1
UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE II
YEAR 2- SE
MESTER 2
THEORY
Version 1: December 2008
NATIONAL DIPLOMA IN
MECHANICAL ENGINEERING TECHNOLOGY
AUTOMOTIVE TECHNOLOGY AND PRACTICE
2
AUTOMOBILE TECHNOLOGY AND PRACTICE MEC.227
CONTENT PAGEWEEK 1
1.1.Introduction Prime movers. 1.2.The steam engine 1.3.The electric engine
1.4.Internal combustion engine.
1.5 Advantages and disadvantages of internal combustion engine as compared to the steam and electric powered vehicles.
1.6. Workshop staff and safety
WEEK 2
2.0. THE FUNDAMENTAL CYCLES OF OPERATION OF PETROL, DIESEL INTERNAL COMBUSTION ENGINES
2.1. Features of the 4-stroke spark ignition engine, 2.2. Futures of the 4 –stroke diesel engine
2.3. The advantages and disadvantages of Spark Ignition over Compression Ignition Engines and vise- visa
2.4 Features of the ‘day type’ two stroke engines: (i) piston (ii) three
ports (iii) con-rod
3
2.5. Compare the advantages and disadvantages of the stroke spark ignition to the 2-stroke compression ignition.
WEEK 3
3,0 THE COMPONENT PART OF AN ATOMOBILE ENGINE
3.1 Definition of terms.
3.1.1.The component parts of an internal combustion engine 3.2 The main functions of the petrol fuel system components
3.3 The main functions of the diesel fuel system components.
WEEK4.0. ENGINE COOLING SYSTEM
4.0.1. Introduction
4.1 Air cooling system
4.2. Water cooling system.WEEK 5
5.0 LUBRICATION SYSTEM
5.1 Engine Lubricating components and their functions.
5. 2. Engine oil Filtration Methods (Bypass or partial flow and Full flow) 5.3. Engine Lubrication methods
4
WEEK 6
6.0 ELECTRICAL SYSTEM
6.1 Introduction
6.2.
Major automobile electrical components.
6.3 The purpose of the battery.
6.4
Constructional details of the alkaline and lead acid batteries.
6.5. Charging and discharging processes of the two types of battery.
6.6. Functions of the alternator/ alternator.
6.7. Simple starting system.
6.8. Coil Ignition System
6.9. The Main Components of the Ignition System and their functions
WEEK 7
7.0 Internal combustion engine fuels and combustion
7.1 Introduction to Operating principles of simple carburetor.
7.2. Fuel injection system
7.3. Petrol engine Fuel line
7.4 Exhaust system.
WEEK 8
8.0.THE TRANSMISSION SYSTEM
8.1.
Diagram showing component parts of transmission system.8.2.
The automobile clutches.
5
8.2.2. Construction
8.5.3 Operation of Clutch
8.3. THE GEARBOX
8.3.1. Speed and load
8.3.2 The Sliding Mesh Four – Speed Gearbox
8.3.3 Construction,
8.3.4 Operation,
WEEK 9
9.0 THE DRIVE LINE (PROPELLER SHAFT)
9.1
Function
9.2
The universal joints
9.3
The final drive.
9.4. Axle shaft arrangement.
WEEK 10
10.0.
THE BODY AND CHASSIS
10.1. Chassis and Vehicle Body (General Objectives)
10.2. Separate chassis-body types.
10.3. Integral type
10.4. Motor vehicle body structure: sub -frame assemblies.
WEEK 11
11.1. Steering System functions.
11.2. The steering System components
11.3. Types of steering gears1.4. Steering Geometry
6
WEEK 12
12.0. Tires and Wheels
12.1 Functions of Tires .
12.2, Types of Tires
12.3. Tyre valve.12.4. Tire pressure.
12.5 Tire and rim sizes
WEEK 13
13.0. THE BRAKING SYSTEM
13.1 Braking system and their operating principles.
13.2 The main parts and Function of hydraulic braking system.
13.3 The operation of drum and disc brakes
13.4 The Master Cylinder and the servo.
WEEK 14
14.0. SUSPENSION SYSTEMS
14.1. Purpose
14.2. Components parts of suspension system
14.3. Types of springs
7 14.4. Torsion bar 14.5. Air spring 14.6. Shock absorbers WEEK 15
15.0. Features of the modern automobile electronic fuel ignition (EFI) system Introduction
15.1 Explain the Electronic fuel injector (EFI) system as it replaces the carburetor
15.2. Description: features of the electronic spark ignition as it replaces the contact-breaker unit. 15.3. Fuel injection and air "flow control
15.4. System identification of fuel injection engine.
WEEK 1
1.0 DESCRIBE THE CONSTRUCTION AND OPARATION OF PRIME MOVERS.
1.1. Introduction The Prime Movers.
The Microsoft Encarta Dictionary of 2008 describes “prime mover” as follows:-
(i) Most important cause of something, or something that initiates a process or activity which is usually an important factor in its continuation.
(ii) A natural or physical energy source, such as wind, solar or electricity that can be harnessed to power a machine.
(iii) An energy converter: a machine that converts energy from a natural or physical source in order to power equipment such as a windmill or turbine.
(iv) Power vehicles, such as steam engine, electric engine and internal combustion engines 1.2. The steam engine
The development (1629) of the steam turbine is credited to the Italian engineer Giovanni Branca, who directed a steam jet against a turbine wheel, which in turn powered a stamp mill. The first recorded patent for a gas turbine was obtained in 1791 by the British inventor John Barber.
8 Fig. 1.1. Early locomotive engine. 1.3. Locomotive steam engine valves. Engine No. 44, a Baldwin 2-8-0 steam locomotive engine built in 1921, has two wheels on the leading truck, eight driving wheels, and no trailing truck. The engine works on the Georgetown Loop Railroad and formerly ran in Central America. Diesel-electric locomotives began to replace steam locomotives in the 1930s and 1940s.
Fig. 1.2 Locomotive steam engine valves (1.3) Electric Engine
9
Fig. 1.3 Electrically driven motor vehicle
Electric Car, automobile propelled by one or more electric motors, drawing power from an onboard source of electricity. Electric cars are mechanically simpler and more durable than gasoline-powered cars. They produce less pollution than do gasoline-powered cars.
(1.4) Internal combustion Engine
The internal combustion engine is a prime mover that uses liquid fuel as its source of energy to cause continuous rotation of a crankshaft. Other types of prime movers are electric powered and steam engines.
