BRAKING SYSTEM

Braking System

An automotive braking system is a group of mechanical, electronic and hydraulically activated components which use friction / heat to stop a moving vehicle.

Principle of Braking

Common principles on which a braking system depends when stopping a car is friction and heat. By applying resistance, or friction, to a turning wheel, a vehicle's brakes cause the wheel to slow down and eventually stop, creating heat as a byproduct. The rate at which a wheel can be slowed depends on several factors including vehicle weight, braking force and total braking surface area.

Energy Conversion

The brake system converts the kinetic energy of vehicle motion into heat

Overview

Brakes convert kinetic energy into heat by creating friction

System must have very high reliability

Brake system is actually an energy conversion device that converts kinetic energy (car in motion possessing momentum) to potential energy (car stopping) through dissipating heat and noise to the surrounding air. The friction surfaces of the brake pads on a disc brake, or the brake shoes on a drum brake convert the forward motion of the vehicle into heat. Heat is what causes the friction surfaces

Requirement of Braking System

Most engines generate a large amount of power to get the car up to speed quickly

To stop the car, an equal amount of power must be expended to stop the car.

Brakes turn the power of the rolling wheel to heat.

The quicker you stop, the more power is required, the more heat is generated

Requirement of brakes:The function of brakes is to stop the vehicle within the required time. The brake of the vehicle absorbs all the energy given by the engine plus that due to the momentum of the vehicle. This energy must then be dissipated. In most of the vehicles, the energy is absorbed by the brakes in the form of heat and dissipated into the stream of air passing around the vehicle.       Apart from stopping the vehicle the brakes should perform the others function too like the vehicle should stop without any jerk and the retardation should be smooth. Also the rate of retardation should be equal to the pedal effort and the effort applied by the driver to stop the vehicle should not be excessive. The rate of wear should also be low. The brake system should not be affected by water, heat, and dust etc. It should require a very low maintenance and durable.

Factors Governing Braking

Pressure The amount of friction generated between moving

surfaces contacting one another depends in part on the pressure exerted on the surfaces.

Coefficient of friction The amount of friction between 2 surfaces (pads and

rotors or shoes and drums) Determined by dividing the force required to pull an

object across a surface by the weight of the object

Friction

Factors Governing Braking

Frictional Contact Surface The amount of surface, or area, that is

in contact. Simply put, bigger brakes stop a car

more quickly than smaller brakes used on the same car.

Heat Dissipation The tremendous heat created by the

rubbing brake surfaces must be conducted away from the pad and rotor (or shoe and drum) and be absorbed by the air.

Importance of Brakes

Loss of brakes can cause accidents, death, and injury

When working on brakes, keep in mind that peoples lives are at stake

If in doubt, seek help

Brake rules

The car must have four wheel brakes operated by a single control. It must have two independent hydraulic circuits with

independent fluid reserves.The brake system must be capable of locking all four

(4) wheels, and stopping the vehicle in a straight line.The braking systems must be protected with scatter

shields from failure of the drive train )A brake pedal over-travel switch must be installed.

This switch must kill the ignition and cut the power to any electrical fuel pumps.The car must be equipped with a red brake light that

illuminates when ever the brakes are applied

How does a braking system work?

When the brake pedal is depressed, the pressure on the brake pedal moves a piston in the master cylinder, forcing the brake fluid from the master cylinder through the brake lines and flexible hoses to the calipers and wheel cylinders. The force applied to the brake pedal produces a proportional force on each of the pistons.

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The calipers and wheel cylinders contain pistons, which are connected to a disc brake pad or brake shoe. Each output piston pushes the attached friction material against the surface of the rotor or wall of the brake drum, thus slowing down the rotation of the wheel. When pressure on the pedal is released, the pads and shoes return to their released positions. This action forces the brake fluid back through the flexible hose and tubing to the master cylinder.

Classification of brakes

In automobile generally two types of brakes are used :-

Mechanical brakesAir brakes & vaccum brakes

Mechanical Brakes

Mechanical brakes all act by generating frictional forces as two surfaces rub against each other. The stopping power or capacity of a brake depends largely on the surface area of frictional surfaces as well as on the actuation force applied. The friction and wear encountered by the working surfaces are severe. Thus, the durability of a brake or service life between maintenance depends heavily on the type of material used to line the shoe or pad.

