Air brake (rail)
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Image:Westinghouse Air Brake piping diagram.jpg Image:Westinghouse Air Brake control handle and valve.jpg 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.
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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.
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.
The triple valve was so named because it performs three functions:
- 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.
- 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.
- 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, this causes the train line to be vented to atmosphere, 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.
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.
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 energised 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 inovations 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.
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. A further enhancement though is the two-pipe system. This has a second main line or main reservoir pipe that is kept charged all the time by the locomotives 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 reservior 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.
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 on a electric train going to Gare de Lyon in Paris.
A different kind of accident nearly happened 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, an this train was driverted to a line with a 30km 900m decent from Katoomba to Emu Plains. The train ran away out of control and was lucky not to have crashed.
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.
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 replacing the air compressor. However the maximum pressure in limited to atmospheric pressure, so that all the equipment has to be larger to compensate.
Other applications
Air brakes are also used in trucks, buses and semi-trailers.
The air brakes on a semi-trailer are connected to a tractor with two lines. One line is called the supply line or the emergency line. It is usually larger and is red or has red fittings. The emergency line provides air pressure to fill the semi-trailer's reservoir tank and the pistons that activate the brakes. The other line is called the service line. It is usually smaller and is blue or has blue fittings.
In normal braking pressing the brake pedal pressurizes the service line. This activates a valve in the trailer which directs air from the reservoir and the emergency line to the brake cylinders where it moves the piston that activates the brakes. When the pedal is let up the service line pressure is decreased. When the service line pressure drops it causes the valve in the trailer to block the air supply from the reservoir while releasing the pressure in the brake cylinder and the brakes are released. The system is a form of servo or amplifier.
If the pressure in the emergency line drops, due to the activation of a valve in the cab, the disconnection of the emergency line coupling or a break in the emergency line, a check valve prevents air from escaping the reservoir and air pressure from the reservoir activates the brakes.
When a semi-trailer is disconnected from the tractor, pressure from the reservoir applies the emergency brakes because the emergency line is disconnected. Eventually air will leak out of the system and there will no longer be air pressure to apply the emergency brakes. Newer trailers are also equipped with spring brakes. When there is air pressure in the reservoir it supplies air to a piston which counteracts the spring. When the air pressure in the reservoir drops, the spring brake cylinder can no longer counteract the spring and the brakes are applied.
External links
Information
- Railway-Technical: Air Brakes
Patents
- Template:US patent -- Steam and air brake -- W. Wright
- Template:US patent -- Air engine -- S. Carson
- Template:US patent -- Steam power brake -- G. Westinghousede:Druckluftbremse (Eisenbahn)