10
1.5. Advantages and disadvantages of internal combustion engine as compared to the steam and electric powered vehicles.
When these three prime movers are compared to each other the following advantages and disadvantages are derived.
Table 1.1
S/N Steam engine Electric powered Internal combustion 1 Cheaper source of energy No harmful combustion
product
Fuel burn at control level
2 Do not require special sealing Less initial cost. Easier control of engine speeds 3 Good for stationary energy supply Quitter in operation Allow for portable engine
design
4 Easier adoptability to chassis
and drive arrangements
5 Higher thermal efficiency
ADVANTEGES DERIVED FROM THE THREE PRIME MOVERS AS COMPARED TO EACH OTHER.
Table 1.2
S/N Steam engine Electric power Internal combustion 1 Bulkiness Source of energy not quite
reliable
Emits carbon monoxide (CO) when engine becomes weak. 2 Engine rev/min. not easily Constant check on battery Effective cooling required.
11
controlled
3 Noisier in operation Not suitable for commercial transport e.g. long distant travels to country sides
Leakage of gas , water or engine oil can lead to engine failure
4 Higher cost of maintenance re
5 Requires good precession
DISADVANTAGES DERIVED FROM THE THREE PRIME MOVERS AS COMPARED TO EACH OTHER.
1.6. Workshop staff and safety • Workshop engineer.
An engineer is a planner, initiator, or supervisor of something, especially something that is achieved with ingenuity or secretiveness. The automobile workshop engineer conduct planning and initiate activities in the shop. The technologist and mechanics carry ‘s out the engineer’s initiative
A number of accidents occur in workshops on daily basis. These accidents can be
avoided, by following safety rules and regulations in work locations. The workshop
engineer could avert accidents by being alert and conscious of what is happening in the
workshop environment. Below are some points to be considered and be observed by staff
and other users of the automatable engineering workshop:
•
A tidy workshop can help in reducing a number of accidents.
•
Tools, components, equipment, and materials must be kept in the appropriate
locations by the technologist in the shop.
•
Always keep the workshop floor clean and free of grease or oil, and as temporary
measure cover the slippery floor with sawdust.
•
Basic Points
The basic points of safety in the work shop are easy to understand
a) Learn the safe way of doing each task.
b) If you do not understand - ask for explanation
c) If you are not taught - ask for instruction
d) Use the safe method against careless actions by yourself or others
e) Practice good housekeeping at all times.
12
f) Co-operate promptly in the event of an accident or fire.
g) Report all accidents to an instructor.
h) Draw your instructor's attention to any potential hazard
WEEK 2
2.0. THE FUNDAMENTAL CYCLES OF OPERATION OF PETROL, DIESEL INTERNAL COMBUSTION ENGINES
Upon completion of this study the students should be able to:-
i. Know the features of the 4 stroke petrol engine and describe its cycles of operation. ii. Know the features of the 4 stroke diesel engine and describe its cycle of operation iii. Compare the advantages and disadvantages of the spark ignition and the compression
ignition engines.
iv. Know the features of the 2-stroke petrol engine and describe its cycle of operation. v. Know the features of the 2-stroke diesel engine and describe its cycle of operation. vi. Compare the advantages and disadvantages of the 2-stroke spark ignition to compression
ignition engines.
2.1. Features of the 4-stroke spark ignition engine,
Main Components of 4 stroke Internal Combustion Engine (Petrol Engine).
13
Figure (2.1) shows-an outline of the engine main components
INDUCTION
COMRESSION
POWER
EXHAUST
Fig. 2.2, The four stroke cycle principles of operation for spark ignition
engine
14
Fig. 2.3. Compression Ignition Engine circle of operation (Otto cycle)
2.3. The advantages and disadvantages of Spark Ignition over Compression Ignition Engines and vise- visa
15 1 2 3 4
Higher risk of fire accident
Higher cost of maintenance as engine service interval is more frequent.
Complications in ignition systems
In- ability to start the engine without battery 1 2
3
Requires starting aids e.g. heater plugs. Produces large volume of smoke with foul odors in some cases prevents clear visibility of other road udders.
Increased weight of parts due to high- pressure requirements.
Table 1
Advantages of SIE over CIE Advantages of CIE over SIE 1
2 3 4
Easier to start in cold conditions Quieter in operation.
Lighter in weight and less initial cost.
Reduced volume in exhaust product emission 1 2 3 4 5 6 7 8 9
Reduced risk of fire accident due to low volatility of diesel fuel.
Long intervals between overhauling and services. Reduced cost of maintenance
Less harmful effect of exhaust products. Engine could run without battery.
More economical as compared to a similar size due to high compression ratio.
Higher thermal efficiency. Greater volumetric efficiency
Injection equipments are more reliable and stable than the electrical ignition system.
16
2.4 Features of the ‘day type’ two stroke engines: (i) piston (ii) three
ports (iii) con-rod
(iv) Crankshaft (v) Crank-case and (vi) Spark plug.
Fig. 2.4. The two stroke cycle: principles of operation
The two stroke engine has no valves but has three ports. The ports are inlet, transfer and the exhaust. The flow of gas through these ports is controlled by the position. When the piston is at bdc its skirt closes the inlet port. The piston travels up the bore (see diagram) as it reaches tdc. It opens the inlet port but closes the transfer and exhaust ports, at the same time compresses the gas in the combustion chamber. At top dead centre (tdc) the spark plug ignites the mixture of petrol
17
and air, burning takes place and expansion occurs, piston moves down the bore on power stroke
Fig. 2.5. The two stroke diesel fueled engine.
2.5. Compare the advantages and disadvantages of the stroke spark ignition to the 2-stroke compression ignition.
(i) Advantages of two stroke compression ignition engine over the two stroke spark ignition engine:
The two stroke CIE do not have to compress the charge in the crank case and discharge through the transfer port to the combustion chamber as occurs with the petrol engine, instead, a blower is used to force air into the culinders, to scavenge the spent gasses and replace them with a fresh charge through the induction poppet valve. Fresh charge can not escape along with the burnt gasses.
18
Unlike the two-stroke petrol engine, the fuel is mot assed to the air until both inlet and exhaust ports are close, therefore one of the drawbacks of two strokes engines, of fuel waste is overcome.
Forced induction promotes smoother slow running and a reduction in the combustion delay period.
Volumetric efficiency is improved which gives a higher power to weight ratio. (ii)Advantages and disadvantages of two-strokes cycle over the 4 strokes engine.
