Baking system is necessary in an automobile for stopping the vehicle. Brakes are applied on the wheels to stop or to slow down the vehicle.
• Various components are used in braking system to convert the momentum momentum (Kinetic energy) in to heat energy using friction.
• Two forms of friction play important role in automotive braking: – Kinetic friction – Static friction Braking action creates kinetic friction kinetic friction in the brakes and static friction between the tire and road to slow the vehicle.
• When brakes are applied, the vehicle’s weight is weight is transferred to the front wheels and is unloaded on the rear unloaded on the rear wheels.
FUNCTIONS OF VEHICLE BRAKING There are two main functions of brakes :
(a) To slow down or stop the vehicle in the shortest possible time at the time of need.
(b) To control the speed of vehicle at turns and also at the time of driving down on a hill slope.
PRINCIPLE OF VEHICLE BRAKING
Braking of a vehicle depends upon the static function that acts between tyres and road surface. Brakes work on the following principle to stop the vehicle : “The kinetic energy due to motion of the vehicle is dissipated in the form of heat energy due to friction between moving parts (wheel or wheel drum) and stationary parts of vehicle (brake shoes)”. The heat energy so generate4d due to application of brakes is dissipated into air. Brakes operate most effectively when they are applied in a manner so that wheels do not lock completely but continue to roll without slipping on the surface of road.
CLASSIFICATION OF BRAKES
•On the Basis of Method of Actuation
Foot brake (also called service brake) operated by foot pedal. Hand brake – it is also called parking brake operated by hand.
•On the Basis of Mode of Operation
(a) Mechanical brakes (b) Hydraulic brakes (c) Air brakes (d) Vacuum brakes (e) Electric brakes.
•On the Basis of Action on Front or Rear Wheels
(a) Front-wheel brakes (b) Rear-wheel brakes.
•On the Basis of Method of Application of Braking Contact
(a) Internally – expanding brakes (b) Externally – contracting brakes.
SHORT NOTES ON MISCELLANEOUS BRAKING SYSTEMS
• Air Brakes Air brakes are applied by the pressure of compressed air. Air pressure applies force on brakes shoes through suitable linkages to operate brakes. An air compressor is used to compress air. This compressor is run by engine power.
• Vacuum Brakes Vacuum brakes are a piston or a diaphragm operating in a cylinder. For application of brakes one side of piston is subjected to atmospheric pressure while the other is applied vacuum by exhausting air from this side. A force acts on the piston due to difference of pressure. This force is used to operate brake through suitable linkages.
• Electric Brakes In electrical brakes an electromagnet is used to actuate a cam to expand the brake shoes. The electromagnet is energized by the current flowing from the battery. When flow of current is stopped the cam and brake shoes return to their original position and brakes are disengaged. Electric brakes are not used in automobiles as service brakes
The brakes which are actuated by the hydraulic pressure (pressure of a fluid) are called hydraulic brakes. Hydraulic brakes are commonly used in the automobiles.
Principle- Hydraulic brakes work on the principle of Pascal’s law which states that “pressure at a point in a fluid is equal in all directions in space”. According to this law when pressure is applied on a fluid it travels equally in all directions so that uniform braking action is applied on all four wheels.
Construction and Working - of Hydraulic Brakes When brake pedal in pressed, the force is transmitted to the brake shoes through a liquid (link). The pedal force is multiplied and transmitted to all brake shoes by a force transmission system. It consists of a master cylinder, four wheel cylinders and pipes carrying a brake fluid from master cylinder to wheel cylinder. The master cylinder is connected to all the four-wheel cylinders by tubing or piping. All cylinders and tubes are fitted with a fluid which acts as a link to transmit pedal force from master cylinder to wheel cylinders.
When brake pedal is pressed to apply the brakes, the piston in the master cylinder forces the brake fluid. This increases the pressure of fluid. This pressure is transmitted in all the pipes and upto all wheel cylinders according to Pascal’s law. This increased pressure forces out the two pistons in the wheel cylinders. These pistons are connected to brake shoes. So, the brake shoes expand out against brake drums. Due to friction between brake linings and drum, wheels slow down and brakes are applied.
Modern motor cars are fitted with disc brakes instead of conventional drum type brakes. In Santro car and Maruti-800, front wheels are provided with disc brakes whereas rear wheel are provided with drum brakes. A disc brake consists of a rotating disc and two friction pads which are actuated by hydraulic braking system as described earlier. The friction pads remain free on each side of disc when brakes are no applied. They rub against disc when brakes are applied to stop the vehicle. These brakes are applied in the same manner as that of hydraulic brakes. But mechanism of stopping vehicle is different than that of drum brakes.
Advantage of Disc Brakes
(a) Main advantage of disc brakes is their resistance to wear as the discs remain cool even after repeated brake applications.
(b) Brake pads are easily replaceable.
(c) The condition of brake pads can be checked without much dismantling of brake system.
Disadvantage of Disc Brakes
(a) More force is needed be applied as the brakes are not self emerging.
(b) Pad wear is more.
(c) Hand brakes are not effective if disc brakes are used in rear wheels also. (Hand brakes are better with mechanical brakes).