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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.
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Figure 2 is generated by rescaling Figure 1 and presenting it as a â€œcolor mapâ€?. Depicted on this figure are faint lines that represent speed sensitive fan wheel modes that both stiffen and, interestingly, soften with increased speed. The frequencies at which these modes should be found during static (nonrotating) modal testing can be found by extrapolating the faint lines to zero rpm.
Figure 1 - Fan Startup Waterfall
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Wheel Modes Crossing Fan 2X
Speed Stiffened Wheel Modes
Speed Softened Wheel Mode
Figure 2 - Color Map Showing Speed Sensitive Fan Wheel Modes
Frequency response functions were acquired using impact testing. A variety of excitation points were investigated. (1,1,1) vector impact at the motor’s drive-side was chosen as the excitation point for reasons of both data quality and stimulation of fan wheel modes of interest. The quality of data issue involved non-linear aspects of measuring the frame’s response when being excited through the bearings from a relatively flexible fan wheel, versus measuring fan wheel’s response as excited by the stiff frame. Figure 3 presents a frequency response function acquired on the fan wheel at Point 25 showing resonances between 1,500 and 1,800 rpm. The location of Point 25 within the utilized modal geometry is depicted in Figure 4. The fan disc (medium gray) is simplistically represented by four points. The hub (dark gray) is also represented by four points, while the shaft is represented by six points; one on either side of the hub, one at each end of the shaft, and one mid-length between the hub and each extreme end of the shaft. The blade area is shown as light gray.
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Figure 3 - Fan Wheel Frequency Response Function at Point 25
Fan Hub Fan Shaft Motor
Figure 4 - Modal Geometry Vibrant Technology
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Modal testing revealed four critical fan wheel wobble modes in the 1,500 cpm to 1,800 cpm frequency range that will be sensitive to rotor speed. Figures 5 through 8 are embedded Avi files presenting animations of fan wheel modes that stiffen with speed and intersect the fan 2X line in the upper portion of the design speed range as shown in Figures 1 and 2. The directions of wobble correspond to asymmetry associated with orientation of the shaftâ€™s keyway. It is this asymmetry that, in part, produces a 2X forcing function during normal fan operation.
Embedded Avi File Double Click on Figure to Start Animation - Right Click to Stop Figure 5 - Horizontal Wobble at 1,538 cpm Vibrant Technology
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Embedded Avi File Double Click on Figure to Start Animation - Right Click to Stop Figure 6 - Vertical Wobble at 1,631 cpm
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Embedded Avi File Double Click on Figure to Start Animation - Right Click to Stop Figure 7 - Wheel Orbit at 1,688 cpm
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Embedded Avi File Double Click on Figure to Start Animation - Right Click to Stop Figure 8 - Horizontal Wobble Coupled with Frame Bending at 1,838 cpm
Because wobble motion is concentrated on the relatively flexible fan disc, problematic frequencies can be easily increased by adding radial stiffening ribs.
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Fan wheel critical speeds excited by two times shaft speed - A Case Study