5-2
VIBXPERT 11.2010
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 difference 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.
The procedures used for bearing
diagnosis (shock pulse, envelope curve,
vibration acceleration) comply with the
ISO norm, 10816-1 (Appendix E3 , E1).
Measurement tasks
Summary of Contents for VIB 5.300 EX Series
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