INTRODUCTION
The brake is one of the most important parts of an automobile as it is must for controlling speed. Whenever the brake is applied, the brake shoe comes in contact with the brake drum and the vehicle slows down. Whenever the contact from the brake drum is removed the vehicle speeds up slowly.
Function of Automobile Brakes
The function of the braking system is to retard the speed of the vehicle or bring it to rest in shortest possible distance when required. A vehicle can be held on an inclined surface against the pull of gravity by the application of brake. It absorbs energy while remaining in action and this absorbed energy appears in the form of heat.
Braking System
The automobiles are fitted with two brakes, the service or foot brake and the emergency or hand brake. The foot brake is used to control the speed of the vehicle and to stop it, when and where desired, by the application of the force on the brake pedal. The hand brake, applied by lever, is used to keep the vehicle from moving when parked. Head brakes are called emergency brakes because they are applied when the service brakes fails. Virtually all vehicles are now equipped with 4-wheel brakes. The front brakes must operate without interfering with the steering action.
Fig.1.1 Layout of a braking system
1.1 CLASSIFICATION OF BRAKING SYSTEM
The brakes for automotive use may be classified according to the following considerations:
1. Purpose
2. Location
3. Method of actuation
1.1.1 Purpose
The brakes may be classified as primary and the parking or the secondary brakes. The service brakes are the main brakes used for stopping the vehicles while in motion, whereas the parking brakes are meant to hold the vehicle on the slope.
1.1.2 Location
The brakes may be located either at the transmission or at the wheels. The wheel brakes are better from the heat dissipation point of view for two reasons. First, the location of the transmission brakes from this view point is very poor and, secondly, there is only brake drum available for heat dissipation. However if the brakes are located in the transmission, the braking torque is automatically divided by differential between two wheels and no special compensation is needed. In case of automobiles, the wheel brakes are used universally.
1.1.3 Method of Actuation
There are different types of brakes like mechanical brakes, hydraulic brakes, electric brakes, vacuum brakes and air brakes. The mechanical brakes have assemblies that consist of mechanical elements for stopping the shafts. The mechanical brakes use levers or linkages to transmit force from one point to another. The Hydraulic brakes require hydraulic power to activate the brake. Hydraulic brakes are assemblies which consist of elements to stop the shafts in equipment drives. The electric brake uses electrically actuated components to stop the shafts. There are different type of electric brakes like band brakes, drum brakes, disc brakes and cone brakes. Electric brakes are used in applications that require fast response times and precise tension control. The vacuum brakes are used in trains. The vacuum is created by operating a value in locomotive, the vacuum actuated braked pistons on each vehicle, and the braking degree can be increased or decreased by the driver. The air brakes are also used in trains, in simple form the air brake is called straight air system, compressed air pushes on a piston in a cylinder. The piston is connected to the brake shoe with the help of mechanical linkage to rub on the wheels to slow down the train.
1.2 FUNCTIONING OF A BRAKE
When brake pedal is steeped on the, we are actually pushing against a plunger in the master cylinder which forces hydraulic oil(brake fluid) through a series of tubes and hoses to the braking unit at each wheel. Since the hydraulic fluid cannot be compressed, pushing fluid through a pipe is just like pushing a steel bar through a pipe. Unlike a steel bar, however, fluid can be directed through many twists and turns on its way to its destinations, arriving with the exact same motion and pressure that is started with. It is very important that the fluid is pure liquid and that there is no air bubbles in it. Air can compress, and air in the system causes sponginess to the pedal and severely reduced braking efficiency. If air is suspected, then the system must be bled to remove the air. There are “bleeder screws†at each wheel cylinder and caliper for this purpose.
1.3 DISC BRAKES
Most modern cars have disc brakes on the front wheels, and some have disc brakes on all four wheels. The most common type of disc brake on modern cars is the single-piston floating caliper. The modern automotive brake system has been refined for over 100 years and become extremely dependable and efficient. From the construction point of view, two categories are the drum brakes and the disc brakes.
The typical brake system consists of disk brakes in front and either disk or drum brakes in the rear connected by a system of tubes and hoses in the rear connected by a system of tubes and hoses that link the brake at each wheel to the master cylinder. Other systems that are connected with the brake system include the parking brakes, power booster and the anti-lock system. On a disk brake, the fluid from the master cylinder is forced into a caliper where it presses against a piston. The piston, in turns, squeezes two brake pads against the disk (rotor) which is attached to the wheel forcing it to slow down or stop.
1.4 DISC v/s DRUM BRAKES
In today’s automobiles world, it is not uncommon to find four-wheel disc brakes as standard equipment on medium-priced, non performance-oriented models. The majority of new vehicles, however, continue to utilize a front-disc/rear-drum brake setup. Major reasons for this include the following:
In case of disc brakes friction surfaces are directly exposed to the cooling air, where as in the drum type, the friction occurs on the internal surfaces, from which heat can be dissipated only after it is passed by conduction through the drums.
There is uniform wear of friction pads compared to the drum brakes.
Disc brakes weigh less than the conventional drum type saving of approximately 20% being possible.
Compared to the drum type, the disc brakes are simple in the design and it is easy to replace the faulty parts.
