When analyzing motors, it is easy to read more into a problem than actually exists. This case history illustrates the need to always look for the simple solution first. While at a power plant working on a large fan, the vibration analyst requested that a large mill motor be examined. The motor had been sent out to motor shops twice. In each instance, the vibration remained high when the motor was put back into operation. Attempts were made to field balance the motor, however, they were unsuccessful. The motor continued to have amplitudes of 8 mils of vibration, all of which was at its operating speed.
The first thing that was checked was the tightness of the motor to its base plate. All the bolts were tight. The next thing that was verified was the tightness of the base plate to the concrete pedestal. The bolts felt tight, however, motion between a bolt head and the base plate was apparent. What had happened was that the bolt had bottomed out. This meant that even though the bolt felt tight
to the mechanic with a wrench, no force was applied between the bolt head and the plate. To solve the problem, a washer was installed. After the installation of the washer, the vibration dropped from 8 mils down to less than 1 mil. Over $30,000 had been spent on this large motor at the repair shops and the problem had been nothing more than the need to add a washer to a bolt.
