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Automotive mechanics (volume i)(part 1, chapter5) friction and bearings

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Chapter 5

Friction and bearings

Friction
Types of friction
Making use of friction
Bearings
Plain bearings
Antifriction bearings
Special types of bearings

Removing and installing bearings
Cleaning and checking bearings
Bearing adjustments
Bearing failures
Antifriction bearing defects
Technical terms
Review questions


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part one introduction to motor vehicles

Friction is a disadvantage in many places and there are
various ways in which it can be reduced. Friction
cannot be eliminated, but it can be greatly reduced by
using bearings and by lubrication. However, friction is
not always a disadvantage because there are places,
such as brakes and clutches, where friction is needed
and where means are used to increase its effects.
This chapter introduces friction and looks at various
types of bearings, where they are used, and how they
are serviced.

Friction
Friction is the force which opposes movement of one
surface over another. It is always present, even
between stationary surfaces, but only becomes
noticeable when one surface is moved over another.
The type of surface has an effect on friction, and rough
surfaces will produce more friction than smooth
surfaces.

While a surface might be classed as being smooth,
it will actually have many small irregularities
(Figure 5.1). If an effort is made to slide one surface
on another, these small hills and hollows will tend
to interlock and oppose movement. Rough surfaces
will obviously drag, and they will have a greater
resistance to movement, or more friction, than
smooth ones.
The machined surfaces of parts are referred to as
having a surface-finish. Machined surfaces are finished
to different degrees of smoothness, depending on the
purpose for which the part is used. The journal of a
crankshaft, which operates in a bearing, is ground to
a fine surface-finish. This reduces friction and wear
between the shaft and its bearing as much as possible.
By comparison, a part that has been machined mainly
for appearance would have a comparatively rough
surface-finish.

Type of material affects friction
While some materials are rough and so produce
friction because of this, there are certain materials
which possess greater frictional properties than
others.
In the operating parts of a motor vehicle, friction is
reduced where it is not wanted, as in bearings and
gears, and is increased where it is needed, as in brakes
and clutches. Different types of materials are used for
these different applications, and parts are either
operated dry to increase friction or lubricated to reduce
friction.

Types of friction
While friction is purely an opposing force, it can be
broken down into five different types: static, limiting,
sliding, rolling and fluid. The types that apply mostly
to motor vehicle parts are sliding, rolling and fluid
friction.
Static friction
Static friction is the friction that holds things stationary
(static). When any article is resting on a level surface,
it will remain there because of static friction. This must
be so, otherwise nothing would ever stay where it was
placed.
Limiting friction
Limiting friction is the friction between two surfaces
when one is about to slide over the other.
If a force is gradually increased to try to slide one
surface on another, then friction also increases and
prevents (limits) movement. However, a point is
reached when the friction can no longer prevent the
surface from sliding. The friction at this point is known
as limiting friction.
Sliding friction

figure 5.1

Surfaces in contact, highly magnified, are not
flat, but have many irregularities which
cause friction

Sliding friction is the resistance to movement that
occurs when one surface is sliding on another. This is a
little less than limiting friction because less force is
needed to keep sliding than to start it. (Try this by
pushing something heavy along the floor or across the
top of a table.)
Sliding friction occurs when a shaft rotates in a
plain bearing, or wherever one part slides in relation to
another.


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chapter five friction and bearings

Rolling friction
Rolling friction is where surfaces are separated by balls
or rollers. Then they do not slide, but roll on each
other. The friction that occurs in this case is called
rolling friction and is less than sliding friction. This
applies to ball and roller bearings, which are used in
various parts of motor vehicles.
■ Ball and roller bearings are used to reduce friction,
and for this reason are often referred to as
antifriction bearings.
Fluid friction
Fluids also have friction, but this is less than the other
types of friction previously mentioned. If two sliding
surfaces are separated by a film of oil, the friction will
be greatly reduced but some friction will still exist.
The friction will not be caused by the surfaces being in
contact, but from the oil between them.
Fluid friction is illustrated in Figure 5.2, which
shows layers of oil between the two surfaces. The
friction within the fluid is caused by one layer of oil
molecules being dragged over another. The oil tends to
adhere to the surfaces, so the layers of oil move at
different speeds, with a still layer of oil closest to the
stationary surface.
■ A fluid can be a liquid or a gas, but liquids have
much greater friction than gases.

