roller bearing used most extensively in the
by air-gap measurements would be more than enough
construction of electric motors and generators used in
to cause unsatisfactory operation of the machine.
the Navy. These bearings are further divided into three
The easiest way of determining the extent of wear
types dependent upon the load it is designed to
in these bearings is to periodically feel the bearing
bear--(1) radial, (2) angular contact, and (3) thrust.
housing while the machine is running to detect any
Figure 3-14 shows examples of these three bearings.
signs of overheating or excessive vibration, and to
The ball bearings on a rotating shaft of an electric
listen to the bearing for the presence of unusual noises.
motor or generator may be subjected to radial thrust
Rapid heating of a bearing may be an indication
and/or angular forces. While every ball bearing is not
o f danger. Bearing temperatures that feel
subjected to all three forces, any combination of one
uncomfortable to the touch could be a sign of
or more may be found depending on the equipment
dangerous overheating, but not necessarily. The
design. Radial loads are the result of forces applied to
bearing may be operating properly if it has taken an
the bearing perpendicular to the shaft; thrust loads are
hour or more to reach that temperature; whereas,
the result of forces applied to the bearing parallel to
serious trouble can be expected if high temperatures
the shaft; and angular loads are the result of a
are reached within the first 10 or 15 minutes of
combination of radial and thrust loads. The load
carried by the bearings in electric motors and
The test for excessive vibration relies to a great
generators is almost entirely due to the weight of the
extent on the experience of the person conducting the
rotating element. For this reason, the method of
test. The person should be thoroughly familiar with
mounting the unit is a major factor in the selection of
the normal vibration of the machines to be able to
the type of bearing installed when they are
correctly detect, identify, and interpret any unusual
constructed. In a vertically mounted unit, the thrust
vibrations. Vibration, like heat and sound, is easily
bearing is used, while the radial bearing is normally
telegraphed. A thorough search is generally required
used in most horizontal units.
to locate the source and determine its cause.
Wear of Bearings.-- Normally it is not necessary
Ball bearings are inherently more noisy in normal
to measure the air gap on machines with ball bearings
operation than sleeve bearings (discussed later). This
because the construction of the machines ensures
fact must be kept in mind by personnel testing for the
proper bearing alignment. Additionally, ball bearing
presence of abnormal bearing noise. A common
wear of sufficient magnitude as to be readily detected
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 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. 3-15, view A), it indicates worn
Figure 3-15.--Checking motor or generator shaft. (A)
Figure 3-14.--Representative types of ball bearings.
Vertical movement; (B) end-play movement.