Page 161 - Intro Predictive Maintenance

152 An Introduction to Predictive Maintenance

assumption that the machine’s dynamics and the resultant vibration profile are con-
stant throughout the entire data acquisition process. This approach hinders the ability
to evaluate real-time relationships between measurement points on the machine-train
and variations in process parameters such as speed, load, pressure, and so on.

Multichannel
Multichannel data provide the best picture of the relationship between measurement
points on a machine-train. Data are acquired simultaneously from all measurement
points on the machine-train. With this type of data, the analyst can establish the rela-
tionship between machine dynamics and vibration profile of the entire machine.

In most cases, a digital tape recorder is used to acquire data from the machine. Because
all measurement points are recorded at the same time, the resultant data can be used
to compare the tri-axial vibration profile of all measurement points. This capability
greatly enhances the analyst’s ability to isolate abnormal machine dynamics and to
determine the root-cause of deviations.

7.7 DATA ACQUISITION

It is important for predictive maintenance programs using vibration analysis to have
accurate, repeatable data. In addition to the type and quality of the transducer, three
key parameters affect data quality: the point of measurement, orientation, and trans-
ducer-mounting techniques.

In a predictive and reliability maintenance program, it is extremely important to keep
good historical records of key parameters. How measurement point locations and ori-
entation to the machine’s shaft were selected should be kept as part of the database.
It is important that every measurement taken throughout the life of the maintenance
program be acquired at exactly the same point and orientation. In addition, the com-
pressive load, or downward force, applied to the transducer should be exactly the same
for each measurement.

7.7.1 Vibration Detectors: Transducers and Cables
A variety of monitoring, trending, and analysis techniques that can and should be used
as part of a total-plant vibration-monitoring program. Initially, such a program depends
on the use of historical trends to detect incipient problems. As the program matures,
however, other techniques such as frequency-domain signature analysis, time-domain
analysis, and operating dynamics analysis are typically added.

An analysis is only as good as the data; therefore, the equipment used to collect the
data is critical and determines the success or failure of a predictive maintenance or
reliability improvement program. The accuracy as well as proper use and mounting
determine whether valid data are collected.