Vacuum brake
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The vacuum brake is a braking system used on trains. It was invented in 1877 in the USA, where it enjoyed only a brief period of popularity, primarily on narrow gauge railroads. The system took a greater hold in the United Kingdom, being used there as the primary form of train braking until the 1970s. Vacuum braking is for all practical purposes now a dead technology; it is not in large-scale use anywhere in the world, supplanted in the main by air brakes.
Vacuum brakes permit the automatic application of brakes down the length of a train from a simple control in the driver's hand. They are also fail safe, since they default to an applied state; power in the form of vacuum is used to release the brakes, so if vacuum is lost due to malfunction or the train breaking apart, the brakes are automatically applied.
Vacuum brakes were a big step forward in train safety. Prior to their invention, a train had to rely on the brakes of the locomotive at the front of a train, and mechanical brakes on the cars and guard's van or brake van (UK) or caboose (US) applied by brakemen to stop an entire train. This limited the braking power of the train and meant only short trains could be stopped safely; furthermore, this system required good communication between the locomotive and the brakemen (normally whistle signals from the locomotive). Since the braking effort was applied irregularly along the train, a great strain was put on couplers, risking train breakup.
Vacuum brakes have now been largely superseded by air brakes which work on a similar principle but use compressed air instead of a vacuum. This allows for more braking power, since the pressure differential between atmospheric pressure and a feasible vacuum is less than that between atmospheric pressure and a realistic brake-pipe pressure.
How they work
The brakes themselves are in the form of metal shoes which press against the train's wheels creating friction which slows the train down.
On a train equipped with vacuum brakes, every wagon or coach is equipped with at least one set of brakes. The default position of each brake shoe is on and the brakes are spring-loaded so that without vacuum, there is pressure applied. Behind each brake shoe is a vacuum cylinder which contains a piston, which draws the brake shoe forwards or backwards, or into the on or off positions.
An airtight pipe runs along the entire length of the train. The air is pumped out of this pipe by a pump in the locomotive to form a vacuum. As a vacuum forms in the vacuum cylinder behind the piston, the piston is pushed backwards by atmospheric pressure, thereby drawing the brake shoe backwards into the off position.
The brakes will automatically move forward to the default "on" position if the vacuum is broken. The train driver can apply the brakes by opening a valve which lets air into the pipe thus breaking the vacuum. If the train breaks up or the pipe develops a leak, the vacuum will again be broken and the brakes will come on.
The vacuum brake was considered preferential to the early air brake in railroad applications largely because it was cheaper to install on a steam locomotive. Air brakes required a steam-powered compressor - bulky, noisy, unsightly and using a lot of power, while the vacuum ejector used to generate vacuum was a much simpler device, having no moving parts.