Advantage:
An important advantage of the two-stroke cycle engine is that it needs approximately only half the cylinder capacity of the four –stroke engine to produce equivalent power for the same number of revolutions of the crankshaft. This result in a smaller lighter power unit, capable of developing smooth torque with low bearing loads
Disadvantage
A serious disadvantage of the two-stroke, spark- ignition engine is that it has a low thermal efficiency. This is due to (a) incomplete scavenging of the exhaust gases and (b) Un-burnt mixture passing out of the combustion chamber with the exhaust gases
WEEK 3 Objectives:
Upon completion of this lesson the students should be able to:-
• List, describe and explain the function of automobile engine component parts
• Define some automotive engineering terms.
3,0 THE COMPONENT PART OF AN ATOMOBILE ENGINE. 3.1 Definition of terms.
• Top or bottom dead centre: the maximum a piston travels to the top or bottom of the cylinder
• Piston stroke : this is the measure of distance moved by the piston from top of the cylinder to the bottom or from the bottom to top.
• Piston displacement: this is the movement of piston from one point to the other
19
• Swept volume: this is the space created by the movement of the piston as it moves from tdc to bdc.
• Mean effective pressure: the average net pressure which, acting on the piston over the full length of its stoke, does the same amount of work as is actually obtained during a complete engine cycle.
• Engine torque: this is the turning moment on the crankshaft. When the piston moves down in power stroke, it transmits torque to the engine crankshaft. The harder the push on the piston the greater the torque applied. Thus the higher the combustion pressure, the greater amount of torque.
Fig. 3.1. explaining the compression ratio of an engine.
• Engine compression ratio: engine compression ratio is the measure of the amount of the mixture as compared to the volume of the cylinder bore when the piston is at b.d.c. and when it has risen to t.d.c.
• Indicated brake power: this is known as the actual power developed in the cylinder of an engine
• Brake power: this is the useful power available at the crankshaft of the engine . it is measured by running the engine against some form of absorption brake (dynamometer)
20
21
Fig.3.2 showing exploded view of an internal combustion engine 3.2 The main functions of the petrol fuel system components .
22
23
Fig 3.4a shows Engine- driven mechanical fuel pump fuel system. Fig. 3.4b shows Electric pump-feed fuel system
3.3 The main functions of the diesel fuel system components. The component parts of the diesel fuel system shown below are:-
i. Tank, which is used for containing diesel fuel for the vehicle use. ii. Fuel pipe and lift pump for lifting fuel from the tank .
iii. Fuel filter: for removing particles from the diesel oil.
iv. Injection pump: this increases fuel pressure to the injector nozzles.
v. Injector nozzles: they allow for the introduction of fuel at high pressure to the combustion chamber.
Fig.3.5. Distributor type fuel injection system
24
WEEK 4
4.0. ENGINE COOLING SYSTEM OBJECTIVES:
Upon completion of this study the students should be able to:-
i. Describe the operation and identify the component parts of air cooling system
ii. Describe the operation and identify the component parts of pressurized cooling system iii. Draw the flow diagram of pressurized air cooling systems.
Introduction
In any moving equipment, power machine or running engines heat is generated because
of:
•
Friction,
•
Power load,
•
Burning of fuel.
Therefore some form of cooling must be provided to take the heat away, this is necessary
to forestall excessive heat accumulation that leads to over heating, in which to following
could occur.
i. Seizure of working parts due to heat expansion,
ii. Excessive wear - the lubricant oil would be burnt
iv.
Pre-ignition in combustion chamber.
25
Most motor - cycle engines compressors, are air cooled. The principle is 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.
Air flow is to be natural or be forced by a fan through ducted passages and over the
finned surfaces (fig. 4.1).
Fig. 4.1) Air cooling system
4.2. Water cooling system.
26
Fig. 4. 2 Thermosyphon cooling system
27
Fig. 4. 3 Pressurized impeller cooling diagram with thermostat and direction of flow.
Fig. 4.4. Radiator pressure cap
(ii) Water pump: This is a small centrifugal pump driven by a belt at the front of the
crankshaft.
28
29
Fig. 4.9 Water cooled engine thermostat positions in the engine
(iii)Pressurized sealed system.
Toping up of water in pressurized sealed cooling system is only occasionally necessary as water get exhausted. Rugged pipes and expansion tanks are provided, such that excess water
30
as a result of increased in its temperature is contained in the reservoir an later used for replenishment when system cools down.
Fig. 4.10 Pressurized sealed cooling system arrangement
WEEK 5
5.0 LUBRICATION SYSTEM
Objectives
Upon completion of this study the students should be able to:- i. Identify and state functions of lubricating components
ii. Use line diagram to explain the operation of the full –flow and bypass oil filtration. iii. State common lubricants and their uses.
5.1 Engine Lubricating components and their functions. Introduction
31
Examination of two metallic components in a machine that has rubbed together for a period of time, shows that.
(i). Heat is generated - this indicates that a part of energy has been used in overcoming friction. (ii). Wear or scoring has occurred due to the high spots on the two surfaces.
The extent of these effects is governed by the friction so if energy losses are to be kept to a minimum, friction should be reduced. Friction may exist in dry or wet. One way to reduce friction is to lubricate the surfaces.
(iii) Components of the Lubrication System
A good example of a pressurized lubrication system is the lubrication of internal combustion engine; see diagram for pressurized lubricating system in (fig. 5.1 )
Fig. 5.1. shows oil lubricating parts and oil circulation format. 1) Oil in sump
2) Oil pump
3) Gear sprocket the drive the oil pump. 4) Dip stick
32
1- Oil pump:
Internal combustion engine oil pumps are usually driven by the camshaft; the pump forces oil from the sump to the main oil gallery.
a) Rotor Type:
It is a pump of displacement type fig. (6.8a) with an internal and external toothed rotors. The inner rotor, has one tooth fewer than the outer rotor, as the rotor revolves the cavities on the section side becomes larger. So that the pump draws in oil. On the discharge side, the cavities become smaller, so that oil forces into the discharge line, and to oil gallery.
Fig. (5.2a) rotor type oil pump
b) Gear-Type:
It conveys the oil from one half of the pump to the other in the gaps between the individual gear teeth and the inner wall of the pump. The gears mesh together to prevent the oil from flowing back. Fig (5.2b)
Fig. (5.2b) Gear type oil pump
2- Relief Valve
To prevent excessive pressure in the lubricating system, a relief valve opens to release part of the oil when pressure goes too high (Fig 5.3)
Fig. (5.3) Relief valve operation
33
The filter removes solid particles generated due to parts wear deposited in the oil. There is a bypass relief valve that opens, to allow unfiltered oil to go directly to the engine when the filter becomes clogged. However the filter is not likely to become clogged if it is replaced regularly (Fig.5.4)
Fig. (5.4) Oil filter type
4- Oil Galleries
These are oil holes drilled in the cylinder block to carry oil from the pump to the main bearings fig. (5.5).
