This slide show accompanies the learner guide "Mechanical Technology Grade 10" by Charles Goodwin, Andre Lategan & Daniel Meyer, published by Future Managers Pty Ltd. For more information visit our website www.futuremanagers.net
There are many aspects to maintenance of
The main purpose of maintenance is to ensure
that machinery always operates at an optimal
Routine maintenance includes inspection,
cleaning, lubricating, adjusting and replacing
Moving parts of machines rubbing against one another
Over time the wear results in these parts working loose
and becoming irregular in shape.
In extreme cases, the whole machine may seize or result
in other breakages.
The effects of excessive wear are costly and can be
dangerous so it is better to perform regular
maintenance tasks to increase the working life of
One of the best ways of reducing friction (which causes
wear) is to use a suitable lubricant.
No matter how smooth metal surfaces may appear, a
microscopic view will reveal very small jagged irregularities.
Oil or grease molecules are able to keep the moving metal
surfaces fractionally apart, drastically reducing the friction-
Not only does oil reduce friction and wear in a machine; it
also moderates temperature as it transfers heat away from
hot areas and reduces corrosion.
A shaft cannot fit perfectly tightly in its bearing for then it would be‒
unable to turn.
There has to be a tiny clearance between the two surfaces.
A film of oil is fed to the bearing at the point where this clearance is
The shaft rotation draws oil round to the point of maximum loading,
where the clearance is smallest, forcing it into an ‘oil wedge’ between
the shaft and bearing and allowing high loads to be taken safely.
Oil wedge effect.
When a shaft starts to rotate in a bearing, oil molecules
adhering to the shaft are carried round with it and more
oil is drawn into the clearance.
As the shaft speed increases, oil is drawn in quickly
and forced into a wedge in the pressure area, lifting the
shaft away from the bearing.
Oil (a liquid) and grease (a semi-solid) are generally
used as lubricants.
Oil in a machine is fed to the bearings under pressure,
and the film of oil keeps the bearing surfaces apart.
This is known as full-film lubrication.
Grease is used for joints which do not keep the bearing
surfaces fully separated this type of lubrication is‒
The steering and suspension of motor vehicles are
examples of grease being used for boundary lubrication.
Oil must be able to cling to a metal surface to provide a
It is the resistance of an oil to flow and is calculated by
the time it takes a set quantity of oil to flow through a
tube of fixed diameter.
The longer the oil takes to flow, the higher the oil’s
viscosity, and the higher the viscosity number which
Simply put, ‘the higher the number, the thicker the oil’.
One of the problems with oil is that its viscosity tends to reduce and it
becomes ‘thinner’ when its temperature is increased (in much the
same way as a lump of margarine melting in a hot frying pan).
When oil becomes ‘too thin’ it looses its ability to reduce friction
Modern engine oils are specially designed as multi-grades to
overcome this problem.
A typical designation is 20W50 SAE.
This means that the oil acts like a SAE 20 oil when it is cold, for easy
cold starting; and like a SAE 50 oil (that is, thicker than a 20 SAE oil
would be) when it is hot.
Motor vehicle gearboxes often use the same kind of oil as engines
(20/50 SAE), but rear-axle oils are usually 90 SAE, with 140 SAE
viscosity oil being used in EP (for ‘extreme pressure’), or Hypoid (for
‘use in hypoid bevel rear axles’).
Oils can also be classified according to their base which is either
mineral oil (refined crude oil) or vegetable oil such as castor oil.
Vegetable oils are used only for special applications such as racing
Most oil firms make synthetic oil.
Synthetic oil is a chemically engineered mineral oil which is
suitable for high-performance and turbo-charged engines.
The viscosity of synthetic oil is usually slightly lower than that of a
conventional mineral oil to achieve similar characteristics when
both oils have reached working temperatures.
Oil manufacturers may also add agents to give oil special
properties, such as corrosion resistance, friction modification and
longevity (for extended use or harsh conditions).
There is a wide variety of grease products on the market.
Grease is highly viscous and very sticky.
It may appear white, brown, black, grey or even red in colour.
The choice of grease will depend on the conditions under which it will
The type of load, temperature and exposure to water which will be
applied to a machine component generally determine the type of
grease to be used.
The grease manufacturer’s recommendation charts will specify the
grease type suitable for the required task.
Never use grease which is not specifically recommended for a certain
Multi-purpose grease, for example, is useful for many applications,
but not all applications.
General purpose grease
Multi-purpose grease can be soap- or
lithium-based. Soap-based grease can
be manufactured as a heavy duty
grease suitable for extreme pressure
applications with good resistance to
water and salt attack.
