Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Brake System
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Brake System
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Subject: Auto Mechanical Works
Class: Senior Secondary 2
Term: 2nd Term
Week: 3
Theme: Transmission And Breaking System
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Sketch the line diagram of brake system and labelthe main parts Describe brake operationand state reasons for adjustment and bleeding Adjust and bleed brakes Calculate braking for ce,power and to rque and statethe major factors that affectbraking efficiency
2. 1. Functions of the Brake System The brake system in a vehicle serves three primary functions:
1. To slow down or stop a moving vehicle: This is the most critical function, converting kinetic energy into heat energy through friction.
2. To hold a vehicle stationary: Essential for parking, especially on inclines, and preventing unintended movement.
3. To control vehicle speed during descent: Helps maintain a safe speed when driving downhill without over-relying on engine braking or overheating the service brakes. 2.
2. Principles of Operation of Brake Systems Modern vehicles primarily use hydraulic brake systems, often assisted by vacuum boosters. Mechanical systems are typically used for parking brakes. 2.2.
1. Mechanical Brake System Principle: Operates using levers, rods, and cables to transmit force directly to the brake shoes or pads. This system relies on mechanical advantage to multiply the force applied by the driver.
Application: Primarily used for parking brakes (handbrake/footbrake) in modern vehicles. Historically, it was used for service brakes on older vehicles.
Components (Parking Brake Example): Handbrake Lever: Driver's input point.
Connecting Rod/Cable: Transmits force from the lever to the wheel brake mechanism.
Equalizer: Distributes force equally to both rear wheels.
Toggle Lever/Actuator: Engages the brake shoes/pads at the wheel.
Operation: When the driver pulls the handbrake lever, a cable mechanism pulls on levers at the rear wheels, forcing the brake shoes against the brake drums (or pads against discs), thus holding the vehicle stationary. 2.2.
2. Hydraulic Brake System Principle: Based on Pascal's Principle, which states that pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel. This allows a small force applied over a small area (master cylinder piston) to generate a much larger force over a larger area (wheel cylinder pistons/caliper pistons), providing significant braking power.
Components: Brake Pedal: Driver's input.
Master Cylinder: Converts pedal force into hydraulic pressure. Contains one or two pistons, reservoirs for brake fluid, and compensated ports. Modern vehicles use a tandem master cylinder for safety, separating front and rear circuits.
Brake Fluid: An incompressible liquid that transmits hydraulic pressure. Must have a high boiling point and resistance to moisture absorption (e.g., DOT 3, DOT 4).
Brake Lines/Hoses: Steel tubes (lines) for rigid connections and flexible rubber hoses for connections to components that move (e.g., calipers on steering knuckles).
Wheel Cylinders (Drum Brakes): Located at each wheel, convert hydraulic pressure back into mechanical force to push brake shoes against the drum.
Brake Calipers (Disc Brakes): Contain pistons that push brake pads against the rotating disc.
Brake Shoes and Linings (Drum Brakes): Shoes are crescent-shaped, with friction material (linings) riveted or bonded to them.
Brake Pads (Disc Brakes): Friction material bonded to a steel backing plate.
Brake Drums (Drum Brakes): A cast-iron rotating component, against which shoes press.
Brake Discs/Rotors (Disc Brakes): A cast-iron or composite rotating component, gripped by pads.
Operation:
1. Driver presses the brake pedal.
2. The pedal force is transmitted to the master cylinder piston(s).
3. The piston(s) push brake fluid, generating hydraulic pressure.
4. This pressure is transmitted equally through brake lines to all wheel cylinders/calipers.
5. At the wheels, the hydraulic pressure forces wheel cylinder pistons (drum brakes) or caliper pistons (disc brakes) outwards.
6. The pistons push brake shoes against drums or brake pads against discs.
7. Friction between the linings/pads and the rotating drums/discs generates a braking force, slowing or stopping the vehicle.
8. When the pedal is released, spring pressure in the master cylinder and wheel cylinders/calipers retracts the pistons, releasing the brakes. 2.2.
3. Servo-Assisted (Vacuum-Assisted)
Brake System Principle: Uses a vacuum booster to multiply the force applied by the driver to the brake pedal, making braking easier and requiring less effort. It works by utilizing the pressure difference between the engine manifold vacuum and atmospheric pressure.
Components: Vacuum Booster (Brake Servo): A large, round unit typically mounted between the brake pedal and the master cylinder. Contains a diaphragm, vacuum check valve, and control valve. * Vacuum Hose: Connects the booster to cylinder and wheel cylinders/calipers retracts the pistons, releasing the brakes. 2.2.
3. Servo-Assisted (Vacuum-Assisted)
Brake System Principle: Uses a vacuum booster to multiply the force applied by the driver to the brake pedal, making braking easier and requiring less effort. It works by utilizing the pressure difference between the engine manifold vacuum and atmospheric pressure.