34
Fig. 5.6 Internal combustion Engine lubricating block diagram.
Network of drillings called oil galleries is used to transport oil from the sump to the bearings an the rocker shaft for lubrication.
5- Oil Coolers
Some engine lubricating systems include an oil cooler to take the excessive heat of the oil. One type is a small radiator mounted on the side of the engine block; Fig. (4.14).
Fig. (5.7) oil cooler
35
Fig. (5.8) Dipstick
A dipstick is used to measure the level of the oil in the oil pan fig. (5.8).Some engines now' have a low - oil level indicator light.
5. 2. Engine oil Filtration Methods (Bypass or partial flow and Full flow)
With the partial-flow system (fig. 5.9a) approximately 10% of the total oil delivery passes through the filter and returns to the sump, while the remainder circulates through the engine bearings and before it reaches the moving parts . The rate of oil flow through the filter is comparatively slow, and a very fine filtering medium can be employed so that its efficiency is high. If a by-pass filter becomes choked, the lubricating oil will no longer be filtered.
Full flow system as shown in (figure 5.9b) is more efficient, it ensures complete and immediate protection from any foreign matter in the oil. The filter is located in the main oil-supply and takes the full delivery of oil from the pump before it reaches the moving parts of the engine. The full-flow filter is fitted with a relief valve to ensure that oil is supplied to the engine when the filters become chocked.
Fig. 5.9a. Partial flow. Fig.5.9b. Full flow. 5.3. Engine Lubrication methods
1. Boundary Lubrication: this is only a thin film of lubricant, a few molecules in thickness, which prevents metal to metal contact, with certain parts, the film breaks down, and the surfaces make occasional contact. Parts such as piston rings and valve trains are subjected to occasional failure of the oil film, and the properties of ‘oiliness’, film strength or load-carrying ability.
36
2. Full fluid film (pressurized) lubrication: full fluid film lubrication builds up quickly to protect rotating parts, such as crankshaft and bearing. This is most frequently used. The system has a pump that draws the oil from the sump usually through a mesh strainer, and forces it at high pressure through the oil lines after passing through a filter in most cases. As Fig. (5.9) shows.
Fig. 5.10
3. Splash lubrication: When crankshaft and its masses hits the engine during rotation, oil is splashed to the interior of the engine. This method is used to lubricate parts which do not require pressure lubrication but just small quantity. Parts such as piston rings and bore requires just small amount of oil for its lubrication.
In this type some machines components are provided with small scoops or discs (Fig. 5.11). Which dip into the oil sump, and scatter the oil throughout the casing.
Fig.(5.11) Oil scoop arrangement in big-end assembly
4. Mix lubrication: this type of lubrication is commonly applied on motor bike and two stroke engines, where engine oil is mixed with the petrol in the tank.
5.4. Common lubricants and their uses
The common lubricants used on motor vehicle are:- i. Engine oil: used for engine lubrication
ii. Gear oil: this is also known as extreme pressure oil, and it is used for lubricating gears in the gearbox or final drive arrangements.
37
iii. Grease: grease is for the lubrication of bearings and gears.
WEEK 6
6.0. Know the minor and major electrical components of a vehicle and describe
their functions.
Objectives :-
Upon completion of this lesson the students should be able to:-
•
List the major electrical components of a vehicle
•
Explain purpose of the battery.
•
Explain the constructional details of the lead-acid battery
•
Explain the constructional details of the alkaline battery
•
Describe the charging and discharging processes of the two types of battery
•
State the functions of the alternator
•
Describe a simple starting system.
•
Draw the component parts of the coil ignition system.
•
Identify the main components of the coil ignition system.
6.1 IntroductionElectricity and electronics play a vital role in the safe and reliable operation of modern
automotive vehicles. Demand range from a simple door switch and courtesy lamp to an engine electrical system so complex that a. it logically follows that anyone who expects to successfully maintain, troubleshoot, and repair today’s vehicles must have a through knowledge of the fundamentals of electricity and electronic.
6.2.
Major automobile electrical components.
The simple diagram ( fig. 6.1) shows how the main items of the electrical
equipment are connected. It can be seen that the electrical system can be broadly
divided into three parts.
(i)
Generator.
(ii)
Storage battery
38
(iii)
Distribution.
Fig. 6.1. Layout of main electrical components.
6.3 The purpose of the battery.
The lead acid battery used on motor vehicle stores electrical energy in the form of chemical energy to start the engine and for other purposes. When the ignition switch is turned on, the battery sends current to the starter motor that turns the engine’s crankshaft. Turning the
crankshaft moves pistons inside the cylinders, compressing fuel vapor for combustion. While the engine is running, an alternator (electric generator) recharges the battery and supplies power to other electrical components
6.4. The constructional details of alkaline and the Lead acid batteries.
The lead acid battery is made up of a number of positive and negative plates, sandwiched together and separated by a corrosion resistant papers, called ‘separators’. The unit is refered to as ‘cell’ and are immersed in a solution of hydrolyte contained in a re-enforce corrosion resistant container. The cells are then connected in series and a negative and positive terminals are
39
6.2. The constructional details of the lead-acid battery
Constructional details of the alkaline battery
Batteries convert chemical energy into electrical energy. In storage batteries, two metal rods, called electrodes, are connected by a circuit and immersed in a liquid, called an electrolyte. The rods chemically react with the electrolyte to produce a flow of electrons through the circuit. The storage batteries of the time were called lead-acid batteries because they had electrodes made of lead and lead dioxide and an electrolyte made of acid. They were heavy, bulky, difficult to recharge, and susceptible to rapid corrosion. To reduce corrosion, Edison decided to use an alkaline solution instead of acid for the electrolyte in his battery. Finding a suitable electrode, however, proved difficult. Edison finally decided on a combination of nickel flake and nickel hydrate for the positive electrode and pure iron for the negative electrode. He used an electrolyte of potassium hydroxide with a small amount of lithium hydroxide
Fig.6.3. Chloralkali Electrolysis
Chloralkali electrolysis is a technique for the industrial production of chlorine and the alkali known as caustic soda (sodium hydroxide) from brine, a solution of common table salt (sodium chloride) in water. Three processes are in use: the diaphragm-cell process, the membrane-cell process, and the mercury-cell process. In the diaphragm-cell process, a porous diaphragm divides the electrolytic cell, which contains brine, into an anode compartment and a cathode
compartment. When an electric current passes through the brine, the salt’s chlorine ions and sodium ions move to the electrodes. Chlorine gas is produced at the anode, and sodium ions at the cathode react with the water, forming caustic soda. Some salt remains in the solution with the caustic soda and can be removed at a later stage. In the membrane-cell process, the
compartments are separated by a membrane rather than a diaphragm. Brine is pumped into the anode compartment, and only sodium ions pass into the cathode compartment, which contains pure water. Thus, the caustic soda produced has very little salt contamination. In the
mercury-40
cell process, mercury, which flows along the bottom of the electrolytic cell, serves as the
cathode. When an electric current passes through the brine, chlorine is produced at the anode and sodium dissolves in the mercury, forming an amalgam of sodium and mercury. The amalgam is then poured into a separate vessel, where it decomposes into sodium and mercury. The sodium reacts with water in the vessel, producing the purest caustic soda, while the mercury returns to the electrolytic cell.
