Kancil,
What I think you are asking is why amplitudes of bearing defects decrease with a decrease in machine speed?
There is a rule of thumb for that when the speed of a machine is doubled the vibration squares. The inverse would thus hold true
ksner,
In my experience it is not unusual for harmonics to show up in cases of severe unbalance.
The unbalance was likely inducing enough energy into the fan? to exagerate any looseness in the machine ie. bearings, base, duct work, piping, motor covers, gaurds etc.
The 3X vibration is...