AUTOMOBILE RUNNING SYSTEM AND POWER TRAIN 4. BRAKING SYSTEMS
The braking system is the most fundamental safety feature on an automobile. A good braking system must be able to stop or slow down the vehicle as and when required, regardless of the condition of road surface and the speed of the vehicle. The brakes are applied on the wheels of a vehicle and if it is needed to stop the vehicle quickly, they must be applied on all the wheels simultaneously. For this reason, automobiles are provided with brakes on all the wheels.
4.0
Objectives After going through this lesson, you will be able to:
i. ii. iii.
Classify brakes Explain the principle of working of hydraulic brakes List the components of hydraulic drum and disc brakes and give the function of each iv. Explain the working of hydraulic drum and disc brakes v. Differentiate between leading & trailing and twin leading shoe drum brake system vi. Describe the working of tandem master cylinder and its advantage over the conventional master cylinder vii. Explain the purpose and working of mechanical brakes
4.1
Introduction
Brakes work on the principle of Conservation of Energy. A moving vehicle possesses kinetic energy which is converted mainly into heat energy on application of brakes. This heat is dissipated into the atmosphere.
• • • • • •
An efficient braking system should fulfill the following requirements: It should be able to stop the vehicle within the smallest possible distance. It must act instantly in case of emergency braking. It must be strong enough to withstand sudden braking force. It should not slip or cause the vehicle to skid out of control. It should have good life. It should not require large amount of effort from the driver.
4.2
Classification
There are primarily two types of brakes: i. The parking brakes, and, ii. The wheel brakes. Parking brakes are of the mechanical type and wheel brakes may either be of the mechanical or of the hydraulic type. Hydraulic wheel brakes may be of the drum or of the disc type whereas mechanical brakes are always of the drum type.
Fig. 4.1: Classification of brakes.
4.2.1 Mechanical Brakes In a motor vehicle that uses mechanical brakes, the road wheel is mounted on a brake drum, inside which are two brake shoes mounted on a back-plate. The brake shoes have brake linings on their outer surface and each shoe has one end pivoted, while the other end is operated by means of a mechanical linkage so that the shoes expand on application of force. When force is applied, the brake Fig. 4.2: Mechanical brakes. lining comes in contact with the brake drum and stops the rotation of the drum. The retractor springs pull the shoes back when the braking force is removed and keep the shoes away from the drum as long as the brakes are not applied. Mechanical brakes are of the internal expanding type and are generally used in two-wheelers as wheel brakes and in four-wheelers as parking brakes and emergency brakes. 4.2.2 Hydraulic Brakes Hydraulic brakes use a confined liquid to transmit the applied braking force. They work on Pascal’s law which states that in a confined space, fluid pressure is transmitted equally in all directions. Hydraulic brakes are the most commonly used type of wheel brakes in four-wheelers and in some two-wheelers, such as newer models of motorcycles. Hydraulic brakes offer several advantages over the mechanical brakes: •
They are simple in construction.
Fig. 4.3: Hydraulic Brakes (a) Construction. (b) Advantage.
• • • •
They transmit equal braking effort in all the wheels. They can be designed to produce high force multiplication. They have a high degree of reliability. They allow an unequal braking force to be applied to the front and the rear wheels, if desired, by varying the diameter of wheel cylinder pistons.
The hydraulic braking system also has a few disadvantages: • The braking system fails if there is a leakage of brake fluid. • Leakage of the brake fluid also spoils the brake shoes.
4.3
Force Multiplication
Fig. 4.4 explains how the effort applied by the driver gets multiplied manifold. The distance from the pedal to the pivot is four times the distance from the cylinder to the pivot, so the force applied at the pedal is increased by a factor of four before it is available at the cylinder. The diameter of the brake wheel cylinder is also three times the diameter of the master cylinder. This further multiplies the force by a factor of nine. Thus, altogether, the system increases the force applied by the foot of the driver by a factor of thirty six.
Self-check Questions 1. Give the classification of brakes. 2. Parking brakes are operated ____________ 3. How do mechanical brakes operate?
Fig. 4.4: Multiplication of force.
4. Hydraulic brakes work on ____________ 5. Unequal braking force is required to be applied on the front and rear wheels. State whether True/False 6. Why is more braking force required at the front wheels? 7. Unequal braking force can be applied by ________________________ 8. How is the braking effort of the driver multiplied?
