Mechanical Solutions, Inc. (MSI) was contracted by a paper mill to solve excessive vibration problems on Induced-Draft and Forced-Draft fans and their respective drive trains.  This case history discusses the motor driven FD fan problem and solution.

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Figure 1. Freeze frame picture from an ODS animation showing the “Rocking” Motion of the FD fan housing.

MSI collected detailed natural excitation vibration data on all components of both fans and their drive trains, including their bases and ducting.  MSI uses a specialized operating force response vibration technique that allows the data to be used to produce very detailed animated operating deflection shapes (ODS) of the fan system.  The ODS animation shows the relative motion of various system components in an exaggerated fashion to help efficiently identify the problem root cause(s).  MSI also performed impact modal testing while the units were operating to determine the natural frequencies and mode shapes.

The FD fan had excessive vibration at 14.85 Hz or at about 1x running speed.  The ODS (Figure 1) revealed a “rocking” motion of the fan housing from front to back.  It appeared that it was pivoting about the back of the housing and the front (suction side) was lifting off the base.  This implied that the connection of the front of the housing to the base was defective, creating a massive “soft foot” along the entire base.

Additionally, the motion of the FD fan motor indicated a “soft foot” at the inboard left foot, generating a casing distortion and electromagnetic unbalance at 2x line frequency (120 Hz).

MSI recommended that the bolts holding the suction side of the FD fan to the foundation be tightened to minimize the back and forth motion at 1x running speed.  The data supported checking the baseplate and foundation for corrosion or cracking at the next opportunity.  Also, eliminating the “soft foot” of the motor by adjusting the bolts on the base and/or re-shimming was recommended.

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