Railway air brake

A railway air brake is a railway brake power braking system with compressed air as the operating medium. Modern trains rely upon a fail-safe air brake system that is based upon a design patented by George Westinghouse on April 13, 1869. The Westinghouse Air Brake Company was subsequently organized to manufacture and sell Westinghouse's invention. In various forms, it has been nearly universally adopted.
The Westinghouse system uses air pressure to charge air reservoirs on each car. Full air pressure causes each car to release the brakes. A subsequent reduction or loss of air pressure causes each car to apply its brakes, using the compressed air stored in its reservoirs.

Overview

Straight air brake

In the air brake's simplest form, referred to as a straight air system, compressed air is directed to a brake cylinder, causing its piston to apply force to mechanical linkage, which linkage is conventionally referred to as the brake rigging. The brake rigging, in turn, is connected to brake shoes that are pressed against the car's wheel treads. The resulting friction slows the car by dissipating its kinetic energy as heat. The brake rigging is often quite elaborate, as it is designed to evenly distribute the brake cylinder's force to multiple wheels.
The source of high-pressure air needed to operate the system is an air compressor mounted in the locomotive, the compressor being driven by a Diesel locomotive's prime mover, or by a cross-compound steam engine on a steam locomotive. Compressors of electric locomotives are usually driven by their own electric motor. The output of the air compressor is stored in a tank, also mounted on the locomotive, this tank being referred to as the main reservoir. Air from the main reservoir is piped to a manually operated brake valve in the locomotive's cab. When the brake valve is opened to apply the brakes, air under pressure is conveyed to the brake mechanism.
A critical weakness of the straight air braking system is that any failure in the piping, such as a blown air hose that results in a loss of pressure, will render the brakes inoperative. For this reason, train brakes do not employ straight air for operation, as there is no redundancy in the event of such a failure. However, straight air is used to operate locomotive brakes, as redundancy is provided by the ability of a locomotive to come to a stop by reversing propulsion in an emergency, a procedure referred to as "plugging".
Locomotive brakes are controlled by an independent brake valve, so-named because the locomotive brakes may be applied or released independently from the train brakes.

Westinghouse air brake

In order to design a braking system without the shortcomings of the straight air system, Westinghouse invented an arrangement in which each piece of railroad rolling stock was equipped with a dual-compartment, compressed-air reservoir and a triple valve, also known as a control valve. A pipe referred to as the brake pipe was fitted to each car to act as a passage for the compressed air needed to make the system function. The brake pipes were fitted with hoses at each end of each car and locomotive for creating a continuous brake pipe connection throughout the train.
Image:RotairValveAriBrakeSRM.jpg|thumb|Rotair Valve from the Westinghouse Air brake Company Unlike the previously described straight air system, the Westinghouse system uses a reduction in brake pipe air pressure to indirectly apply the brakes.
In his patent application, Westinghouse refers to his 'triple-valve device' because of the three component valvular parts comprising it: the diaphragm-operated poppet valve feeding reservoir air to the brake cylinder, the reservoir charging valve, and the brake cylinder release valve. Westinghouse soon improved the device by removing the poppet valve action. These three components became the piston valve, the slide valve, and the graduating valve.
The Westinghouse system functions as follows:

When brake pipe pressure is reduced below car reservoir pressure , the triple valve will close the brake cylinder exhaust port and open a port connecting the service compartment of the reservoir to the cylinder, charging the latter with air from the former and causing a brake application. Cylinder charging will continue until brake pipe and reservoir pressures have equalized, at which time the triple valve will seal the reservoir-to-cylinder port to maintain cylinder pressure.
When brake pipe pressure is increased above car reservoir pressure, the triple valve will open the brake cylinder exhaust port, venting the cylinder to the atmosphere and hence releasing the brakes. Simultaneously, the triple valve will open a port from the reservoir to the brake pipe, causing both reservoir compartments to be recharged. When reservoir and brake pipe pressures have equalized, the triple valve will close the port connecting the brake pipe to the reservoir. The reservoir will be sealed off from both the brake pipe and the brake cylinder, and should be able to maintain pressure until needed again.
When brake pipe pressure is reduced below car reservoir pressure, an emergency brake application will occur. The triple valve will open an unlapped port connecting the emergency compartment of the car's reservoir to the brake cylinder. The resulting sudden application of full reservoir pressure to the brake cylinder will produce the maximum amount of braking force that is possible. At the same time, the triple valve will locally vent the brake pipe to the atmosphere, which behavior will increase the rate at which the sudden pressure loss will propagate throughout the train. Local venting action is necessary because without it, the rate at which brake pipe pressure can be reduced through the automatic brake valve or a blown or disconnected air hose might not be fast enough to trigger an emergency response on more than a few cars. If the pressure loss was due to, for example, a blown air hose at the front of a 100-car freight train and there was no local venting, the triple valves of many of the cars farther back in the train might not produce an emergency response, or the response might be significantly delayed. Cars nearest to the front would forcefully apply their brakes well before the cars farther back, causing a "run-in", an abrupt and violent bunching of train slack that could lead to a derailment.

