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the same measurement Point, allowing you to see how the Point’s
vibration values are “trending” over time. This trend comparison
between present and past readings is easier to analyze when the
values are plotted in a “trend plot”. A trend plot displays current
and past values plotted over time. Measurement records should also
include a baseline (known good) reading. The baseline value may be
acquired after an overhaul or when other indicators show that the
machine is running well. Subsequent measurements are compared to
the baseline to determine machinery changes.
15.3 Comparison with Other Machinery
When several similar machines are used under the same operating
conditions, evaluation can be carried out by measuring all machines at
the same Points and comparing the results.
16.0 EVALUATING ACCELERATION ENVELOPE MEASUREMENTS
Use the same techniques of comparison as for Overall Vibration readings.
Remember, acceleration envelope is an advanced “early warning” of
a developing problem. High values do not necessarily indicate bearing
failure. They can also indicate:
A. Lack of lubricati
on or decreasing oil viscosity due to high-bearing
temperature caused by overload or external heat source
B. Breaking of the lubricant film by excessive imbalance, misalignment,
or housing deformation. Loss of boundary lubrication
C. A rubbing seal or cover
D. Gear mesh interaction (bad lubrication, defects)
E. Dirt or particles in the lubricant, or a seal or filter problem
16.1 Trend Comparison
Use Trend Comparison similar to Overall Vibration to establish
severity levels. Accelerating Envelope readings tend to decrease as
Overall Vibration readings increase. This happens when the defect in
the bearing is becoming more severe and the frequency is generates
becomes lower, which makes it better read with the Velocity‑type
readings.
