06-11-2012, 12:27 PM
Vacuum brake
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ABSTRACT
The vacuum brake is a braking system employed on trains and introduced in the mid-1860s. A variant, the automatic vacuum brake system, became almost universal in British train equipment and in those countries influenced by British practice. Vacuum brakes also enjoyed a brief period of adoption in the USA, primarily on narrow gauge railroads. However, its limitations caused it to be progressively superseded by compressed air systems starting in the United Kingdom from the 1970s onward. The vacuum brake system is now obsolete; it is not in large-scale usage anywhere in the world, other than in South Africa, supplanted in the main by air brakes.
Overview
In the earliest days of railways, trains were slowed or stopped by the application of manually applied brakes on the locomotive and in brake vehicles through the train, and later by steam power brakes on locomotives. This was clearly unsatisfactory, but the technology of the time did not easily offer an improvement. A chain braking system was developed, requiring a chain to be coupled throughout the train, but it was impossible to arrange equal braking effort down the length of the train.
A major advance was the adoption of a vacuum braking system in which flexible pipes were connected between all the vehicles of the train, and brakes on each vehicle could be controlled from the locomotive. The earliest pattern was a simple vacuum brake, in which vacuum was created by operation of a valve on the locomotive; the vacuum actuated brake pistons on each vehicle, and the degree of braking could be increased or decreased by the driver. Vacuum, rather than compressed air, was preferred because steam locomotives can be fitted with ejectors, which are simple venturi devices that create vacuum without the use of moving parts.
However, the simple vacuum system had the major defect that in the event of one of the hoses connecting the vehicles becoming displaced (by the train accidentally dividing, or by careless coupling of the hoses, or otherwise) the vacuum brake on the entire train was useless.
How the automatic vacuum brake works
Air at atmospheric pressure from the train pipe is admitted below the piston, which is forced up
In its simplest form, the automatic vacuum brake consists of a continuous pipe -- the train pipe -- running throughout the length of the train. In normal running a partial vacuum is maintained in the train pipe, and the brakes are released. When air is admitted to the train pipe, the air pressure acts against pistons in cylinders in each vehicle. A vacuum is sustained on the other face of the pistons, so that a net force is applied. A mechanical linkage transmits this force to brake shoes which act by friction on the treads of the wheels.
The fittings to achieve this are therefore:
• a train pipe: a steel pipe running the length of each vehicle, with flexible vacuum hoses at each end of the vehicles, and coupled between adjacent vehicles; at the end of the train, the final hose is seated on an air-tight plug;
• an ejector on the locomotive, to create vacuum in the train pipe;
• controls for the driver to bring the ejector into action, and to admit air to the train pipe; these may be separate controls or a combined brake valve;
• a brake cylinder on each vehicle containing a piston, connected by rigging to the brake shoes on the vehicle; and
• a vacuum (pressure) gauge on the locomotive to indicate to the driver the degree of vacuum in the train pipe.
Practical considerations
The automatic vacuum brake as described represented a very considerable technical advance in train braking. In practice steam locomotives had two ejectors, a small ejector for running purposes (to exhaust air that had leaked into the train pipe) and a large ejector to release brake applications. Later Great Western Railway practice was to use a vacuum pump instead of the small ejector.
Dual brakes
Vehicles can be fitted with dual brakes, vacuum and air, provided that there is room to fit the duplicated equipment. It is much easier to fit one kind of brake with a pipe for continuity of the other. Train crew need to take note that the wrong-fitted wagons do not contribute to the braking effort and make allowances on down grades to suit. Many of the earlier classes of diesel locomotive used on British Railways were fitted with dual systems to enable full usage of BR's rolling stock inherited from the private companies which had different systems depending on which company the stock originated from.
Air brakes need a tap to seal the hose at the ends of the train. If these taps are incorrectly closed, a loss of brake force may occur, leading to a dangerous runaway. With vacuum brakes, the end of the hose can be plugged into a stopper which seals the hose by suction. It is much harder to block the hose pipe compared to air brakes.
Present-day use of vacuum brakes
Today's largest operators of trains equipped with vacuum brakes are the Railways of India and Spoornet (South Africa), however there are also trains with air brakes and dual brakes in use. South African Railways (Spoornet) operates more than 1 000 electric multiple unit cars, which are fitted with air compressed brakes. The electro-vacuum system uses a 2-inch (51 mm) train pipe and basic automatic vacuum brake system, with the addition of electrically-controlled application and release valves in each vehicle. The application and release valves greatly increase the rate of train pipe vacuum destruction and creation. This, in turn, greatly increases the speed of brake application and release. The performance of electro-vacuum brakes on SAR EMUs is equivalent to electro-pneumatic braked EMUs of a similar age.
Other African railways are believed to continue to use the vacuum brake. Other operators of vacuum brakes are narrow gauge railways in Central Europe, the largest of which is the Rhaetian Railway.
Vacuum brakes have been entirely superseded on the National Rail system in the UK, although they are still in use on most heritage railways. They are also to be found on a number (though increasingly fewer) main line vintage special trains.
Iarnród Éireann (the national rail operator in the Republic of Ireland) ran vacuum-braked British Railways Mark 2 stock on passenger trains until the end of March 2008[2] and still operates vacuum-braked revenue freight (at least in the case of Tara Mines ore traffic).