Using Ultrasound to Identify Oven Motor Bearing Failure

An inspection with an ultrasound instrument was carried out on 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 rotate the oven and are rotating at a speed of around 7-10 rpm. Meaning we are talking about a case of extreme slow speed bearings, which is usually a challenge to inspect.

An ultrasonic instrument was used to inspect all 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. Also, 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.

After the results from the ultrasonic inspection, 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 indeed the presence of metal particles, confirming the damage as indicated by the ultrasound instrument.

The next step was naturally scheduling an outage to replace the bearing, which was in a very bad condition as it can be seen in the images. Part of the outer race came away as it was opened. It was also noticeable that one of the rollers had moved 90 degrees. The cage had been totally damaged too.

This find has saved the company a significant amount of money, as it was necessary to get cranes in to replace such a big bearing, a job that took up to 6 hours. Luckily this was done during a planned outage, avoiding the costs of unplanned downtime.