Page 131 - An Introduction To Predictive Maintenance

Vibration Monitoring and Analysis 121

vibration profiles. Many vibrations are normal for a piece of rotating or moving
machinery. Examples of these are normal rotations of shafts and other rotors, contact
with bearings, gear-mesh, and so on. Specific problems with machinery generate
abnormal, yet identifiable, vibrations. Examples of these are loose bolts, misaligned
shafts, worn bearings, leaks, and incipient metal fatigue.

Predictive maintenance using vibration signature analysis is based on the following
facts, which form the basis of the methods used to identify and quantify the root causes
of failure:

• All common machinery problems and failure modes have distinct vibration
frequency components that can be isolated and identified.
• A frequency-domain vibration signature is generally used for analysis
because it consists of discrete peaks, each representing a specific vibration
source.
• There is a cause, referred to as a forcing function, for every frequency com-
ponent in a machine-train’s vibration signature.
• When the signature of a machine is compared over time, it will repeat until
some event changes the vibration pattern (i.e., the amplitude of each distinct
vibration component will remain constant until the operating dynamics of
the machine-train change).

Although an increase or decrease in amplitude may indicate degradation of the
machine-train, this is not always the case. Variations in load, operating practices, and
a variety of other normal changes also change the amplitude of one or more frequency
components within the vibration signature. In addition, it is important to note that a
lower amplitude does not necessarily indicate an improvement in the mechanical con-
dition of the machine-train. Therefore, it is important that the source of all amplitude
variations be clearly understood.

7.2.4 Vibration-Measuring Equipment
Vibration data are obtained by the following procedure: (1) mounting a transducer
onto the machinery at various locations, typically machine housing and bearing caps,
and (2) using a portable data-gathering device, referred to as a vibration monitor or
analyzer, to connect to the transducer to obtain vibration readings.

Transducers
The transducer most commonly used to obtain vibration measurements is an
accelerometer. It incorporates piezoelectric (i.e., pressure-sensitive) films to convert
mechanical energy into electrical signals. The device generally incorporates a weight
suspended between two piezoelectric films. The weight moves in response to vibra-
tion and squeezes the piezoelectric films, which sends an electrical signal each time
the weight squeezes it.