Page 294 - Intro Predictive Maintenance

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FAILURE-MODE ANALYSIS

All of the analysis techniques discussed to this point have been methods to determine
if a potential problem exists within the machine-train or its associated systems.
Failure-mode analysis is the next step required to specifically pinpoint the failure mode
and identify which machine-train component is degrading.

Although failure-mode analysis identifies the number and symptoms of machine-train
problems, it does not always identify the true root-cause of problems. Visual inspec-
tion, additional testing, or other techniques such as operating dynamics analysis must
verify root-cause.

Failure-mode analysis is based on the assumption that certain failure modes are
common to all machine-trains and all applications. It also assumes that the vibration
patterns for each of these failure modes, when adjusted for process-system dynamics,
are absolute and identifiable.

Two types of information are required to perform failure-mode analysis: (1) machine-
train vibration signatures, both FFTs and time traces; and (2) practical knowledge of
machine dynamics and failure modes. Several failure-mode charts are available
that describe the symptoms or abnormal vibration profiles that indicate potential prob-
lems exist. An example is the following description of the imbalance failure mode,
which was obtained from a failure-mode chart: Single-plane imbalance generates a
dominant fundamental (1¥) frequency component with no harmonics (2¥, 3¥, etc.).
Note, however, that the failure-mode charts are simplistic because many other
machine-train problems also excite, or increase the amplitude of, the fundamental (1¥)
frequency component. In a normal vibration signature, 60 to 70 percent of the total
overall, or broadband, energy is contained in the 1¥ frequency component. Any devia-
tion from a state of equilibrium increases the energy level at this fundamental shaft
speed.

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