 

Air Brakes

The compressed air is used for obtaining brake application. The brake pipe and feed pipe run throughout the length of the coach. Brake pipe and feed pipe on consecutive coaches in the train are coupled to one another by means of respective hose couplings to form a continuous air passage from the locomotive to the rear end of the train. The compressed air is supplied to the brake pipe and feed pipe from the locomotive. The magnitude of braking force increases in steps with the corresponding reduction in brake pipe pressure and vice-versa.

Air Brakes

Vacuum Brakes

The vacuum brake system derives its brake force from the atmospheric pressure acting on the lower side of the piston in the vacuum brake cylinder while a vacuum is maintained above the piston. The train pipe runs throughout the length of the coach and connected with consecutive coaches by hose coupling. The vacuum is created in the train pipe and the vacuum cylinder by the ejector or exhauster mounted on the locomotive.

 

Vacuum Brakes

Drum Brakes

With drum brakes, fluid is forced into the wheel cylinder, which pushes the brake shoes out so that the friction linings are pressed against the drum, which is attached to the wheel, causing the wheel to stop.

Drum Brake Parts

Brake Shoes (5, 12) Backing Plate (14) Return Springs (3, 13) Hold Down Springs

(11) Self Adjusting Parts

(4, 7, 8, 10) Wheel Cylinder Parking Brake (6, 9) Drums

PARTS OF DRUM BRAKING SYSTEM

Brake ShoesLike the disk pads, brake shoes consist of a steel shoe with the friction material or lining riveted or bonded to it.  Also like disk pads, the linings eventually wear out and must be replaced.  If the linings are allowed to wear through to the bare metal shoe, they will cause severe damage to the brake drum.

Backing PlateThe backing plate is what holds everything together.  It attaches to the axle and forms a solid surface for the wheel cylinder, brake shoes and assorted hardware.  It rarely causes any problems.

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Brake DrumBrake drums are made of iron and have a machined surface on the inside where the shoes make contact.   Just as with disk rotors, brake drums will show signs of wear as the brake linings seat themselves against the machined surface of the drum.  When new shoes are installed, the brake drum should be machined smooth. Brake drums have a maximum diameter specification that is stamped on the outside of the drum. When a drum is machined, it must never exceed that measurement.   If the surface cannot be machined within that limit, the drum must be replaced.

Drum Brakes

Expanding shoes create force on the inner surface of the drum

Used on the rear of some trucks and SUV’s

Self-energizing design requires less activation force

Require periodic adjustment

Drum Brakes

Disc Brake Parts

Calipers (4) Brake Pads Rotors (3)

Construction

Disc brakes use a cast iron rotor, inboard of the vehicle wheel.

Both sides of the rotor are machined smooth for the brake pads to rub against.

Usually the 2 surfaces are separated by a finned centre section for better cooling (ventilated rotors).

The pads are attached to metal shoes, which are actuated by pistons housed in the brake calliper.

Disc Brake Operation

A piston inside the caliper pushes the brake pads toward each other

The rotor is located between the pads

The squeezing effect causes friction which slows the car

Disk Brakes

Caliper squeezes pads to create force on the surfaces of the rotor.

Used in most automotive applications.

Benefits: Simple design Self adjusting Rotor venting allows

faster heat dissipation

2 Types of disk brake calipers

Floating caliper Piston pushes

pad against the inner rotor surface, reaction force causes the caliper to slide and contact the outer surface

2 Types of disk brake calipers

Fixed caliper has at least one piston on each side

Each piston drives it’s corresponding pad into contact with the rotor

Disc Brake Material

A disc brake usually made of cast iron or ceramic composites (including carbon, Kevlar and silica).

DESIGN

The design of the disc varies somewhat. Some are simply solid cast iron, but others are hollowed out with fins or vanes joining together the disc's two contact surfaces. This "ventilated" disc design helps to dissipate the generated heat and is commonly used on the more-heavily-loaded front discs.Many higher performance brakes have holes drilled through them. This is known as cross-drilling A cross-drilled disc on a

modern motorcycle

Parking Brake for Rear Wheels

Power Assistance

Power booster reduces pedal force required to stop vehicle

Engine vacuum is used to actuate a large diaphragm

When the pedal is depressed, vacuum is applied to only one side of the diaphragm and force is applied to the master cylinder push rod

Booster stores sufficient vacuum for several brake applications if the engine fails.

Power Booster in closed position

Vacuum Assist Units

Uses engine vacuum to add to the force of you foot when applying brakes

Leaking seals can cause lose of assist (hard brakes) or lose of brake fluid