Air brake (rail)
From Wikipedia, the free encyclopedia
On railways and trams an air brake is a brake operated by compressed air. A safer air brake was patented by George Westinghouse on March 5, 1872. Westinghouse's invention revolutionized the railroad industry, making stopping reliable and thus permitting trains to travel at higher speeds. Westinghouse made many alterations to improve his invention leading to various forms of the automatic brake. The United States Congress passed the Safety Appliance Act in 1893 making the use of some automatic brake system mandatory. By 1905, over 2,000,000 freight, passenger, mail, baggage and express railroad cars and 89,000 locomotives in the United States were equipped with the Westinghouse Automatic Brake.
Contents |
[edit] Overview
In the air brake's simplest form, called the straight air system, compressed air pushes on a piston in a cylinder. The piston is connected to a brake shoe which can rub on the train wheel, using the resulting friction to slow the train. The pressurized air comes from an air compressor in the locomotive and is sent from car to car by a train line made up of pipes beneath each car and hoses between cars. The principal problem with the straight air braking system is that any separation between hoses and pipes causes loss of air pressure and hence the loss of the force applying the brakes. This deficiency could easily cause a runaway train. Straight air brakes are still used on locomotives, although as a dual circuit system, usually with each bogie having its own circuit.
In order to design a system without the shortcomings of the straight air system, Westinghouse invented a system wherein each piece of railroad rolling stock was equipped with an air reservoir and a triple valve, also known as a control valve.
The triple valve is often described as being so named because it performs three functions. This is a widespread myth, as the triple valve simply performs two functions: it applies the brakes and releases them. In so doing, it supports certain other actions (i.e. it 'holds' or maintains the application, and it permits the reservoir to be recharged during the release). 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. When he soon improved the device by removing the poppet valve action, these three components became the piston valve, the slide valve, and the graduating valve.
- If the pressure in the train line is lower than that of the reservoir, the brake cylinder exhaust portal is closed and air from the car's reservoir is fed into the brake cylinder to apply the brakes.
- If the pressure in the train line is higher than that of the reservoir, the triple valve connects the train line to the reservoir feed, causing the air pressure in the reservoir to increase. The triple valve also causes the brake cylinder to be exhausted to atmosphere, releasing the brakes.
- As the pressure in the train line and that of the reservoir equalize, the triple valve closes, causing the air pressure in the reservoir and brake cylinder to be maintained at the current level.
Unlike the straight air system, the Westinghouse system uses a reduction in air pressure in the train line to apply the brakes. When a train's engineer applies the brake by operating the locomotive brake valve, the train line vents to atmosphere, reducing the train line pressure and in turn triggering the triple valve on each car to feed air into its brake cylinder. When the engineer releases the brake, the locomotive brake valve portal to atmosphere is closed, allowing the train line to be recharged by the locomotive's compressor. The subsequent increase of train line pressure causes the triple valves on each car to discharge the brake cylinder's contents to atmosphere, releasing the brakes and recharging the reservoirs.
Under the Westinghouse system, therefore, brakes are applied by reducing train line pressure and released by increasing train line pressure. The Westinghouse system is thus fail safe — any failure in the train line, including a separation ("break-in-two") of the train, will cause a loss of train line pressure, causing the brakes to be applied and bringing the train to a stop.
Modern air brake systems are in effect two braking systems combined:
- The service brake system, which applies and releases the brakes during normal operations, and
- The emergency brake system, which applies the brakes rapidly in the event of a brake pipe failure.
When the train brakes are applied during normal operations, the engineer makes a "service application" or a "service rate reduction," which means that the train line pressure reduces at a controlled rate. It takes several seconds for the train line pressure to reduce and consequently takes several seconds for the brakes to apply throughout the train. In the event the train needs to make an emergency stop, the engineer can make an "emergency application," which immediately vents all of the train line pressure to atmosphere, resulting in a rapid application of the train's brakes. An emergency application also results when the train line comes apart or otherwise fails, as all air will also be immediately vented to atmosphere.
In addition, an emergency application brings in an additional component of each car's air brake system: the emergency portion. The triple valve is divided into two portions: the service portion, which contains the mechanism used during brake applications made during service reductions, and the emergency portion, which senses the immediate, rapid release of train line pressure. In addition, each car's air brake reservoir is divided into two portions--the service portion and the emergency portion--and is known as the "dual-compartment reservoir." Normal service applications transfer air pressure from the service portion to the brake cylinder, while emergency applications cause the triple valve to direct all air in both the service portion and the emergency portion of the dual-compartment reservoir to the brake cylinder, resulting in a 20-30% stronger application.
The emergency portion of each triple valve is activated by the extremely rapid rate of reduction of train line pressure. Due to the length of trains and the small diameter of the train line, the rate of reduction is high near the front of the train (in the case of an engineer-initiated emergency application) or near the break in the train line (in the case of the train line coming apart). 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 train line's pressure directly to atmosphere. This serves to propagate the emergency application rapidly along the entire length of the train.
