258 Auto Suspension and Steering
Wheel Bearing Design
Wheel bearings must be carefully selected to deal
with types of loads, maximum bearing speed, where the
bearing will be used on the vehicle, and what the vehicle
is used for. Overall bearing size, as well as the size of the
rolling elements, must be determined to give the longest
service without unnecessary weight and size.
Another factor that must be calculated carefully is
bearing preload. Preload is the amount of pressure placed
on the bearing before it is put into service. In other words,
preload is how tightly the rolling elements and races fit
together before other loads are placed on the bearing. Too
little preload will cause the rolling elements to rock and
vibrate, damaging the bearing. Insufficient preload also
intensifies the effect of shock loads. Too much preload will
press the rolling elements too tightly against the races, cre-
ating unnecessary friction and heat.
Bearing Loads
Any load on the bearing tends to push the bearing
parts together. The resulting pressure increases friction and
heat. There are two types of loads placed on wheel
bearings: radial loads and axial loads. Radial loads are
caused by the weight of the vehicle passing through the
spindle or axle, through the bearing and hub, and to the
rim and tire. As the vehicle moves, bearing rotation causes
centrifugal force, which tends to make the bearing rollers
move outward. This combination of vehicle weight and
centrifugal force produces a radial load. Radial loading
occurs at a right angle to the bearing and shaft. This is
shown in Figure 12-19.
One revolution of propeller shaft
30°
1200
1100
1000
900
800
0 90 180 270 360 Deceleration Deceleration Acceleration Acceleration
Conventional universal joint
showing fluctuations
Constant velocity universal
joint, no fluctuations
Speeds of
driven shaft
for driving
shaft speed of
1000 RPM
Figure 12-17. Speed fluctuation chart. As a universal joint turns, the angle formed by the cross changes. It never divides the angle
equally between the two shafts, (except for one brief moment as the cross angle shifts from one extreme to the other). This causes
an acceleration-deceleration fluctuation that transmits torque in a jerky fashion. The dotted line illustrates the even speed that is
achieved when a constant velocity (later) universal joint is used. The undulating (waving) line represents the speed fluctuation pres-
ent when a conventional universal joint is used. (General Motors)
Roller
Cage
Inner
race
Grease
cap
Nut lock
Grease Hub
cavity
Inner
race
Seal
Spindle
Hub
Cotter
pin
Outer race
Outer race
Figure 12-18. Cutaway of a roller bearing and front hub used on
a two-wheel drive truck. (Dodge)
Ball bearing
A
B
C
D
E
F
G
H
I
J
K
L
LOAD
Figure 12-19. A radial-loaded ball bearing. Ball “A” is carrying
the greatest load, while balls E, F, G, H, and I are carrying the
smallest load. (Federal-Mogul)