Air (Pnuematic) Brake System

A

REPORT

ON

AIR (PNUEMATIC) BRAKE SYSTEM

BY

Submitted by:

Sudeep Singh(ME Final Year)

Satendra Bharti(ME Final Year)

Pramod Sonker(ME Final Year)

Ahsaan Ullah(ME Final Year)

Abstract:

An air braking unit is used in an air braking system. The air braking unit is arranged to be positioned, in use, at a vehicle wheel, and comprises an inlet for receiving, in use, compressed air from a central source. At least one first valve is arranged to selectively allow compressed air from the inlet to enter a wheel brake chamber in use.

At least one second valve is arranged to selectively allow air from the brake chamber to be released via an outlet to the atmosphere in use and control means controls the first and second valves to operate to selectively control the air pressure in the brake chamber in use.

CONTENTS

1.Introduction

2.History and development

3.Principles of Operation and Construction

4.Components

5.Advantages and Disadvantages

6.Literature Survey

7.Scope of Air Brake System

8.Future Aspects

9.Summary

10.References

1.1INTRODUCTION

1.1.1Air Brake

A friction type of energy-conversion mechanism used to retard, stop, or hold a vehicle or other moving element. The activating force is applied by a difference in air pressure. With an air brake, a slight effort by the operator can quickly apply full braking force.

The air brake, operated by compressed air, is used in buses; heavy-duty trucks, tractors, and trailers; and off-road equipment. The air brake is required by law on locomotives and railroad cars. The wheel-brake mechanism is usually either a drum or a disk brake. The choice of an air brake instead of a mechanical, hydraulic, or electrical brake depends partly on the availability of an air supply and the method of brake control.

In a motor vehicle, the air-brake system consists of three subsystems: the air-supply, air-delivery, and parking/emergency systems. The air-supply system includes the compressor, reservoirs, governor, pressure gage, low-pressure indicator, and safety valve. The engine-driven compressor takes in air and compresses it for use by the brakes and other air-operated components. The compressor is controlled by a governor that maintains air compression within a preselected range. The compressed air is stored in reservoirs.

The air-delivery system includes a foot-operated brake valve, one or more relay valves, the quick-release valve, and the brake chambers. The system delivers compressed air from the air reservoirs to the brake chambers, while controlling the pressure of the air. The amount of braking is thereby regulated. In the brake chambers, the air pressure is converted into a mechanical force to apply the brakes.

As the pressure increases in each brake chamber, movement of the diaphragm pushrod forces the friction element against the rotating surface to provide braking. When the driver releases the brake valve, the quick-release valve and the relay valve release the compressed air from the brake chambers. The parking/emergency system includes a parking-brake control valve and spring brake chambers. These chambers contain a strong spring to mechanically apply the brakes (if the brakes are properly adjusted) when air pressure is not available.

During normal vehicle operation, the spring is held compressed by system air pressure acting on a diaphragm. For emergency stopping, the air-brake system is split into a front brake system and a rear brake system. If air pressure is lost in the front brake system, the rear brake system will continue to operate. However, the supply air will be depleted after several brake applications. Loss of air pressure in the rear brake system makes the front brake system responsible for stopping the vehicle, until the supply air is depleted.

Air Brake continued…

Either of two kinds of braking systems,the first, used by trains, trucks, and buses, operates by a piston driven by compressed air from reservoirs connected tobrakecylinders. When air pressure in the brake pipe is reduced, air is automatically admitted into the brake cylinder. The first practical air brake for railroads was invented in the 1860s byGeorge Westinghouse. The second type, used by aircraft and race cars, consists of a flap or surface that can be mechanically projected into the airstream to increase the resistance of the vehicle to air and lower its speed.

1.1.2Air Brake Implementations

1. Brake operated by compressed air, esp. in heavy vehicles and trains

2.An articulated flap or small parachute for reducing the speed of an aircraft

3.Rotary fan or propeller connected to a shaft to reduce its speed

Collins Discovery Encyclopedia, 1st ion © HarperCollins Publishers 2005

Air brake (Mechanical Engineering)

An energy-conversion mechanism activated by air pressure and used to retard, stop, or hold a vehicle or, generally, any moving element.

1.2History and Development

1.2.1The straight air brake

The first form of the air brake consisted of an air pump, a main reservoir, and an engineer's valve on the locomotive, and of a train pipe and brake cylinder on each car. One problem with this first form of the air brake was that braking was applied to the first cars in a train much sooner than to the rear cars, resulting in shocks and damages when the rear cars bunted against the cars ahead of them. The main objection however was that it was not an automatic brake, i.e. even a minor mishap like a broken coupling left the entire train without any brake power at all.