6.5. Charging and discharging processes of the two types of battery.
Battery charging methods vary, based on several considerations(i) Electrical capacity of battery being serviced (ii) Temperature of the electrolyte.
(iii)Battery state of charge. (iv) Batter age and condition.
Battery charging methods include high or fast and slow or trickling charging. Fast rate charging provides high charging rate for a short time and should be limited to 60 amperes for
12v batteries. Battery may be discharged when charging system becomes faulty or constant operation of the starter for too long a time, probably because of engine fault.
6.6. Functions of the alternator.
The automotive generator or alternator is an electromagnetic device that converts
the mechanical energy supplied by the engine into electrical energy. In operation,
the generator or alternator maintains the storage battery in fully charged condition
and supplies electrical power for the ignition system and accessory equipment.
Fig. 6.4 Alternator charging system circuit
6.7. Simple starting system.
41
The automotive starting motor (fig. 7.5 ) Is an electromagnetic device that comverts electrical energy into mechanical energy. It is designed specifically for cranking internal combustion engines at speeds which will permit starting.
Fig. 6.5.
6.8. Coil Ignition System
(i) Ignition System is the arrangement put together to provides the high quality spark needed to ignite fuel in a gasoline internal-combustion engine. The ignition system produces, distributes, and regulates electric sparking that ignites fuel vapor in the combustion chambers.
42
(ii)Types of Ignition Systems
Electric sparking is the most popular ignition system used in modern gasoline engines, but the manner of producing and regulating the spark has changed with new technology. Computers control the ignition systems in modern automobiles, although many older vehicles still rely on mechanically operated and controlled ignition systems. Two broad categories of ignition systems are defined by whether or not a battery is used to store electricity for starting the engine. Most automobile engines have battery-powered ignition systems. Ignition systems without batteries rely on a generator called a magneto.
The purpose of the ignition system is to ignite the mixture of air and petrol at the correct
timing
Fig. (6.7) ignition system of petrol engine
6.9. The Main Components of the Ignition System and their functions
1- Battery: Battery is used on motor vehicle stores electrical energy in the form of chemical energy to start the engine and for other purposes. When the ignition switch is turned on, the battery sends current to the starter motor that turns the engine’s crankshaft. Turning the
crankshaft moves pistons inside the cylinders, compressing fuel vapor for combustion. While the engine is running, an alternator (electric generator) recharges the battery and supplies power to other electrical components.
43
2- Ignition Switch: It allows current to flow from the battery to the coil, it also operates
the starter motor.
3- Distributor: The distributor routes high-voltage pulses to individual cylinders in the correct
sequence and with precise timing. It also houses a mechanical switching system involvingbreaker points, that open to interrupt the flow of electric current. A rotating shaft in the
distributor moves the pivot arm, causing the two metal points to contact each other and then move apart. When the points touch each other, low-voltage current flows through them to a transformer called the coil. When the points separate, they break the low-voltage circuit to the coil. In an eight-cylinder engine running at 3000 revolutions per minute (rpm), the breaker points open and close about 200 times per second. Inside the coil interruptions in the low-voltage circuit (12 volts, normally) create pulses of 20,000 volts or more.
4- Contact breaker point ( CB ): A mechanically operated device, which breaks the low
tension circuit when the spark is required
5- Condenser or Capacitor: is a device that temporarily stores electric charge. In the ignition
system a capacitor helps produce a sharply defined cutoff of current when the breaker points open. The capacitor also absorbs the surge of high-voltage electricity as it moves from the coil tothe points. In so doing the capacitor minimizes arcing across the breaker points when they open, greatly increasing their service life.
6- Spark plug: is made of a material that conducts electricity encased in a ceramic body. Its
threaded base screws into the top of an engine cylinder. Two electrodes on the base of the spark plug project into the combustion chamber. High-voltage current passes from the top of the spark plug to electrodes on its base. The current then arcs, or jumps the gap, between the electrodes, igniting fuel vapor in the combustion chamber.7- High tension wires: these special wires, connects the distributor to spark with Plug
8. Ignition Coil. When the breaker points opens, the low-voltage current stops and the magnetic field collapses, inducing a high-voltage surge in the secondary winding.A wire conductor carries the pulses from the coil to the distributor, which routes them through other wires to individual spark plugs. .
44 WEEK 7
AUTOMOTIVE TECHNOLOGY AND PRACTICE MEC. 227 7.0 Internal combustion engine fuels and combustion
Objectives:
Upon completion of this lesson the students should be able to:- i. . Describe operating principles of a simple carburetor ii. List main parts of the fuel injection system
iii. Describe the operating principles of fuel injection engine iv. Draw the lay out of petrol fuel line
v. Identify exhaust system and state its functions 7.1 Introduction:
Operating principles of simple carburetor
Carburetor, is a device that mixes fuel and air for burning in an internal-combustion engine. A carburetor atomizes (converts into a vapor of tiny droplets) liquid gasoline. An airflow carries the atomized gasoline to the engine’s cylinders, where the gas is ignited.
45
Fig. 7.1 Simple carburetor showing its parts
Fig. 7.2 Simple carburetor showing exaggerated venturi.
The basic carburetor is built around a hollow tube called a throat, or barrel. Downward motion of the engine’s pistons creates a partial vacuum inside the cylinders that draws air into the
carburetor’s throat and past a nozzle that sprays fuel. The mixture of air and fuel produced inside the carburetor is delivered to the cylinders for combustion.
