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How Brakes Work

admin | Nov 13, 2011 | Comments 1 898 views

How Brakes Work Introduction

A vehicle’s brakes cause the vehicle to decelerate or stop by converting the vehicle’s kinetic (motion) energy into heat energy through friction. The heat is dissipated to the atmosphere.

Kinetic (motion) energy

Brakes can be classified into disc brakes and drum brakes. In a disc brake, pads squeeze a disc that rotates together with the wheel, thereby slowing the disc and wheel. In a drum brake, shoes expand onto the inside surface of a drum that rotates together with the wheel, thereby slowing the drum and wheel. Vehicles use disc brakes on front wheels, and disc or drum brakes on rear wheels.

Disc Brake and Drum Brake

Brake discs and drums are made from cast iron. The friction materials applied to them in the form of brake shoes and pads were originally made from asbestos but for health-related reasons are now usually made from sintered alloy substances.

A vehicle’s brake system consists of service brake and parking brake. The service brakes are activated hydraulically by the brake pedal and are used while the vehicle is moving. The parking brake is activated mechanically by a parking brake lever or pedal, and it is used to hold the vehicle in position while stationary.

Service brake and Parking brake

2. Brake System Construction

Major component parts of brake system are as follows:

Brake System Major Parts

1 Brake pedal
2 Rear brake caliper
3 Rear brake disc
4 Parking brake lever
5 Front brake disc
6 Front brake caliper
7 Proportioning valve
8 Master cylinder
9 Brake booster

3. Service Brake

Master Cylinder
The master cylinder of a brake system converts pedal force into hydraulic pressure. When the brake pedal is depressed, pistons in the master cylinder are activated, causing pressure to act on the brake fluid. When the brake pedal is released, return springs move the pistons back to their original positions.

vehicles utilize tandem master cylinders. This type of master cylinder serves two independent hydraulic lines. Since both hydraulic lines are independent, fluid loss or other abnormalities in one line do not cause all the brakes to fail.

The master cylinder is fitted with a brake fluid reservoir. Fluid in the reservoir compensates for variations in the fluid level that accompany movement of the pistons and for permanent changes in the quantity of fluid in the brake lines that occur as the brake pads become worn. (When the brake pads are worn, the caliper piston must stroke longer to contact the pads to the disc. Thus more fluid is required in the brake line).

Brake Master Cylinder

1 Reservoir cap
2 Reservoir
3 Master cylinder
4 Secondary piston
5 Return spring
6 Rear right brake
7 Front left brake
8 Rear left brake
9 Front right brake
10 Return spring
11 Primary piston

Disc Brake
A disc brake consists of a brake disc, brake caliper and two brake pads. The brake disc is sandwiched by the brake pads and the pads are held inside the brake caliper. The brake hydraulic pressure created by the master cylinder is sent to the brake caliper and the caliper presses the pads against the disc.

Most brake discs are solid, but some vehicle models utilize ventilated discs (each of which contains radial vanes between its rubbing surfaces for optimum cooling performance) on their front brakes. (The front brakes are subjected to relatively heavy loading.) On certain models, ventilated discs are also used on the rear brakes.

Drum Brake
A drum brake consists of a brake drum, wheel cylinder, two brake shoes and a backing plate. The wheel cylinder and the brake shoes are mounted on the backing plate, which is fixed on the trailing arm and the brake drum is mounted on the rear hub and covering over them. The brake hydraulic pressure created by the master cylinder is sent to the wheel cylinder and the cylinder expands the shoes against the drum.

Proportioning Control Valve (PCV)

When a vehicle’s brake pedal is depressed, the vehicle undergoes a nosedive. At this time, the increased load on the front wheels makes the front brakes less prone to locking. On the other hand, the rear brakes become more prone to locking since the load on the rear wheels is simultaneously reduced.

Locking of the rear wheels would cause instability and could cause the vehicle to spin, so a proportioning control valve (PCV) is incorporated to adjust the brake pressure supplied to the front and rear brakes. When the brake hydraulic pressure exceeds a predetermined level, the PCV limits the amount of pressure supplied to the rear brakes, thus making them less likely to lock. Vehicles utilize dual PCVs, which are designed for operation with divided-line brake circuits. On some models, this function is built in the ABS hydraulic unit.

Brake Booster

1 Valve piston
2 To rear wheel cylinder
3 To rear wheel cylinder
4 Valve piston
5 From master cylinder
6 From master cylinder

The brake booster reduces the amount of force required to depress the brake pedal. The type used in vehicles is a diaphragm-type vacuum servo brake booster. This type boosts the force applied to the brake pedal utilizing the pressure differential between the atmospheric pressure and the vacuum pressure in the engine’s intake manifold. The boosted force is applied to the pistons in the master cylinder.

Brake Hoses and Pipes

Metal pipes are used to connect brake system components that are fixed to a vehicle’s chassis. Rubber hoses are used to connect components that move vertically due to operation of the suspension system. Vehicle uses an X-split divided-line connection arrangement to prevent a single abnormality from causing all the brakes to fail. In this arrangement, one line serves the front-right and rear-left brakes and the other line serves the front-left and rear-right brakes.