Friction and bearings 69 Friction 70 Types of friction 70 Making use of friction 71 Bearings 71 Plain bearings 73 Antifriction bearings 74 Special types of bearings 76 Removing and installing bearings 77 Cleaning and checking bearings 78 Bearing adjustments 79 Bearing failures 80 Antifriction bearing defects 81 Technical terms 83 Review questions 83
Trang 1Friction and bearings
Chapter 5
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
Trang 2Friction 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
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.
figure 5.1 Surfaces in contact, highly magnified, are not
flat, but have many irregularities which cause friction
Trang 3Rolling 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.
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
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.
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
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
Trang 4General 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.
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.
figure 5.3 Locations with different coefficients of friction
(a) Brake disc and pads (b) lubricated plain bearing (c) roller bearing
disc pad lubricant plain bearing shaft roller
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
Trang 53 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.
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.
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
sus-pension have to move in relation to the body or the
wheel hub.
■ Refer to the chapters on suspension and steering in
Part four.
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.
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 connecting-rod 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.
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
figure 5.7 Rear end of a crankshaft showing a main
bearing, oil seal and needle roller bearing
Trang 6Antifriction 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.
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 Ball and roller bearings
(a) point contact of a ball bearing (b) line
Trang 7Types of ball bearings
Following are descriptions of the four types of ball
bearings shown in Figure 5.10.
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.
Roller bearings
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 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
Trang 8between 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.
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.
figure 5.12 Needle rollers in the bore of a gear – the
rollers run directly on the shaft and in the gear
figure 5.13 Tapered roller bearing
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-to-back, 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
Trang 9housing is removed, and so it is prelubricated and
sealed during manufacture The lubricant in the
bear-ing 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
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.
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.
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
figure 5.15 The clutch release bearing is a special thrust
bearing
figure 5.16 Use of a puller with a forcing screw – this
special tool can be adjusted to fit behind the bearing cone
Trang 10adequately 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.
figure 5.18 A drift being used to replace a bearing on a
shaft
figure 5.17 Using a press
(a) removing a shaft from a bearing (b)
press-ing a bearpress-ing on to a shaft with a tubular presspress-ing tool
press ram
tubular pressing tool shaft
bearing