7.2. Fuel injection system
(i) Electronic IgnitionElectronic ignition systems use semiconductors and other solid-state electronic components to switch current flows on and off in the coil, eliminating the need for breaker points. Automobile manufacturers began installing electronic ignition systems in the 1970s and 1980s in an effort to produce cleaner, more efficient engines.
(ii) Computer Electronic Ignition is a devices that detect the position of the crankshaft and trigger electrical impulses at the correct moments. Some systems integrate ignition coils, mounted either above each spark plug or to the side of the engine, that produce 40,000 volts or more. The higher voltage produces a hotter spark with cleaner burning, longer plug life, and improved fuel economy. Computers monitor and control the entire ignition process, adjusting ignition timing and fuel delivery for the specific driving conditions, vehicle speed, and strain on the engine.
46
Fig.7.3. Fuel electronic injection system
1. Ignition key 2. Security control 3.swetch 4.switch body 5. Relay 6. Electroinc Control Unit
7. Plug 8.Injector nozzle. 9.Pump
This system operates on ectromechanical principles in that injection pump and nozzles are used in conjuction with the distributor and the plugs. This ignition system in more reliable as compared to the two known traditional mean of providing fuel and its ignition in the combustion chambet.
47
7.4 Fuel injection system main parts
The fuel-injection system replaces the carburetor in most new vehicles to provide a more efficient fuel delivery system. Electronic sensors respond to varying engine speeds and driving conditions by changing the ratio of fuel to air. The sensors send a fine mist of fuel from the supply through a fuel-injection nozzle into a combustion chamber, where it is mixed with air. The mixture of fuel and air triggers ignition.
(iii) Fuel injection principles of operation
In fuel injection system, the air is directed into the combustion chamber, through the intake manifold with uniform volume and velocity. The fuel is injected directly into the combustion chamber or at intake valve under calibrated pressure. This process ensures precise fuel control for all conditions of operation, which allow for better handling of leaner mixtures than can be found in conventional carburetor.
Another feature of fuel injection, is its ability to ram air into the combustion chambers of the engine.
48
1. Injector nozzle 2. Piston 3. Vaporized atomized fuel
Fig 7.5 Fuel injection system, showing two methods of injecting fuel into combustion chamber.
7.3. Petrol engine Fuel line
The main purpose of fuel system is, to supply fuel to the engine cylinders in a vaporized
form, to ease combustion. Fig. (3.4) shows a simple fuel system used in the motor
vehicles.
Fuel system main components and other purpose are briefly described as follows:
1- Fuel tank: to store fuel
2- Fuel pump: to draw fuel from fuel tank to the carburetor.
3- Fuel filter: to filter the fuel from small foreign particles
4- Carburetor: to meter and mix the fuel at correct air-fuel ratio and to atomize the fuel
into fine particles so as to burn it quickly.
49
Fig. (7.6) fuel injection system components
7.4 Exhaust system.
Each exhaust stroke emits a sound wave composed of higher and lower frequencies of compression vibration. These frequencies of combustion vibration can be damped down by passing the gases through an absorption type of silencer in which a perforated steel tube in surrounded by glass or wire wool. The gases pass through the tube in an unobstructed flow, while the high frequency sound waves pass through the tube in an unobstructed flow, while the high frequency sound waves pass through the perforations to be damped down by the wool. Exhaust system consists of a steel drop pipe connected to the manifold.
50
Fig 7. 7 A silencer muffler showing the internal components..
WEEK 8
8.0.THE TRANSMISSION SYSTEM
General Objectives:
1. Know the general arrangements and layout of the various types of transmission system.
2. Understand the constructional details and operations of friction clutches used in road vehicles.
3. Understand the constructional details and operations of manually-operated gear-boxes.
4. Know the components, and describe the constructional details and operation of the propeller and drive shafts.
Understand the methods of bearing mountings, adjustment and lubrication requirements of final drive.
8.1. Diagram showing component parts of Transmission system.
51 5.
8.2 The automobile clutch
The main function of the clutch is to interrupt the transmission of crankshaft torque to the gearbox. Different trains of gears, providing different combinations of speed and toque, must be used to suit different driving and load conditions but it is almost impossible to engage, or release gears when they are transmitted torque. It is also practically impossible to engage a rotating gear, under torque, with a stationary or slower running gear and it certainly cannot be done without damage.
The clutch is also designed to absorb the shock of engaging two shafts running at different speeds, and to absorb small torque irregularities.
52
8.2.1 Types of clutches
The most widely used form of clutch is the friction type. This may be:
(a) The cone clutch, which is now only used in the synchromesh units of gearboxes, and in overdrives and some epicyclic gearboxes;
(b) The single-plate clutch (multi spring or diaphragm spring) which is used in most cars and small commercial vehicles.
(c) The multi-plate clutch, which is, used in motorcycles and in some racing cars and tractor, and also in special types of very heavy commercial and civil engineering vehicles.
The single and multi-plate friction clutches are usually dry types but some wet types are still in use. In these cork-insert or phosphor-bronze plates are fitted between steel plates, all the plates being immersed in oil.
Other forms of clutch are coming into wider use and generally form a part of the pre-selector, two pedal, or fully automatic transmission systems. These are the centrifugal and magnetic clutches, the fluid flywheel, and the hydraulic torque converter.
8.2.2 Single plate multi spring clutch Construction
53
Fig. 8.3. Single plate clutch
The single-plate clutch consists of a centre plate which is clamped between two other plates. These two outer plates are driven by the engine crankshaft, and in turn drive the centre plate which is mounted upon the splined gearbox input shaft. The rear face of the flywheel is used as one driving plate and the second, or pressure, plate is mounted inside the clutch body which is bolted to the flywheel. The pressure plate is forced towards the flywheel by a set of strong springs which are arranged radially inside the body. Three levers, or fingers, are carried on pivots suspended from the case of the body, and are so arranged as to be able to pries the pressure plate away from the flywheel by the inward movement of a carbon or ball thrust-release bearing. The bearing is mounted upon a forked shaft and is moved forward by the depression of the clutch pedal. The connection between the pedal and the shaft may be made by means of rods, cables, chain, or by a hydraulic system. 8.2.3 Operation of the single plate Clutch
Basically, the clutch consists of three parts. These are the engine flywheel, a friction disk, and a pressure plate. When the engine is running, the flywheel is rotating. The pressure plate is attached to the flywheel so the pressure plate also rotates. The friction disk is located between the two. When the clutch is released, the driver has pushed down on the clutch pedal. This action forces the pressure plate to move away from the friction disk. There are now air gaps between the flywheel and the friction disk, and between the friction disk and the pressure plate. No power can be transmitted through the clutch.
