Sorry for my bad English in advance.
I have six identical fans. The construction is motor coupling and fan.
One of them had huge vibrations on 001V had 38 velocity on 30 Hz frequency and 002V 30 velocity also on 30 Hz the motor speed is around the same frequency so I assume it is motor speed.
The vibration is only on vertical axis and vibrations are only on 30 Hz frequency, no harmonics. So I assumed it must be imbalance.
We Inspected fan, for broken vanes but only found excessive dirt on vanes. So we removed it and it helped significantly the 001V dropped down to 20 and 002V dropped 15, still it more that we want so we will try to balance it, but the thing bothers me that the balancing will only help for short period of time. I would like to know the root cause and prevent this thing for the other 5 fans.