However, this type of grease has a low
melting point and may fail in high
temperature applications (e.g.: disc
brake hub wheel-bearings).
High melting point lithium
grease is suitable for use in
machine components where
a large amount of heat is
Motor vehicles with disc
brakes cause wheel bearings
to get hotter than those with
drum brakes and therefore
require grease with high heat
Graphite grease is a thick
paste of non-melting
petroleum greases blended
with a high quality lubricating
grade of natural fine graphite
It is water resistant and ideal
for leaf springs where metal-
to-metal contact is best
It is also for used on brake
known as copper slip)is a
high purity anti-seize
It comprises copper and
graphite particles in a high
melting point carrier.
It has a golden-bronze
colour and is used for the
prevention of seizing in high
heat (>1 000 °C) and
Special rubber compatible
grease is designed for use
on hydraulic brake and
clutch components where
hardening or swelling must
It can also be used on
rubber seals and O-rings.
Machine components are lubricated in many ways.
Accessible components are lubricated manually, with
oil or grease, using various devices or automatic
Some components like sealed bearings are
manufactured with a grease reservoir and require no
additional lubrication for their working life.
Grease is normally applied by means of a
It could be hand, battery or pneumatically
Pneumatically-driven grease guns are usually
used in automotive service workshops.
Manual grease gun
Battery-operated grease gun
Grease is usually applied to the machine component
through a grease nipple or a grease cup, which feeds
the grease into the required joint.
There many different types of grease nipple, but they
all have the same basic purpose.
The grease gun is attached to the grease nipple and
grease is pumped into the joint.
Care should be taken to ensure that the grease has
sufficiently penetrated the entire joint and that pockets of air
have not become trapped in the joint.
The circle in Figure 7.12 indicates the position of a grease
nipple on a steering joint of a motor vehicle.
Once the grease gun has been attached to the nipple, grease
can be pumped into the joint.
A steering joint
Take note how the grease is forced right
through the joint concerned and emerges on
the sides of the joint.
Grease can also be applied to machines
by means of a grease cup.
There are many variations which either
inject the grease by tightening the thread
on the grease cup by hand or
The automatic types are either spring
loaded or electronically timed to apply
grease at set intervals.
These devices save the tedium of
frequent greasing and ensure the correct
volume of grease is always applied.
A spring-loaded grease
cup on a machine
An electronically timed
measured-shot grease cup
Oil can be applied to machine parts in three main ways.
The application is broadly categorised as non-mechanical,
mechanical and forced system lubrication.
Mechanical and non-mechanical lubrication systems are
usually ‘total loss’ systems where fresh oil is continually
In forced lubrication systems, the oil is circulated throughout
the machine under pressure and filtered for re-use.
Oil can be directly applied to machine
parts by means of an oil can.
It can also be applied by means of a
‘filler cup’ which is gravity fed and
dispenses oil over a period of time.
Filler cups need to be topped up
periodically to prevent the machine
parts from running dry.
Some machines have
centrally located lubrication
devices which feed oil or
grease via tubes
(hydraulically) to the various
lubrication points as can be
seen in the Figures.
The lubricant can be
delivered via hand or
A hand operated
A centralised lubrication system
The internal combustion engine is a very good example of a
forced lubrication system.
Similar principles apply to most major machine lubrication
These and other engines have hundreds of moving parts, which
operate under enormous loads and very high heat.
Because of this, machines need very efficient lubrication
The diagram below gives a very simplified, schematic
representation of how oil is circulated under pressure through
the engine in order to lubricate all the bearing surfaces and
The most popular type of lubrication system is the wet-sump
type. This is so called because the engine sump is kept wet by
being used as a reservoir for the lubricating oil.
In operation, the engine oil is drawn from the sump by the oil
To prevent large items being drawn up into the pump the oil
must first pass through the strainer.
From the pump the oil is passed through the filter and then to
the main oil gallery.
The lubrication system of an internal combustion engine
The pump sends the oil to the main gallery under a pressure of
approximately 400 kPa.
To ensure that this pressure is not exceeded, and so to prevent
damage to oil seals and bearings, an oil pressure relief valve is
fitted into the main gallery.
From the main gallery the oil goes under pressure to the main
bearings and through drillings in the crankshaft to the big-end
At one end of the engine there is a long vertical drilling; this
takes oil up to the camshaft and valve gear.
Each component in the system needs to be properly
maintained, as failure in any of the components will result in
major damage to the machine.
Oil pumps are usually inspected at engine overhauls
and replaced when excessive wear is detected.
This contains a pair of meshing gears revolving in a closely
Oil fed in at one side is carried round the edge and out at
the other side.
