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Brake is one of most important device of a vehicle used for retarding or stopping the vehicle within smallest possible distance, in consistant with safety and without wheel skidding.
A brake is a mechanical device that inhibits motion by absorbing energy from a moving system.
The brakes also used to hold the vehicle at rest on an inclined road against the pull of gravity.
Need of Brake
1) The brakes must be strong enough to stop the vehicle within a short distance in an emergency but this should be consistent with safety. This is possible only when there is no wheel skidding and driver has proper control over the vehicle during emergency.
2) The brake must have good antifade characteristics i.e. the effectiveness of the brake should be remain constant with prolong application.
Principle of Braking System
1) The brakes works on the principle of friction.
2) The friction is basically the property of material in contact and depends upon the relative smoothness of the surface in contact.
3) In braking, two surfaces come in contact in motion which cause the kinetic energy to convert into heat energy due to friction.
4) The force of friction or retarding force created between the two mating surface ( brake lining and the brake drum ) depend upon the pressure or force exerted on the shoes.
5) The retarding force applied by the brake at the wheel depends upon the co-efficient of friction between the road and the tyre surface and the component of the weight on the wheel retarding force.
Type of Brakes
- Mechanical brakes are obsolete now as a service brake and these are still used on rear wheel in cars as a parking or emergency brake.
- The drum type manually operated mechanical brake system shown in fig. In a motor vehicle, the wheel is attached to the drum.
- The drum encloses the complete brake mechanism and protect it from dust and moisture. The inner side of drum is open. The backing plate at the open side of the brake drum completes the brake enclosure and hold the brake assembly.
- The backing plate is attached to the vehicle axle housing and acts as a base or frame for fastening the brake shoe and the operating cam mechanism with it linkage. The wheel attaching bolts on the brake drum connects the wheel and drum.
- The brake shoe are hinged to back plate at one end by an anchor join while other end rest on cam or toggle. This cam can be turn by camshaft which passes through hole in backing plate.
- The camshaft can be operated by brake padel through linkage. The brake shoe are pull inward or held by retracting spring.
- When brake pedal is pressed, the cam turn by the expanding brake shoe outward, against the retractor spring force.
- The brake lining comes in contact with brake drum causes friction between them.
- This force of friction opposes the direction of rotation and reduce the speed or stop the vehicle when brake pedal release, the retracting spring pull the brake shoe inward which turn the cam and brakes are release. This type of brakes are called internal expanding brake.
- As compared to mechanical brakes, these brake system is silent, flexible and self lubricating. Also this system is highly efficient, durable and simple in design.
- The hydraulic braking system is based on pascal law which states that the fluid transmit pressure equally in all directions without any loss.
- This system is designed in such a way that even when brakes are in released position, a small pressure must be maintained in pipeline to protect the entering of air in the system when cup of wheel cylinder are kept expanded.
- This system consist of master cylinder, wheel clinder, steel pipe, flexible hose, brake linkage and a check valve at the end of master cylinder.
- When the operator presses the brake pedal, this force is transmitted to the piston in master cylinder through linkage.
- The piston in master cylinder moves by compressing the return spring. As bypass port is covered, further movement of piston builds up pressure in the compression chamber.
- When sufficient pressure is built up, the fluid checks the valve deflect and the fluid under pressure in the pipe line enters in wheel cylinder.
- As soon as the fluid enters in wheel cylinder, it exerts a pressure on two piston to move the piston outward. This outward motion of piston causes the brake shoe expand creating tension in retracting spring.
- The brake shoe lining is pressed tightly against the internal surface of brake drum. This friction between the brake lining and drum, slow down or stop the rotation of the drum and hence the vehicle slows down or stops.
- As brake pedal is released, the retracting spring pulls the brake shoe inward to original position. This causes the piston in wheel cylinder to push back. Due to this the brake fluid flows in reverse direction i.e. to the master cylinder and to fluid reservoir.
- As the pressure drops in the fluid line, the fluid checks valve at the end of master cylinder closes.
Pneumatic or Air Braking System
- Pneumatic braking system is used in medium and heavy duty vehicles.
- It consists of air filte, air compressor, unloader valve, reservoir, brake valve and brake chamber with steel tube and flexible hoses.
- The compressor is operated by engine. The compressor suck the air from atmosphere through air filter and compressed it to high pressure and discharge to the reservoir through oil separator.
- The pressure gauge is fitted to reservoir to indicated the pressure in the tank. The reservoir contain enough compressed air for several braking operation. The air from reservoir tank is supplied to brake valve and to other accessories.
- There is no braking effect until the brake pedal is not pressed as brake valve prevent the passege of air to the brake chamber.
- When brake pedal is pressed, the brake valve changes its position and brake valve open.
- The compressed air from brake valve flow to brake chamber acts on the flexible diaphragms in brake chamber.
- The diaphragm pushes the rods connected with the levers of brake gear cam. The cam turn and expend the brake shoe with make frictional contact with brake drum thus braking the wheels.
- When brake pedal is released, the supply of compressed air is cut off from the brake chamber and they are connected to atmosphere.
- The pressure from the chamber drops and brake shoes are returned to their initial position and wheel run free.
- The brake valve is equipped with servo-mechanism which ensure that the braking force on shoe proportional to the applied force on the pedal and also import relative reaction to the movement of the pedal. So driver can sense the degree of brake application.
- An unloader is located between the compressor and reservoir air pressure line. The unloader valve is relieves the compressor of its pumping load once the unloader the cut out pressure is obtained and seal the reservoir.
- When the compressure is built up a pressure depending upon the setting of the adjusting screw.
- The unloader then delivers the air discharge by the compressure to the atmosphere.Thus allowing the compressor to run light while the reservoir contain sufficient supply of air.
- The air filter prevent the dust and foreign material entering the operating system.These are mounted on the chassis and have a drain plug to allow the condensate to be easily removed.
Disc and Drum Brake
- In disc brakes friction surfaces are directly exposed to the cooling air while in drum type brake, the friction occurs on the internal surfaces, from which is dissipated only after it passed by conduction through the drum.
- In disc brake frictional pad are flat while in drum brake friction lining are curve shape.
- The design of disc brake is such that there is no loss of efficiency due to expansion. As in drum brake system became hot, expansion of a drum of internally friction surface apart, causing loss of effective pedal travel.
- On the other hand brake disc expansion merely changes the relative position on the friction surfaces slightly without tending to increase the clearance.
- Spot type disc brakes weightless than the conventional drum type counterpart a saving of approximately 20 % being possible.
- Disc brakes have better antifade characteristics than drum brake.
- The sensitivity of a brake to changes in the friction coefficient at the rubbing surfaces can be shown by plotting brake factor against friction coefficient.
- Total frictional area of pads in spot brake is very less compared to drum brake i.e. 1:4. This means the pressure intensity in disc brake is greater than drum type.
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