figure 5.2

Fluid friction is the friction between layers of
liquid moving at different speeds or, as in the
diagram, between layers a, b, c, d and e

71

the brakes are applied. The heat is transferred into the
drum or disc and other brake parts and then dissipated
into the surrounding atmosphere. Brake lining and pad
material is often referred to as friction material
because of its high-friction properties.
Clutches also depend on friction for their operation.
The clutch plate is faced with friction material that is
held between the cast iron surfaces of the clutch
pressure plate and the flywheel.
Tyres depend on the friction between the rubber
tread and the road surface. The term traction is used to
denote the friction effect when the wheels are driving,
and adhesion is the friction effect when cornering.
Coefficient of friction
The coefficient of friction is a way of measuring the
friction of two materials that are in contact. Different
pairs of materials will have different coefficients – the
higher the number, the greater the friction effect
between the materials.
With brakes, the two materials to be considered are
the composition friction material of which the pad or
lining is made, and the cast iron of the disc or drum.
The coefficient of these two materials is around 0.3,
which is quite high.
Figure 5.3 shows three locations where there are
different coefficients of friction. Brakes operate dry,
with a high coefficient of friction, about 0.3. Plain
bearing surfaces and their shafts are lubricated so that
they have a low coefficient of friction, around 0.01.
With rollers between surfaces, the friction could be
further reduced to give an even lower coefficient of
friction of around 0.001.
■ If there is any difficulty understanding coefficient of
friction, just think of it as a way of measuring the
friction between two surfaces. The greater the
friction, the higher the coefficient.

Bearings
Making use of friction
Brakes use composition brake pads or linings, which
are forced against cast iron discs or drums. The
composition material and cast iron operating together
have a high coefficient of friction, which is needed for
brake materials.
Both materials are capable of withstanding the
effects of the heat that is generated by friction when

Bearings are used in many parts of a motor vehicle.
Where loads are light, bearings are very simple and
they are lubricated by simple means. Where loads are
heavy and constant, bearings are much more important
and their lubrication is critical. A drilled hole, which
carries a shaft, is a simple bearing which is quite
suitable for some purposes. For other applications,
antifriction bearings or special bearing materials with
pressure lubrication are required.


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disc

pad

(a)

figure 5.3

lubricant

plain bearing

shaft

roller

(b)

(c)

Locations with different coefficients of friction
(a) Brake disc and pads (b) lubricated plain bearing (c) roller bearing

General types of bearings
Bearings can be divided into two general types: plain
bearings, where the shaft runs directly on the bearing
surface, and antifriction bearings, which have balls or
rollers as part of the bearing. All bearings have some
friction, although friction is reduced by the balls and
rollers in ball-type and roller-type bearings, and by
special antifriction metals in many plain bearings.
Application of friction to bearings
An appreciation of friction and how it applies in actual
bearings is helpful when problems arise and faults are
being diagnosed.
Plain bearings are subjected to sliding friction,
which is reduced by lubrication. When the shaft is
stationary in the bearings, or under heavy loads, the oil
is squeezed out, and wear is caused by friction.
Ball and roller bearings have much less friction
than plain bearings. However, when a ball or roller
moves across a surface, it tends to form a groove. This
offers rolling resistance. With a ball or roller bearing,
rolling resistance tends to occur with both the inner
and outer race surfaces and, though it may be minute, a
cold-flow of the metal surface results. This causes the
races to become deformed under load and produce
friction (Figure 5.4).
Normally, deformation is very small, but if the
bearing is excessively loaded, it can cause damage to
the hardened surfaces and pitting will result, with
subsequent bearing failure.
Bearing loading
There are three types of loads that bearings may have
to carry. These are shown in Figure 5.5.

figure 5.4

Deformation caused by a ball or roller in a
bearing (exaggerated) produces friction

figure 5.5

Three types of loads can be applied to
bearings

1. Radial load. The load is applied at right angles to
the shaft and the bearing carries the load along its
radius.
2. Thrust load. In this case, the load is applied
lengthwise to the shaft, and the bearing accepts this
as a side thrust.