4.4 4.4.1
Components and Working Hydraulic Drum Brakes
Fig. 4.5: Components of drum brake (a) Master cylinder. (b) Wheel cylinder. (c) Brake shoe. (d) Retractor spring. (e) Back plate. (f) Brake drum.
In a hydraulic braking system, the applied force is transmitted by a fluid known as the brake fluid. A cylinder, called the wheel cylinder, contains the brake fluid and is attached to the two brake shoes. When the brake pedal is pressed, it forces the hydraulic fluid under pressure from the master cylinder through pipelines to the wheel or slave cylinder. This fluid pushes the piston inside the wheel cylinder outwards, thereby expanding the brake shoes which contact the drum and slow down its rotation. This results in the slowing down of the speed of the rotation of the wheel and, consequently, the speed of the vehicle. A drum brake consists of two curved brake shoes, each with a lining, that press against the inside of a drum. The main components of the drum brake and their functions are: • • • • •
•
Master cylinder: There is one master cylinder for the entire system and it is used to create fluid pressure when the brake pedal is pressed. Wheel cylinders: There are one or two wheel cylinders in each wheel and they push out and expand the brake shoes. Brake shoes: There are two brake shoes in each wheel and they carry the brake lining which contacts the rotating drum and retards its motion. Retractor spring: This sprint pulls the brake shoes back after the braking force has been withdrawn. Back plate: This plate has all the components, such as the brake shoes, fulcrum pin, wheel cylinder(s), retractor spring and the brake adjuster (if provided), etc. mounted on it. Brake drum. The wheel is mounted on the brake drum and it rotates or stops with this drum.
Types of drum braking systems There are two different types of drum braking systems: i. ii.
The leading and trailing shoe system, and, The twin leading shoe system.
The leading and trailing shoe system has both the brake shoes hinged on the same pivot and the leading shoe exerts more force than the trailing shoe. In the twin leading shoe system, the shoes pivot against the drum from opposite points. Since both the shoes are leading in this case, they exert an equal force. The leading and trailing
Fig. 4.6: Types of drum braking systems (a) Leading and trailing shoe system. (b) Twin leading shoe system.
shoe system is a cheaper and a better alternative for rear wheels since it is equally effective, irrespective of whether the car is moving forward or backwards.
4.4.2 Hydraulic Disc Brakes Disc brakes are a relatively newer development as compared to drum brakes. Almost all cars nowadays use disc brakes in front wheels while some use disc brakes in all the wheels. A disc brake consists of a disc, which is joined to the wheel. The disc has two friction pads on both sides, which are pushed towards the disc when the brakes are applied. The friction pads are pushed, on application of the brakes, by hydraulically operated pistons contained in the two halves of an assembly called the caliper. The pads retard and stop the motion of the disc and thereby that of the road wheel.
Fig. 4.7: Working of disk brakes (a) Pedal released. (b) Pedal pressed.
• •
•
The main components of the disc brake and their functions are: Master cylinder: The master cylinder creates pressure in the fluid when the brake pedal is pressed. Disc: The disc rotates along with the wheel and slows down or stops its motion when the brakes are applied. Caliper. The caliper is in two halves and is fork-like with each half containing a friction pad bonded to a steel plate, a piston, and a cylinder in which the piston moves.
The disc breaks used in cars are usually vented to be able to dissipate the heat generated by the friction between the pads and the disc. Vented disc brakes have a set of vanes between the two sides of the disc and these vanes pump air through the
Fig. 4.8: Parts of disk brakes.
disc to provide cooling.
• • •
Disc brakes offer certain advantages over drum brakes. These are: Better braking efficiency, due to uniform pressure distribution over the disc than can be applied over the drum. Efficient cooling and heat dissipation since the disc is exposed to atmosphere. Expansion of the disc due to heat does not impair brake effectiveness. This is because the disc moves closer towards the pads when heated as against the case in drum brakes, where the drum moves away from the brake shoes when heated.
Self-check Questions 9. ___ _No. of master cylinder and ____________ No. of wheel cylinder are used in a car. 10. Function of master cylinder is to __________________ when the brake pedal is pressed. 11. _____ brake shoes are used in each wheel brake 12. __________ shoe exerts more force on the drum 13. Disc brakes are used in _________ wheels or all wheels
4.5
Tandem Master Cylinder
A disc brake along with a tandem master cylinder is standard fitment on most cars nowadays. A tandem master cylinder consists of two master cylinders in one assembly. The assembly has two circuits, each of which is designed to apply fluid pressure on two wheels. The tandem master cylinder arrangement improves the safety of the vehicle because even if there is failure in one circuit due to fluid leakage, the other circuit would still function in the normal manner and slow down and stop the vehicle.
