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The braking system of the 1873 year steam locomotive
Brakes are used on the vehicles of railway trains to slow them, or to keep them standing when parked. While the principle is familiar from road vehicle usage, operational features are more complex because of the need to control trains, i.e. multiple vehicles running together, and to be effective on vehicles left without a prime mover.

Early days

In the earliest days of railways, braking technology was primitive. The first trains had brakes operative on the locomotive tender and on vehicles in the train, where “porters” or, in the United States brakemen, travelling for the purpose on those vehicles operated the brakes. Some railways fitted a special deep-noted brake whistle to locomotives to indicate to the porters the necessity to apply the brakes. All the brakes at this stage of development were applied by operation of a screw and linkage to brake blocks applied to wheel treads, and these brakes could be used when vehicles were parked. In the earliest times, the porters travelled in crude shelters outside the vehicles, but “assistant guards” who travelled inside passenger vehicles, and who had access to a brake wheel at their posts supplanted them.

The braking effort achievable was limited, and an early development was the application of a steam brake to locomotives, where boiler pressure could be applied to brake blocks on the locomotive wheels.

As train speeds increased, it became essential to provide some more powerful braking system capable of instant application and release by the train driver, described as a continuous brake because it would be effective continuously along the length of the train.

However there was no clear technical solution to the problem, because of the necessity of achieving a reasonably uniform rate of braking effort throughout a train, and because of the necessity to add and remove vehicles from the train at frequent points on the journey. (At these dates, unit trains were a rarity).

The chief types of solution were:

  • The chain brake, such as the Heberlein brake, in which a chain was connected continuously along the train.
When pulled tight it activated a friction clutch that used the rotation of the wheels to tighten a brake system at that point; this system has severe limitations in length of train capable of being handled, and of achieving good adjustment.
  • The simple vacuum system. An ejector on the locomotive created a vacuum in a continuous pipe along the train, and the vacuum operated brake cylinders on every vehicle. This system was very cheap and effective, but it had the major weakness that it became inoperative if the train became divided or if the train pipe was ruptured.
  • The automatic vacuum brake. This system was similar to the simple vacuum system, except that the creation of vacuum in the train pipe exhausted vacuum reservoirs on every vehicle and released the brakes. If the driver applied the brake, his driver's brake valve admitted atmospheric air to the train pipe, and this atmospheric pressure applied the brakes against the vacuum in the vacuum reservoirs. Being an automatic brake, this system applies braking effort if the train becomes divided or if the train pipe is ruptured. Its disadvantage is that the large vacuum reservoirs were required on every vehicle, and their bulk and the rather complex mechanisms were seen as objectionable.
Rotair Valve Westinghouse Air brake Company
  • The Westinghouse air brake system. In this system, air reservoirs are provided on every vehicle and the locomotive charges the train pipe with a positive air pressure, which releases the vehicle brakes and charges the air reservoirs on the vehicles. If the driver applies the brakes, his brake valve releases air from the train pipe, and triple valves at each vehicle detect the pressure loss and admit air from the air reservoirs to brake cylinders, applying the brakes. The Westinghouse system uses smaller air reservoirs and brake cylinders than the corresponding vacuum equipment, because a moderately high air pressure can be used. However, an air compressor is required to generate the compressed air and in the earlier days of railways, this required a large reciprocating steam air compressor, and this was regarded by many engineers as highly undesirable.


Note: there are a number of variants and developments of all these systems.

Later British practice

In British practice, only passenger trains were fitted with continuous brakes until about 1930, and goods and mineral trains ran at slower speed, and relied on the brake force from the locomotive and tender, and the brake van – a heavy vehicle provided at the rear of the train and occupied by a guard.

Goods and mineral vehicles were provided with hand brakes, by which the brakes could be applied by a hand lever operated by staff on the ground. These hand brakes were used where necessary when vehicles were parked, but also when these trains needed to descend a steep gradient; the train then stopped before descending, and the guard walked forward to pin down the handles of sufficient brakes to give adequate braking effort. Early goods vehicles had brake handles on one side only, and random alignment of the vehicles gave the guard sufficient braking, but from about 1930 so-called "either-side" brake handles were provided. These trains, not fitted with continuous brakes were described as "unfitted" trains and they survived in British practice until about 1985. However from about 1930 semi-fitted trains were introduced, in which some goods vehicles were fitted with continuous brakes, and a proportion of such vehicles marshalled next to the locomotive gave sufficient brake power to run at somewhat higher speeds than unfitted trains.

In the early days of diesel locomotives, a purpose-built brake tender was attached to the locomotive to increase braking effort when hauling unfitted trains. The brake tender was low, so that the driver could still see the line and signals ahead if the brake tender was propelled (pushed) ahead of the locomotive, which was often the case.

Continuous brakes

As train loads, gradients and speeds increased, braking became a problem. In the late 19th century, significantly better continuous brakes started to appear. The earliest type of continuous brake was the chain brakewhich used a chain, running the length of the train, to operate brakes on all vehicles simultaneously.