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chapter five friction and bearings

3. Combination load. This is a combined radial and
thrust load, and certain bearings are designed for
the purpose. Many bearings designed for radial
loads will also accept light thrust loads. This
applies to most ball bearings.

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Split-sleeve bearings
Split sleeve bearings are plain bearings that are made
in two halves (Figure 5.6). They are referred to as
split-sleeve bearings, bearing shells or bearing
inserts.

Plain bearings
A simple plain-type bearing can be a hole drilled in a
casting or other part. This type of bearing is usually
confined to pins or shafts that have limited movement,
or that rotate at slow speed. A smear of oil or grease
might be all the lubrication that is needed for a simple
bearing.
Plain bearings are also used in locations where a
shaft is rotating at high speeds. In such cases, the
bearing and its shaft are provided with pressure
lubrication – engine camshaft bearings are an
example.
In some engines, the camshaft is mounted in plain
bearings which are formed by accurately boring holes
in the cylinder head. The steel camshaft journals then
run directly on the aluminium alloy of which the
cylinder head is made. The aluminium alloy provides a
suitable bearing surface for this purpose.
■ Refer to the section ‘Lubrication of engine
bearings’ in Chapter 11: Engine-lubricating
systems for information on how engine bearings
are lubricated.

figure 5.6

Plain bearings can be designed to carry both
radial and thrust loads – thrust washers are
a form of bearing that take thrusts

Normally, split-sleeve bearings will accept radial
loads only, but when made with flanges, they will
accept both radial loads and thrust loads. A thrust
washer is also shown – thrust washers are a form of
bearing which take thrust loads only. They are
sometimes split so that they can be installed over a
shaft.
Figure 5.7 shows part of an engine crankshaft and
one of its bearings. This bearing will accept radial
loads and also thrust loads. Crankshaft and connectingrod bearings are of split-sleeve design. Because of the
shape of the crankshaft, the bearings must be made in
two pieces to enable them to be installed on the
crankshaft journals.

Sleeve bearings and bushes
Sleeve bearings are plain bearings, in the form of a
sleeve, which are pressed into holes bored in castings
or other parts. Some sleeve bearings consist of a steel
tubular backing with an antifriction metal lining.
Camshaft bearings for overhead-valve engines are
sleeve-type bearings, which fit into bores machined in
the cylinder block.
For other applications, such as pins and smaller
shafts, bronze or nylon bushes are used. Bushes are
sleeves which are used as bearings.
Rubber bushes are used in suspension parts and
these are a form of bearing. Some have inner and
outer metal sleeves with rubber between, others are
plain rubber. They are used where parts of the suspension have to move in relation to the body or the
wheel hub.
■ Refer to the chapters on suspension and steering in
Part four.

figure 5.7

Rear end of a crankshaft showing a main
bearing, oil seal and needle roller bearing


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Antifriction metal
Steel-backed bearings are lined with softer metal,
sometimes referred to as antifriction metal. The steel
backing provides rigidity and the lining provides a
good bearing surface. The antifriction metal is an alloy
(mixture) which may contain lead, tin, copper or
aluminium. Sometimes these alloys are referred to as
white metal because of their colour. Engine bearings
can have three thin layers of bearing metal on a steel
backing.
■ Bearings with antifriction metals must be well
lubricated, which is why engine bearings receive a
constant supply of oil from the lubricating system.

