Balancing problems with beating.

Dear experts,

We are having two  Motor mounted single impeller centrifugal fans. Generally only one out of two is running.

These fans are sharing same frame and hence when both fans are running we experience beating phenomenon.

During our schedule monitoring we found high vibration in a fan 'A'. Diagnosis revealed balancing problem. Now the problem is that to balance the fan it is required to be run along with fan 'B'. But, when both fan are running amplitude on 'A' varies from 4 mm/sec to 18 mm/sec.

Can anyone suggest how to proceed for balancing

Original Post

Hi Rohit

Few questions

  1. Motor Kw
  2. RPM
  3. Base structure type, is the fan mounted on hollow metal base frame? A pic of the same would give a better idea.
  4. How much is the vib in Fan A when fan B is not running & vice versa? Values please
  5. Is there a rpm difference of both A & B. did you measure?
  6. Is there a rise in vibration or this high vibration is pertaining long.
  7. How did you diagnose the unbalance?



Kishore sir,

Yes these are small fans. Generally as per history these fan require 4-5 grams hence static balancing does not give that much accuracy.

Akhil Sir please find the details below

1. Fans are scanner air fan for HRSG.

2. 2.2 KW; 2980 rpm

3. motor and fan assembly is mounted on a hollow CS frame. Frame isolates the assembly from foundation by vibration isolation pads.

4. Both fans generally runs around 7mm/sec rms value.

5. Fan 'A' (the problematic one) has reached value up to 15 mm/sec in DE axial direction and 11 mm/sec in radial direction. Fan sucks air from atmosphere and hence dirt deposition is common. also the phase analysis shows unbalance symptoms. 

6. When fan 'B' is running alone. Vibrations on fan 'A' in stop condition are 6mm/sec rms.

7. RPM of both fans is not measured specifically but both are motor driven with same rated rpm. However, due to load fluctuation during both fan running there may be some rpm deviation but that was not measured.


Photos (2)

There are two measurement methods that many analyzers cannot do for balancing:

1) Very high resolution filter to separate the two fan speeds

2) Time synchronous averaging before measuring 1xSS amplitude and phase

Another effective method (capable on CSI 2120) is to record speed, amplitude and phase during coast-down. Pick amplitude and phase values at some consistent speed to make the balance calculations. This method can also work for "electrical bumping" when fan does not reach full speed.

Of course the low-tech balance method is to replace the fan; when you have a spare!


What is the frequency of the beating?  That will provide info about the speed difference of the fans.


"the phase analysis shows unbalance symptoms."   If you are not measuring either fan's rpm, what are you seeing for phase during the beating?

" both are motor driven ...." - are the fan blades mounted direct on the motor shaft? or are belt drives involved? 

What types of motor? Induction,

Dear Dan

Phase readings were fluctuating by 30 degrees, but that much tolerances are always kept for phase measurement hence. However with such fluctuations we cannot add weight to the rotor.

Fan is directly mounted on motor shaft. it is an induction motor 2.2 KW


Can you not shut one of the fans down long enough to take the reference readings on the fan left running?

And do this on the trial runs also?

It should not take more than a minute or less to gather the data for each of the data runs on the fan you are trying to balance.

Thanks and Have a Great Day,


If you can draw (produce values even) polar plots of the 1X over time, pick the center of the curve drawn (probably messy).  Use this for the data points, before and after balance trial to calculate the correction.  Take a lot of date (time) to get a good 'average.'

I used to use this method when I had to plot with a plotter.  In those days the ink would sometime tear a hole (holes) in the paper.  Of course one can do this numerically as well with just the amplitude and phase data.

If your analyzer can track shaft speed and filtered 1xSS and phase angle (like my CSI-2120-2), then I would get balance measurements during coast-down. Just pick data from the same speed for the Reference and Trial/Trim runs. Stay away from full speed, since speed change is faster, and amplitude/phase is less stable/repeatable. Stay away from very low speed, since vibration level is typically very low. I would look for stable amplitude and phase data between half and three quarter speed trying to avoid resonance. You could also choose two or more speeds, calculate the balance correction, and then compare to the calculations for the other speeds. Advantages of this coast-down method is that is not affected by vibrations from the other fan, and low speed phase data can be used to find the heavy spot to optimize trial weight location.


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