Lesson Note on Components of a Transmission Unit (Gear Box): Sliding Mesh Gearbox and Reverse Mechanism for SS2

Class: Senior Secondary School 2 (SS2 / SSS2)
Term: First Term
Week: 7
Age: 16 years
Duration: 45 minutes
Subject: Auto Mechanic Work
Curriculum Theme: Power Transmission Systems
Previous Lesson: Components of a Transmission Unit (Clutch): Construction and Operation of a Single Plate Clutch.
Topic: COMPONENTS OF A TRANSMISSION UNIT (GEAR BOX)— OPERATION OF A SLIDING MESH GEARBOX AND REVERSE MECHANISM.
Subject Matter: Meaning of COMPONENTS OF A TRANSMISSION UNIT (GEAR BOX) (definition of gearbox in transmission), operation of a sliding mesh gearbox (gear engagement, speed selection), reverse mechanism (idler gear action, reverse drive).

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

• Define a gearbox and state its function in a vehicle.
• Explain the operation of a sliding mesh gearbox.
• Describe the mechanism for reverse gear engagement.

Affective Domain:

• Appreciate the importance of a gearbox for efficient vehicle operation.
• Develop an interest in understanding vehicle transmission systems.

Psychomotor Domain:

• Identify major components of a gearbox.
• Observe and describe the engagement of gears in a sliding mesh gearbox.
• Observe and describe the action of the idler gear for reverse.

Social Domain:

• Collaborate with peers during observation of the gearbox.
• Communicate observations and understanding effectively.

Reference Materials

The following resources were used in planning this lesson:

• Senior Secondary Schools Education Curriculum
• State Unified Scheme of Work
• Auto Mechanic Work for Senior Secondary Schools Textbook

Instructional Materials

The teacher will teach this lesson with the aid of:

• A live vehicle
• A dismantled gearbox
• Posters showing gearbox diagrams
• Charts illustrating gear engagement

Rationale for the Lesson

Understanding the components and operation of a gearbox is important for pupils to comprehend how a vehicle’s speed and torque are managed. This knowledge is essential for diagnosing transmission problems and performing basic maintenance, which are valuable skills in auto mechanics.

Prerequisite/Previous Knowledge

Pupils should have basic knowledge of simple machines, rotational motion, and the main parts of a vehicle.

Lesson Content/Board Summary

COMPONENTS OF A TRANSMISSION UNIT (GEAR BOX)— OPERATION OF A SLIDING MESH GEARBOX AND REVERSE MECHANISM

Meaning of Transmission Unit (Gearbox)

A gearbox, also known as a transmission unit, is a mechanical device in a vehicle that changes the speed and torque of the engine output to the driving wheels. It allows the driver to select different gear ratios, enabling the vehicle to move at various speeds and overcome different resistances.

Components of a Gearbox

The following are major components found in a gearbox:

• Input Shaft: Connects to the engine via the clutch.
• Output Shaft: Connects to the propeller shaft, which drives the wheels.
• Lay Shaft (Countershaft): A secondary shaft that carries gears which mesh with gears on the input and output shafts.
• Gears: Toothed wheels of various sizes that transmit power and change speed/torque.
• Selector Forks: Mechanisms that move gears or dog clutches to engage desired gear ratios.
• Gear Lever: Allows the driver to select gears.
• Reverse Idler Gear: A small gear used specifically for reversing the direction of rotation.
• Casing: The housing that encloses and protects all the internal components, usually filled with lubricating oil.
• Bearings: Support the rotating shafts and reduce friction.

Operation of a Sliding Mesh Gearbox

In a sliding mesh gearbox, the gears on the main shaft can be slid along the shaft by a selector fork to mesh directly with gears on the lay shaft. This direct meshing creates different gear ratios for various speeds.

• Neutral: No gears are engaged on the main shaft, allowing the engine to run without moving the vehicle.
• First Gear: A large gear on the main shaft slides to mesh with a small gear on the lay shaft, providing high torque for starting.
• Higher Gears: Smaller gears on the main shaft mesh with progressively larger gears on the lay shaft, reducing torque but increasing speed.
• Gear Engagement: The driver moves the gear lever, which actuates a selector fork. The selector fork then slides the appropriate gear along the main shaft until its teeth engage with the corresponding gear on the lay shaft.

Reverse Mechanism

The reverse mechanism uses an additional gear called an “idler gear” to reverse the direction of rotation of the output shaft.

