Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Gear Box
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Gear Box
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Subject: Auto Mechanical Works
Class: Senior Secondary 2
Term: 3rd Term
Week: 2
Theme: Transmission And Breaking System
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Sketch and describe the operation of differentkinds of gear box Carry out simplecalculation in volvingengine speed, gear ratio,and road speed Sketch and describe the operation of different gearlinkages
teeth and a driven gear with 40 teeth. `GR = 40 teeth / 20 teeth = 2:1` This means for every 2 revolutions of the driving gear, the driven gear makes 1 revolution. Torque is multiplied, speed is reduced.
Example 2: Calculating Wheel Speed An engine is running at 3000 RP
M. The gearbox is in 3rd gear with a ratio of 1.5:1, and the final drive ratio is 4:
1. Overall Gear Ratio (OGR) = Gearbox Ratio × Final Drive Ratio = 1.5 × 4 = 6:1 Wheel Speed (RPM) = Engine Speed / OGR = 3000 RPM / 6 = 500 RPM Example 3: Calculating Road Speed Using the values from Example 2, assume the vehicle has tyres with a diameter of 0.6 meters (approximately 23.6 inches). Wheel Circumference = π × D = 3.142 × 0.6 m = 1.8852 meters Road Speed (km/h) = (Wheel Speed (RPM) × Wheel Circumference (m) × 60) / 1000 Road Speed (km/h) = (500 × 1.8852 × 60) / 1000 Road Speed (km/h) = 56556 / 1000 = 56.56 km/h (approximately) 2.
4. Gear Linkages Gear linkages are the mechanical connections that transmit the driver's input from the gear selector lever to the internal selector forks within the gearbox.
1. Rod-Type Linkage: Description: Uses solid metal rods to connect the gear lever to the gearbox selector shafts. These rods are usually rigid and provide a direct mechanical connection.
Operation: When the driver moves the gear lever, the movement is directly transferred through a series of rods and pivot points to the selector shafts inside the gearbox, which then move the selector forks to engage the desired gear.
Sketch: (Teacher should draw a simplified top-down view showing a gear lever in the cabin, connected by a series of rigid rods with pivot points to the side of a gearbox. Illustrate how lever movement translates to gearbox internal shaft movement. Label gear lever, pivot points, connecting rods, gearbox.)
Advantage: Direct feel, robust, relatively simple, less prone to stretching.
Disadvantage: Can transmit more vibration and noise, requires precise alignment, more complex routing for front-wheel drive (FWD) vehicles where the gearbox is often transverse.
2. Cable-Type Linkage: Description: Employs flexible cables (similar to bicycle brake cables, but much stronger) to connect the gear lever to the gearbox. Each cable consists of an inner wire operating within an outer sheath.
Operation: When the driver moves the gear lever, one cable typically controls the fore-aft movement (gear selection along a gate, e.g., 1st to 2nd), and the other cable controls the side-to-side movement (selecting the gate, e.g., 1st/2nd gate to 3rd/4th gate). These cable movements pull or push the selector levers on the gearbox.
Sketch: (Teacher should draw a simplified view showing a gear lever connected by two flexible cables routing towards the gearbox. Illustrate how one cable handles selection and the other handles engagement. Label gear lever, flexible cables, gearbox selector points.)
Advantage: More flexible routing, reduced noise and vibration transfer, commonly used in FWD vehicles due to ease of packaging.
Disadvantage: Can feel less direct, cables can stretch or fray over time, potentially leading to sloppy shifting or difficulty in engaging gears. --- 2.
1. Introduction to the Gear Box A gearbox (also known as a transmission) is a mechanical device found in motor vehicles that allows the driver to select the appropriate gear ratio between the engine and the drive wheels.
Its primary functions are: Torque Multiplication: To increase the torque delivered to the wheels for starting, accelerating, or climbing hills, as the engine's torque output is often insufficient at low speeds.
Speed Variation: To vary the speed of the drive wheels relative to the engine speed.
Reverse Gear: To allow the vehicle to move backwards.
Neutral Position: To disconnect the engine from the drive wheels when stationary or idling. 2.
2. Types of Gear Boxes and Their Operation A. Manual Gearboxes (Manual Transmission - MT) Manual gearboxes require the driver to manually select gears using a gear lever and clutch pedal. They are generally categorized into three main types based on their internal design:
1. Sliding Mesh Gearbox: Description: This is the oldest and simplest type. Gears on the main shaft slide along splines to engage directly with corresponding gears on the layshaft. Only one set of gears is in mesh at any time.
Operation: To change gear, the driver disengages the clutch, moves the gear lever, which slides a gear on the main shaft into mesh with the appropriate gear on the layshaft. This requires precise timing and 'double de-clutching' to match gear speeds, making it difficult and prone to gear clash.
Sketch: (Teacher should draw a simple schematic showing two parallel shafts, with gears on the main shaft sliding to engage fixed gears on the layshaft. Label main shaft, layshaft, input shaft, output shaft, sliding gears, fixed gears.)
Advantage: Simple design, robust.
Disadvantage: Difficult to shift, noisy, prone to gear clash and wear. Rarely found in modern vehicles.
