125 kW Motor used for driving Vertical Pump (multistage) have a bearing  (Rolling Element Bearing) that has its Ball Spin Frequency same at that of Line Frequency of 50 Hz. Is this a serious problem ???

Original Post

I've never really considered it an issue as ball spin is fairly rare. It's more of a nuisance. There are usually other indications that would show if you have bearing issues going on, such as an increase in your demod reading, impacting in the waveform, etc. If it's not exactly 50 hz, you could take an extremely high resolution reading focused on 50hz and it would seperate them.

Thank Gravel for your advice. The vibration has been dominated by 1X (more than 95%). I hardly see anything  at 50 Hz. However the amplitude has been very high. 18 mm/s RMS at motor inboard and 24 mm/s at Motor Outboard, and at Bearing Casing (Motor Side) it 7 mm/s RMS. COuld this be a problem of directional force or stiffness. The vibration measurement was not carried out on the unit until recently. 

Well, a high 1X is one of the most common problems for vertical pumps. You will have to do some phase analysis to try and figure out the problem. It could be anything from pipe strain, the base, resonance, worn bushings, etc, etc.  

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