Antifriction bearings
Antifriction bearings include ball and roller bearings of
various designs. As previously indicated, friction is
greatly reduced because of the rolling action of the
balls or rollers. Steel balls have point (or spot) contact
with the surfaces of the bearing, and rollers have line
contact (Figure 5.8). Because of this, roller bearings
have a little more friction than ball bearings. However,
size for size, roller bearings are able to carry a greater
load than ball bearings.

Ball and roller bearings
(a) point contact of a ball bearing (b) line
contact of a roller bearing

Three basic designs of antifriction bearings are
shown in Figure 5.9. These are ball bearings, roller
bearings and tapered roller bearings, although there are
a number of variations to these basic designs.
Ball bearings
The ball bearing in Figure 5.9 is a single-row ball
bearing, which can also be referred to as a ballrace or
annular bearing. It consists of an inner and an outer
race with grooves or tracks in which the balls roll.
The balls are held in place by a cage or retainer,
which spaces them evenly around the bearing. This
type of bearing cannot be dismantled and is not
adjustable. It will carry radial loads and light thrust
loads.
This is one of the most commonly used types of
antifriction bearings.

figure 5.8

figure 5.9

Three common types of antifriction bearings


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Types of ball bearings

Roller bearings

Following are descriptions of the four types of ball
bearings shown in Figure 5.10.

There are three basic types of roller bearings: straight
roller bearings, needle roller bearings and tapered
roller bearings, although there are a number of
variations. Straight and needle rollers of different
designs are illustrated in Figure 5.11.

1. Single-row ball bearing. This is similar to the
ballrace previously described, but it has a circlip in
its outer race. The circlip acts as a retainer to locate
the bearing in its housing.
Some ball bearings are designed with deep
grooves. A deep groove on one side of the outer
race and on the opposite side of the inner race
enables the bearing to accept thrust loads in one
direction.
2. Double-row ball bearing. This has two rows of
balls to enable it to carry heavy radial loads. It will
also accept light thrust loads in either direction.
3. Thrust bearing. A ball bearing of this type will
accept heavy thrust loads, but cannot accept radial
loads.
4. Cup-and-cone bearing. This has an inner cone and
an outer cup, with steel balls in a cage between
them. The three parts of the bearing are separate.
These bearings must be used in pairs and they have
to be adjusted when they are installed. A pair of
bearings will accept both radial and thrust loads.
Tapered roller bearings can carry a greater load and
so are generally used instead of cup-and-cone ball
bearings.

figure 5.10

Types of ball bearings

1. Straight roller bearing. This design is known as a
straight roller bearing, cylindrical roller bearing,
or plain roller bearing. It has parallel rollers
which run in grooves in the inner and outer
races. The surfaces on which they roll are referred
to as raceways. The rollers are held in place by a
cage.
Straight roller bearings are used in similar
locations to ball bearings. They are used to carry
heavy radial loads, although some designs, with
suitable flanges, will carry light thrust loads in one
direction. While the parts of most bearings cannot
be dismantled, some bearings are made without
flanges so that the parts can be separated.
2. Roller assembly. This consists of a number of
straight rollers held in a cage. A roller assembly has
no inner or outer race of its own, but is fitted

figure 5.11

Types of roller bearings


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between a hardened shaft and the bore of a gear or
similar part. Roller assemblies will accept radial
loads only.
3. Caged needle rollers. Small roller bearings are
referred to as needle rollers, or needle bearings
because of their size. They can be used loose, or
held in some form of cage as shown in the
illustration. Like roller assemblies, they have no
races and are used between a hardened shaft and a
hardened bore.
4. Needle thrust bearing. A needle thrust bearing
has its needles mounted radially in a washer-type
retainer. It is, in fact, used as a thrust washer.
It can be installed on a shaft between two
hardened surfaces to take the thrust load between
parts, or used with a hardened steel washer on
each side.
5. Loose needle rollers. Needle rollers can be used
without a retainer as shown in Figure 5.12, where a
number of rollers have been installed in the bore of
a gear to provide a bearing. The rollers operate
directly on a hardened shaft. Thrust washers are
fitted at each end of the gear to retain the needles in
place.

