Vibration
Monitoring of Journal Bearings
A shaft supported by journal bearings will move relative to the bearing
housing as various forces are imposed onto the shaft. A proper vibration
transducer should monitor the relative motion between the shaft and the
bearing. Higher vibration frequencies are not of prime concern since they
would not be transmitted through the oil film reliably.
The only sensor available that can measure
relative measurements of the shaft is the non-contacting pickup, sometimes
called a displacement, eddy current, or
proximity pickup. This type of sensor measures the relative vibration of
the shaft and, also, the relative position of the shaft with respect to the
bearing clearances. High frequencies such as blade passage and cavitation
would be attenuated by the lubricant. Case mounted sensors would not provide
an accurate indication of the vibration due to the inherent damping offered
by the lubricant between the shaft and the bearing. For more information
about installation and theory of operation of NCPUs, see the STI Application
Notes: Eddy Current Transducer
Installation, Part 1-Radial Vibration.
The exception to this rule is in the case of
large heavy machines (fans in particular) mounted on light-weight
foundations. Because the machine has a greater mass than the footing, the
vibration will be transmitted through the journal bearing to the bearing
housing and footing, where an accelerometer can be mounted. This is helpful
in the case of large fans because proximity probes are often a cost
prohibitive item in proposing a on-line monitoring system for non-critical,
low cost large fans.
A typical vibration monitoring system for one
journal bearing would consist of:
| Item |
Qty. |
P/N |
Description |
| 1 |
2 |
Probes |
Proximity Probe
Systems |
| 2 |
2 |
CMCP540A |
Vibration
Displacement Monitors |
Vibration Monitoring of
Rolling Element Bearings
Rolling element bearings, by their design and installation, provide a very
good signal transmission path from the vibration source to the outer bearing
housing. Also, these bearings require monitoring of the unique bearing
frequencies generated by the various parts of the bearing, in addition to
the rotor fault frequencies. This style of bearing is typically monitored
using a case mounted transducer: an
accelerometer or velocity pickup. A displacement sensor observing the
shaft relative vibration would show little, if any, vibration due to the
vibration node created by the bearing.
Using signal integration techniques specific
frequency ranges relating to certain defects can be emphasized. Acceleration
signals, obtained from case mounted sensors, emphasize high frequency
sources, while displacement signals emphasize lower frequency sources, with
velocity signals falling between the extremes. Recent innovations for
determining bearing condition are Acceleration Enveloping, Spectral Emitted
Energy (SEE), and Spike Energy measure high frequency resonances generated
by bearing defects. As a trended variable, in conjunction with other
parameters such as displacement, velocity or acceleration, they can give the
earliest indication of bearing defects.
As time progresses the earliest indication of
failure are obtained from filtered high frequency signals because these
signals are generated by the resonance of the bearing and by bearing
component defects. During the early stages of failure the other three
parameters may not generate enough signal to be detected because these
parameters emphasize progressively lower frequency ranges. As failure
continues and the damaged bearing generates the individual bearing defect
frequencies, the other parameters register signals in order from higher to
lower frequencies.
A typical vibration monitoring system for one
rolling element bearing would consist of:
| Item |
Qty. |
P/N |
Description |
| 1 |
1 |
CMCP1100 |
Low Cost
Accelerometer |
| 2 |
1 |
CMCP530A |
Vibration
Acceleration Monitor |
For additional reading on vibration monitoring of journal
and rolling element bearings visit:
STI's Application Notes |
