Vacuum brake
From Wikipedia, the free encyclopedia
The vacuum brake is a braking system used on trains. It was first introduced in the mid 1860s but the most widely adopted system 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.
Contents |
[edit] Overview
Vacuum brakes permit the automatic application of brakes down the length of a train from a simple control in the driver's hand, although when first introduced from the 1860s the system (the Eames brake) was only the simple (or "straight") type, with no “fail safe” capability. The opposition on the grounds of cost (particularly by the LNWR and its chairman Richard Moon) to the fitting of the automatic type of brake meant that it took a serious accident at Armagh in 1889 before legislation compelled the automatic system. The latter is fail safe, since it defaults to the 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. Furthermore, as the brake pipe runs the full length of the train, additional release valves can be provided where necessary: for example connected to the passenger alarms or in the guards’ compartments.
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.
[edit] 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 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.
However railway company rulebooks in the UK now stipulate the opposite procedure if a derailment of part of the train is suspected: the driver’s instructions are not to apply the brakes from the locomotive, but to allow the guard to use a brake valve (or apply a hand brake) towards the rear of the train to keep the couplings tight and the whole train in line. This instruction follows the Shipton-on-Cherwell train crash of 24 December 1874.
[edit] Coupling
Vacuum brakes have no sealing taps at the ends of wagons, which cannot therefore be accidentally and dangerously turned off. Instead a domed plug with a rubber sealing ring (called a dummy in the UK) is fixed to the buffer bars of locomotives and brake vans: at each end of the train the flexible connecting pipe is pushed onto this and the vacuum holds it in place. This also acts as a safety feature during coupling: the operator's first action is to strike the pipe off the dummy, which destroys the vacuum and confirms that the locomotive brakes are set. However the suction of the vacuum brake can cause foreign objects (in one case a mouse) to get sucked into the piping, causing some of the brakes to fail in a dangerous fashion.
An incompatibility can be found when, during a journey, a locomotive is exchanged for one that cannot achieve the same degree of vacuum. The brake pipe will have been disconnected and the brakes fully applied, but the new locomotive cannot release them. When this happens the vacuum reservoir on each vehicle has to be relieved manually (a cord-operated valve being provided for the purpose) before the new locomotive can balance the pressures. This was a recognised procedure when the initial locomotive had been provided by the Great Western Railway: that company specified that its brakes would operate at twenty-five inches of mercury while most other companies used only twenty-one, but smaller variations between individual locomotives could also cause the problem.
[edit] Dual brakes
Since vacuum brakes are obsolete on railways there is a need to convert to air brakes. Fortunately, it is possible to fit rolling stock with dual brake pipes, one for vacuum and one for air, and it is also possible subject to space being available to fit the both kinds of brakes to the wagons. Incompatibility is a kind of break of gauge, but a less serious one.
[edit] Use on road vehicles
Vacuum brakes are not used on road vehicles because a continuous bleed of air is harder to maintain, also, there is not as much room for large diameter cylinders. Steam powered road vehicles tended much smaller, but quicker reacting, boilers to enable more frequent stops. Vacuum braking would cause the water to drop below a safe level.
[edit] Vacuum brakes today
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 operators of vacuum brakes are narrow gauge railways in Central Europe, largest of them is Ferrovia Retica. Great Britain used to prefer vacuum brakes, but has now converted to air brakes.
[edit] Railways with vacuum brakes
- Angola (CFB)
- Botswana
- Malawi
- Mozambique
- Namibia
- Nigeria
- Senegal
- Swaziland
- South Africa
- Zambia
- Zimbabwe
Several southern African countries, particularly South Africa, are slowly converting freight wagons to either dual-brakes or to airbrakes only.
There are many Heritage Railways in the UK which use vacuum brakes. See http://www.heritagerailways.com/ for details.
[edit] References
- British Transport Commission, London (1957:142). Handbook for Railway Steam Locomotive Enginemen.
