Page 293 - Intro Predictive Maintenance

284 An Introduction to Predictive Maintenance

include a full description of the machine-train and process system parameters. For
example, abnormal strip tension or traction in a continuous-process line changes
the load distribution on the process rolls that transport a strip through the line. This
abnormal loading induces a form of misalignment that is visible in the roll and its
drive-train’s vibration profile.

Analysis of shaft deflection is a fundamental diagnostic tool. If the analyst can estab-
lish the specific direction and approximate severity of shaft displacement, it is much
easier to isolate the forcing function. For example, when the discharge valve on an
end-suction centrifugal pump is restricted, the pump’s shaft is displaced in a direction
opposite to the discharge volute. Such deflection is caused by the back-pressure gen-
erated by the partially closed valve. Most of the failure modes and abnormal operat-
ing dynamics that affect machine reliability force the shaft from its true centerline.
By using common-shaft diagnostics, the analyst can detect deviations from normal
operating condition and isolate the probable forcing function.

We have used centrifugal pumps to illustrate the basics of operating dynamics analy-
sis, but these same concepts are applicable to all plant machinery, equipment, and
systems. The same concepts can be used for both dynamic and static plant systems
with equal results. In every case, the first step is a thorough understanding of the design
precepts of the system, then understanding the installation and application. It is imper-
ative that all deviations created by the installation, application, or mode of operation
must be fully understood and used to analyze the dynamics of the system.