Wear of Bearings
Normally, it is not necessary to measure the air gap
on machines with ball bearings because the construction
of the machines ensures proper bearing alignment.
Additionally, ball bearing wear of sufficient magnitude
Figure 7-2.--Checking motor or generator shaft. (A) Vertical
as to be readily detected by air-gap measurements would
movements: (B) End-play movement.
be more than enough to cause unsatisfactory operation
of the machine.
Lubrication
The easiest way of determining the extent of wear
A common cause of motor and generator failure is
in these bearings is to periodically feel the bearing
overlubrication. Forcing too much grease into the
housing while the machine is running to detect any signs
bearing housing seals and onto the stationary windings
of overheating or excessive vibration, and to listen to the
and rotating parts of the machine will cause overheating
bearing for the presence of unusual noises.
and deterioration of insulation, eventually resulting in
electrical grounds and shorts. Overheating will also
Rapid heating of a bearing maybe an indication of
cause rapid deterioration of the grease and the eventual
danger. Bearing temperatures that feel uncomfortable
destruction of a bearing. To avoid overlubrication, add
to the touch could be a sign of dangerous overheating,
new lubricant only when necessary.
but not necessarily. The bearing may be operating
properly if it has taken an hour or more to reach that
The frequency that new grease must be added
temperature; whereas, serious trouble can be expected
depends on the service of the machine and the tightness
if high temperatures are reached within the first 10 or
of the housing seals, and the requirements should be
15 minutes of operation.
determined for each machine by the engineering officer
or PMS requirements. A large quantity of grease
The test for excessive vibration relies to a great
coming through the shaft extension end of the housing
extent on the experience of the person conducting the
usually indicates excessive leakage inside the machine.
test. The person should be thoroughly familiar with the
To prevent greasing by unauthorized personnel,
normal vibration of the machines to be able to correctly
grease cups are removed from motors and generators.
detect, identify, and interpret any unusual vibrations.
Pipe plugs are inserted in place of the grease cups. The
Vibration, like heat and sound, is easily telegraphed. A
pipe plugs are replaced temporarily with grease cups
thorough search is generally required to locate the
during lubrication (fig. 7-3). (Removable grease cups
source and determine its cause.
Ball bearings are inherently more noisy in normal
operation than sleeve bearings (discussed later). This
fact must be kept in mind by personnel testing for the
presence of abnormal bearing noise. A common method
for sound testing is to place the blade of a screwdriver
against the bearing housing and the handle against the
ear. If a loud, irregular grinding, clicking, or scraping
noise is heard, trouble is indicated. As before, the
degree of reliance in the results of this test depends on
the experience of the person conducting the test.
Checking the movement of a motor or generator
shaft can also give an indication of the amount of
bearing wear. If the motor shaft has excessive vertical
movement (fig. 7-2, view A), it indicates worn bearings.
Figure 7-2, view B, shows how to get a rough
approximation of motor or generator end-play
movement. You can correct excessive end-play, as
described in the applicable technical manual, by adding
Figure 7-3.--Grease-lubricated ball bearings.
bearing shims.
7-3