figure 5.12

figure 5.13

Tapered roller bearing

Special types of bearings
There are some locations where special bearings would
be more suitable than standard bearings. Bearings can
be provided with seals, or shields, or made so that they
are self-aligning.
Bearings with shields and seals
For special applications, bearings are made with
shields or seals. These arrangements are shown in
Figure 5.14.
Shields can be on one or both sides of a bearing.
They are used to keep out dirt and to restrict the flow
of lubricant through the bearing.
Seals are used for bearings which are prepacked
with lubricant during manufacture. Bearings of this
type are usually used in locations where the bearing is
not readily accessible, and the seal is needed to retain
the lubricant for the life of the bearing.

Needle rollers in the bore of a gear – the
rollers run directly on the shaft and in the
gear

6. Tapered roller bearings. Tapered roller bearings
can be separated into two parts (Figure 5.13). The
inner race, complete with the rollers and retainer, is
known as the cone, and the outer race is called the
cup. The cup and cone are held together when the
bearing is installed and adjusted.
Some tapered roller bearings are designed to be
used on their own, but standard tapered roller
bearings are used in pairs. When mounted back-toback, they can carry heavy radial loads as well as
thrust loads in both directions.

figure 5.14

Sections through a ballrace show the seals
and shields

Clutch release bearing
The clutch release bearing, which is located inside
the clutch housing, is a special thrust bearing
(Figure 5.15). It is accessible only when the clutch


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bearings are a tight fit, force is needed, but this must
be correctly applied. The rule to observe is that force
should never be applied through the balls or rollers of
the bearing. This will render the bearing unserviceable,
probably by forcing it apart.
■ Force must always be applied to the inner part of
a bearing when removing it from a shaft, or to the
outer part of a bearing when removing it from a
housing.

figure 5.15

The clutch release bearing is a special thrust
bearing

Many manufacturers recommend special tools for
removing and replacing bearings, and these should be
used when available. Figure 5.16 shows a special
puller being used to remove a taper roller bearing from
a differential carrier. The puller is designed to fit under
the bearing cone, not under the retainer and rollers.

housing is removed, and so it is prelubricated and
sealed during manufacture. The lubricant in the bearing would normally last until the other parts of the
clutch need servicing.
Self-aligning bearings
Shafts and bearings must be in correct alignment,
otherwise the bearings will be overloaded and will
suffer premature failure. However, for special purposes
where alignment is difficult, self-aligning ball bearings
are used. These have a wide groove in the outer race
which allows the inner race and balls to tilt, so that the
bearing aligns itself to suit the alignment of the shaft.
Other bearings
There are many other types of bearings used in various
ways.
Steel pins may have bushes of bronze, rubber or
nylon. Other bushes may be of steel with steel pins.
Generally, a soft and a hard metal are used together for
a plain bearing and shaft, though there are many
examples of hardened steel parts working together, but
these are well lubricated.
Sintered bronze bushes are used in smaller
components such as starters. In the manufacture of
these, powdered metal is fused together to form a very
porous material which will retain oil to provide good
lubrication.

Removing and installing bearings
During dismantling and repair of components, bearings
have to be removed from shafts and housings. Where

figure 5.16

Use of a puller with a forcing screw – this
special tool can be adjusted to fit behind the

bearing cone

Universal puller kits are also available. These
contain many fittings and adaptors to suit a variety of
jobs. In many instances, an arbor press or a hydraulic
press is used for removing and replacing bearings.
The following is general information on bearing
removal and replacement.
Using a press
Figure 5.17 shows how a press is used to remove and
replace a bearing.
To remove a bearing, the inner race is placed on
metal press plates or on a pressing tool. This must


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press ram

tubular
pressing
tool
shaft

bearing

(a)

(b)

