Chapter 12 Driveline and Wheel Components 261
Some bearings are packed with grease when
manufactured and are permanently sealed. The wheel
bearings on front-wheel drive vehicles are usually made in
this way. Some solid rear axle bearings are also greased for
life. If the seals fail, the bearing must be replaced.
A common lubrication method used on solid rear
axles of rear-wheel drive vehicles is to splash the bearing
with gear oil as the bearing rotates. Oil in the rear axle
housing is thrown upward by the action of the ring gear
and other parts, and is thrown sideways onto the outer
bearings as the vehicle is turned. On some solid rear axles
in large trucks the bearing operates partially or completely
submerged in oil. This lubrication method is uncommon
on light trucks and cars.
Types of Lubricant
The two main classes of wheel bearing lubricants are
wheel bearing greases and gear oils. Most wheel bearings
are lubricated by grease. Some solid rear axle wheel
bearings are lubricated with gear oil.
Greases are semisolid lubricants, thick enough to stay
in place and provide lubrication, but thin enough to flow
between parts. Quality greases resist water entry and
corrosion, help the bearing to absorb shock loads, and do
not melt when hot. The wheel bearings on most modern
vehicles call for lithium grease, often called EP lithium
grease (for extreme pressure) or lithium soap. Lithium is a
metallic element that provides additional lubrication and
corrosion protection. Some older vehicles used what is
known as long-fiber grease. This grease contained addi-
tives that kept it from flying out of the bearings at high
speeds and temperatures. In most cases, it is acceptable to
use lithium grease in older vehicles. Lithium grease should
always be used when specified by the manufacturer.
Wheel bearings used in solid rear axles are sometimes
lubricated with the gear oil used to lubricate other rear axle
parts. This oil is commonly known as hypoid oil. Gear oil
is similar in viscosity (weight) to motor oil, but it contains
high-pressure additives that resist being squeezed from
between the gears of the ring and pinion. Gear oils are
classified according to weight (which does not correspond
to motor oil weight) and by their GL number. The GL (Gear
Load) number is set by the American Petroleum Institute
(API), with GL 1 being the lowest grade intended for light
duty applications. Most cars and light trucks require a
GL 4 oil as a minimum, with GL 5 and 6 for more severe
conditions.
Bearing Seals
Bearing seals have two purposes: to prevent lubricant
from leaking out and to prevent dirt and water from enter-
ing. Most oil seals are constructed as shown in Figure
12-28. The flexible rubber part of the seal is called the lip.
The lip contacts the rotating shaft to seal it. The lip is
crimped or bonded to a steel outer shell. A garter spring is
installed behind the seal lip to hold it tightly to the shaft. A
small amount of lubricant enters the lip area and forms an
oil film between the lip and shaft. This oil film reduces
seal and shaft wear, and provides additional sealing. If
the seal is in good condition, the oil film will not leak
past the seal lip.
The portion of the oil seal behind the lip is designed
so that any pressure behind the seal tends to press the lip
tightly onto the shaft. This helps prevent leaks when pres-
sures are high, such as when the grease in a bearing
assembly becomes hot and expands. When pressure is low
(and leaks are unlikely), the lower tension on the lip
reduces seal and shaft wear.
Most oil seals are pressed into the hub or axle as
shown in Figure 12-29. Some seals have a felt wiper that
absorbs small amounts of lubricant that get past the lip.
Seals used on front-wheel drive vehicles often have a dust
shield. The dust shield protects the seal against dirt
and water.
Hubs and Axle Flanges
The wheel hub is the mounting surface for the wheel
rim and tire. It connects the rotating wheel assembly to the
Nut lock
Cotter pin
“Pull” through
and “wrap” tight
Figure 12-26. The hub nut is secured in its correct position with
a nut lock and a new cotter pin. (Chrysler)
Locknut
Groove
More than
4 mm
(0.16′′)
Figure 12-27. This wheel bearing locknut is staked into position
after proper bearing adjustment. The nut lip is staked (forced)
into the spindle or drive axle groove with a pointed tool. No
cotter pin is needed. (Mazda)