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Shock pulses can sometimes spread through a machine
housing without significant damping. This means that the
shock pulses from the bearing with the highest shock
pulse level can, under unfavourable circumstances, inter-
fere with the readings on all the other bearings.
The problem is aggravated when the bearings are of dif-
ferent sizes and rotating at different speeds, as in a gear
box. A bearing with high rotational speed has a high dBi
value and generates relatively strong pulses even when
its operating condition is good. The same shock pulse
level measured on a bearing with a low dBi may indicate
bad bearing condition.
In such cases, you must proceed as follows:
1 Take a reading with dBi set to “--” on all bearings.
This will reveal the strongest shock pulse source on
the machine. In the example in the figure, you get
a reading of 53 dBsv for bearing A and 47 dBsv for
bearing B.
NOTE! When taking readings with dBi set to “--”
the evaluation in green - yellow- red does not
apply!
See also chapters “Normalized shock pulse
values with dBi” and “Input data”.
2 Work out the direction of possible cross talk. You
know that the stronger source can mask the signal
of the weaker source. In this case, cross talk must
go from bearing A to bearing B.
3 Subtract the dBi values from the dBsv values. In the
example, you get 26 dB
N
for bearing A, 40 dB
N
for
bearing B.
You can now draw two conclusions: The reading for
bearing A, coming from the stronger source, is probably
accurate. The bearing condition is reduced (26 dB = yel-
low zone) but not seriously so.
The reading from bearing B is either true or false. If true,
it indicates bad bearing condition (40 dB = red zone), but
you cannot confirm that with the instrument before con-
dition gets worse and bearing B becomes the stronger
shock pulse source. Your solution is to take frequent
readings and compare the results from both bearings.
1
Readings with dBi = “--” reveal
the stronger source
3
The reading from the
stronger source is
normally
accurate
The reading from the
weaker source cannot
be confirmed
Cross talk !?
2
Cross talk must go from the
stronger to the weaker source
Readings on gear boxes