Page 298 - Intro Predictive Maintenance

Failure-Mode Analysis 289

Figure 14–1 Single-plane imbalance.

Multiplane. Multiplane mechanical imbalance generates multiple harmonics of
running speed. The actual number of harmonics depends on the number of imbalance
points, the severity of imbalance, and the phase angle between imbalance points.

Figure 14–2 illustrates a case of multiplane imbalance in which there are four out-of-
phase imbalance points. The resultant vibration profile contains dominant frequencies
at 1¥, 2¥, 3¥, and 4¥. The actual amplitude of each of these components is determined
by the amount of imbalance at each of the four points, but the 1¥ component should
always be higher than any subsequent harmonics.

Lift/Gravity Differential
Lift, which is designed into a machine-train’s rotating elements to compensate for the
effects of gravity acting on the rotor, is another source of imbalance. Because lift does
not always equal gravity, some imbalance always exists in machine-trains. The vibra-
tion component caused by the lift/gravity differential effect appears at the fundamen-
tal or 1¥ frequency.

Other
All failure modes create some form of imbalance in a machine, as do aerodynamic
instability, hydraulic instability, and process loading. The process loading of most