Bearin
Clutch
Splined
Input
shaft
54
When the driver releases the clutch pedal, power can flow through the clutch. Springs in the clutch force the pressure plate against the friction disk. This action clamps the friction disk tightly between the flywheel and the pressure plate. Now, the pressure plate and friction disk rotate with the flywheel. The friction disk is assembled on a splined shaft that carries the rotary motion to the transmission. This shaft is called the Clutch shaft, or transmission input shaft
.
Fig.8. 4
55
THE GEARBOX
8.3.1. Speed and load
The petrol engine can only operate efficiently within a limited range of engine speeds, usually between 2000 and 4000 crankshaft revolutions per minute. The power produced by the engine is available at the crankshaft as a combination of speed and torque. This power will be capable of propelling the vehicle against a certain maximum load or resistance; any load in excess of this maximum will result in slowing down the engine. It will, therefore, produce less and less power until it is brought to a standstill or stalled.
The loads imposed upon the engine will vary with the weight being moved the nature of the road, i.e. the level, uphill or downhill. The greatest amount of power is required when first moving the vehicle form rest.
Fig. 8.5 Speed and loading.
As power is the speed multiplied by the torque (p = S x T) it follows that if the speed is reduced the torque can be increased. By placing a train of gears between the crankshaft and the driving road wheels the turning power of the wheels can be increased by reducing their speed. This enables an engine of a given power output to overcome a greater load –
Pinion A
Pinion B
F
Pinion A
C = F x r
F x 2r = 2C
Pinion B
r
2r
56
but at a lower speed. In practice three or four alternative gear trains are used which give a choice of speeds and torques to suit all conditions of vehicles speed and engine loading. A neutral position must be available to allow the running of the engine while the vehicle is stationary and a reversing gear train must also be available. All the various gear trains and their selector mechanisms are built into a gearbox which is fitted between the clutch and the final drive mechanism in the rear axle.
8.3.2
The Sliding Mesh Four – Speed Gearbox
Fig.8. 6. Transmission gearbox
In construction and arrangement this gearbox is generally similar to the three – speed type but there are a few important differences. These are:
(1) The incorporation of an extra gear train makes available an extra series of intermediate torques, which enables the engine to overcome the loads acting against it without either being overworked or having to operate at excessive speeds.
57
(2) The reverse idler gear has two sets of teeth, of different diameter, and is engaged by being moved bodily along its own shaft i.e. it is not permanently engaged with the lay shaft.
(3) The reverse idler gear has its own selector shaft and fork, and the gear lever has five different positions. The reverse gear selector mechanical is so arranged that
reverse cannot be selected by accident. This is usually accomplished by having to use extra force, or an unusual lifting or side movement of the gear lever.
8.3.3 Construction,
The input and output shafts lie on the same axis and, although the forward end of the output shaft is supported in a bush fitted inside the input shaft, there is no direct connection between them. These shafts are supported and located by ball bearings mounted in the end walls of the gearbox case.
The lay shaft axis is parallel with the other two shafts and lies under or to one side of them, the largest lay shaft gear wheel, or pinion, being permanently engaged with the integral pinion of the input shaft. The lay shaft rotates upon plain bushes or needle-roller bearings which are supported by a non-rotating shaft. End-float is controlled by phosphor bronze spacer washers.
The lay shaft has four integral pinions which have spur teeth. The output shaft is splined and carries splined pinions which provide the third, second and first gear ratios. The movement of the gear lever, acting through the selector shafts and forks, causes the selected pinion to slide along the output shaft and be meshed with one of the lay shaft pinion.
8.3.4 Operation, Fig.8.6
First or bottom gear:
The selector fork moves the double-output pinion (6 and 8) to the rear to engage (8) with the rear lay shaft pinion (7). The torque is transmitted through input (1) to lay shaft pinion (2), then lay shaft pinion (7) to output pinion (8). This ratio proves the greatest forward speed reduction and torque increase.Second gear:
The selector fork moves the double-output pinion (6) and (8) forward to engage pinion (6) with the third lay shaft gear (5). The torque is transmitted through input (1) to lay shaft pinion (2), then lay shaft pinion (5) to output pinion (6). This ratio provides more speed but less torque increase than that of the first gear.58
Fig.8. 6
Third gear:
The selector fork of the third – and top-gear selector shaft moves the output pinion to the rear to engage with the second lay shaft pinion. The torque is transmitted through input to lay shaft and from lay shaft to output. This ratio proves more speed but less torque increase than the first and second gear ratios.Top gear:
The selector fork moves the output forward to engage with input pinion (1) by means of dogs. The input and output shafts now rotate as one shaft and the output speed and torque are the same as that of the crankshaft.N.B:
Note that bottom gear provides the greatest forward speed reduction and the greatest torque increase. As the other ratios are engaged the output speed is increased while the output torque is reduced until, when top gear is engaged, the input and output speeds and torques are the same as those of the crankshaft.
Reverse gear:
The output remain in the neutral position, that is between lay shaft and between lay shaft the reverse selector shaft and fork move the double reverse idler pinion to engage with lay shaft and output at the same time. The torque is now transmitted through input to lay shaft and from lay shaft (7) to reverse idler . Then from reverse idler59
to output. In many gearboxes the reverse ratio provides the greatest reduction in speed and the greatest increase in torque
WEEK 9
9.0 THE DRIVE- LINE (PROPELLER AND SHAFT)
9.1
Function
The propeller shaft is used to connect the output shaft of the gearbox to the pinion shaft of the final drive mechanism in the rear axle. As the suspension system operates, the rear axle rises and falls continuously. It also moves backwards and forwards as it rises and falls in an arc, having as its centre the forward shackle pin of the rear spring. In addition, the pinion nose itself is forced upward when the engine torque is applied to the pinion, and is forced down when the brakes are applied. The propeller shaft must be so designed as to transmit the torque from the gearbox to the final drive smoothly and continuously in spite of all these different movements.
Arrangement
The propeller shaft is a tubular steel unit with a Hook joint at each end. The joints consists of two U-shaped steel forgings or ‘yokes’ which are connected at 90o to each other by a four-legged cross or ‘spider’. Needle roller or rubber bearings may be used to support the spider legs in the forgings. These U-joints, or universal joints, allow the smooth transmission of torque even though the gearbox and pinion shafts are never in exact alignment.
60
Fig.9.1 The main types of propeller shaft.
