We have overhung centrifugal blowers (Fan: backward curved) equipped in our plant. Centrifugal blowers are with arrangement 4 & 8.  Some are of 2900 rpm and other 1450 rpm. I will appreciate your help in following query: Is it possible to determine exact source of imbalance in centrifugal fans.  In frequency spectrum analysis how we could find and differentiate  the imbalance is due to :

1. Fan unbalance  or

2. Motor bearing fit loose in housing  or

3. Fan hub loose fit on shaft

I will appreciate your support in this regard.


Mayur M. Nagothanekar

Dy. Manager,

Supreme Petrochem Ltd

Original Post







Yes you can identify the exact spot of unbalance on the fan wheel with a balancing program http://www.irdbalancing.com/ird-balancing.html

Fan unbalance with show dominant 1x rpm in radial direction in spectrum, time waveform will appear sinusoidal,  typically horizontal is twice the amplitude as vertical. Phase shift from horz to vert should be 90 degrees

Looseness will show up in spectrum as multiples of 1x turning speed, it can also show up as a pure 1x peak....you should see at least 7-10 harmonics or more, if you only see 3-4, it could be base/structural related.


If the problem is repeatable start to start, why not just balance it?  I've seen a number of people search for the right spot to balance - if that is part of your question.  However, the place to balance is anywhere that is accessible (including safe to place a weight) and for which the rotor responds.

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