Page 171 - Intro Predictive Maintenance

162 An Introduction to Predictive Maintenance

vibration or energy from a frequency range of zero to the user-selected maximum fre-
quency, F MAX. Broadband data are overall vibration measurements expressed in units
such as velocity-PK, acceleration-RMS, and so on. This type of data, however, does
not provide any indication of the specific frequency components that make up the
machine’s vibration signature. As a result, specific machine-train problems cannot be
isolated and identified.

The only useful function of broadband analysis is long-term trending of the gross
overall condition of machinery. Typically, sets of alert/alarm limits are established to
monitor the overall condition of the machine-trains in a predictive maintenance
program; however, this approach has limited value and, when used exclusively,
severely limits the ability to achieve the full benefit of a comprehensive program.

Narrowband
Like broadband analysis, narrowband analysis also monitors the overall energy, but
for a user-selected band of frequency components. The ability to select specific groups
of frequencies, or narrowbands, increases the usefulness of the data. Using this tech-
nique can drastically reduce the labor required to monitor machine-trains and improve
the accuracy of detecting incipient problems.

Unlike broadband data, narrowband data provide the ability to directly monitor, trend,
and alarm specific machine-train components automatically by using a microproces-
sor for a window of frequencies unique to specific machine components. For example,
a narrowband window can be established to directly monitor the energy of a gear set
that consists of the primary gear-mesh frequency and corresponding sidebands.

7.8.2 Comparative Analysis
Comparative analysis directly compares two or more data sets in order to detect
changes in the operating condition of mechanical or process systems. This type of
analysis is limited to the direct comparison of the time-domain or frequency-domain
signature generated by a machine. The method does not determine the actual dynam-
ics of the system. Typically, the following data are used for this purpose: baseline data,
known machine condition, or industrial reference data.

Note that great care must be taken when comparing machinery vibration data to indus-
try standards or baseline data. The analyst must make sure the frequency and ampli-
tude are expressed in units and running speeds that are consistent with the standard
or baseline data. The use of a microprocessor-based system with software that auto-
matically converts and displays the desired terms solves this problem.

Baseline Data
Reference or baseline data sets should be acquired for each machine-train or process
system to be included in a predictive maintenance program when the machine is