Leading machine condition monitoring specialist, Alstom MSc
was called in to investigate a long-standing high vibration issue with a bypass precipitation fan at a cement plant.
High vibration amplitudes were causing a forced reduced shaft running speed, which was impacting production capacity.
The machine was a motor sitting on a fabricated steel base with a belt driven overhung fan. The permanently monitored fan support bearings were running at about 12 mm/sec RMS, close to the trip limit of 15 mm/sec. Vibration frequency components identified in the spectrum included high 1x fan speed in the radial and axial directions, 2x, 3x and 4x fan speed and motor shaft speed.
After confirming that factors such as pulley misalignment, looseness and imbalance were not related to the high vibration, a more in-depth investigation was conducted to investigate critical speed and cross channel phase relationships across the machine in the form of an Operational Deflection Shape (ODS) that determines the nature of the machinery vibration. An ODS animation can show the motion of hundreds of points of a complex structure simultaneously, and at different frequencies, enabling the motion of a machine to be visualised.
The motor and its base were rocking out-of-phase with the fan pedestal. This was due to the cantilevered nature of the motor support allowing the shaft centrelines to move relative to each other and resulting in operational misalignment of the pulleys.
It was decided to increase the support of the cantilevered section supporting the motor. Subsequent cross channel phase measurements confirmed that the motor and fan were now ‘in-phase’. Balancing was then performed resulting in substantial improvement in overall vibration amplitudes.