Components: Vacuum Booster (Brake Servo): A large, round unit typically mounted between the brake pedal and the master cylinder. Contains a diaphragm, vacuum check valve, and control valve.
Vacuum Hose: Connects the booster to the engine intake manifold (source of vacuum).
Operation:
1. The booster is normally under vacuum on both sides of its diaphragm.
2. When the driver presses the brake pedal, a control valve opens, allowing atmospheric pressure to enter one side of the diaphragm while the other side remains under vacuum.
3. The pressure difference creates a strong force on the diaphragm, which in turn pushes the master cylinder piston, significantly augmenting the driver's pedal effort.
4. This amplified force results in more effective braking with less pedal effort. 2.
3. Line Diagram of Brake System (Teacher to draw on board or project. The sketch should show the following key components and their connections.) Brake Pedal Pushrod Vacuum Booster (Servo) Master Cylinder (with fluid reservoir) Brake Lines (to front and rear wheels) Front Calipers (with Disc and Pads) Rear Wheel Cylinders (with Drum and Shoes) Parking Brake Lever and Cable (connecting to rear wheels) ``` Driver's Foot | V [Brake Pedal] | V [Pushrod] ----> [Vacuum Booster (Servo)] | | V | (Vacuum from Engine Manifold) [Master Cylinder] | ^ | ^ | | | | (Brake Fluid Reservoir) | | | | | V | V [Brake Line (Front)] [Brake Line (Rear)] | | V V [Front Right Caliper] [Rear Right Wheel Cylinder] (with Disc & Pads) (with Drum & Shoes) | | V V [Front Left Caliper] [Rear Left Wheel Cylinder] (with Disc & Pads) (with Drum & Shoes) [Parking Brake Lever] | V [Parking Brake Cable (to Rear Wheels)] ``` 2.
4. Reasons for Brake Adjustment and Bleeding 2.4.
1. Brake Adjustment Purpose: To maintain the correct clearance between the brake shoes/pads and the drum/disc, and to ensure consistent pedal travel.
Reasons for Adjustment: Brake Linings Wear: As brake linings wear down, the clearance increases, leading to excessive brake pedal travel and reduced braking efficiency. Adjustment compensates for this wear.
Uneven Braking: Improperly adjusted brakes can cause one wheel to brake more effectively than another, leading to the vehicle pulling to one side during braking.
Increased Pedal Travel: A spongy or low brake pedal can indicate excessive clearance, requiring adjustment.
Parking Brake Effectiveness: The parking brake cable often requires adjustment to ensure it properly engages and holds the vehicle.
Types of Adjusters: Manual adjusters (older systems, often for parking brake) and automatic adjusters (common in modern drum brakes, adjust during reverse braking or pedal application). 2.4.
2. Brake Bleeding Purpose: To remove air bubbles from the hydraulic brake system.
Reasons for Bleeding: Air in the System: Air is compressible, unlike brake fluid. If air enters the brake lines, pressing the pedal will first compress the air instead of transmitting pressure to the wheels, resulting in a spongy or soft brake pedal and severely reduced or lost braking power.
After Component Replacement: Whenever a hydraulic component (e.g., master cylinder, wheel cylinder, caliper, brake line) is opened or replaced, air inevitably enters the system.
Fluid Contamination/Change: When replacing old brake fluid or flushing the system, bleeding is necessary to ensure no air remains.
Brake Pedal Sponginess: A common symptom of air in the brake lines. 2.
5. Brake Adjustment and Bleeding Procedures (
Note: These are general procedures. Specific vehicle models may have variations.) 2.5.
1. Procedure for Brake Adjustment (
Example: Rear Drum Brakes with Star Wheel Adjuster)
1. Safety First: Park the vehicle on a level surface, engage the parking brake, and place chocks behind the front wheels. Raise the rear of the vehicle using a jack and secure it with jack stands. Remove grip but also cause tire bounce.
5. Vehicle Speed: Kinetic energy increases quadratically with speed (KE = 0.5 * mv^2). Doubling speed quadruples the energy to dissipate, vastly increasing stopping distance.
6. Vehicle Weight/Load: Heavier vehicles have more inertia and require greater braking force and longer distances to stop. Overloading a vehicle (common in Nigeria with commercial vehicles like trucks and buses) severely compromises braking efficiency.
7. Driver Reaction Time: The time it takes for the driver to perceive a hazard and apply the brakes directly adds to the overall stopping distance.
8. Brake Balance: Uneven braking force distribution among wheels can lead to instability (vehicle pulling) and reduced overall efficiency. force generated by the friction between the brake linings/pads and the drums/discs, which opposes the motion of the vehicle.
Formula: F_b = μ N F_b = Braking Force (Newtons, N) μ (mu) = Coefficient of friction between the braking surfaces (dimensionless, typically 0.3 to 0.5 for brakes). N = Normal force (the force pressing the brake surfaces together, Newtons, N). This is usually the clamping force of the caliper or the outward force of the wheel cylinder.
