Page 109 - Intro Predictive Maintenance

100 An Introduction to Predictive Maintenance

6.1.1 Technology Limitations

Computer-based systems have several limitations. In addition, some system charac-
teristics, particularly simplified data acquisition and analysis, provide both advantages
and disadvantages.

Simplified Data Acquisition and Analysis
While providing many advantages, simplified data acquisition and analysis can also
be a liability. If the database is improperly configured, the automated capabilities
of these analyzers will yield faulty diagnostics that can allow catastrophic failure of
critical plant machinery.

Because technician involvement is reduced to a minimum, the normal tendency is to
use untrained or partially trained personnel for this repetitive function. Unfortunately,
the lack of training results in less awareness and knowledge of visual and audible clues
that can, and should be, an integral part of the monitoring program.

Single-Channel Data
Most of the microprocessor-based vibration-monitoring systems collect single-
channel, steady-state data that cannot be used for all applications. Single-channel data
are limited to the analysis of simple machinery that operates at relatively constant
speed.

Although most microprocessor-based instruments are limited to a single input channel,
in some cases, a second channel is incorporated in the analyzer; however, this second
channel generally is limited to input from a tachometer, or a once-per-revolution input
signal. This second channel cannot be used for vibration data capture.

This limitation prohibits the use of most microprocessor-based vibration analyzers for
complex machinery or machines with variable speeds. Single-channel data acquisi-
tion technology assumes the vibration profile generated by a machine-train remains
constant throughout the data acquisition process. This is generally true in applications
where machine speed remains relatively constant (i.e., within 5 to 10rpm). In this
case, its use does not severely limit diagnostic accuracy and can be effectively used
in a predictive maintenance program.

Steady-State Data
Most of the microprocessor-based instruments are designed to handle steady-state
vibration data. Few have the ability to reliably capture transient events such as
rapid speed or load changes. As a result, their use is limited in situations where these
changes occur.

In addition, vibration data collected with a microprocessor-based analyzer are
filtered and conditioned to eliminate nonrecurring events and their associated vibra-