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The headphone is a means to verify and trace shock pulse
sources. The signal from a bearing should be highest on the
bearing housing. If you get a higher signal outside of the
bearing housing (across an interface in the material), you
are most likely measuring shock pulses from another bearing
or some other source. Typical for bearing signals is that the
stronger shock pulses, best heard a few dB below the peak
level, appear at random intervals.
The codes refer to the Evaluation Guide, or the Flowchart
on pages 34-35, which must be used to further evaluate the
bearing condition.
If the instrument displays “2/3” or “4/5”, use the head-
phones to determine the condition code.
1 For a good bearing, the dBm is within the green zone.
dBm and dBc are close together.
2 The shock pulse pattern from a damaged bearing contains
strong pulses in the red zone, a random sequence, and a
large difference between dBm and dBc. When you lubricate
the bearing, the values should drop but rise again.
3 A dry running bearing has a high carpet value very close
to the dBm. When you lubricate the bearing, the values
should drop and stay low. A similar pattern is caused by pump
cavitation, in which case readings on the pump housing are
stronger than those taken on the bearing housing, and are
not influenced by lubricating the bearing.
4 A regular pattern, containing bursts of strong pulses in a
rhythmic sequence, is caused by e.g. scraping parts.
5 Individual pulses in a regular sequence are cause by click-
ing valves, knocking parts, regular load shocks.
6 A sudden drop in the shock pulse level is suspicious.
Check your measuring equipment. If the reading is correct,
you may have a slipping bearing ring.
Shock pulse patterns – condition codes
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