9.2. UNIVERSAL JOINT
These are used to connect two shafts when their centre lines intersect.
Fig. 9.2 The universal joint (yoke)
Types of universal joints
The three main types of universal joint used in vehicle construction are: (1) The cross type such as the ‘Hardy-Spicer’
(2) The ring type such as the ‘Lay rub’
61
9.3. THE FINAL DRIVE
This is generally referred to as the differential but includes the crown wheel and pinion or gear assembly having the same functions.
Functions
The crown wheel and pinion assembly is used
a) To change the direction of the drive through a right angle, and
b) To increase the available torque by reducing the speed [power =torque times speed]. The ratios used in cars are about 4; 1 while those of commercial vehicles may be as high as 10; 1.
The differential is a second gear assembly which is bolted to the side of the crown wheel, or inside a worm-wheel and which rotates with it.
This unit allows the half-shafts to rotate at different speeds but under the same torque, and only comes into operation when the vehicle is cornering. Its function or purpose is to reduce the tendency for the tires to be dragged sideways instead of rolling around the curved path. It also reduces the stresses imposed upon the shafts and bearings and reduces tire wear. Skidding is also much less liable to occur.
62
Fig 9.3 The deferential units
Construction
Crown wheel and pinion
These are hardened and tempered steel bevel gears which are arranged with their axes at right angles. The larger is the crown wheel and this carries the differential assembly. The pinion is the smaller and is integral with a short shaft to which is bolted the propeller shaft. The complete final drive gear assembly is mounted in a strong steel casting which is bolted into the rear axle case. A tubular part of the casting, called the pinion nose, supports the pinion and its integral shaft either in double thrust ball bearings or in opposed taper- roller bearings. Some designs may use a plain roller bearing at the inner end of the pinion. The differential case is formed into two arms which carry the bearings used to support and locate the crown wheel. These bearings may be both ball or tapper-roller types and their thrust directions are opposed. Provision is made to adjust the meshing of the gears either by screwed sleeves, shims, or by pre-loading jigs and shims.
63
Figure 9.4. Rear wheel and front wheel drives
9.8. Differential, Fig
This consists of a case [which may be in two parts] which bolted or riveted to the side of the crown wheel and rotates with it. Two or four planet wheels are mounted upon a spider shaft and are fitted inside the case in such a way that the spider shaft is turned end over end. Also fitted inside the case, and meshing with the plane wheels, are two sun wheels which are internally splined, and which support and drive the inner ends of the half-shafts. The gear teeth and the spider shaft are the most highly stressed part of the assembly and are those most liable to fracture.
64
Fig. 9.5 diagram showing planet and sun gears
Differential principle, Fig. 9.5
This is similar to the simple bar type of brake compensator. In Fig 8.3 the end of the beam are fitted into slots in the circumference of the discs. If a force is applied to the centre of
65
the beam and at a tangent to the discs [at right angles to their radii], and if each disc offers the same resistance to being turned, then the reaction forces acting on each disc will rotate at the same speeds, and two torques or turning moments will be the same. In the practical differential the discs are the sun wheels on the half-shafts and the beam is the spider shaft and it planet wheels. When one disc does offer more resistance to being turned than the other, the beam is forced to pivot about its centre. The disc with the greatest resistance will hold back while the other is push forward by the pivoting of the beam. Under the of a continuous tangential force at the centre of the beam one is slower and one faster in rotation than the tangential force; i.e. the revolution per minute lost by the disc with the greater resistance are gained by the other. The reaction forces on the discs are the same because the force available is divided equally by the beam. The radii are the same so the toque acting on each is the (torque= force * radius) – although their speeds are now different.
.
Operation
Vehicle running straight
,
Fig
. 9.6
The driving torque of the propeller shaft and of the pinion is increased by his speed reduction between the pinion and the crown wheel. The direction of the drive is turned bodily through a right angle. T he differential spider is rotated end over end, carrying the planet wheels with it although they do not rotate on the spider. The road wheels, half-shafts and sun wheels offer the same resistance to being turned and the differential gearing does not therefore operate.
66
Vehicle cornering. Fig. 9. 7
During a turn the outer wheel has to move along an arc of greater radius than the inner wheel, and to do this in the same time it must be speeded up. The inner wheel is slowed down as the vehicle turns and this increase the resistance of its sun wheel. The spider shaft is still being turned end over at crown wheel speed, and as the inner sun wheel slows the planet wheels are forced to rotate on the spider shaft and about the inner sun wheel. In so doing the speed of the outer sun, and the outer road wheel, is increased by the same proportion as the speed of the inner sun is reduced.
The torque is still divided equally between the two half-shafts but their speeds are different. Note. The differential system only operates when there is a difference between the resistances to turning of the road wheels. When one wheel loses its grip on a poor surface its resistance is reduce to zero. The planet gear wheels therefore rotate on their spider and run around the sun of the opposite wheel. This remains stationary and the slipping wheel is driven by all available torque. Vehicles which, have to operate over poor ground (e.g. Tractors, civil engineering and military vehicles) are often fitted with a devise which puts the differential gearing out of operation as required. In effect the two half-shafts joined together so that one wheel can drive when the other slips.
Fig. 9.7.Deferential action: vehicle cornering.
67
Three main methods are used to support the half-shafts in the rear axle case. In all of them the inner ends of the shafts are splined into, and supported by, the sun wheels of the differential assembly (see Fig.). The differences lie in the arrangement of the hub bearings in relation to both the case and shaft, and in the forces or loads imposed upon the shaft itself.
Semi-floating, Fig.
The hub and the half-shaft are, in effect, a one-piece unit although they may in be splined or fitted together by means of a taper, key and lock-nut. The bearing is carried on the shaft and is located by a nut or a sleeve. The outer track of the bearing is fitted a recess in the axle case and is located by a retainer plats bolted to the end flange of the axle case. This retainer usually encloses a spring-loaded oil seal and often in corporate an oil or grease trap to prevent excess lubricant ruining the brake linings.
Three quarter floating, Fig.9.8
The bearing is mounted on the casing and is held against a shoulder by a lock-nut and tab washer. The hub is made in two parts, the inner part fitting over the bearing and also enclosing a spring-loaded oil seal. The outer part may be integral with the half-shaft, be a splined and interference fit upon it, or be secured to the shaft by a taper, key and lock-nut. The brake drum may be integral with the hub outer half or secured to it by countersunk-headed set screw. The back-plate mounting flange is nearer to the centre of the axle than in the semi-floating designs.
