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26
A) Maximum value dBm
B) Difference between dBm and dBc
C) Rhythm of the strongest pulses
A shock pulse pattern is a sequence of either random
or rhythmical strong pulses (dBm level) above a carpet
of very rapid weaker pulses (dBc level). You have to be
aware of:
•
the dBm value
•
the difference between dBm and dBc
•
the rhythm of the strongest pulses.
The rhythm of the strongest pulses is best discerned by
listening with the headphone at a setting a few dB below
the dBm level. Typical for bearing signals is a
random
sequence of strong pulses (no discernable rhythm).
Rhythmical shocks can come from a bearing but are
more often a sign of interference. Typical patterns are
described on the next pages.
The Bearing Checker recognizes the pattern of the
reading taken and determines which of the six patterns
below is a match. The matching number is displayed in
the upper left corner of the Bearing display when the
measurement is completed. This number corresponds
to the pattern numbers below.
There may be times when the instrument displays a
combination of the numbers “
2/3”
or “
4/5”
, in which
case the instrument can not distinguish between the two
codes. Use the headphones and listen to the pattern to
determine condition code.
1 Pattern from a good bearing
A bearing in good condition should have a dBm value
below 20 and a dBc value approximately 5 to 10 dB lower.
Once you have verified the reading, there is no need for
any further evaluation.
The maximum value can be lower than 0. However, be
suspicious when the measured value is very low. The
cause is often a bad measuring point or an incorrectly
installed adapter or transducer. If the reading is very
low, check the installation. Measure on other parts of
the bearing housing and try to pick up a stronger signal.
Another possible reason for a very low reading is that
there is no load on the bearing. This can happen with well
balanced fans and similar rotating machines.
Typical shock pulse patterns from rolling bearings
A
B
C
dBm
dBc
Evaluation code