ITT i-ALERT2, Application Manual

Page: 19 / 64

i-ALERT2 Application Guide 19 of 64
KURTOSIS
Monitoring of Kurtosis is a somewhat unique feature in condition monitoring
equipment that separates the i-ALERT2 monitor from traditional overall
vibration meters. Kurtosis is a mathematical function that indicates how
“peaky” a data set is. It is related to Crest Factor which is another commonly
used vibration parameter used to assess machine condition. Kurtosis will tend
to increase in the event the machinery starts to experience increases in
Impacting and is relatively insensitive to changes in speed or load. Impacting
can come from many sources such as bearing faults, cavitation, or mechanical
looseness. It is recommended that the user first establish baseline levels of
Kurtosis and then adjust warning/alarm limits accordingly as some machinery
will naturally have widely varying levels of Kurtosis in their vibration data sets.
Kurtosis values around 3 are considered “Normal.”
Kurtosis is subject to the same limitations as Crest Factor when determining
bearing failure progression. If there is no significant impacting such as with
long shallow spalls in a bearings race or inadequate lubrication the Kurtosis
measure will not change significantly.
Table 4: Condition based on overall vibration velocity in RMS
Figure 10: Trend of Kurtosis