2. Constant Mesh Gearbox: Description: All gears on the main shaft are constantly in mesh with their corresponding gears on the layshaft.
However, the gears on the main shaft rotate freely on the shaft until they are locked to the main shaft by dog clutches.
Operation: The driver disengages the clutch, then moves the gear lever. This shifts a dog clutch (or selector fork) along splines on the main shaft to engage the desired gear, locking it to the main shaft. The gears are always spinning, reducing gear clash during engagement compared to sliding mesh.
Sketch: (Teacher should draw a schematic showing main shaft and layshaft with all gears constantly meshed. Illustrate dog clutches sliding on splines on the main shaft to engage specific gears. Label main shaft, layshaft, input shaft, output shaft, constantly meshed gears, dog clutches, selector forks.)
Advantage: Easier and smoother shifting than sliding mesh, less gear clash.
Disadvantage: Still requires driver skill to synchronize speeds, can be noisy without synchromesh.
3. Synchromesh Gearbox (Synchronizer Gearbox): Description: This is an improvement over the constant mesh design and is the most common type of manual gearbox in modern vehicles. It incorporates synchronizer units (synchros) that automatically match the rotational speed of the engaging gears before the dog clutch engages.
Operation: When the driver selects a gear, the selector fork moves a synchro hub towards the desired gear. A friction cone on the synchro ring first makes contact with a mating cone on the gear, bringing their speeds into synchronization. Once speeds are matched, the dog clutch then slides fully and locks the gear to the main shaft smoothly and silently.
Sketch: (Teacher should draw a detailed schematic focusing on a single gear pair and its synchromesh unit. Show the main shaft, a freely rotating gear, a synchro hub splined to the main shaft, a synchro ring with friction cone, and a dog clutch.
Illustrate the engagement sequence: cone contact, speed synchronization, then dog clutch engagement. Label all components clearly.)
Advantage: Very smooth, quiet, and easy gear changes with minimal gear clash. High durability.
Disadvantage: More complex and expensive to manufacture than older manual types.
B. Automatic Gearbox (Automatic Transmission - AT)
Description: An automatic gearbox automatically selects the appropriate gear ratio without driver its synchromesh unit. Show the main shaft, a freely rotating gear, a synchro hub splined to the main shaft, a synchro ring with friction cone, and a dog clutch.
Illustrate the engagement sequence: cone contact, speed synchronization, then dog clutch engagement. Label all components clearly.)
Advantage: Very smooth, quiet, and easy gear changes with minimal gear clash. High durability.
Disadvantage: More complex and expensive to manufacture than older manual types.
B. Automatic Gearbox (Automatic Transmission - AT)
Description: An automatic gearbox automatically selects the appropriate gear ratio without driver intervention for gear changes.
Operation (Brief): Uses a torque converter instead of a clutch to transmit power from the engine. Power then goes through a system of planetary gear sets, which provide different gear ratios. Gear changes are controlled by a hydraulic system (or increasingly, electronic controls) that actuates clutch packs and brake bands to lock or unlock components of the planetary gear sets.
Sketch: (Teacher should draw a very simplified block diagram showing: Engine -> Torque Converter -> Planetary Gear Set(s) -> Output Shaft. Briefly label key components but focus on the functional flow rather than internal detail given the scope.)
Advantage: Easier to drive, smoother acceleration, less driver fatigue, especially in heavy traffic common in cities like Lagos or Abuja.
Disadvantage: Typically less fuel-efficient than manual transmissions (though modern ATs are closing the gap), more complex, heavier, and more expensive to repair. 2.
3. Gear Ratio, Engine Speed, and Road Speed Calculations
A. Gear Ratio (GR): The gear ratio is the ratio of the number of teeth on the driven gear to the number of teeth on the driving gear, or the ratio of their rotational speeds.
Formula: GR = (Number of teeth on Driven Gear) / (Number of teeth on Driving Gear) GR = (Speed of Driving Gear) / (Speed of Driven Gear)
A gear ratio greater than 1:1 means torque is multiplied (lower gear).
A gear ratio less than 1:1 means speed is multiplied (overdrive gear).
B. Engine Speed (ES): Measured in Revolutions Per Minute (RPM). This is the rotational speed of the engine crankshaft.
C. Road Speed (RS): The actual speed of the vehicle, typically measured in km/h or mph. This depends on engine speed, overall gear ratio (gearbox ratio x final drive ratio), and tyre size.
