JSS 3 / Basic 9
Previous lesson :
The pupils have previous knowledge of
in Basic Science in their previous lesson
Behavioral Objectives : At the end of the lesson , pupils should be able to
- Define Gears, gearing and related calculations
Instructional Materials :
- Wall charts
- Related Online Video
- Flash Cards
Methods of Teaching :
- Class Discussion
- Group Discussion
- Asking Questions
- Role Modelling
- Role Delegation
Reference Materials :
- Scheme of Work
- Online Information
- 9 Year Basic Education Curriculum
Topic: MECHANICAL ENERGY TRANSMISSION SYSTEM (Linear Motion)
Lever arrangement to produce linear motion
Use of slides and slots in mechanical systems
Shows a push-pull link mechanism, which executes basically linear motion. Arms A and B have pivots as shown below. If for example, we push the driving arm A to the right, the driven arm B will also be pushed to the right. If we pull arm A to the left, the driven arm B will also be pulled to the left. Hence, we refer to the mechanism as a “push-pull” linkage.
Mechanical Systems Performing
The two images below show systems executing pure rotary motion. The hand-drill performs reversible rotary motion. This means the handle and the drilling bit can move clockwise or anticlockwise as may be required. The same thing applies to the worm gear shown in fig 2. This is a device which has a threaded shaft (worm) that meshes with a gear wheel (worm wheel). It is used to transfer rotary motion between shafts that are right angles to one another.
Transmission of Motion
Power for the propulsion of a car is derived from the engine. The generated power is usually available at the crankshaft as a combination of speed (2,000 to 4,000 crankshaft revolutions per minute for a petrol engine) and torque.
The figures above showing different arrangement of engine and transmission.
The power must be made available to the road wheels. The figure above show different transmission systems for vehicles. The transmission system for a vehicle with a front-engine, rear-wheel drive (a) of the figures above consists of a clutch, a gearbox, a propeller shaft and a rear axle. This arrangement is very popular. However, most cars are now coming with front-engine front-wheel drive (b) of the images above. Rear-engine rear-wheel drive (c) above is found in a typical Volkswagen Beetle car. In the transmission system of the Range Rover, torque splitter splits the torque generated by the engine between the rear axle and the front axle. Hence, it is referred to as .a four-wheel drive vehicle.
It is worth noting that the transmission system in figures (b) and (c) above do not have propeller shafts as in the case of others. The elimination of propeller shaft gives more space for a compact like the Volkswagen Beetle.
Common to all transmission systems are the gearbox and the clutch. The gearbox is placed between the engine crankshaft and the driving road wheels.
It is however, practically impossible to engage a rotating gear which is transmitting torque, with a stationary or slower-running gear as is often required during gear change. Such as attempt can only damage the gears. The main function of a clutch is to interrupt the transmission of a crankshaft torque to the gearbox. The clutch is designed to join together or disconnected two shafts running at different speeds. In the case of a motor vehicle transmission, the two shafts are the engine crankshaft and the gearbox shaft.
(a) of the figures above shows a mechanical clutch control system. The hydraulic system consists a clutch pedal, a master cylinder, a slave cylinder, flexible hose and piping.
Mechanical clutch control system
The control arm is rotated clockwise or anti-clockwise to disengage or engage the clutch. This is achieved by a release mechanism operated by the driver pressing the clutch pedal. The fig above shows a mechanical system consisting of a clutch pedal and linkages to increase and transmit the pedal force. The hydraulic fluid transmits and increases the pedal force exerted by the driver. The increased force is then used to turn the control arm.
Control of Rotary Motion – Brakes
A moving vehicle and its occupants or a moving bicycle and its rider possess some kinetic energy. When a moving vehicle is brought to a stop, the kinetic energy is reduced to zero. Where has the original kinetic energy gone? In a vehicle or bicycle, it is the brakes, which absorb the energy by friction, convert it into heat, which is then dissipated to its surrounding. Energy is continually absorbed as the vehicle slows down until it finally comes to a stop. In addition the brakes must bring the vehicle to a stop in a smooth manner and also in a straight line. There must be no skidding.
How do Brakes Work?
There are basically two types of friction brakes in use, namely the drum brakes and the disc brakes. The drum brakes consist of two brake pads or linings which can be forced outwards against the inner surface of the rotating rum fixed to the wheel. The shoes are mounted on a back plate rigidly attached to a non-rotating part of the axle. The disc brakes as shown below, consists basically of a caliper that houses friction pads which are loaded inwards against each side of a rotating disc, fixed to the wheel. The brake system for a bicycle is shown (b) below
Flexible connecting pipes, a servo-unit and wheel cylinder assemblies. The wheel cylinders operate the shoes and linings or the pads. When the driver presses the brake pedal, the force applied to the pedal is magnified by a simple linkage mechanism. The force is then transmitted by the hydraulic system through pipelines and flexible hosing to each brake.
It functions properly provided there is no air in the connecting pipes and the flexible hosing. This is because when air is compressed, it does not transmit the pressure as in the case of brake fluid. Such air must be expelled by “bleeding the system”. Next time you go to a motor vehicle repair garage, ask the mechanic to show you how a hydraulic braking system is bled.
In case the primary hydraulic braking system fails, a secondary independent system is usually provided for the driver to fall back on. The secondary system is usually referred to as the hand brake or parking brake. Generally, it uses the same pads and linings as the primary system, but the pads and linings are pushed by a mechanical linkage operated by hand. This works essentially like the bicycle brakes down shown in fig (b) above.
The topic is presented step by step
The class teacher revises the previous topics
He introduces the new topic
The class teacher allows the pupils to give their own examples and he corrects them when the needs arise
The class teacher wraps up or conclude the lesson by giving out short note to summarize the topic that he or she has just taught.
The class teacher also goes round to make sure that the notes are well copied or well written by the pupils.
He or she does the necessary corrections when and where the needs arise.
- Define Gears, gearing and related calculations
- How do you reduce the friction effect of gears?
- List uses of gears
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