Vacuum brake
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 using atmospheric pressure; 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. 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.
Vacuum brakes have now been largely superseded by air brakes which work on a similar principle but use compressed air instead of a vacuum.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
Each vehicle is fitted with at least one brake cylinder. These are of large diameter, as only the force from atmospheric pressure is used to apply the brakes (see below). They are enclosed in an airtight case (with a gland for the piston rod) and are mounted vertically with the piston rod beneath. Connection to the brakes is by brake rigging. The brakes themselves take the form of cast iron (or, rarely, wooden) blocks which are pivoted to bear on the train wheels.
In sequence, the method of operation is: On coupling up the locomotive, air is exhausted from a pipe running the length of the train by a large steam-operated ejector ("making a brake" – UK). Each cylinder is arranged so that this vacuum initially acts equally on both sides of the brake piston. The piston, which is of cast iron, drops under gravity and releases the brakes. The large ejector is then turned off to save steam and a small ejector (or light pump worked from the crosshead) is employed to maintain the vacuum.
If air is admitted to the train pipe, either under the control of the driver or because of an equipment failure, a one-way ball valve at each brake cylinder (or sometimes a one-way piston sealing ring, as found in a bicycle pump) prevents it from entering the upper part of the cylinder. The pressure differential (effectively, the air at atmospheric pressure entering the train pipe) closes the valve and forces the piston upwards, thus applying the brake. The airtight case, which is open to the upper chamber, is acting as a “vacuum reservoir” for the upper part of the cylinder, ensuring fail-safe operation.
To release the brake the driver's brake valve is closed and the vacuum restored by either the small or large ejector, as necessary.
To discharge the vacuum under fine control the driver is provided with a graduable brake valve (which usually also incorporates the controls to admit steam to the ejectors) and a vacuum gauge. The running “pressure” is usually in the range of twenty-one to twenty-five inches of mercury (a perfect vacuum is regarded as thirty-one inches of mercury). During braking the driver will watch the gauge to estimate how firmly he/she is applying the brakes – a light application sees the vacuum drop to about fifteen inches and a heavy one to about ten. Of course in an emergency application ("dropping the handle" – UK) the train pipe vacuum can be completely destroyed. During all normal braking, anticipation and careful attention to the pressure is required as, even with the large ejector turned on, it takes some time to restore the vacuum, release the brakes and allow the train to proceed. It is good practice to restore the vacuum slowly when the train is in motion, to avoid releasing the brakes at the front of the train before those at the rear: neglecting this will cause undue strain on the couplings.