Aim
To study and prepare repot on the constructional details, working principles and operation of Automotive Brake Systems.
a) Hydraulic and Pneumatic Brake Systems
b) Drum Brake System
c) Disc Brake System
Theory
Labelled Diagram, Constructional Details, Working Principle and Operation of the above Steering Systems
PRINCIPLE
It goes without saying that brakes are one of the most important control components of vehicle. They are required to stop the vehicle within the smallest possible distance and this is done by converting the kinetic energy of the vehicle into the heat energy which is dissipated into the atmosphere.
BRAKING REQUIREMENTS
1 The 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 driver must have proper control over the vehicle during emergency braking and the vehicle must not skid.
2 The brakes must have good antifade characteristics i.e. their effectiveness should not decrease with constant prolonged application e.g. while descending hills. This requirement demands that the cooling of the brakes should be very efficient.
HYDRAULIC BRAKES
Most of the cars today use hydraulically operated foot brakes on all the four wheels with an additional hand brake mechanically operated on the rear wheels. An outline of the hydraulic braking system is shown in fig. the main component in this is the master cylinder which contains reservoir for the brake fluid. Master cylinder is operated by the brake pedal and is further connected to the wheel cylinders in each wheel through steel pipe lines, unions and flexible hoses. In case of Hindustan Ambassador car, on front wheels each brake shoe is operated by separate wheel cylinder (thus making the brake two shoe leading) whereas in case of rear wheels there is only one cylinder on each wheel which operates both the shoes (thus giving one leading and one training shoe brakes.) As the rear wheel cylinders are also operated mechanically with the hand brake, they are made floating. Further, all the shoes in the Ambassador car are of the floating anchor type.
The system is so designed that even when the brakes are in the released position, a small pressure of about 50 kPa is maintained in the pipe lines to ensure that the cups of the wheel cylinder are kept expanded. This prevents the air from entering the wheel cylinders when the brakes are released. Besides, this pressure also serves the following purposes.
(i) it keeps the free travel of the pedal minimum by opposing the brake shoe retraction springs.
(ii) During bleeding, it does not allow the fluid pumped into the line to return, thus quickly purging air from the system.
DRUM BRAKES
In this type of brakes, a brake drum is attached concentric to the axle hub whereas on the axle casing is mounted a back plate. In case of front axle, the back plate is bolted to the steering knuckle. The back plate is made of pressed steel sheet and is ribed to increase rigidity and to provide support for the expander, anchor and brake shoes. It also protects the drum and shoe assembly from mud and dust. Moreover, it absorbs the complete torque reaction of the shoes due to which reason it is sometimes also called torque plate. Two brake shoes are anchored on the back plate as shown in fig. Friction linings are mounted on the brake shoes. One or two retractor springs are used which serve to keep the brake shoes away from the drum when the brakes are not applied. The brake shoes are anchored at one end, whereas on the other ends force F is applied by means of some brake actuating mechanism which forces the brake shoe against the revolving drum, thereby applying the brakes. An adjuster is also provided to compensate for wear of friction lining with use. The relative braking torque obtained at the shoes for the same force applied at the pedal varies depending upon whether the expander (cam or toggle lever) is fixed to the back plate or it is floating, whether the anchor is fixed or floating and whether the shoes are leading or trailing.
DISC BRAKES
As shown in fig. a disc brake consists of a cast iron disc bolted to the wheel hub and a stationary housing called caliper. The caliper is connected to some stationary part of the vehicle, like the axle casing or the sub axle and is cast in two parts, each part containing a piston. In between each piston and disc, there is friction pad held in position by retaining pins, spring plates etc. Passages are drilled in the caliper for the fluid to enter or leave each housing. These passages are also connected to another one for bleeding. Each cylinder and contains a rubber sealing ring between the cylinder and the piston.
When the brakes are applied, hydraulically actuated pistons move the friction pads into contact with the disc, applying equal and opposite forces on the later. On releasing the brakes, the rubber sealing rings act as return springs and retract the pistons and the friction pads away from the disc.
For a brake of this type
T = 2µpaR
Where
µ = coefficient of friction
p = fluid pressure
a = cross sectional area of one piston
R = distance of the longitudinal axis of the piston from the wheel axis
BRAKE SYSTEM FOR MARUTI (SUZUKI) 800 CAR
The front wheel brakes are of the disc type, whereas for rear wheels drum type brakes (leading trailing shoes) are employed. Parking brake is mechanically operated by a wire and link system and works on the rear wheels only. Same brake shoes are used for service and parking brakes. The layout of the system is shown in fig.
A tandem master cylinder is employed. The hydraulic pressure produced there is applied to two independent circuits. One circuit is for front left and rear right brakes, whereas the other is for front right and rear left brakes. Due to this reason, the braking system in the Maruti has greater safety because even if a pressure leak occurs in the brake line of one circuit, the other braking circuit works, due to which a certain degree of braking is still available to the vehicle.
Reference Books
1 Automotive Mechanics – Crouse/Anglin
2 Automobile Engineering – Dr Kirpal Singh
Viva Questions
1 What is the principle of automotive brake
1 What do the brakes do to the energy as they stop a moving car
2 What are the primary and secondary brakes
3 Why brakes with more than 80% efficiency are not used in automobiles
4 What is fading of brakes
5 On what factors does the force of adherion between the road wheels and the road depends
6 How does skidding takes place
7 What is leading shoe
8 What is the difference between power assisted and power operated brakes
9 What is the advantage of a two shoe trailing brake
10 Name important components of a disc brake
11 What is the advantage of a swinging caliper type disc brake
12 Name important components of a drum brake
13 What is a split hydraulic brake system
14 What is the function of a metering valve in the braking system
15 What are the main constituents of a braking fluid
16 What are the advantages of hydraulic brakes over mechanical brakes
17 What is the purpose of brake shoe adjuster
18 Explain clearly requirements of automobile brake
19 Discuss in detail the braking system employed in case of Maruti 800 cars
20 What is tandem master cylinder. Define.
21 Describe the principles of various techniques employed to prevent skidding
22 Draw a neat sketch showing the linkage to operate brake master cylinder
23 What are the advantages of using split brake system
24 What are the essential characteristics required of a good braking fluid.