Page 113 - An Introduction To Predictive Maintenance

Predictive Maintenance Techniques 103

Overlap Averaging

Many of the microprocessor-based vibration-monitoring systems offer the ability to
increase their data acquisition speed. This option is referred to as overlap averaging.
Although this approach increases speed, it is not generally recommended for vibra-
tion analysis. Overlap averaging reduces the data accuracy and must be used with
caution. Its use should be avoided except where fast transients or other unique
machine-train characteristics require an artificial means of reducing the data acquisi-
tion and processing time.

When sampling time is limited, a better approach is to reduce or eliminate averaging
altogether in favor of acquiring a single data block, or sample. This reduces the acqui-
sition time to its absolute minimum. In most cases, the single-sample time interval is
less than the minimum time required to obtain two or more data blocks using the
maximum overlap-averaging sampling technique. In addition, single-sample data are
more accurate.

Table 6–1 describes overlap-averaging options. Note that the approach described in
this table assumes that the vibration profile of monitored machines is constant.

Excluding Machine Dynamics

Perhaps the most serious diagnostic error made by typical vibration-monitoring pro-
grams is the exclusive use of vibration-based failure modes as the diagnostic logic.

Table 6–1 Overlap Averaging Options

Overlap, % Description
0 No overlap. Data trace update rate is the same as the block-processing rate.
This rate is governed by the physical requirements that are internally
driven by the frequency range of the requested data.
25 Terminates data acquisition when 75% of each block of new data is acquired.
The last 25% of the previous sample (of the 75%) will be added to the new
sample before processing is begun. Therefore, 75% of each sample is new.
As a result, accuracy may be reduced by as much as 25% for each data set.
50 The last 50% of the previous block is added to a new 50% or half-block of
data for each sample. When the required number of samples is acquired
and processed, the analyzer averages the data set. Accuracy may be
reduced to 50%.
75 Each block of data is limited to 25% new data and the last 75% of the
previous block.
90 Each block contains 10% new data and the last 90% of the previous block.
Accuracy of average data using 90% overlap is uncertain. Since each block
used to create the average contains only 10% of actual data and 90% of a
block that was extrapolated from a 10% sample, the result cannot be
representative of the real vibration generated by the machine-train.
Source: Integrated Systems, Inc.