Vacuum brake
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 countries influenced by British practice. Vacuum brakes also enjoyed a brief period of adoption in the United States, primarily on narrow-gauge railroads. Their limitations caused them to be progressively superseded by compressed air systems starting in the United Kingdom from the 1970s onward.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, largely supplanted by air brakes.
Introduction
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, given the slow and unreliable response times and extremely limited braking power that could be exerted, but the existing technology did not 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 along the entire 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 scheme 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; venturi devices that create vacuum without moving parts.
The simple vacuum system had the major defect that in the event of one of the hoses connecting the vehicles becoming displaced the vacuum brake on the entire train was useless.
In response to this obvious defect, the automatic vacuum brake was subsequently developed. It was designed to apply fully if the train became divided or if a hose became displaced. The automatic vacuum brake was slightly more expensive to manufacture and install than the simple system due to it requiring a higher number of machined parts, and incurred higher running costs since the ejector ran continuously to maintain the vacuum when the train was running, rather than only being used when braking as in the simple system.
Opposition to the fitting of the automatic type of brake on the grounds of cost meant that it took a serious accident at Armagh in 1889 before legislation compelled the adoption of the automatic system. In this accident at Armagh, a portion of a train was detached from the locomotive on a steep gradient and ran away, killing 80 people. The train was fitted with the simple vacuum brake, which was useless on the disconnected portion of the train. It was clear that if the vehicles had been fitted with an automatic continuous brake, the accident would almost certainly not have happened, and the public concern at the scale of the accident prompted legislation mandating the use of a continuous automatic brake on all passenger trains.
In continental Europe, the vacuum brake was sometimes called the Hardy brake, after John George Hardy of the Vacuum Brake Co, 7 Hohenstaufengasse, Vienna.
Operation
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 at atmospheric 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 on the treads of the wheels.The fittings to achieve this are:
- 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 gauge on the locomotive to indicate to the driver the degree of vacuum in the train pipe.
When the vehicles have been at rest, so that the brake is not charged, the brake pistons will have dropped to their lower position in the absence of a pressure differential.
When a locomotive is coupled to the vehicles, the driver moves the brake control to the "release" position and air is exhausted from the train pipe, creating a partial vacuum. Air in the upper part of the brake cylinders is also exhausted from the train pipe, through a non-return ball valve.
If the driver now moves his control to the "brake" position, air is admitted to the train pipe. According to the driver's manipulation of the control, some or all of the vacuum will be destroyed in the process. The ball valve closes and there is a higher air pressure under the brake pistons than above it, and the pressure differential forces the piston upwards, applying the brakes. The driver can control the amount of braking effort by admitting more or less air to the train pipe.
Practical considerations
The automatic vacuum brake as described represented a considerable technical advance in train braking. In practice steam locomotives had two ejectors, a small ejector for running purposes and a large ejector to release brake applications. The small ejector used much less steam than the large ejector but could not generate vacuum in the train pipe sufficiently quickly for operational purposes, especially in a long train. Later Great Western Railway practice was to use a vacuum pump instead of the small ejector – the pump was fitted to one of the engine crossheads and so did not use any steam, with the disadvantage that it only operated when the locomotive was in motion. The GWR favoured this due to the use of braking systems working on a vacuum level higher than other railways which would have required a relatively large and steam-hungry "small" ejector.Image:Graduable brake valve.jpg|thumb|Graduable brake valve and the small and large ejector cocks from a GWR locomotive
Most steam locomotives of the period used straightforward live steam brakes on their own wheels, with the vacuum brake being solely used on the train. In such a case the two systems were usually operated proportionately by a single control, whereby the reduction in vacuum in the train brake system would open the valve feeding steam to the engine brake. It was unusual for any form of dedicated control to be provided solely for the steam brake – even when running with no train the driver controlled the engine's steam brakes by adjusting the vacuum brake system using the ejectors on the engine and the "head end" of the train pipe. This allowed the driver of the lead engine direct control over the brakes on any trailing locomotive when double heading.
With the introduction of diesel and electric locomotives by British Railways from the early 1950s, this same basic arrangement was carried over. BR's Modernisation Plan of 1955 called for, amongst other things, a long-term aim to switch to air brakes for both passenger and freight stock. The standard Mark 1 coaching stock had been designed and procured before the decision to switch to modern traction and air brakes had been taken, so the majority of the stock was fitted with traditional vacuum brakes. Air-braked goods wagons were introduced steadily from the mid-1960s and the Mark 2a coaching stock with air brakes was built from 1967. Diesel and electric locomotives naturally could not use the traditional steam-driven ejector to generate vacuum. Smaller locomotives had exhausters or vacuum pumps driven directly by their prime mover while larger ones had similar machines mounted separately and driven by dedicated electric motors. It was normal practice on mainline locomotives to fit two exhausters for redundancy. Just as steam locomotives had a small and large ejector, the diesels and electrics had their brake controls set up to run one exhauster continuously to generate and maintain the vacuum in the system, with the second one being started when the brake handle was set to its 'Release' position to provide a quicker response. A switch in the locomotive cab allowed the driver to choose which exhauster would serve each function.
Release valves are provided on the brake cylinders; when operated, usually by manually pulling a cord near the cylinder, air is admitted to the upper part of the brake cylinder on that vehicle. This is necessary to release the brake on a vehicle that has been uncoupled from a train and now requires to be moved without having a brake connection to another locomotive, for example if it is to be shunted.
In the UK the pre-nationalisation railway companies standardised around systems operating on a vacuum of, with the exception of the Great Western Railway, which used. Sea level air pressure is about, depending on atmospheric conditions.
This difference in standards could cause problems on long-distance cross-country services when a GWR locomotive was replaced with another company's engine, as the new engine's large ejector would sometimes not be able to fully release the brakes on the train. In this case the release valves on each vehicle in the train would have to be released by hand, before the brake was recharged at 21 inches. This time-consuming process was frequently seen at large GWR stations such as Bristol Temple Meads.
The provision of a train pipe running throughout the train enabled the automatic vacuum brake to be operated in emergency from any position in the train. Every guard's compartment had a brake valve, and the passenger communication apparatus also admitted air into the train pipe at the end of coaches so equipped.
When a locomotive is first coupled to a train, or if a vehicle is detached or added, a brake continuity test is carried out, to ensure that the brake pipes are connected throughout the entire length of the train.