Using a press
(a) removing a shaft from a bearing (b) pressing a bearing on to a shaft with a tubular pressing tool

figure 5.18

A drift being used to replace a bearing on a
shaft

figure 5.17

adequately support the inner race on the bed of the
press. The shaft is then pressed out of the bearing as
shown in Figure 5.17(a). The outer race should not
carry any load during this operation.
The bearing can be replaced by using a piece of tube
which fits over the shaft and against the inner race as
shown in Figure 5.17(b). The tube can also be used with
a hammer to carefully drive the bearing on to the shaft.
Using a hammer and drift
A soft steel drift (or punch) and hammer are often used
to tap a bearing from a housing. A drift can also be
used to remove a bearing from a shaft or to replace a
bearing on a shaft (Figure 5.18).
The end of the drift should be shaped to fit against
the shaft and flat against the bearing race as shown.
Drifting should be carried out alternately on opposite
sides of the bearing to keep it straight, and care should
be taken to prevent damage to the cage.
A mild steel drift should be used – a hard steel
punch is not suitable. A brass drift is unsuitable for
bearings because chips of brass tend to break off and
these could become lodged in the bearing.
Mounting compound
A bearing-mounting compound can be applied to a
bearing when it is being installed. This is used to

prevent unwanted movement between a bearing and its
shaft, or between a bearing and its housing. The
compound retains the bearing but still enables it to be
removed with normal bearing-removal tools. The
compound can also be used for bushes, sleeves and oil
seals.

Cleaning and checking bearings
Any component that is to be dismantled should be
cleaned externally before dismantling is commenced.
This will prevent dirt and grit from contaminating the
internal parts, such as bearings. Bearings contaminated
with dirt will be difficult to clean.
The following are points that should be observed
when cleaning bearings:
1. Wash bearings in clean solvent and then dry them
with air. Bearings with hard grease may require
soaking.
2. Lubricate bearings immediately after cleaning.
3. Turn the bearing slowly by hand and check for any
roughness or unevenness (Figure 5.19). With thrust
bearings, apply pressure in the thrust direction.
4. Examine the balls or rollers and the bearing
surfaces for defects. Look closely while rotating the
bearing so that all the surfaces are examined.
5. Hold ball bearings stationary while drying with
compressed air. Do not spin the bearing – this is
dangerous and detrimental to the bearing.


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chapter five friction and bearings

figure 5.20

figure 5.19

Checking the condition of a bearing

79

Packing a bearing with grease
(a) roller bearing (b) forcing grease into the
bearing

MAZDA

Bearing adjustments
6. Sealed bearings should not be washed. Wipe the
outside clean and check for roughness as previously
described.
7. If a bearing is to be used again, but not straight
away, it should be lubricated and then wrapped in
oiled paper. This will keep out dirt and prevent
rust.
Bearing lubrication
Bearings such as those in transmissions are either
completely immersed or partly immersed in oil, and so
receive ample lubrication. Where a bearing has oil to
the level of some of its balls or rollers, they will carry
oil to the parts of the bearing above the oil level and so
lubricate all parts of the bearing.
Engine bearings are supplied with oil by the
lubricating system, which pumps oil to all the engine
bearings and also to other parts that require
lubrication.
Some wheel bearings must be packed with grease
before they are installed. This must be done correctly
because the bearings depend on this grease to provide
them with lubrication. In the case of wheel bearings,
this must last for many thousands of kilometres.
Packing bearings with grease
A bearing is packed by forcing grease in between the
rollers or balls and the races, so that the space between
the races is completely filled with grease.
This can be achieved by pushing grease into one
side of the bearing with the fingers until it begins to
come out the other side, or by forcing grease into the
bearing from the palm of the hand as shown in
Figure 5.20. The aim, when packing the bearing, is to
fill the bearing with grease and not merely coat its
outside.