4.6
• • •
Fig. 4.9: Tandem master cylinder.
Brake Fluid The brake fluid is a special fluid of specification SAE 1703J and it must meet the following requirements: It must have a high boiling point and a low freezing point. It must be chemically stable. It must be non-corrosive and must not attack rubber or metal parts.
Self-check Questions 14. Tandem master cylinder supplies brake fluid to ___ wheels 15. Give the specification of brake fluid.
4.7
Assignments
4.7.1 Class Assignment 1. Discuss why brakes become inefficient on wet roads. 4.7.2 Home Assignment 1. Study the brakes of a bicycle.
4.8
Summing Up
The braking system is the most fundamental safety feature on an automobile. It works on the principle of conservation of energy. Kinetic energy of a moving vehicle is converted mainly into heat energy on the application of brakes and this heat is dissipated into the atmosphere. Brakes are classified as parking brakes and wheel brakes. Parking brakes are of the mechanical type and wheel brakes may either be of the mechanical or of the hydraulic type. Hydraulic wheel brakes may be of the drum or of the disc type whereas mechanical brakes are always of the drum type. Hydraulic brakes use a confined liquid to transmit the applied braking force. They work on Pascal’s law. Hydraulic brake fluid is a special fluid of specification SAE 1703J. A disc brake along with a tandem master cylinder is standard fitment on most cars now. The tandem master cylinder arrangement improves safety of the vehicle because even if there is failure in one circuit, due to fluid leakage, the other circuit would still function in the normal manner and slow down and stop the vehicle
4.9 1. 2. 3. 4. 5. 6. 7. 8.
Possible Answers to Self-check Questions See Fig. 4.2. mechanically by rotation of a cam which pushes brake shoes out against a drum Pascal’s law True because of weight transfer when the car stops suddenly varying the diameter of wheel cylinder pistons by leverage
9. 1, 1 or 2 (1 in leading and trailing; 2 in twin leading) 10. create fluid pressure 11. 2 12. Leading 13. front 14. 2 15. SAE 1703J
4.10
Terminal Questions
1. 2. 3. 4. 5. 6. 7.
Give the classification of brakes. How do mechanical brakes operate? How do hydraulic brakes work? Why is more braking force required at the front wheels? How is this achieved? How is the braking effort of the driver multiplied? List the components of the hydraulic drum brakes and give the function of each. Differentiate between the leading & trailing shoe and the twin leading shoe system. Which is better? Why? 8. List the components of the hydraulic disc brakes and give the function of each. 9. Why are disc brakes better? 10. What is the difference between a tandem master cylinder and a conventional master cylinder? 11. List the requirements of hydraulic brake fluid.
4.11
References and Suggested Further Reading
1. AA BOOK OF THE CAR. Drive Publications Ltd, London. 2. Maruti 800 Service Manual 3. http://www.howstuffworks.com
4.12
Glossary
Brake fluid
Special fluid of specification SAE 1703J
Disc brakes
Braking system in which a disc is joined to the wheel and is pressed between brake pads to retard and stop the motion of the disc and thereby that of the road wheel
Hydraulic brakes
Brakes that are applied by hydraulic force
Leading and trailing shoe system
Both the brake shoes being hinged on the same pivot, the leading shoe exerts more force than the trailing shoe
Master cylinder
Main cylinder which creates pressure in the fluid to operate all wheel brakes, when brake pedal is pressed
Mechanical brakes
Brakes that are applied by mechanical force
Parking brakes
Brakes that are applied in a stationary vehicle to prevent it from moving downhill or being pushed.
Pascal’s law
States that in a confined fluid, pressure is transmitted equally in all directions
Principle of conservation of energy
States that energy cannot be created or destroyed
Tandem master cylinder
Master cylinder having two circuits, each of which is designed to apply fluid pressure to two wheels. This improves the safety as in case of failure in one circuit due to fluid leakage, the other circuit would still function in the normal manner and slow down and stop the vehicle
Twin leading shoe system
Both shoes pivot against the drum from opposite points. Since both the shoes are leading in this case, they exert an equal force
Wheel brakes
Brakes that are applied in a moving vehicle to slow down or stop the vehicle