[edit] Enhancements
Electro-pneumatic or EP brakes are a new type of air brake that will allow for immediate application of brakes throughout the train instead of the sequential application for the current type. Electro-pneumatic brakes are currently in testing in North America and South Africa in captive service ore and coal trains. EP brakes have been in use in German high speed trains (most notably the ICE) since the late 1980s.
Passenger trains have had for a long time a 3-wire version of the Electro-pneumatic brake, which gives 7 levels of braking force. In most cases the system is not failsafe, with the wires being energized in sequence to apply the brakes, but the conventional automatic air brake is also provided to act as a fail safe, and in most cases can be used independently in the event of a failure of the EP brakes.
Later systems replace the automatic air brake with an electrical wire (in the UK, at least, known as a "round the train wire") that has to be kept energised to keep the brakes off.
More recent innovations are Electronically Controlled brakes where the brakes of all the wagons and locomotives are connected by a kind of Local Area Network, which allows individual control of the brakes on each wagon, and the reporting back of performance of each wagon's brakes.
[edit] Long steep grades
The Westinghouse air brake is not completely foolproof when it comes to operating trains down long steep gradients. If the engineer applies the brakes too often, the pressure in the reservoirs can drop too low such that the brakes do not apply strongly enough to keep the train under control. To prevent this, engineers will use an alternative brake called the Dynamic brake (rheostatic brake in the UK) to slow the train, using the air brakes only as necessary.
In an emergency, the engineer can usually still make an emergency application, as the emergency portion of the dual-compartment reservoir is not affected by normal service reductions. The triple valves sense an emergency reduction based on the rate of train line pressure reductions, so as long as the train line's air is vented immediately to atmosphere, the train's triple valves will still apply the brakes in emergency. However, if the engineer has completely exhausted the air brake system such that the pressure of the train line is extremely low (because all of the air is still being used to recharge the empty reservoirs), the reduction, while rapid, will not be enough to trip the triple valves into emergency.
A further enhancement though is the two-pipe system. This has a second main line or main reservoir pipe (so called because it is connected directly to the locomotive's main reservoir, which stores the compressed air used to charge up the train line) that is kept charged all the time by the locomotive's compressor, and through non-return valves charges the reservoirs on the rest of the train. This not only eliminates the above problems, but also allows the brakes to release faster, since the brake pipe only has to recharge itself, and not the reservoirs. The main reservoir can also be used to supply air for auxiliary systems such as doors or air suspension systems. Nearly all passenger trains (all in the UK and USA), and many freights, now have the two-pipe system.
[edit] Accidents
The air brake can fail if one of the cocks where the pipes of each carriage are joined together is accidentally closed. In this case, the brakes on the wagons behind the closed cock will fail to respond to the driver's command. This happened in 1953 to the Federal Express, a Pennsylvania Railroad train pulling in to Washington DC's Union Station, causing the train to crash into the passenger concourse and fall through the floor.
There are a number of safeguards that are usually taken to prevent this sort of accident happening. Railroads have strict government-approved procedures for testing the air brake systems when making up trains in a yard or picking up cars en route. These generally involve connecting the air brake hoses, charging up the brake system, setting the brakes and manually inspecting the cars to ensure the brakes are applied, and then releasing the brakes and manually inspecting the cars to ensure the brakes are released. Particular attention is usually paid to the rearmost car of the train, either by manual inspection or via an automated end-of-train device, to ensure that brake pipe continuity exists throughout the entire train. When brake pipe continuity exists throughout the train but the brakes do not apply or release on one or more cars, this indicates that the cars' triple valves are malfunctioning. Depending on the location of the air test, the repair facilities available, and regulations governing the number of inoperative brakes permitted in a train, the car may be set out for repair or taken to the next terminal where it can be repaired.
A different kind of accident nearly happened in Australia when a train with the wrong kind of brake shoe was diverted due to a derailment to an extremely steep line. The train in question had brake shoes that lost their grip when overheated, and this train was diverted to a line with a 30km 900m descent from Katoomba to Emu Plains. The train ran away out of control and was lucky not to have crashed.
[edit] Standardisation
The airbrake of 2006 is not identical with the original airbrake as there have been slight changes in the design of the triple valve, which are not completely compatible between versions, and which must therefore be introduced in phases. That said, the basic air brake used on railways worldwide are remarkably compatible.
[edit] Vacuum brakes
The main competitor to the air brake is the vacuum brake, which operates on negative pressure. The vacuum brake is a little simpler than the air brake, with an ejector with no moving parts replacing the air compressor. Disconnection taps at the ends of cars are not required as the loose hoses are sucked onto a mounting block.
However the maximum pressure is limited to atmospheric pressure, so that all the equipment has to be larger and heavier to compensate. This disadvantage is made worse at high altitude.
[edit] See also
[edit] References
- Air Brake and Train Handling Manual. Copyright 2006 Alaska Railroad Corporation
- Air Brake and Train Handling Manual. Copyright 2003 BNSF Railway Company
[edit] External links
Information
- Railway-Technical: Air Brakes
Patents
- U.S. Patent 12263 -- Steam and air brake -- W. Wright
- U.S. Patent 16220 -- Air engine -- S. Carson
- U.S. Patent 88929 -- Steam power brake -- G. Westinghouse