1.2.2The plain automatic air brake

In 1872, George Westinghouse invented the automatic air brake by inventing the triple valve and by equipping each car with its own air cylinder. Air pressure is maintained in the auxiliary reservoirs and in the train pipe at all times when the brakes are not applied. Equilibrium of air pressure is maintained in the train pipe and in the auxiliary air cylinders.

To apply the brakes to all of the cars at about the same time, pressure is released from the train pipe, causing the triple valve on each car to apply the brakes. To release the brakes on each car, pressure is increased in the train pipe until an excess pressure above that of the pressure in each auxiliary cylinder is reached, which throws the triple valve so as to close the inlet to the brake cylinder and open the inlet to the auxiliary reservoir from the train pipe, thus allowing the equilibrium of the two pressures to be reached.

1.2.3The quick action triple valve

Although the plain automatic air brake was a great improvement over the straight air brake, in an emergency the system still applied the brakes to the last cars in a train later than to the first cars in a train. To remedy that condition, George Westinghouse invented the quick action triple valve in 1887. It automatically vents air from the brake pipe locally on each car, which applies the brakes more quickly.

1.2.4Electric railways

For the air brake to be employed on electric railways requires an air compressor that is powered by electricity. Powerful electric locomotives were produced by the Westinghouse Electric & Manufacturing Company and by other companies.

1.3BASIC PRINCIPLE OF OPERATION

1.3.1Basic Principles

Pneumatics can be described as the way of transferring mechanical energy from one point to another using air pressure. Gas laws describe the volume-temperature-pressure relationships for gases under a variety of conditions. Boyle's Law states that the absolute pressure of a contained gas is inversely proportional to its volume provided its temperature remains constant. Gay-Lussac's Law states that the absolute pressure of a confined gas is proportional to its temperature provided its volume remains constant. Charle's Law states that the volume of a confined gas is proportional to its temperature provided its pressure remains constant.(Principles of Pneumatics | eHow.com)

Compressed air systems have many uses. They can inflate tires, operate blowers used to clean out electronic systems and power tools like pneumatic nail drivers. Some air compressors are so big they must be hauled around with a truck, while some are so small they fit on a work bench. They all have similar parts and operate the same way.

1.3.2The Tank

The tank holds the compressed air. In one sense the whole function of an air ` system is to put more air into the tank. These usually consist of a strong cylinder (with rounded ends) with three ports. The place where the air comes in usually has a valve to insure the flow of air moves one way, a place where the air goes out, and a place where the pressure gauge attaches so you can tell how much pressure the tank is experiencing.

1.3.3The Pump

The pump is forcing the air into the tank. Typically the air compressor system automatically controls the pump--turning it on when the pressure in the tank is low and turning it off when the pressure in the tank is high. In more expensive systems, the pump speed can be changed depending on the pressure in the tank. This ensures the pressure in the tank is more consistent. The pump can be electric or gasoline powered. The gasoline powered pumps are used in outdoor locations where electricity is not available.

1.3.4Valves

There is a one-way valve (called a check valve) between the pump and the tank. This enables the air to go from the pump to the tank but not the other way. There is usually a check valve on the output from the tank as well, in addition to an emergency shutoff valve to save the compressed air if something goes wrong with the system. This speeds up recovery time once the problem is fixed, so you do not have to waste time compressing more air.

1.3.5Gauges

There is a gauge showing how much pressure is currently in the tank. If this is too much, the pump must be shut off. It is also handy to see when you have enough pressure to operate the pneumatic tools that the air compressor system is driving. Some of the gauges are not visible--such as those which turn the pump on and off and the gauge that controls the emergency relief pressure valve

1.4Components

A basic air brake system capable of stopping a vehicle has five main components:

1.A compressor to pump air with a governor to control it.

2.A reservoir or tank to store the compressed air.

3.A foot valve to regulate the flow of compressed air from the reservoir when it is needed for braking.

4.Brake chambers and slack adjusters to transfer the force exerted by the compressed air to Mechanical linkages.

5.Brake linings and drums or rotors to create the friction required to stop the wheels.

It is necessary to understand how each of these components works before studying their functions in the air brake system.

1.4.1Compressor and Governor

Compressed air is used to transmit force in an air brake system. The source of the compressed air is a compressor. A compressor is designed to pump air into a reservoir which results in pressurized air.

The compressor is driven by the vehicle’s engine, either by belts and pulleys or shafts and gears. In vehicles where the compressor is driven by belts, they should bechecked regularly for cracks and tension. Also, check the compressor for broken mounting brackets or loose bolts.

