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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 the brakes on the guard's van or brake van (UK) or caboose (US) at the back 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 rear of the train (normally whistle signals from the locomotive). Since the braking effort was applied from the ends of 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Vacuum brakes have now been largely superseded by air brakes which work on a similar principle but use compressed air instead of a vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also. 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.
The brakes themselves are in the form of metal shoes which press against the train's wheels creating frictionIn physics, friction is the resistive force that occurs when two surfaces travel along each other when forced together. It causes physical deformation and heat buildup. The frictional force is a function of the force pressing the surfaces together and the 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 pistonIn general, a piston is a sliding plug that fits closely inside the bore of a cylinder. Its purpose is either to change the volume enclosed by the cylinder, or to exert a force on a fluid inside the cylinder. A piston in an internal combustion engine usua, 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 pumpA pump is a mechanical device used to move liquids or gases. Also the heart is an essential pump in humans and animals to move the blood around. The earliest pump was described by Archimedes around 300 BC and is known as the Archimedes screw pump. Pumps w in the locomotive to form a vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also. 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 valveA valve is a mechanical device that regulates the flow of fluids (either gases, fluidised solids, slurries or liquids) by opening, closing, or partially obstructing various passageways. There are a number of names for particular types of valve: Ball cock, 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 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.
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