Vacuum brake
The vacuum brake is a braking system used on trains. It was first introduced in the mid 1860s and a variant, the automatic vacuum brake system became almost universal in British train equipment, and in those countries influenced by British practice.
It enjoyed a brief period of adoption in the USA, primarily on narrow gauge railroads.
Its limitations caused it to be progressively superseded by compressed air systems, in the United Kingdom from the 1970's.
The vacuum brake system is now obsolescent; it is not in large-scale use anywhere in the world, 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.
The automatic vacuum brake had been developed: it was designed to apply fully if the train becomes divided or if a hose becomes displaced, but the railway operators resisted its adoption at first, as it involved considerably more components than the simple system and cost more.
In a serious accident at Armagh, a portion of a train was detached from the locomotive on a steep gradient and ran away, killing 88 people. 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.
How the automatic vacuum brake works
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.
In the diagram, the piston is shown in red; it is connected at the bottom to the brake rigging, and if the piston is pulled up, the brakes are applied.
The brake cylinder is contained in a larger housing -- this gives a reserve of vacuum as the piston operates. The cylinder rocks slightly in operation to maintain alignment with the brake rigging cranks, so it is supported in trunnion bearings, and the vacuum pipe connection to it is flexible. The piston in the brake cylinder has a flexible piston ring that allows air to pass from the upper part of the cylinder to the lower part if necessary.
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 (as air will have leaked slowly into the upper part of the cylinder, destroying the vacuum).
When a locomotive is coupled to the vehicles, the driver moves his 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 via the train pipe. In the diagram, the green area represents vacuum.
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. At this point there is a higher air pressure (less vacuum -- indicated in blue in the diagram) under the brake pistons than above it, and the pressure differential forces the piston upwards, applying the brakes. The driver can control the severity of the braking effort by admitting more or less air to 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.
The driver's brake valve was usually combined with the steam brake control on the locomotive.
The ejectors on steam locomotives are set to create a certain degree of vacuum in the train pipe; in British practice a full release is 21 inches of mercury. An absolute vacuum is about 30 inches of mercury, depending on atmospheric conditions; the Great Western Railway adopted 25 inches of mercury as its standard degree of vacuum.
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.
When a locomotive is replaced by another locomotive to continue the journey, it occasionally happens that the second locomotive is adjusted to achieve a slightly lower degree of vacuum than the previous locomotive, or a Great Western Railway locomotive is to be replaced by another company's locomotive operating at a lower degree of vacuum. In this case the second locomotive may be unable to release the train brakes fully. The release valves on the vehicles had to be operated in such a case, on every coach -- a time consuming operation frequently witnessed at Bristol Temple Meads and elsewhere.
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 (usually called "the communication cord" in lay terminology) 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..
Limitations
The progress represented by the automatic vacuum brake nonetheless carried some limitations; chief among these were:
- the practical limit on the degree of vacuum attainable means that a very large brake piston and cylinder are required to generate the force necessary on the brake blocks; when a proportion of the British ordinary wagon fleet was fitted with vacuum brakes in the 1950's, the physical dimensions of the brake cylinder prevented the wagons from operating in some private sidings that had tight clearances;
- for the same reason, on a very long train, a considerable volume of air has to be admitted to the train pipe to make a full brake application, and a considerable volume has to be exhausted to release the brake (if for example a signal at danger is suddenly lowered and the driver requires to resume speed); while the air is traveling along the train pipe, the brake pistons at the head of the train have responded to the brake application or release, but those at the tail will respond much later, leading to undesirable longitudinal forces in the train. In extreme cases this has led to breaking couplings and causing the train to divide.
- the existence of vacuum in the train pipe can cause debris to be sucked in. An accident took place near Ilford in the 1950's, due to inadequate braking effort in the train. A rolled newspaper was discovered in the train pipe, effectively isolating the rear part of the train from the driver's control. The blockage should have been detected if a proper brake continuity test had been carried out before the train started its journey.
A development introduced in the 1950's was the direct admission valve, fitted to every brake cylinder. These valves responded to a rise in train pipe pressure as the brake was applied, and admitted atmospheric air directly to the underside of the brake cylinder.
American and continental European practice had long favoured compressed air brake systems, the leading pattern being a proprietary Westinghouse system. This has a number of advantages, including smaller brake cylinders (because a higher air pressure could be used) and a somewhat more responsive braking effort. However the system requires an air pump. On steam engines this was usually a reciprocating steam pump, and it was quite bulky. Its distinctive shape and the characteristic puffing sound when the brake is released (as the train pipe has to be recharged with air) make steam locomotives fitted with the Westinghouse brake unmistakable, for example in old films.
In the UK, the Great Eastern Railway, the North Eastern Railway and the Caledonian Railway adopted the Westinghouse system. Inevitably this led to compatibility problems in exchanging traffic with other lines. It was possible to provide through pipes for the braking system not fitted to any particular vehicle so that it could run in a train using the "other" system, allowing through control of the fitted vehicles behind it, but of course with no braking effort of its own.
Vacuum brakes in 2007
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. Other African railways are believed to continue to use the vacuum brake. Other operators of vacuum brakes are narrow gauge railways in Central Europe, largest of them is Ferrovia Retica.
Vacuum brakes have been entirely superseded on the National Rail system in the UK, although they are still in use on some heritage railways.
References
- British Transport Commission, London (1957:142). Handbook for Railway Steam Locomotive Enginemen.