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Brakes are used on the cars of railway trains to enable deceleration, control acceleration (downhill) or to keep them standing when parked. While the basic principle is familiar from road vehicle usage, operational features are more complex because of the need to control multiple linked carriages and to be effective on vehicles left without a prime mover. Clasp brakes are one type of brakes historically used on trains. == 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. It was also unreliable, as the application of brakes by guards depended upon them hearing and responding quickly to a whistle for brakes. 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. In the UK, the Abbots Ripton rail accident in January 1876 was aggravated by the long stopping distances of express trains without continuous brakes, which -it became clear- in adverse conditions could considerably exceed those assumed when positioning signals. This had become apparent from the trials on railway brakes carried out at Newark in the previous year, to assist a Royal Commission then considering railway accidents. In the words of a contemporary railway official, these showed that under normal conditions it required a distance of 800 to 1200 yards to bring a train to rest when travelling at 45½ to 48½ mph, this being much below the ordinary travelling speed of the fastest express trains. Railway officials were not prepared for this result and the necessity for a great deal more brake power was at once admitted〔T E Harrison (Chief Engineer of the North Eastern Railway at the time, document of December 1877 quoted (page 193) in F.A.S.Brown ''Great Northern Railway Engineers'' Volume One: 1846–1881, George Allen & Unwin, London, 1966: (for those who feel the Victorians should have metric conversions backfitted: at speeds of - stopping distances were - )〕 More precise results from the Newark trials are not to hand; trials conducted after Abbots Ripton reported the following (for an express train roughly matching one of those involved, like it on a 1 in 200 fall, but unlike it braking under favorable conditions)〔(), ''Report of the Court of Inquiry into the Circumstances Attending the Double Collision on the Great Northern Railway which occurred at Abbotts Ripton on the 21st January 1876'', HMSO, 1876〕 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. * Hydraulic brakes. As with (passenger) car brakes; actuating pressure to apply brakes was transmitted hydraulically. These found some favor in the UK (e.g. with the Midland and Great Eastern Railways), but even in the UK problems were found with the water used as brake fluid freezing 〔According to (C) Hamilton Ellis, ''Nineteenth Century Railway Carriages'', Modern Transport, London, 1949 The Midland supplied both the hydraulic-braked trains trialed at Newark (see below) Ellis goes on to note '' op cit'' p 58 Freezing possibilities told against the hydraulic brakes, though the Great Eastern Railway, which used them for a while, overcame this by the use of salt water〕 * 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. * The simple vacuum system. An ejector on the locomotive created a vacuum in a continuous pipe along the train, allowing the external air pressure to operate 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. Note: there are a number of variants and developments of all these systems. The Newark trials showed the braking performance of the Westinghouse air-brakes to be distinctly superior〔data below from (C) Hamilton Ellis, ''Nineteenth Century Railway Carriages'', Modern Transport, London, 1949 - ranked in order of merit after allowing for weight of train - italicised systems were not truly continuous〕 but for other reasons〔simplicity of engineering as a technical reason; but there seem to have been strong non-technical reasons to do with Westinghouse's salesmanship〕 it was the vacuum system that was generally adopted on UK railways. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Railway brake」の詳細全文を読む スポンサード リンク
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