The chain brake was soon superseded by air operated or vacuum operated brakes. These brakes used hoses connecting all the wagons of a train, so the driver could apply or release the brakes with a single valve in the locomotive.

These continuous brakes can be simple or automatic, the essential difference being what happens should the train break in two. With simple brakes, pressure is needed to apply the brakes, and all braking power is lost if the continuous hose is broken for any reason. Simple non-automatic brakes are thus useless when things really go wrong, as is shown with the Armagh rail disastermarker.

Automatic brakes on the other hand use the air or vacuum pressure to hold the brakes off against a reservoir carried on each vehicle, which applies the brakes if pressure/vacuum is lost in the train pipe. Automatic brakes are thus largely "fail safe", though faulty closure of hose taps can lead to accidents such as the Gare de Lyon accidentmarker.

The standard Westinghouse Air Brake has the additional enhancement of a triple valve, and local reservoirs on each wagon that enable the brakes to be applied fully with only a slight reduction in air pressure, reducing the time that it takes to release the brakes as not all pressure is voided to the atmosphere.

Non-automatic brakes still have a role on engines and first few wagons, as they can be used to control the whole train without having to apply the automatic brakes.

Types

Air versus vacuum brakes

In the early part of the 20th century, many British railways employed vacuum brakes rather than the air brakes used in America and much of the rest of the world. The main advantage of vacuum was that the vacuum can be created by a steam ejector with no moving parts (and which could be powered by the steam of a steam locomotive), whereas an air brake system requires a noisy and complicated compressor.

However, air brakes can be made much more effective than vacuum brakes for a given size of brake cylinder. An air brake compressor is usually capable of generating a pressure of vs only for vacuum. With a vacuum system, the maximum pressure differential is atmospheric pressure ( at sea level, less at altitude). Therefore, an air brake system can use a much smaller brake cylinder than a vacuum system to generate the same braking force. This advantage of air brakes increases at high altitude, e.g. Peru and Switzerland where today vacuum brakes are used by secondary railways. The much higher effectiveness of air brakes and the demise of the steam locomotive have seen the air brake become ubiquitous; however, vacuum braking is still in use in India, in Argentina and in South Africa, but this will be declining in near future.

Air brake enhancements

One enhancement of the automatic air brake is to have a second air hose (the main reservoir or main line) along the train to recharge the air reservoirs on each wagon. This air pressure can also be used to operate loading and unloading doors on wheat wagons and coal and ballast wagons. On passenger coaches, the main reservoir pipe is also used to supply air to operate doors and air suspension.

Electropneumatic brakes

A higher performing EP brake has a train pipe delivering air to all the reservoirs on the train, with the brakes controlled electrically with a 3-wire control circuit. This can give seven levels of braking, from mild to severe, and allows the driver greater control over the level of braking used, which greatly increases passenger comfort. It also allows for faster brake application, as the electrical control signal is propagated effectively instantly to all vehicles in the train, whereas the change in air pressure which activates the brakes in a conventional system can take several seconds or tens of seconds to propagate fully to the rear of the train. This system is not however used on freight trains due to cost.

The system adopted on the Southern Region of British Railways in 1950 is more fully described at Electro-pneumatic brake system on British railway trains

Electronically controlled pneumatic brakes

Electronically controlled pneumatic brakes (ECP) are a development of the late 20th Century to deal with very long and heavy freight trains, and are a development of the EP brake with even higher level of control. In addition, information about the operation of the brakes on each wagon can be returned to the driver's control panel.

With ECP, a power and control line is installed from wagon to wagon from the front of the train to the rear. Electrical control signals are propagated effectively instantaneously, as opposed to changes in air pressure which propagate at a rather slow speed limited in practice by the resistance to air flow of the pipework, so that the brakes on all wagons can be applied simultaneously rather than from front to rear. This prevents wagons at the rear "shoving" wagons at the front, and results in reduced stopping distance and less equipment wear.

There are two brands of ECP brakes under development, one by New York Air Brake and the other by Wabtec. A single standard is desirable, and it is intended that the two types be interchangeable.

Examples



Reversibility

Brake connections between wagons may be simplified if wagons always point the same way, such as in Tasmaniamarker. An exception would be made for locomotives which are often turned on turntable or triangles.

On the new Fortescue railway opened in 2008, wagons are operated in sets, although their direction changes at the balloon loop at the port. The ECP connections are on one side only and are unidirectional.

Accidents with brakes







  • Chapel-en-le-Frith, Great Britain (1957) – broken steam pipe made it impossible for crew to apply brakes.
  • Federal Express (1953) - valve closed by badly designed bufferplate.






See also





Manufacturers



References

  1. (Cc) Glossary for the LNWR Society
  2. http://news.bbc.co.uk/2/hi/africa/6928227.stm
  3. MZT Hepos
  4. Nabtesco
  5. http://www.railway-technology.com/contractors/brakes/dellner/enquiry.asp
  6. http://www.aflink.co.za/Rail/rail.html
  7. Hanning & Kahl


Sources



External links


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