Page 132 - An Introduction To Predictive Maintenance

122 An Introduction to Predictive Maintenance

Portable Vibration Analyzers

The portable vibration analyzer incorporates a microprocessor that allows it to math-
ematically convert the electrical signal to acceleration per unit time, perform an FFT,
and store the data. It can be programmed to generate alarms and displays of the data.
The data stored by the analyzer can be downloaded to a PC or a more powerful com-
puter to perform more sophisticated analyses, data storage and retrieval, and report
generation.

7.3 VIBRATION SOURCES
All machinery with moving parts generates mechanical forces during normal opera-
tion. As the mechanical condition of the machine changes because of wear, changes
in the operating environment, load variations, and so on, so do these forces. Under-
standing machinery dynamics and how forces create unique vibration frequency com-
ponents is the key to understanding vibration sources.

Vibration does not just happen. There is a physical cause, referred to as a forcing func-
tion, and each component of a vibration signature has its own forcing function. The
components that make up a signature are reflected as discrete peaks in the FFT or
frequency-domain plot.

The vibration profile that results from motion is the result of a force imbalance. By
definition, balance occurs in moving systems when all forces generated by, and acting
on, the machine are in a state of equilibrium. In real-world applications, however,
there is always some level of imbalance, and all machines vibrate to some extent. This
section discusses the more common sources of vibration for rotating machinery, as
well as for machinery undergoing reciprocating and/or linear motion.

7.3.1 Rotating Machinery
A rotating machine has one or more machine elements that turn with a shaft, such as
rolling-element bearings, impellers, and other rotors. In a perfectly balanced machine,
all rotors turn true on their centerline and all forces are equal. In industrial machin-
ery, however, it is common for an imbalance of these forces to occur. In addition to
imbalance generated by a rotating element, vibration may be caused by instability in
the media flowing through the rotating machine.

Rotor Imbalance
Mechanical imbalance is not the only form of imbalance that affects rotating elements.
It is the condition where more weight is on one side of a centerline of a rotor than on
the other. In many cases, rotor imbalance is the result of an imbalance between cen-
tripetal forces generated by the rotation. The source of rotor vibration can also be an
imbalance between the lift generated by the rotor and gravity.