Most tapered roller bearings have to be adjusted during
installation. There are two types of adjustments: screw
adjustments and shim adjustments.
Screw adjustments
The front-wheel bearings of a rear-wheel-drive vehicle
are an example of bearings with a screw adjustment
(Figure 5.21). The bearings are mounted back-to-back,
with the cups in the wheel hub, and the cones on the
axle spindle.
The spindle has a slotted nut for bearing adjustment. When adjusting, the nut is first tightened until all
clearance in the bearing is removed. The nut is then
backed off approximately one-sixth of a turn to
provide the rollers with a small running clearance.
■ As the nut is being tightened, the wheel hub should
be rotated so that the bearings seat correctly.
Shim adjustments
Adjusting shims or spacers can be fitted behind the cup
or cone of a roller bearing. The shims can be selected
for thickness so that the required clearance within the
bearing is obtained. Adding shims will reduce
clearance and provide a tighter adjustment, while
removing shims will increase clearance and give a
looser adjustment.
Figure 5.22 shows one location where shims are
used. The shims are fitted between the bearing cups
and the housings and used to adjust the differential
case bearings. These are tapered roller bearings which,
in this location, are usually given a preload. This is
done by selecting shims of suitable thickness.
Unitised bearings
Some bearings are referred to as unitised bearings.
They are used for wheel bearings. The outer race, inner


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figure 5.21

A conventional front wheel, hub and bearings for a rear-wheel-drive vehicle

FORD

preload, this could cause one bearing to be loose and
the pinion would have insufficient bearing support.
■ Some wheel bearings are tapered roller bearings,
but they are not preloaded.

Bearing failures
figure 5.22

Differential case bearings with adjusting
shims TOYOTA

race and balls or rollers are designed as a unit and do
not require adjustment.
Shims, selective-fit snap rings, or spring-type
washers are often used with these bearings. They are
not used to adjust the actual bearing itself, but are used
to position the bearing on a shaft or in a housing, or to
remove end-float from a shaft. In some instances, the
outer race has a groove to take a snap ring and this is
used to retain the bearing in its housing.
Bearing preload
Preload is an initial load that is placed on the bearings
by adjustment, to ensure that the balls or rollers are
fully in contact with their bearing surfaces when
operating.
Preloading of bearings is only done where the
bearings are taking a thrust as well as a radial load.
The pinion bearings in a rear-axle assembly, for
example, are preloaded. These are tapered roller
bearings and the thrust from the actual working load of
the gears tends to force one bearing hard into its cup
and the other away from its cup. Without the initial

Ball and roller bearings have a long life provided they
are not overloaded, maladjusted, badly fitted or
allowed to run short of lubricant.
When a component is being dismantled, a tight
bearing should not be removed from its shaft unless it
has to be renewed, or removal is an essential part of
the dismantling procedure. Bearings can be damaged
by needlessly removing them from shafts merely to
clean and replace them.
■ Suitable bearing-removing tools are essential and it
is important that they are used properly to prevent
damage.
Causes of bearing failure
Careful examination of bearings is necessary when
seeking the reasons for a bearing failure. The
following are some of the causes and how these can be
recognised.
Abrasion
Entry of dirt and grit into bearings causes premature
wear by lapping the surfaces, which take on a mat
finish instead of remaining bright.
Lack of lubrication
This results in heat which causes discolouration of the
balls and rollers, and also of the bearing surfaces.


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chapter five friction and bearings

Particles of metal may be picked up from the
separators.
In plain bearings and bushes, scoring and excessive
wear will occur from shortage of lubrication. Complete
bearing failure will result if engine bearings are
operated without oil.
Pins and bushes with limited movement between
bearing surfaces will seize if not correctly lubricated.
Corrosion
Water or moisture will cause corrosion and this shows
as pit marks or rust areas.
Careless handling or incorrect storing of ball and
roller bearings after cleaning can cause surface
corrosion. Bearings should be lubricated and wrapped
in oiled paper, even for short storage periods.
Faulty fitting
Ball and roller bearings often have an interference fit
between the bearing and shaft. This must not be
excessive, otherwise the inner race is forced to expand
and binding of the bearing will occur, with subsequent
failure.
Any burrs or damage to the shaft will cause
distortion of a section of the race. The distorted area
will be overloaded and failure will commence there.
The shaft should be clean and smooth to allow the
bearing to fit correctly.
These precautions also apply to the fit of the
bearing outer race in its housing. Both shaft and
housing should be examined prior to bearing
installation.
Faulty adjustments
There are three types of adjustment: too loose, too tight
and correct. Reference should be made to the
manufacturer’s specifications, as some bearings are
preloaded and others are not.
A general rule is that bearings are adjusted to have
neither looseness nor tightness – either condition can
cause broken and chipped balls or rollers, and scored
bearing surfaces.

