VMI VIBER X4, Руководство пользователя

Страница: 34 / 58

Vibration Measurement Instruments
Bearing Condition
The bearing condition value is a sum average value, the RMS value, of all high frequency vibrations in the
set frequency interval. This value is an acceleration average with the unit “ g ”.
Interpretation of the Bearing Condition Value
The bearing condition value is an indirect method of measuring the status of anti-friction bearings. A high
value indicates that further frequency analysis needs to be performed.
When the balls or rollers rotate inside a bearing, a wide band noise and vibration arise. The noise or vibra-
tions are increased if the bearing is poorly lubricated, overloaded or has a damaged surface. Both listening
and reading the BC value are good ways of analyzing the bearings.
Find the machine speed in the diagram, follow this line up to the judgment lines and read
the value on the left axis.
The figure above is a guide to interpreting the bearing condition value. If vibrations from other causes (e.g.
flow surge and sometimes gear mesh forces) have vibrations within the frequency range, this can indicate a
high bearing condition value without the bearing being damaged. A high bearing condition value can also
be acquired if the bearing is poorly lubricated or is overloaded due to a large misalignment or a large belt
tension.
If the selected frequency band includes low frequencies, the bearing condition value would include vibra-
tions
due to unbalance, misalignment etc., and not those purely from bearing vibrations, and would therefore be
difficult to interpret. If the selected frequency band only includes very high frequency vibrations (above 20
kHz), a special vibration transducer would be needed that is very rigidly and closely mounted to the bearing
because the machine structure works as a mechanical filter for high frequencies.
Normal machinery vibrations from unbalance, misalignment etc., have few vibrations above 3200 Hz.
NOTE 1 A high bearing condition value should always be used to request further frequency analysis.
NOTE 2 High bearing condition values can appear in gear boxes, grinding machines, converting machines
with cutters and similar machines without any bearing faults. This is because these machines “naturally”
produce high frequencies and the bearing condition value can be misinterpreted. Try to make measurements
when the machine is unloaded.
34