Fan Wheel Critical Speeds Case Study

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www.StructuralTechnology.com  PO Box 564  171 East Third Street  Zoar, OH 44697  (330) 874-4117

CASE HISTORY FAN WHEEL CRITICAL SPEEDS EXCITED BY TWO TIMES SHAFT SPEED Chris D Powell Structural Technology Corporation www.StructuralTechnology.com

A 100 hp air handling unit exhibited high vibration at two times fan shaft rotation (2X) near the upper end of its variable speed range. The fan was a 32” diameter single inlet design with nine blades.

The problem was investigated by first generating a startup waterfall to identify if 2X vibration was due to poor mechanical health or if it was being enhanced by structural resonances. The latter were identified in the waterfall. However, impact testing did not find the same frequencies as in the waterfall. Modal testing was performed using ME’scopeVES. The Figure 1 waterfall plot is an overlay of axial (green), horizontal (blue), and vertical (yellow) velocity spectra acquired on the fan’s drive-side bearing during startup. This figure shows an axial resonance at 1,800 cpm followed by two horizontal resonances at 2,200 cpm and 2,500 cpm, all being excited along the 2X fan line.

The resonance at 1,800 cpm was confirmed using impact testing, but no peaks appeared at either 2,200 or 2,500 cpm, indicating the latter two frequencies were most likely associated with speed sensitive rotor modes.

Vibrant Technology

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Fan Wheel Critical Speeds Case Study by Vibrant Technology - Issuu