Due to its design, the Westinghouse system is inherently fail-safe, in that any uncommanded loss of brake pipe pressure, such as the aforementioned blown air hose, will cause an immediate brake application.

Modern systems

Modern air brake systems serve two functions:

braking applies and releases the brakes during normal operations.
braking rapidly applies the brakes in the event of a brake pipe failure or an emergency application by the engine operator or passenger emergency alarm/cord/handle.

When the train brakes are applied during normal operation, the engine operator makes a "service application" or a "service rate reduction", which means that the brake pipe pressure reduces at a controlled rate. It takes several seconds for the brake pipe pressure to reduce and consequently takes several seconds for the brakes to apply throughout the train. The speed of pressure changes during a service reduction is limited by the compressed air's ability to overcome the flow resistance of the relatively-small-diameter pipe and numerous elbows throughout the length of the train, and the relatively-small exhaust port on the head-end locomotive, which means the brakes of the rear-most cars will apply sometime after those of the forward-most cars apply, so some slack run-in can be expected. The gradual reduction in brake pipe pressure will mitigate this effect.
Modern locomotives employ two air brake systems. The system which controls the brake pipe is called the automatic brake and provides service and emergency braking control for the entire train. The locomotive at the head of the train have a secondary system called the independent brake. The independent brake is a "straight air" system that makes brake applications on the head-of-train locomotive consist independently of the automatic brake, providing for more nuanced train control. The two braking systems may interact differently as a matter of preference by the locomotive builder or the railroad. In some systems, the automatic and independent applications will be additive; in some systems the greater of the two will apply to the locomotive consist. The independent system also provides a bail off mechanism, which releases the brakes on the lead locomotives without affecting the brake application on the rest of the train.
In the event the train needs to make an emergency stop, the engine operator can make an "emergency application," which will rapidly vent all of the brake pipe pressure to atmosphere, resulting in a faster application of the train's brakes. An emergency application also results when the integrity of the brake pipe is lost, as all air will also be immediately vented to atmosphere.
An emergency brake application brings in an additional component of each car's air brake system. The triple valve is divided into two portions: the service section, which contains the mechanism used during brake applications made during service reductions, and the emergency section, which senses the faster emergency reduction of train line pressure. In addition, each car's air brake reservoir is divided into two sections—the service portion and the emergency portion—and is known as the "dual-compartment reservoir". Normal service applications transfer air pressure from the service section to the brake cylinder, while emergency applications cause the triple valve to direct all air in both the sections of the dual-compartment reservoir to the brake cylinder, resulting in a 20 to 30 percent stronger application.
The emergency portion of each triple valve is activated by the higher rate of reduction of brake pipe pressure. Due to the length of trains and the small diameter of the brake pipe, the rate of reduction is highest near the front of the train or near the break in the brake pipe. Farther away from the source of the emergency application, the rate of reduction can be reduced to the point where triple valves will not detect the application as an emergency reduction. To prevent this, each triple valve's emergency portion contains an auxiliary vent port, which, when activated by an emergency application, also locally vents the brake pipe's pressure directly to atmosphere. This serves to more rapidly vent the brake pipe and hasten the propagation of the emergency reduction rate along the entire length of the train.
Use of distributed power somewhat mitigates the time-lag problem with long trains, because a telemetered radio signal from the engine operator in the front locomotive commands the distant units to initiate brake pressure reductions that propagate quickly through nearby cars.