SPM A2010 - Unnormalized Readings on Gear Boxes

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Comparative Readings on Gear Boxes
Shock pulses can sometimes spread through a ma-
chine housing without significant damping. This means
that the shock pulses from the bearing with the high-
est shock pulse level can, under unfavourable circum-
stances, interfere with the readings on all the other
bearings.
The problem is aggravated when the bearings are of
different sizes and rotating at different speeds, as in a
gear box. A bearing with high rotational speed has a
high NORM No. and generates relatively strong pulses
even when its operating condition is good. The same
shock pulse level measured on a bearing with a low
NORM No. may indicate bad bearing condition.
In such cases, you have to proceed as follows:
1 Ignore the condition codes. Compare the LR/HR
readings of all bearings. This will reveal the
strongest shock pulse source on the machine. In
the example in figure 41A, you get a LR/HR
reading of 51/46 for bearing A and 48/43 for
bearing B.
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 (fig.
41B).
3 Check the readings against the values calculated
by LUBMASTER. In the example, bearing A is 3
dB above normal while the values for bearing B
are approximately 19 dB above normal.
You can now draw two conclusions: The reading for
bearing A, coming from the stronger source, is prob-
ably true. The bearing's condition is good.
The reading from bearing B is either true or false. If
true, it indicates very bad bearing condition, but you
cannot confirm that with the instrument before condi-
tion gets worse and bearing B becomes the stronger
shock pulse source. However, as long the readings on
bearing B follow the same trend as those on bearing
A you can assume that they are, in fact, due to cross-
talk and can be ignored.
1 Unnormalized readings reveal the
stronger source
Fig. 41
3 The reading from the stronger source
is normally true
The reading from the weaker source
cannot be confirmed
Cross talk !?
2 Cross talk must go from the
stronger to the weaker source
NORM 46 NORM 32
51/46 48/43
51/46 48/43
48/44 29/25
+ 3 ! + 19 ??