抄録
A remarkable vibration at the bearing housing of electric motor had been detected on the large cooling tower in some chemical plant when the cooling tower had been in the trial run. This motor drove the cooling fan with long drive shaft and this fan was an axial flow fan which diameter was 5m. The dominant frequency of the vibration was 17.5Hz when the rotational speed had been some special condition and the vibration had not been extreme on the other rotational condition. The mechanism of this vibration trouble had been made clear. The dominant frequency of vibration was same as the blade passing frequency of the cooling fan and as the frequency of circumference natural vibration mode of the fan casing. The periodical velocity fluctuation in the wake flow with fan blades excited the natural vibration of fan casing. And the base of motor with drive shaft were excited by the vibration of the fan casing as the base connected to the bottom of fan casing. But the bending vibration of the long drive shaft was not the cause of the vibration of motor and bearing.Finally, the stiffness of fan casing was changed to strong for controlling the natural vibration.
