Page 104 - An Introduction To Predictive Maintenance

94 An Introduction to Predictive Maintenance

Bearings

Large generators typically use Babbitt bearings, which are nonrotating, lined metal
sleeves (also referred to as fluid-film bearings) that depend on a lubricating film to
prevent wear; however, these bearings are subjected to abnormal wear each time a
generator is shut off or started. In these situations, the entire weight of the rotating
element rests directly on the lower half of the bearings. When the generator is started,
the shaft climbs the Babbitt liner until gravity forces the shaft to drop to the bottom
of the bearing. This alternating action of climb and fall is repeated until the shaft speed
increases to the point that a fluid-film is created between the shaft and Babbitt liner.

Subharmonic frequencies (i.e., less than the actual shaft speed) are the primary eval-
uation tool for fluid-film bearings, and they must be monitored closely. A narrowband
window that captures the full range of vibration frequency components between elec-
tronic noise and running speed is an absolute necessity.

Casing and Shaft
Most generators have relatively soft support structures. Therefore, they require shaft
vibration-monitoring measurement points in addition to standard casing measurement
points. This requires the addition of permanently mounted proximity, or displacement,
transducers that can measure actual shaft movement.

The third (3X) harmonic of running speed is a critical monitoring parameter. Most, if
not all, generators tend to move in the axial plane as part of their normal dynamics.
Increases in axial movement, which appear in the third harmonic, are early indicators
of problems.

Line Frequency
Many electrical problems cause an increase in the amplitude of line frequency, typi-
cally 60Hz, and its harmonics. Therefore, a narrowband should be established to
monitor the 60, 120, and 180Hz frequency components.

Running Speed
Actual running speed remains relatively constant on most generators. While load
changes create slight variations in actual speed, the change in speed is minor. Gener-
ally, a narrowband window with lower and upper limits of ±10 percent of design speed
is sufficient.

5.3.4 Process Rolls
Process rolls are commonly found in paper machines and other continuous process
applications. Process rolls generate few unique vibration frequencies. In most cases,
the only vibration frequencies generated are running speed and bearing rotational fre-