Measurements were showing high vibration up to 10 mm/sec PK at the two bearings of the ID fan. Analysis was found that itís a case of resonance where it was found that the resonating frequency of the system coincides with the frequency of the turning speed. Thus, the imbalance of the fan was amplified by the resonance effect. An ODS (Operating Deflection Shape) analysis was done on the fan to see the points of structure weakness so that we can increase the stiffness of the weakened point and thus shift the critical frequency away from the frequency of the turning speed.
The ODS analysis showed that the structure that is carrying the bearings is swinging in horizontal direction. A recommendation was suggested to weld a 45 degree webs inside and outside the structure to increase its stiffness in the horizontal direction as shown in figure. The inside webs were only put, as there was no space to put the outside webs. A balancing job was done later after this modification and the vibration was reduced up to 0.8 mm/sec as shown below.