This case study involves a super dryer for wood chips. The large bearings were rotating between 7 and 10 rpm. There was a bank of four motors with eight bearings in total, and they were rotating at a slow speed to keep the drum rotating as the wood chips came in one end damp and out the other end dry. If the dryer were to stop, heat would build up and cause physical damage, including possible fire.

We were on the site six months after installation—a perfect time to collect baselines. We expected to see 0 dB, and that’s what we found for seven of the bearings. Because of the size and slow speed, there was very little friction, especially since they were only six months old and had been greased correctly.

As we were listening to the eighth bearing, we heard a faint ticking sound. The display indicated 2 dB. The guys on site were not worried, as it was only 2 dB. I rephrased this to say that seven of the bearings were 0 dB and the eighth was 200% higher than the rest. They admitted that this was not a good sign, but they didn’t seem to believe me when I said they had a severely damaged bearing.

I asked them to consider further tests to justify my recommendations. Vibration analysis did not pick much up, so we took a grease sample. I knew they would find metal particles in the lubrication to confirm the problem. I went away for a week and they called me with the results of the grease sample: they had found a very large metal particle. Note that this is a case where multiple technologies are useful.

They hired a crane to take the drum off and replace the bearing. Then they sent me some pictures of the bearing.

As they took it apart, part of the outer race fell off. There was a huge crack down the side. The cage was damaged and one of the rolling elements had moved 90°. Though it wasn’t emitting a relatively high level of sound, we could faintly hear that rolling element sliding over the cracks in our recording, and we could see the gap in the time waveform.

As for the root cause, the drum had been lowered onto the motors by a crane and it had impacted one side in particular, causing damage.