Page 170 - Intro Predictive Maintenance

Vibration Monitoring and Analysis 161

In most cases, a defective cable or transducer generates a signature that contains a
ski-slope profile, which begins at the lowest visible frequency and drops rapidly
to the noise floor of the signature. If this profile is generated by defective com-
ponents, it will not contain any of the normal rotational frequencies generated by the
machine-train.

With the exception of mechanical rub, defective cables and transducers are the only
sources of this ski-slope profile. When mechanical rub is present, the ski slope will
also contain the normal rotational frequencies generated by the machine-train. In some
cases, it is necessary to turn off the auto-scale function in order to see the rotational
frequencies, but they will be evident. If no rotational components are present, the cable
and transducer should be replaced.

Additional Data
Data obtained from a vibration analyzer are not all that are required to evaluate
machine-train or system condition. Variables, such as load, have a direct effect on the
vibration profile of machinery and must be considered. Therefore, additional data
should be acquired to augment the vibration profiles.

Most microprocessor-based vibration analyzers are capable of directly acquiring
process variables and other inputs. The software and firmware provided with these
systems generally support preprogrammed routes that include almost any direct or
manual data input. These routes should include all data required to effectively analyze
the operating condition of each machine-train and its process system.

7.8 VIBRATION ANALYSES TECHNIQUES
Techniques used in vibration analysis are trending, both broadband and narrowband;
comparative analysis; and signature analysis.

7.8.1 Trending
Most vibration-monitoring programs rely heavily on historical vibration-level ampli-
tude trends as their dominant analysis tool. This approach is valid if the vibration data
are normalized to remove the influence of variables, such as load, on the recorded
vibration energy levels. Valid trend data provide an indication of change over time
within the monitored machine. As stated in preceding sections, a change in vibration
amplitude indicates a corresponding change in operating condition that can be a useful
diagnostic tool.

Broadband
Broadband analysis techniques have been used to monitor the overall mechanical con-
dition of machinery for more than 20 years. The technique is based on the overall