81

Antifriction bearing defects
There are a number of defects that can occur in
antifriction bearings. When inspecting bearings, look
for signs that will enable a defect to be identified. The
top illustrations in Figure 5.23 show the parts of roller
and ball bearings that should be inspected.
Following are the terms used to describe bearing
defects. The lower part of the figure illustrates some of
these.
1. Galling. Wear of the bearing surfaces with some
small pitting. Caused by poor lubricant or lack of
lubrication.
2. Spalling. Badly pitted surfaces on both the inner
and outer raceways and possibly on the balls or
rollers. This is due to the metal being overstressed
and is known as metal fatigue. It could be caused
by a loose adjustment, which allows impacts or
shock loading of the bearing surfaces. Overloading
of the bearing is another likely cause.
3. Corrosion. Etch marks on the surfaces, or any part
of the bearing. Caused by the presence of water or
moisture.
4. Pitting. Pit marks in the bearing surfaces. This
could be an advanced state of corrosion caused by
water or moisture.
5. Discolouration. The surfaces of the bearing are
coloured from heat. This is most likely due to lack
of lubrication. Continued operation would cause
galling and spalling. Discolouration could also be
due to the bearing being adjusted too tightly. This
would produce overheating.
6. Fretting. Rub marks on the outside of the outer race
and the bore of the inner race. Marks on the outer
race are caused by movement of the bearing in its
housing (too loose). Rub marks on the inner race
are caused by the bearing being too loose on its
shaft.
7. Cracked races. An inner race could crack if it was
too tight on its shaft, and an outer race could crack
if it was too tight in its housing. The tight condition
of the bearing would also cause overheating.
8. Brinelling. This is a series of indentations or marks
on the raceways and is caused by shock loading or
poor installation.


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part one introduction to motor vehicles

figure 5.23

Types of bearing defects


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chapter five friction and bearings

Technical terms
Friction, static friction, sliding friction, rolling
friction, fluid friction, surface-finish, traction,
adhesion, coefficient of friction, radial, thrust, plain
bearing, antifriction metal, thrust washer, alloy,
antifriction bearing, point contact, line contact,
radial load, thrust load, flange, deformation, selfaligning, sintered, bronze, solvent, packing bearings,
preload, abrasion, corrosion, galling, spalling,
fretting, brinelling.

8.

What is a plain bearing?

9.

What are antifriction bearings?

10.

What is a thrust bearing?

11.

What are the advantages of double-row ball
bearings?

12.

Name four types of roller bearings.

13.

What are needle roller bearings?

14.

Why are sealed bearings used?

15.

When would self-aligning bearings be used?

16.

Consider how a bearing should be supported
when pressing it from a shaft.

17.

Explain how a ball bearing would be cleaned
and checked.

18.

Where are tapered roller bearings likely to be
found?

19.

What is meant by bearing preload?

20.

Describe briefly how the front-wheel bearings of
a rear-wheel-drive vehicle would be adjusted.

21.

List some of the terms associated with bearing
defects.

Review questions
1.

What is friction?

2.

What is meant by fluid friction?

3.

Where is friction an advantage?

4.

Name some of the locations where friction is
needed.

5.

What is meant by coefficient of friction?

6.

How can friction be reduced?

7.

Name the three types of loads that bearings can
carry.

83


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