Page 24 - An Introduction To Predictive Maintenance

14 An Introduction to Predictive Maintenance

When the first microprocessor-based vibration meter was developed in the early
1980s, the ability to acquire multiple blocks of raw data and then calculate an average
vibration value was incorporated to eliminate the potential for spurious signals or bad
data resulting from impacts or other transients that might distort the vibration signa-
ture. Generally, one to three blocks of data are adequate to acquire an accurate vibra-
tion signature. Today, most programs are set up to acquire 8 to 12 blocks of data from
each measurement point. These data are then averaged and stored for analysis.

This methodology poses two problems. First, this approach distorts the data that will
ultimately be used to determine whether corrective maintenance actions are necessary.
When multiple blocks of data are used to create an average, transient events, such as
impacts and periodic changes in the vibration profile, are excluded from the stored
average that is the basis for analysis. As a result, the analyst is unable to evaluate the
impact on operating condition that these transients may cause.

The second problem is time. Each block of data, depending on the speed of the
machine, requires between 5 and 60 seconds of acquisition time. As a result, the time
required for data acquisition is increased by orders of magnitude. For example, a data
set, using 3 blocks, may take 15 seconds. The same data set using 12 blocks will then
take 60 seconds. The difference of 45 seconds may not sound like much until you
multiply it by the 400 measure points that are acquired in a typical day (5 labor hours
per day) or 8,000 points in a typical month (100 labor hours per month).

Single-channel vibration instruments cannot provide all of the functions needed to
evaluate the operating condition of critical production systems. Because these instru-
ments are limited to steady-state analysis techniques, a successful predictive mainte-
nance program must also include the ability to acquire and analyze both multichannel
and transient vibration data. The ideal solution to this requirement is to include a
multichannel real-time analyzer. These instruments are designed to acquire, store, and
display real-time vibration data from multiple data points on the machine-train. These
data provide the means for analysts to evaluate the dynamics of the machine and
greatly improve their ability to detect incipient problems long before they become a
potential problem.

Real-time analyzers are expensive, and some programs in smaller plants may not be
able to justify the additional $50,000 to $100,000 cost. Although not as accurate as
using a real-time analyzer, these programs can purchase a multichannel, digital tape
recorder that can be used for real-time data acquisition. Several eight-channel digital
recorders on the market range in price from $5,000 to $10,000 and have the dynamic
range needed for accurate data acquisition. The tape-recorded data can be played back
through most commercially available single-channel vibration instruments for analy-
sis. Care must be taken to ensure that each channel of data is synchronized, but this
methodology can be used effectively.

Operating Dynamics Analysis. Vibration data should never be used in a vacuum.
Because the dynamic forces within the monitored machine and the system that it is a