Page 302 - Intro Predictive Maintenance

Failure-Mode Analysis 293

Figure 14–5 Three types of misalignment.

peak or series of peaks, the overall energy contained in the vibration signature is
increased. Unfortunately, the increase in noise floor cannot always be used to detect
mechanical looseness. Some vibration instruments lack sufficient dynamic range to
detect changes in the signature’s noise floor.

14.1.4 Misalignment
This condition is virtually always present in machine-trains. Generally, we assume
that misalignment exists between shafts that are connected by a coupling, V-belt, or
other intermediate drive; however, it can also exist between bearings of a solid shaft
and at other points within the machine.

How misalignment appears in the vibration signature depends on the type of mis-
alignment. Figure 14–5 illustrates three types of misalignment (i.e., internal, offset,
and angular). These three types excite the fundamental (1¥) frequency component
because they create an apparent imbalance condition in the machine.

Internal (i.e., bearing) and offset misalignment also excites the second (2¥) har-
monic frequency. The shaft creates two high spots as it turns through one complete
revolution. These two high spots create the first (1¥) and second harmonic (2¥)
components.

Angular misalignment can take several signature forms and excites the fundamental
(1¥) and secondary (2¥) components. It can excite the third (3¥) harmonic frequency
depending on the actual phase relationship of the angular misalignment. It also creates
a strong axial vibration.