ultrasound. The service provider can then prioritize based on the ultrasound findings.

Another scenario in which ultrasound may be used first over vibration analysis is with the monitoring of slow speed bearings. Slow speed bearing monitoring with ultrasound is easier than you might think.  Because most high-end ultrasound instruments have a wide sensitivity range and frequency tuning, it is possible to 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. But these conditions are precisely what UE System’s UltraTrak 850s Smart Analog Sensorwas designed for. This simple and easy ultrasound technology

• continuously monitors friction, impact, and turbulence.
• issues automated warnings in the event of a bearing or lubrication failure.
• identified steam trap issues and valve leakage.
• connects to existing monitor warning systems

This sensor can be used on all bearing speeds – including slow speed bearings as low as 1 RPM. All of these factors combine to create a device that supports real-time data trending and alerting, making it possible to mark and address important issues faster and more easily.

Case Example: Using Ultrasound to Identify Oven Motor Bearing Failure

An inspection with an ultrasound instrument was conducted at a site with a newly installed oven dryer. This was a large drum oven, about 20 meters long by 5 meters wide. It was rotated by 4 large motors, each of them having two large sets of bearings. These motors rotated the oven at a speed of around 7-10 RPM.

An ultrasonic instrument was used to inspect all the bearings – almost all of them presented a nice and smooth sound and a 0dB reading, except for one. On one of the bearings from this set, the ultrasonic instrument was displaying 2dB instead of 0 and the sound heard from the headphones was different: it was not smooth as in the other bearings and it presented a repetitive “knocking” sound. This gave the inspector an indication that something might have been wrong with this specific bearing.

Large drum oven bearing with a crack detected using ultrasonic tools

After the results from the ultrasonic inspection were collected, a grease sample was taken to confirm if there was any damage on the bearing – in which case the grease sample would show metal contamination. The results from the grease analysis showed the presence of metal particles, confirming the damage as indicated by the ultrasound instrument. The next step was scheduling an outage to replace the bearing, which was in a very bad condition, as can be seen in the images.

Another view of the cracked drum oven bearing

How to Use Ultrasound to Analyze Slow-Speed Bearings

Ultrasound technology is very useful when trying to monitor the condition of slow speed bearings, and an ultrasonic instrument or sensor can provide maintenance personnel with a warning of failure, even with extremely slow speeds.

With bearings rotating at normal speeds, the ultrasonic inspection can be performed by comparing changes in dB values. This establishes that a bearing with a certain value above a decibel baseline will need lubrication or is in a failure state, depending on how many decibels it is above the baseline.

However, with slow speed bearings, comparing dB levels and establishing alarms is not enough. In many situations, the difference in the dB levels can be non-existent and the inspector might think there is nothing wrong.

For slow-speed bearings, one must rely on the sound quality and pattern. For this, it is necessary to use an ultrasonic instrument with sound recording capabilities, like the OnTrak SmartLube systemfrom UE Systems, 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 analyzed on a 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.

The spectrum analysis of this oven motor bearing shows clearly where the hits the crack as the knock stops briefly. Thus, the sound pattern, being the most reliable source of information when determining the condition of a slow-speed bearing using ultrasound. indicates an already existing problem.

Sound Spectrum of the damaged bearing, where the peaks in amplitude give the inspector a clear sign of damage.

On the other hand, the spectrum of a recorded sound from one of the “good” bearings shows a very different picture – a very uniform spectrum with almost no changes in the amplitude.

Sound Spectrum of a “good” bearing. Very uniform and no changes in amplitude.

This find not only saved the company a significant amount of money, but also time. Because they were able to detect the damaged bearing, they were able to schedule the replacement during a planned outage, avoiding the costs of unplanned downtime. With the power of ultrasound technology, your site can catch problems before they become major issues, avoiding unplanned shutdowns and helping your bottom line.