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Slow speed bearings produce low levels of ultrasonic noise – but an ultrasound instrument can still reveal faults by allowing the bearing’s signature in the audible frequency range to be analysed.

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 20m long by 5m wide. It was rotated by four large motors, each of them having two large sets of bearings. The motors were rotating at a speed of around 7-10 rpm, so that this was a case of extreme slow speed bearings.

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, the ultrasonic instrument was displaying 2dB instead of 0db. Also, the audible sound heard through the headphones was different: it was not smooth as in the other bearings and it presented a repetitive “knocking” sound.

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