Shock Pulse Measurement - Fluke VIBXPERT II User Manual

Data collector & vibration analyzer
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5-2
Measurement tasks
The procedures used for bearing di-
agnosis (shock pulse, envelope curve,
vibration acceleration) comply with the
ISO norm, 10816-1 (Appendix E3 , E1).

Shock pulse measurement

Shock pulse signal levels are indicated as a combination of 'Carpet
value' (background level, indicative of lubrication condition) and
'Maximum value' (transient peak level, indicative of damage), both
expressed in logarithmic [dB] terms. Bearing condition is determined
by comparing normalized signal levels and their difference with
reference values.
These characteristic overall values are typically affected by
various external factors (e.g. rolling velocity, i.e. bearing size and
rpm, signal damping, lubrication). To evaluate the condition of
the bearing objectively and to allow comparison between values
measured on different bearings requires either a comparison
measurement in the good condition or a normalization of the
measured values.
Normalization
This process considers the following individual factors:
- Bearing size, RPM
These bearing-specific factors are combined in the so-called '
initial value' or the 'dBi value'. The dBi value is calculated from the
rotational speed and the bore diameter of the bearing.
- Signal damping, lubrication, load, ...
These unspecific external influencing factors must be determined
empirically within the framework of a 'normalization adjustment'.
Together with the dBi value, the 'adjustment value' obtained in this
way, or even the dBa value, results in the so-called 'adjusted initial
value' (dBia value).
Consequently, the normalized shock pulse value (dBn) works out to:
dBn = dBsv – dBi – dBa = dBsv – dBia
Normalization adjustment
Under ideal conditions (dBa=0), the normalized carpet value of a
bearing in good condition equals 5 dBn. The 'true' dBa value equals
the dif ference from the measured value, only with the initial value
(dBi) normalized carpet value.
Example
Normalized measurement on a new bearing: 9 dBn.
The adjustment value is therefore:
9 dBn - 5 dBn = 4 dBa.

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