The inner rotor meshes internally with the outer one,
which has one more lobe. The two have different axes of
rotation, so the spaces vary in size, causing oil to be drawn
in and forced out.
The oil filter catches small particles of dirt and metal in the oil,
which could otherwise cause damage inside the engine.
Eventually, the filter starts to clog up and the oil can’t flow
through it as easily.
If the filter clogs up completely, a bypass valve will open so that
some oil can still reach the engine, but dirt and metal particles
will be circulating as well.
A new filter should always be fitted whenever the engine oil is
Always make sure that the correct filter for the particular
engine is used.
Engine oil is typically replaced after 10 000 km of driving.
The cartridge type oil filter
Excessive wear, overheating, seizing and distortion is generally
referred to as machine failure.
We will not be studying it in any great depth but we will however look
at a few of the basic causes.
Excessive wear can take on different forms in different components.
For example, the type of wear on a bearing will not be the same as
that on a gear.
Excessive wear can be caused by prolonged wear and tear, as
machine parts are designed for a certain life-span.
However machine parts’ life-spans can be greatly increased if they
are maintained correctly.
Failure can occur if parts are overloaded or there is a
breakdown in the lubrication or the cooling system.
As wear increases in machine parts, the effectiveness of the
lubrication and cooling systems lessens and the parts tend to
As parts overheat, they expand and apply greater pressure to
their surrounding parts which in turn creates more friction and
even more heat.
If machine parts are allowed to deteriorate to the extent that
they overheat, they will eventually seize.
This means that they will jam together, get distorted or even
disintegrate and result in very costly repairs.
Most motor-cycle engines are air-cooled.
The principle is to fin the cylinder so as to increase the area
of the hot surface exposed to the flow of cool air.
This method of cooling is cheap, lightweight and is not
subject to troubles such as leakage and freezing problems.
In this system the outer surfaces of the cylinder and
head are enclosed in a casing or jacket, leaving a
space between cylinder and jacket through which a
suitable liquid can be circulated.
The coolant used is water, to which is normally
added an anti-freeze solution.
The basic system consists of a water jacket which is
connected by a rubber hose to a header tank.
This tank forms part of the radiator, a heat exchanger
made by connecting two tanks, header and lower, with a
number of finned tubes to provide a large surface area for
the disposal of the unwanted heat.
Air flow over the tubes and fins carries away the heat
radiated from the hot coolant and so lowers the
temperature of the coolant as it passes down the tubes.
A bottom hose connects the lower tank to the jacket to
provide a return path to the engine for the coolant.
There are hundreds of different types of failure that
can affect machine parts.
We include a few examples of failure in bearings and
gears, as well as their possible causes.
Visit a scrap yard in your local area and try to
identify some of the failures in the scrapped
engines as shown in the in the table.
Make a list and sketch all the faults you are able
to identify and try to explain the cause of the
Erosions and fracture lines on the raceway of a
Flaked or shelled balls
Overloading, extended operation and material
If balls have assumed their temper colour (unable
to depict by means of a diagram) it is likely that they
have overheated due to lack of sufficient
Severe abrasion of a gear tooth
Abrasion may be caused by contamination of the oil
by metallic debris, extraneous grit (particularly
siliceous dust) or other hard particles capable of
bridging the oil film between the working surfaces
during meshing of the teeth.
Scuffing results from high heat and loads causing
the oil film between gears to break down.
Hydraulic control systems are control systems
in which remote operation is obtained by the
transmission of pressure from a pedal or lever
through thehydraulic fluid.
Hydraulic systems are used to do work.
They are mostly used in cranes and other lifting
Failure in these systems leads to a loss in control of the
It is obvious that any failure in these systems may have life-
It is very important to ensure thorough, scheduled maintenance
is carried out on these systems to ensure correct working at all
Hydraulic fluid reservoir levels should be periodically checked,
as should lines and hoses.
If any moisture is detected around the lines and hoses,
immediate action should be taken to repair or replace the
Pneumatic systems are control systems
in which remote operation is obtained
by the transmission of compressed air through
valves and switches.
They are most commonly used in factory
It is very important to ensure the air pressure is
correctly regulated and that condensed water
vapour is removed through water traps.
Cables are routinely used in all types of machinery for control
Some examples are accelerator cables and hand brake cables
in motor vehicles.
It is imperative that cables are also kept in good condition, free
from barbs and well lubricated.
Cables may also stretch with age and use, and may have to be
The performance of routine maintenance may involve many
One of the most common tasks in the maintenance of a motor
vehicle is replacing old engine oil.