D. Formulas for Calculation: The fundamental relationship can be expressed as: `Road Speed (RS) = (Engine Speed (ES) × Wheel Circumference) / (Overall Gear Ratio × 60)` Where: ES is in RPM Wheel Circumference = π × Tyre Diameter (in meters) Overall Gear Ratio (OGR) = Gearbox Ratio (GR) × Final Drive Ratio (FDR) 60 is used to convert minutes to hours if speed is in km/h and circumference in meters. A more direct approach for instructional purposes: `ES / (OGR × RS_factor) = RS` (where RS_factor converts units appropriately) Or, more commonly, to find road speed given engine speed: `Road Speed (km/h) = (Engine Speed (RPM) × Tyre Diameter (m) × π) / (Overall Gear Ratio × 1000 / 60)` Simplifying: `Road Speed (km/h) = (ES × D_tyre × π × 60) / (OGR × 1000)` Let's use a common intermediate step: Wheel Speed (WS) `Wheel Speed (RPM) = Engine Speed (RPM) / Overall Gear Ratio (OGR)` Then, to convert Wheel Speed to Road Speed: `Road Speed (km/h) = (Wheel Speed (RPM) × Wheel Circumference (m) × 60) / 1000` `Wheel Circumference (m) = π × Tyre Diameter (m)` Example 1: Calculating Gear Ratio A gear train has a driving gear with 20 teeth and a driven gear with 40 teeth. `GR = 40 teeth / 20 teeth = 2:1` This means for every 2 revolutions of the driving gear, the driven gear makes 1 revolution. Torque is multiplied, speed is reduced.
Example 2: Calculating Wheel Speed An engine is running at 3000 RP
M. The gearbox is in 3rd gear with a ratio of 1.5:1, and the final drive ratio is 4:
1. Overall Gear Ratio (OGR) = Gearbox Ratio × Final Drive Ratio = 1.5 × 4 = 6:1 Wheel Speed (RPM) = 3.
1. Teacher Activities Introduction (10 minutes): Review previous knowledge on engine components and power transmission.
Pose questions: "What happens when a driver wants to move a vehicle from rest? Why can't the engine just connect directly to the wheels?" Elicit responses leading to the need for a gearbox. Introduce the topic "Gear Box" and state the lesson objectives clearly. Concept Explanation and Demonstration (30 minutes): Explain the purpose and functions of a gearbox using a real gearbox (if available) or detailed diagrams/models. Describe the operation of different manual gearbox types (Sliding Mesh, Constant Mesh, Synchromesh) using sketches on the board or projected diagrams. Emphasize the evolution and advantages of synchromesh. Briefly explain the basic principle of an automatic gearbox for comparison. Introduce the concepts of gear ratio, engine speed, and road speed. Work through Example 1 (Gear Ratio calculation) and Example 2 (Wheel Speed calculation) on the board, explaining each step. Introduce gear linkages (Rod-type and Cable-type), demonstrating their mechanisms with sketches.
Guided Practice Facilitation (20 minutes): Present Example 3 (Road Speed calculation) for students to attempt in pairs or individually first, then guide them through the solution on the board, ensuring understanding of units. Pose short conceptual questions about gearbox types or linkages. Activity Monitoring and Feedback (10 minutes): Circulate among students during problem-solving and sketching activities, providing individual support and constructive feedback. Address common misconceptions or difficulties observed.
Conclusion (5 minutes): Summarize key learning points. Assign independent practice and hint at the next lesson. 3.
2. Student Activities Engagement: Respond to teacher's introductory questions, drawing on prior knowledge. Pay close attention during explanations and demonstrations.
Active Learning: Observe and analyze diagrams or physical gearbox components. Take notes on definitions, types, and operational descriptions. Actively participate in class discussions. Sketch the internal mechanisms of different gearboxes (Synchromesh emphasis) and gear linkages as explained by the teacher.
Problem Solving: Attempt calculation problems during guided practice, applying the formulas learned. Collaborate with peers on problem-solving tasks.
Questioning and Clarification: Ask questions for clarification on any confusing concepts, calculations, or operational details. ---
Vehicle Maintenance and Entrepreneurship in Nigeria: Understanding the gearbox is fundamental for mechanics operating in the informal and formal automotive repair sectors across Nigeria. Knowledge of synchromesh operation helps in diagnosing shifting problems (e.g., grinding gears) in manual transmissions common in many private and commercial vehicles. Being able to perform calculations related to gear ratios allows technicians to advise customers on optimal tyre sizes or differential changes for specific applications (e.g., heavy-duty hauling vs. fuel efficiency), directly impacting vehicle performance and economy for Nigerian drivers and businesses. This knowledge can lead to specialized gearbox repair services, fostering entrepreneurship.
Driving Efficiency and Safety: For drivers in Nigeria, especially those operating commercial vehicles like "Danfo" buses, "Keke Napep" tricycles, or trucks, understanding how to select the correct gear based on engine speed, load, and road conditions (e.g., climbing a bridge like the 3rd Mainland Bridge or navigating congested traffic in Abuja) is crucial. Correct gear selection directly impacts fuel consumption, reducing operational costs, and enhances vehicle control, contributing to road safety. The principles of gear ratio help explain why a vehicle struggles uphill in a high gear and why downshifting is necessary. Local Automotive Assembly and Component Sourcing: As Nigeria strives to develop its automotive industry (e.g., Innoson Vehicle Manufacturing, Peugeot Automobile Nigeria), a deep understanding of gearbox technology is vital. Engineers and technicians involved in vehicle assembly, quality control, or even potential local component manufacturing need to apply these principles. Knowledge of different gear linkage types, for instance, helps in optimizing cabin ergonomics and ensuring reliability in locally assembled vehicles, tailoring designs to suit Nigerian environmental and usage demands. ---