Example 1: A brake caliper applies a clamping force of 500 N on a brake pad. If the coefficient of friction between the pad and the disc is 0.4, calculate the braking force generated by that pad. N = 500 N μ = 0.4 F_b = 0.4 500 N = 200
N. Commentary: This is the force for one pad. Total braking force would be for all pads/shoes. 2.6.
2. Braking Power (P_b)
Definition: The rate at which work is done by the brakes to slow down the vehicle, or the rate at which kinetic energy is dissipated as heat.
Formula: P_b = F_b v P_b = Braking Power (Watts, W) F_b = Total Braking Force (Newtons, N) v = Velocity of the vehicle (meters per second, m/s).
Example 2: A vehicle is braking with a total braking force of 5000 N while traveling at a speed of 10 m/s. Calculate the braking power. F_b = 5000 N v = 10 m/s P_b = 5000 N 10 m/s = 50,000 W or 50 k
W. Commentary: Braking power is highest at higher speeds, leading to significant heat generation. 2.6.
3. Braking Torque (T_b)
Definition: The rotational force applied by the brakes to the wheels, causing them to decelerate.
Formula: T_b = F_b r T_b = Braking Torque (Newton-meters, Nm) F_b = Braking Force applied at the friction surface (Newtons, N) r = Effective radius from the center of rotation to the point where the braking force is applied (meters, m). For disc brakes, this is often the effective radius of the pad's contact point. For drum brakes, it's the drum's inner radius.
Example 3: A braking force of 200 N is applied to a brake disc at an effective radius of 0.15 meters. Calculate the braking torque. F_b = 200 N r = 0.15 m T_b = 200 N 0.15 m = 30 Nm.
Commentary: Torque at the wheel then translates to torque at the road through the tire radius. 2.
7. Major Factors Affecting Braking Efficiency
1. Coefficient of Friction (μ): Between brake linings/pads and drums/discs. Higher μ means better braking. Affected by material choice, temperature, and moisture.
2. Condition of Brake Components: Wear: Worn brake pads/linings, scored discs/drums reduce friction and heat dissipation.
Fluid Level/Condition: Low fluid leads to air entry. Old/contaminated fluid can boil or corrode components.
Air in System: Causes a spongy pedal and reduces effective pressure transmission.
Leaks: Loss of hydraulic pressure.
Component Malfunction: Seized calipers, worn wheel cylinders, faulty master cylinder.
3. Tire Condition: Tread Depth: Worn tires reduce grip with the road, leading to skidding and longer stopping distances, even with perfect brakes.
Tire Pressure: Incorrect pressure affects the tire contact patch and grip.
4. Road Surface Condition: Dry vs.
Wet/Slippery: Water, oil, gravel, sand significantly reduce the coefficient of friction between tires and road, increasing stopping distances. Rough vs.
Smooth: Rougher surfaces can provide more grip but also cause tire bounce.
5. Vehicle Speed: Kinetic energy increases quadratically with speed (KE = 0.5 * mv^2). Doubling speed quadruples the energy to dissipate, vastly increasing stopping distance.
6. Vehicle Weight/Load: Heavier vehicles have more inertia and require greater braking force and longer distances to stop. Overloading a vehicle (common in Nigeria with commercial vehicles like trucks and buses) severely compromises braking efficiency.
7. Driver Reaction Time: The time it takes for the driver to perceive a hazard and apply the brakes directly adds to the overall stopping distance.
Road Safety and Accident Prevention: Understanding brake systems is paramount for improving road safety in Nigeria. Learners can appreciate how regular brake maintenance (e.g., checking fluid, replacing worn pads/shoes, bleeding air) directly prevents brake failures and reduces the risk of accidents, especially for commercial vehicles like "Okada" (motorcycles), "Keke NAPEP" (tricycles), and buses that are crucial for public transport. This knowledge empowers them to advocate for or perform necessary maintenance.
Entrepreneurship and Automotive Service: The skills acquired in sketching, describing operation, adjusting, and bleeding brakes are directly applicable to careers as automotive technicians or entrepreneurs in local mechanic workshops (e.g., a "panel beater" who also performs brake services). There is a constant demand for skilled individuals who can reliably service brake systems for the diverse range of vehicles on Nigerian roads, from sedans to heavy-duty trucks transporting goods. Vehicle Inspection and Pre-Purchase Checks: For individuals looking to buy a used vehicle in Nigeria, knowledge of brake systems is crucial for pre-purchase inspections. A learner can identify signs of poor brake maintenance (e.g., soft pedal, squealing brakes, fluid leaks, uneven wear) which can indicate potential safety hazards or costly repairs, empowering them to make informed decisions and avoid unsafe vehicles.