• Idler Gear Action: When reverse gear is selected, the idler gear is moved into mesh between a gear on the lay shaft and a gear on the main shaft.
• Reverse Drive: The idler gear introduces an extra stage of meshing, causing the main shaft to rotate in the opposite direction to the input shaft, thereby making the vehicle move backwards.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils and asks them how a vehicle changes speed, and how it moves backward. The teacher then introduces the topic: Components of a Transmission Unit (Gearbox) and its operation.
Pupils’ Activity: Pupils respond to the questions and listen attentively to the topic introduction.
Learning Point: Pupils are prepared for the lesson and understand its relevance.

Step 2: Meaning of Transmission Unit (Gearbox)

Time: 8 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines a gearbox, explains its primary function in a vehicle, and its importance in controlling speed and torque. The teacher writes the definition on the board.
Pupils’ Activity: Pupils listen, ask questions for clarity, and copy the definition into their notes.
Learning Point: Pupils understand what a gearbox is and why it is essential.

Step 3: Components of a Gearbox

Time: 8 minutes
Teaching Skill: Identification/Demonstration
Teacher’s Activity: Using a dismantled gearbox, posters, and charts, the teacher identifies and points out the major components such as the input shaft, output shaft, lay shaft, various gears, selector forks, and the reverse idler gear. The teacher briefly explains the function of each component.
Pupils’ Activity: Pupils observe the components, identify them from the dismantled gearbox and charts, and listen to the explanations.
Learning Point: Pupils can identify and recall the names of key gearbox components.

Step 4: Operation of a Sliding Mesh Gearbox

Time: 8 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher demonstrates the operation of a sliding mesh gearbox using the dismantled unit or a working model. The teacher shows how different gears slide and engage to achieve different speeds (e.g., first, second, third, etc.) and explains the gear ratios involved for each speed.
Pupils’ Activity: Pupils observe the demonstration, paying attention to how gears engage and disengage. They may ask questions about the process.
Learning Point: Pupils understand the principle of gear engagement and speed selection in a sliding mesh gearbox.

Step 5: Reverse Mechanism

Time: 6 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher specifically focuses on the reverse mechanism, explaining the role of the idler gear. The teacher demonstrates how the idler gear is engaged between the lay shaft gear and the main shaft gear to reverse the direction of rotation, leading to the vehicle moving backward.
Pupils’ Activity: Pupils observe the demonstration of the reverse mechanism and understand how the idler gear functions to achieve reverse motion.
Learning Point: Pupils grasp the concept and function of the reverse idler gear.

Step 6: Practical Observation/Dismantling Review

Time: 5 minutes
Teaching Skill: Guided Observation/Practical Application
Teacher’s Activity: The teacher guides pupils to observe the dismantled gearbox again, reinforcing the identification of parts and how they would interact during operation. The teacher encourages pupils to describe parts and their functions.
Pupils’ Activity: Pupils actively observe, identify parts, and describe their understanding of the operation based on the demonstration.
Learning Point: Pupils consolidate their understanding of gearbox components and their functions through practical observation.

Step 7: Evaluation/Review

Time: 5 minutes
Teaching Skill: Questioning/Assessment
Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. What is a gearbox and what is its main function in a vehicle?
2. Name three major components of a gearbox.
3. How is speed selected in a sliding mesh gearbox?
4. Explain the role of the idler gear in the reverse mechanism.

Pupils’ Activity: Pupils answer orally and in writing.
Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Summarization/Assignment
Teacher’s Activity: The teacher summarizes the key points of the lesson, emphasizing the importance of the gearbox for vehicle control. The teacher then gives pupils homework to draw a simple diagram of a sliding mesh gearbox and label its parts.
Pupils’ Activity: Pupils listen to the summary and copy down the homework assignment.
Learning Point: Pupils review the lesson content and have an assignment to reinforce learning.

Lesson Keywords

• Gearbox – A mechanical device that changes the speed and torque of the engine output.
• Transmission Unit – Another name for a gearbox.
• Sliding Mesh – A type of gearbox where gears slide along a shaft to engage.
• Idler Gear – An extra gear used to reverse the direction of rotation.
• Selector Fork – A component that moves gears into engagement.
• Reverse Mechanism – The system that allows a vehicle to move backward.

Differentiation

For pupils who grasp concepts quickly, the teacher can ask them to explain the difference between a sliding mesh gearbox and a constant mesh gearbox (if they have prior knowledge or can infer). For pupils needing more support, the teacher will provide additional visual aids and closer guidance during the practical observation, focusing on identifying fewer key components.

Note for teachers using this lesson plan

Ensure that the dismantled gearbox is clean and safe for pupils to handle and observe. Emphasize the practical application of this knowledge in real-world vehicle maintenance and repair. If a live vehicle is available, demonstrate the gear changes and feel of the gear lever for different speeds and reverse.