Hello everybody.

I am not a great expert in vibration analysis. Just a bit of experience, on the field, with our rotating machines. I try to understand more the behaviour of the machines, with the vibrations. It helps for the maintenance

I am using a Vibrotest 80 from Bruel and Kjaer. With an "overall" recording, we have the possibility to see the acceleration and the BCU.

I understand that the BCU is the Bearing Condition Unit. But it is correlated to the acceleration ? If the bearing is worn, the BCU is high, and the acceleration is high ? Actually, I though this, until I find a case where the BCU is very low, and the acceleration is very high. Therefore, I am lost

Do you have an explaination ?

Thanks a lot for your help


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Original Post

Thank you for your response

The machine is quite particular. It is a centrifuge. To be simple, it is a big salad spinner for sugar industry. The rotation speed is about 1100 RPM (18.3 hz), for a wei

I generally record the values directly on the bearing housing really closed to the bearings.

When I check the report of the recording :

Trigger Type,Free Run, ,CH1 Detection A,rms, ,
Trigger Level,2, ,CH1 High-Pass A,10, ,
Slope,+, ,CH1 Low-Pass A,1000, ,
Transducer Units,N/A, ,CH1 Overall B,Active, ,
Sensitivity,N/A, ,CH1 Units B,BCUdc, ,
Transducer Offset,N/A, ,CH1 Detection B,env, ,
Display Units,N/A, ,CH1 High-Pass B,16000.1, ,
High Pass Filter,N/A, ,CH1 Low-Pass B,63000.1, ,

As it is the record for ISO 10816, it is a low pass filter 1000 Hz for A (acceleration in particular). But I see that Channel B (BCU) has a High pass 16000 and low pass 63000.

It is probably the explaination right ?


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Glad to see you know the machine ! Yeah generally the customer balance the basket on-site, directly on the machine. It is quite bad, because there are 2 balancing plans normally, and there is another rotating part around the basket, to be balanced separatly.

Oh really ? You thing that the BCU is useless ? Because I used to use it a lot, and generally, it works. When the bearings are old, the BCU is high, and when the bearing are new, the BCU is low. Therefore I am lost now :'(

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