Air Brakes
Introduction
The air brake is the standard, fail-safe, train brake used by railways all over the world. In spite of what you might think, there is no mystery to it. It is based on the simple physical properties of compressed air. So here is a simplified description of the air brake system.
See also the Brakes Glossary, E-P Brakes, Vacuum Brakes.
Contents
Basics - The Principal Parts of the Air Brake System - Operation on Each Vehicle - Release - Application - Lap - Additional Features of the Air Brake - Emergency Air Brake - Emergency Reservoirs - Distributors - Two-Pipe Systems - Self-Lapping Brake Valves - Other Air Operated Systems - Comment.
Basics
A moving train contains energy, known as kinetic energy, which needs to be removed from the train in order to cause it to stop. The simplest way of doing this is to convert the energy into heat. The conversion is usually done by applying a contact material to the rotating wheels or to discs attached to the axles. The material creates friction and converts the kinetic energy into heat. The wheels slow down and eventually the train stops. The material used for braking is normally in the form of a block or pad.
The vast majority of the world's trains are equipped with braking systems which use compressed air as the force to push blocks on to wheels or pads on to discs. These systems are known as "air brakes" or "pneumatic brakes". The compressed air is transmitted along the train through a "brake pipe". Changing the level of air pressure in the pipe causes a change in the state of the brake on each vehicle. It can apply the brake, release it or hold it "on" after a partial application. The system is in widespread use throughout the world.
The Principal Parts of the Air Brake System
The diagram left (click for full size view) shows the principal parts of the air brake system and these are described below.
Compressor
The pump which draws air from atmosphere and compresses it for use on the train. Its principal use is is for the air brake system, although compressed air has a number of other uses on trains. See Auxiliary Equipment.
Main Reservoir
Storage tank for compressed air for braking and other pneumatic systems.
Driver's Brake Valve
The means by which the driver controls the brake. The brake valve will have (at least) the following positions: "Release", "Running", "Lap" and "Application" and "Emergency". There may also be a "Shut Down" position, which locks the valve out of use.
The "Release" position connects the main reservoir to the brake pipe . This raises the air pressure in the brake pipe as quickly as possible to get a rapid release after the driver gets the signal to start the train.
In the "Running" position, the feed valve is selected. This allows a slow feed to be maintained into the brake pipe to counteract any small leaks or losses in the brake pipe, connections and hoses.
"Lap" is used to shut off the connection between the main reservoir and the brake pipe and to close off the connection to atmosphere after a brake application has been made. It can only be used to provide a partial application. A partial release is not possible with the common forms of air brake, particularly those used on US freight trains.
"Application" closes off the connection from the main reservoir and opens the brake pipe to atmosphere. The brake pipe pressure is reduced as air escapes. The driver (and any observer in the know) can often hear the air escaping.
Most driver's brake valves were fitted with an "Emergency" position. Its operation is the same as the "Application" position, except that the opening to atmosphere is larger to give a quicker application.
Feed Valve
To ensure that brake pipe pressure remains at the required level, a feed valve is connected between the main reservoir and the brake pipe when the "Running" position is selected. This valve is set to a specific operating pressure. Different railways use different pressures but they generally range between 65 and 90 psi (4.5 to 6.2 bar).
Equalising Reservoir
This is a small pilot reservoir used to help the driver select the right pressure in the brake pipe when making an application. When an application is made, moving the brake valve handle to the application position does not discharge the brake pipe directly, it lets air out of the equalising reservoir. The equalising reservoir is connected to a relay valve (called the "equalising discharge valve" and not shown in my diagram) which detects the drop in pressure and automatically lets air escape from the brake pipe until the pressure in the pipe is the same as that in the equalising reservoir.
The equalising reservoir overcomes the difficulties which can result from a long brake pipe. A long pipe will mean that small changes in pressure selected by the driver to get a low rate of braking will not be seen on his gauge until the change in pressure has stabilised along the whole train. The equalising reservoir and associated relay valve allows the driver to select a brake pipe pressure without having to wait for the actual pressure to settle down along a long brake pipe before he gets an accurate reading.
The pipe running the length of the train, which transmits the variations in pressure required to control the brake on each vehicle. It is connected between vehicles by flexible hoses, which can be uncoupled to allow vehicles to be separated. The use of the air system makes the brake "fail safe", i.e. loss of air in the brake pipe will cause the brake to apply. Brake pipe pressure loss can be through a number of causes as follows:
- A controlled reduction of pressure by the driver
- A rapid reduction by the driver using the emergency position on his brake valve
- A rapid reduction by the conductor (guard) who has an emergency valve at his position
- A rapid reduction by passengers (on some railways) using an emergency system to open a valve
- A rapid reduction through a burst pipe or hose
- A rapid reduction when the hoses part as a result of the train becoming parted or derailed.
Angle Cocks
At the ends of each vehicle, "angle cocks" are provided to allow the ends of the brake pipe hoses to be sealed when the vehicle is uncoupled. The cocks prevent the air being lost from the brake pipe.
Coupled Hoses
The brake pipe is carried between adjacent vehicles through flexible hoses. The hoses can be sealed at the outer ends of the train by closing the angle cocks.
Brake Cylinder
Each vehicle has at least one brake cylinder. Sometimes two or more are provided. The movement of the piston contained inside the cylinder operates the brakes through links called "rigging". The rigging applies the blocks to the wheels. Some modern systems use disc brakes. The piston inside the brake cylinder moves in accordance with the change in air pressure in the cylinder.
Auxiliary Reservoir
The operation of the air brake on each vehicle relies on the difference in pressure between one side of the triple valve piston and the other. In order to ensure there is always a source of air available to operate the brake, an "auxiliary reservoir" is connected to one side of the piston by way of the triple valve. The flow of air into and out of the auxiliary reservoir is controlled by the triple valve.
Brake Block
This is the friction material which is pressed against the surface of the wheel tread by the upward movement of the brake cylinder piston. Often made of cast iron or some composition material, brake blocks are the main source of wear in the brake system and require regular inspection to see that they are changed when required.
Many modern braking systems use air operated disc brakes. These operate to the same principles as those used on road vehicles.
Brake Rigging
This is the system by which the movement of the brake cylinder piston transmits pressure to the brake blocks on each wheel. Rigging can often be complex, especially under a passenger car with two blocks to each wheel, making a total of sixteen. Rigging requires careful adjustment to ensure all the blocks operated from one cylinder provide an even rate of application to each wheel. If you change one block, you have to check and adjust all the blocks on that axle.
Triple Valve
The operation of the brake on each vehicle is controlled by the "triple valve", so called because it originally comprised three valves - a "slide valve", incorporating a "graduating valve" and a "regulating valve". It also has functions - to release the brake, to apply it and to hold it at the current level of application. The triple valve contains a slide valve which detects changes in the brake pipe pressure and rearranges the connections inside the valve accordingly. It either:
- recharges the auxiliary reservoir and opens the brake cylinder exhaust,
- closes the brake cylinder exhaust and allows the auxiliary reservoir air to feed into the brake cylinder
- or holds the air pressures in the auxiliary reservoir and brake cylinder at the current level.
The triple valve is now usually replaced by a distributor - a more sophisticated version with built-in refinements like graduated release.