The compressor is in constant drive with the engine. Whenever the engine is running, so is the compressor. When pressure in the system is adequate, anywhere from a low of 80 psi to a high of 135 psi it is not necessary for the compressor to pump air. A governor controls theminimum and maximum air pressure in the system by controlling when the compressor pumps air. This is known as “loading” andunloading” stage respectively. Most compressors have two cylinders similar to an engine’s cylinders. When the system pressure reaches itsmaximum, which is between 115 and 135 psi, the governor places thecompressor in the “unloading” stage.

The compressor must be able to build reservoir air pressure from 50 to 90 psi within three minutes. If unable to do so the compressor requires servicing. A compressor may not be able to build air pressure from 50 to 90 psi within three minutes if the air filter is plugged orif the belt is slipping. If these were not at fault the compressor could be faulty.

The governor must place the compressor in the“loading” stage at no lower than 80 psi. During the“unloading” stage, the compressor is able to cool.

Usually compressors are lubricated from the enginelubrication system, although some compressors areself-lubricating and require regular checks of thelubricant level.

It is very important the air that enters the system bekept as clean as possible. The air must first passthrough a filter to remove any dust particles. The airfilter must be cleaned regularly. A dirty filter willrestrict the flow of air into the compressor, reducingits efficiency. Some vehicles have the inlet port ofthe compressor connected to the intake manifoldand receive air that has been filtered by the engineair cleaner.

A piston type compressor operates on the sameprinciple as the intake and compression strokes ofan engine.

Intake stroke: The downward stroke of the pistoncreates a vacuum within the cylinder which causesthe inlet valve to open. This causes atmospheric airto flow past the inlet valve into the cylinder.

Usually compressors are lubricated from the engine lubrication system, although some compressors are self-lubricating and require regular checks of the lubricant level.

It is very important the air that enters the system be kept as clean as possible. The air must first pass through a filter to remove any dust particles. The air filter must be cleaned regularly. A dirty filter will restrict the flow of air into the compressor, reducing its efficiency. Some vehicles have the inlet port of the compressor connected to the intake manifold and receive air that has been filtered by the engineair cleaner.

A piston type compressor operates on the same principle as the intake and compression strokes of an engine.

· Intake stroke: The downward stroke of the piston creates a vacuum within the cylinder which causes the inlet valve to open. This causes atmospheric air to flow past the inlet valve into the cylinder.

Compression stroke: The upward motion of the piston compresses the air in the cylinder. The rising pressure cannot escape past the inlet valve (which the compressed air has closed). As the piston nears the top of the stroke, the pressurized air is forced past the discharge valve and into the discharge line leading to the reservoir.

1.4.2Reservoirs

Reservoirs or tanks hold a supply of compressed air.The number and size of the reservoirs on a vehiclewill depend on the number of brake chambers andtheir size, along with the parking brakeconfiguration. Most vehicles are equipped withmore than one reservoir. This gives the system alarger volume of main reservoir air. The firstreservoir after the compressor is referred to as thesupply or wet reservoir. The other reservoirs areknown as primary and secondary or dryreservoirs. When air is compressed, itbecomes hot. The heated air cools in the reservoir,forming condensation. It is in this reservoir that mostof the water is condensed from the incoming air.

1.4.3Air Dryer

An air dryer may be installed between thecompressor and the wet reservoir to help removemoisture from the compressed air. It may be partiallyfilled with a high moisture-absorbent desiccant andan oil filter, or it may be hollow with baffles designedto assist in separating the moisture from the air.

Both types of air dryers use air pressure to purge oreject the accumulated contaminants from theirdesiccant bed. The purge valve has a heaterelement, which prevents the moisture from freezingin cold climate operation. The wiring connected tothe heater should be inspected for loose ordisconnected wires. They are also equipped with asafety valve.

1.4.4Safety Valve

A safety valve protects reservoirs from becomingover pressurized and bursting if the governormalfunctioned and did not place the compressor inthe unloading stage. The valve consists of a springloadedball that will allow air to exhaust from the reservoir into the atmosphere. The valve’s pressuresetting is determined by the force of the spring. Asafety valve is normally set at 150 psi. If the pressurein the system rises to approximately 150 psi, thepressure would force the ball off its seat, allowingthe pressure to exhaust through the exhaust port inthe spring cage. When reservoir pressure issufficiently reduced to approximately 135 psi, thespring will force the ball back onto its seat, sealingoff the reservoir pressure. Not all safety valves havea manual release feature.