let the equipment dry before flushing with fresh
perpendicular to the shaft. Thrust loads are the
water. For complete information on approved
result of forces applied to the bearing parallel to
solvents and washing and drying procedures, refer
the shaft. Angular loads are the result of a
to NSTM, chapter 300.
combination of radial and thrust loads. The load
carried by the bearings in electric motors and
BEARINGS
generators is almost entirely due to the weight of
the rotating element. For this reason, the method
Bearings are designed to allow a rotating
of mounting the unit is a major factor in the
armature or rotor to turn freely within a motor
selection of the type of bearing installed when the
or generator housing. Shaft bearings must be
unit is constructed. In a vertically mounted unit,
properly maintained to reduce the heat caused by
the thrust bearing is used, while the radial bear-
ing is normally used in most horizontal units.
The two common types of bearings found in
motors and generators are antifriction bearings
WEAR. --Normally, it is not necessary to
and friction bearings.
measure the air gap on machines with ball bear-
ings, because the construction of the machines en-
Antifriction Bearings
sures proper bearing alignment. Additionally, ball
bearing wear of sufficient magnitude as to be
There are two types of antifriction bearings--
readily detected by air-gap measurements would
ball and roller. Basically, both types consist of
be more than enough to cause unsatisfactory
two hardened steel rings, hardened steel rollers
operation of the machine.
or balls, and separators. The annular, ring-shaped
The easiest way of determining the extent of
ball bearing is the type of roller bearing used most
wear in these bearings is to periodically feel the
extensively in the construction of electric motors
bearing housing while the machine is running to
and generators used in the Navy. These bearings
detect any signs of overheating or excessive vibra-
are further divided into three types dependent
tion and to listen to the bearing for the presence
upon the load it is designed to bear-- (1) radial,
of unusual noise.
(2) axial thrust, and (3) angular contact. Examples
Rapid heating of a bearing may be an indica-
of these three bearings are shown in figure 10-5.
tion of danger. Bearing temperature that feels
The ball bearing on a rotating shaft of an
uncomfortable to the touch could be a sign of
electric motor or generator may be subjected to
dangerous overheating, but not necessarily. The
a radial, thrust, and/or angular force. While every
bearing may be operating properly if it has taken
ball bearing is not subjected to all three forces,
an hour or more to reach that temperature, but
any combination of one or more may be found
serious trouble can be expected if a high
depending on the equipment design. Radial loads
temperature is reached within the first 10 or 15
are the result of forces applied to the bearing
minutes of operation.
The test for excessive vibration relies a great
extent on the experience of the person conducting
the test. The person should be thoroughly familiar
with the normal vibration of the machine to be
able to detect, identify, and interpret correctly any
unusual vibrations. Vibration, like heat and
sound, is easily telegraphed. A thorough search
is generally required to locate its source and to
determine its cause.
Ball bearings are inherently more noisy in
normal operation than sleeve bearings. If you are
testing for the presence of abnormal bearing noise,
keep this in mind. A common method for sound
testing is to place one end of a screwdriver against
the bearing housing and the other end 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
Figure 10-5.--Representative types of ball bearings.
10-6
