Page 358 - Improving Machinery Reliability

324 Improving Machinery Reliability

Conversely, a load connected to the motor by a properly aligned coupling will
impose almost no added load on the motor bearings. Vertical applications, and espe-
cially pump motors, place significant thrust loads on the motor shaft bearings as
well. Medium-horsepower motors for such applications are generally equipped with
deep-grooved ball bearings.
Larger vertically mounted motors (125 HP and above) driving pumps, fans, aera-
tors, mixers, autoclaves, cooling towers, and similar applications may require grease-
lubricated, angular-contact ball bearings, oil-lubricated spherical roller bearings, or
sliding bearings that are oil-lubricated and water-cooled to handle extremely high
thrust loads.
Bearing life (Llo Life) is defined by the Anti-Friction Bearing Manufacturers
Association (AFBMA) as the life, in millions of revolutions or hours of operation
that 90% of a group of bearings will complete at a given load and speed before
fatigue failure occurs.
Motor bearings are normally selected with an Llo design life adequate to deliver
approximately five years (40,000 to 50,000 hours) of continuous operation. Average
motor bearing life is approximately five times design life and median bearing life is
approximately seven times design life. A properly designed and maintained bearing
system will normally outlast the insulation and will not be the determining factor in
the life of the motor.
Whenever possible, motor bearings should be configured to allow re-lubrication.
The housings should include a relief path for the spent lubricant. Preventive mainte-
nance requires regular application of the same type of grease initially applied at the
factory. Polyurea greases, for example, are not compatible with lithium greases and
mixing may lead to loss of lubrication.
Care should also be taken not to introduce contamination when greasing. Contam-
inants create friction between the bearing elements and cause heat build-up. This
heat can melt the grease, allowing it to run out, causing both bearings and motor to
overheat. Motors operating in very hot or very cold environments may require spe-
cial lubricants. Motor manufacturers occasionally select bearings that are lubricated
for life at the time of manufacture. The mountings include contamination barriers
such as shields and slingers to prevent the entry of liquid and solid contaminants, as
well as sealing surfaces to prevent the loss of the factory installed lubricant. Need-
less to say, life-time lubricated bearings may limit both the anticipated and actually
achieved life of the motor.
Some “lubricated-for-life” cartridge bearing mountings are, indeed, furnished with
provisions for adding lubrication when the motor environment is particularly dirty,
wet, or corrosive. The addition of a small amount of lubricant (!4 ounce every three
months) is merely intended to refresh the lubricant seal between the shaft and the
bearing cartridge or the motor end bracket. Keep in mind that the added grease, how-
ever, does not reach the bearing elements since the double seals or shields prevent
the flow of lubricant into the bearing.
Oil mist and other types of oil lubrication systems are used on large-horsepower
motors and the manufacturer’s recommendations for lubricant selection and system
maintenance may have to be followed closely.