1.5 DRUM BRAKES
Early automotive brake systems, after the era of hand levers, used a drum design at all four wheels. They were called drum brakes because the components were housed in a round drum that rotated along with the wheel. Inside was a set of shoes that, when the brake pedal was pressed, would force the shoes against the drum and slow the wheel. Fluid was used to transfer the movement of the brake pedal into the movement of the brakes shoes, while the shoes themselves were made of a heat-resistant friction material similar to that used on clutch plates.
Fig 1.2 Parts of drum brake
The brake assembly at the wheel is comprised of the following main components.
Backing plate, brake shoes, brake lining, wheel cylinders, drum, springs, etc. Backing plates are pressed steels discs placed on the axle ends on which the brake shoes are mounted. Brake shoes are curved members, usually curved two numbers for each wheel and the backing plated so as to be concentric with wheels. They are provided at one end and are moved outward at the other end when brakes are applied. Brake lining of special asbestos compound are either bonded or riveted to the faces of these shoes. This lining comes in contact with the metal brake drum when the brakes are applied. Friction between the lining and the brake drum resists the turning of the wheels. The linings withstand high heat and dragging force that develops when the brake shoe are pressed against the brake drum. In the internal expanding shoes, the brake lining comes in contact with the inside surface of the brake drum and in internal contacting type the lining contacts on the outside surface of the brake drum and in internal contacting type the lining contacts on the outside surface of the drum. The pivot pins at the end of the shoes are called anchors. Wheel cylinders are used to spread the shoe outward against the drum by mechanical operation. When the pressure on the brake pedal is released, the brake retracting springs return the shoes to their off position.
Brake drums are made of cast iron or alloy or chrome nickel. The walls of the drum are very thin and are centrifugal castings. The brake drum assembly is mounted on the wheel blots between the hub and the wheel, so as to revolve with the wheel. The drum surrounds the brake shoes. The surface of the rim must be smooth, round and concentric with the shoe surface. For better dissipation of heat, fins are provided outside the brake drum.
1.6 TYPES OF BRAKE DRUM
The brakes are classified according to the application of the brake shoe to the revolving brake drum as the internal-expanding or external-contraction type. They are also classified as mechanical or hydraulic, depending upon the way of transferring of brake force from hand lever or foot pedal to the brake shoes; either by means of mechanical linkage or by hydraulic pressure. In modern vehicles the service brakes are generally hydraulically operated, internal- expanding type and the hand brake are mechanically operated internal expanding or external-contracting type.
There are different types of brake systems. Some of them are fixed expander type, floating expander type and floating anchor type. Fixed expander type, it is observed in the practice that a leading shoe tip is dragged along the drum even when there is no application of force, while the tip of the trailing shoe is thrown off the brake drum. Thus the net force applied on the leading shoe becomes more than the net force applied on the trailing shoe and an unequal effect is produced. Moreover the coefficient of friction gets decreased. Floating expander type, in this type the expander is not fixed on the back plate, but is kept floating, in this way the braking effect is automatically balanced. Floating anchor type, The two shoes are linked together at the floating anchor and have a common fixed anchor. In this both the shoes become leading.
1.7 COMPONENTS OF DRUM BRAKES
Drum brakes consist of a backing plate, brake shoes, brake drum, wheel cylinder, return springs and an automatic or self-adjusting system. When we apply the brakes, brake fluid is forced under pressure into the wheel cylinder, in turn, pushes the brakes shoes into contact with the machined surface on the inside of the drum. When the pressure is released, return springs pull the shoes back to their rest position. As the brake linings wear, the shoes must travel a greater distance to reach the drum. When the distance reaches a certain point, a self-adjusting mechanism automatically reacts by adjusting the rest position of the shoes so that they are closer to the drum.
1.7.1 Brake Shoes
Brake shoes consist of a steel shoe with the friction material or lining riveted on 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.
1.7.2 Backing Plate
The 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.
1.7.3 Brake Drum
Brake drums are made of iron and have a machined surface on the inside where the shoes make contact. Just a with disk rotors, brake drums will show signs of wear as the brake linings seat themselves against the machined surface of the drum.
1.7.4 Wheel Cylinder
The wheel cylinder consists of a cylinder that had two pistons, one on each side. Each piston has a rubber seal and a shaft that connects the piston with a brake shoe. When brake pressure is applied, the pistons are forced out pushing the shoes into contact with the drum.
1.7.5 Return Springs
Return springs pull the brake shoes back to their rest position after the pressure is released from the wheel cylinder. If the springs are weak and do not return the shoes all the way, it will cause premature lining wear because the linings will remain in contact with the drum.
1.7.6 Leverage
The pedal is designated in such a way that it can multiply the force from out leg several times before any force is even transmitted to the brake fluid.
Fig 1.3 Brake Drum Parts
1.8 BRAKING REQUIREMENTS
The brake system consists of disc brakes and drum brakes. The disc brakes are used for front wheels and for the rear wheel either disc or drum is used and it is connected by a system of tubes and hoses that link each brake to the master cylinder.
Brakes must be strong enough to stop the vehicle within a minimum distance in an emergency. But this should also be consistent with safety.
The brakes must have good anti fade characteristics e.g. while descending hills. This requirement demands the cooling of brakes should be very efficient.
