|
ABSTRACT
Rolling bearing vibration detection. Roller bearing vibration
measurement is one of the major condition monitoring tools in
regular use. By measuring the velocity, acceleration and frequencies
emitted from a rolling bearing it is possible to tell its condition
and the likelihood of imminent failure. The technology is well
established and well proven but requires a good understanding
of equipment vibration characteristics before a user can draw
good interpretations from the results.
Keywords: out of balance, vibration analysis, bearing life
Method of Bearing Vibration Monitoring
All bearings, even those in perfect condition, produce 'noise'
as the elements roll over the raceways and rub against the internal
cage and flanges. This noise is generated at high frequency and
low amplitude. The bearing housing amplifies the noise to a point
where a sensitive accelerometer can 'hear' them. By careful detection
and filtering, the noise signal can be amplified and represented
as a frequency series in real-time.
Most bearing
faults occur with the rolling elements, cage or raceways. The
frequency of the fault has a direct relationship with the geometry
of the bearing and the relative speed of each individual raceway.
Software libraries of noises for bearings in ideal condition are
available for the vast number of bearings. By comparing the actual
noise to the ideal bearing signature it is possible to highlight
problems with the in-service bearing. The method of presenting
the data is either by visually representing it on an oscilloscope
screen (monitor screen) as waves or as a number on a digital display.
The Four
Stages of Bearing Failure
A roller bearing progresses through four stages to failure. Vibration
analysis permits the monitoring of the bearing's progression through
each stage and to estimate when failure will actually occur. In
the case of a raceway failure these would be the four progressive
stages.
- The bearing
is new and has no defects. This is the time to record its frequency
'signature' and normal operating acceleration and velocity values.
- If examined
at this stage there would be no visible defects. However under
the surface of the raceway sub-surface defects have started.
The frequency signature has changed, the overall base level
noise has risen and the velocity spectrum (graph) has risen
higher.
- At this
point the raceway shows visible signs of surface failure. The
extent of the failure increases and grows with more metal coming
off in minute sheets (delaminating). The velocity spectrum is
much higher and much more background noise has developed. Within
the background noise particular frequencies start to standout
(side bands) and indicate failure is fast approaching.
- If the
bearing is still in service everyone knows it is time to change
it out because they can hear it. More vibration frequencies
appear and more velocity side bands develop. Readings start
to indicate amplitude changes and the noise moves into the range
of human hearing.
A Great
Range of Uses
Bearing vibration analysis can detect lubrication failures, misalignment,
out of tolerance running, rubbing, improper gear teeth meshing,
out of balance, bent shafts, loose components, worn parts, faulty
couplings, improper operating conditions (like pump cavitation)
and deflecting support structures. However to be able to analyze
the presence of these type of problems requires a highly skilled
person with much experience and exposure to bearing vibration
signatures at various stages of failure.
|
