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A steam locomotive is a locomotive powered by steam. The term usually refers to its use on railways, but can also refer to a "road locomotive" such as a traction engine or steamroller.
Steam locomotives dominated railway usage from the start of the 19th century, until the middle of the 20th Century. They were gradually improved and developed in their over 150 years of development and use. Starting in about 1930 other types of engines were developed and steam locomotives were gradually superseded by diesel and electric locomotives.
Origins
The earliest railways employed horses to draw carts along railed tracks.
As the development of steam engines progressed through the 1700s, various attempts were made to apply them to road and railway use. William Murdoch a Scottish inventor built a prototype steam road locomotive in 1784. The first-known working model of a steam rail locomotive was designed and constructed by John Fitch in the United States in 1794, although it did not lead to further developments.
The first full scale working railway steam locomotive was built by British inventor Richard Trevithick, on 21 February 1804 the world's first railway journey took place as Trevithick's unnamed steam locomotive hauled a train along the tramway of the Penydarren ironworks, near Merthyr Tydfil in Wales Accompanied with Andrew Vivian, it ran with mixed success. Then followed the successful twin cylinder locomotive by Christopher Blackett's team built at Wylam in 1811, closely followed by Matthew Murrays' rack locomotive for the edge railed Middleton Railway in 1812. These early efforts culminated in 1829 with the Rainhill Trials and the opening of the Liverpool and Manchester Railway a year later making exclusive use of steam power for both passenger and freight trains.
The United States started developing steam locomotives in 1829 with the Baltimore and Ohio Railroad's Tom Thumb. This was the first locomotive to run in America, although it was intended as a demonstration of the potential of steam traction, rather than as a revenue-earning locomotive. The first successful steam railway in the US was the South Carolina Railroad whose inaugural train ran on December 25, 1830 hauled by the Best Friend of Charleston. Many of the earliest locomotives for American railroads were imported from England, including the Stourbridge Lion and the John Bull, but a domestic locomotive manufacturing industry was quickly established, with locomotives like the DeWitt Clinton being built in the 1830s.
Basic form
Boiler
The typical steam locomotive employs a steel fire-tube boiler that contains pressurized water and steam. A firebox is normally located in the rear of the boiler (chimney in front). The firebox has a water filled steel chamber surrounding the top and sides of the flame in the firebox. If wood or coal is used to make the fire in the firebox it is built on a set of grates where ashes may be separated from the burning fuel. These ashes have to be periodically removed from the engine. If wood or coal are the fuel used in the firebox there is a door at the rear of the firebox that is opened to add more fuel. If oil is used there nearly always is a door for adjusting the air flow, maintenance or for cleaning the oil jets. To extract even more heat, the smoke and hot air from the combustibles in the firebox travel horizontally several feet (cm) down a hollow bundle of parallel tubes buried in the water in the boiler and welded to the front of the boiler. The combination of heat extracted in the firebox and tubes in the boiler are used to convert the water to pressurized steam in the boiler. To minimize heat loss from the boiler it is normally surrounded with layers of insulation. The water and steam in the boiler is kept pressurized to raise the boiling temperature of the water and generate high pressure steam. The amount of pressure in the boiler is monitored by the engineer or fireman by a gauge mounted in the cab. Excess steam pressure can be released manually or may blow a safety valve. Too much pressure may cause the boiler to burst potentially killing the crew as well as disabling the engine.
At the front of the boiler is the smokebox, where steam is ejected into the chimney (US: "smoke stack") drawing the smoke and hot air through the fire tubes in the boiler and out the top of the chimney. The combustion in a typical steam engine is not very complete leading to a prodigious amount of smoke and often sparks being produced. This made these engines very dirty to live around as well as being an acute hazard while passing through a forest, tunnel or snow shed.
The steam generated in the boiler is used to drive the locomotive and for other purposes (whistles, brakes, pumps, air flow etc.) as needed. This constant use of water (steam) requires that the boiler have water continually pumped (usually automatically) into it to keep its water level up. The source of this water, an unpressurized water tank, must be periodically refilled at water stops. The water level in the boiler is normally monitored with a transparent tube or gauge. If the boiler runs low on water the fire in the firebox may melt a hole in the steel in the boiler—causing a boiler explosion. One of the hazards of boilers for many years was the failure of the steel boiler causing an explosion which could kill the crew. Start up on a large engine may take an hour or more of preliminary heating of the water in the boiler before it is ready to go. Scale may build up in boiler that prevents good heat transfer. Corrosion eventually makes the boiler unsafe and it has to be rebuilt or replaced. In a wreck or accident the boiler may burst, potentially hurting or killing the crew. These are several of the serious disadvantages of a steam engine which have led to its eventual replacement by other engines which require less maintenance and are safer and cleaner.
Steam circuit
The steam generated in the boiler fills the steam space above the water in the partially-filled boiler. Its maximum working pressure is limited by spring-loaded safety valves. It is then collected either in a perforated tube fitted above the water level or from a dome that often houses the regulator valve, or throttle, the purpose of which is to control the amount of steam leaving the boiler. The steam then either travels directly along and down a steam pipe to the engine unit or may first pass into the wet header of a superheater, the role of the latter being to improve thermal efficiency and eliminate water droplets suspended in the "saturated steam", the state in which it leaves the boiler. On leaving the superheater, the steam exits the dry header of the superheater and passing down a steam pipe entering the steam chests adjacent to the cylinders of a reciprocating engine. Inside each steam chest is a sliding valve that distributes the steam via ports that connect the steam chest to the ends of the cylinder space. The role of the valves is twofold: admission of each fresh dose of steam and exhaust of the used steam once it has done its work.
The cylinders are double acting, with steam admitted to each side of the piston in turn. In a two-cylinder locomotive, one cylinder is located on each side of the locomotive. The cranks are set 90° out of phase. During a full rotation of the driving wheel, steam provides four power strokes; each cylinder receives two injections of steam per revolution. The first stroke is to the front of the piston and the second stroke to the rear of the piston; hence two working strokes. Consequently two deliveries of steam onto each piston face in two cylinders generates a full revolution of the driving wheel. Each piston is connected to the driving axle on each side by a connecting rod, the driving wheels are connected together by coupling rods to transmit power from the main driver to the other wheels. At the two "dead centres", when the connecting rod is on the same axis as the crankpin on the driving wheel, it will be noted that no turning force can be applied. Since the two sides are 90° out of phase, only one side can be at dead centre at a time.
Each piston transmits power directly through a connecting rod (US: main rod) and a crankpin (US: wristpin) on the driving wheel (US main driver) or to a crank on a driving axle. The movement of the valves in the steam chest is controlled through a set of rods and linkages called the valve gear, actuated from the driving axle or else from the crankpin; the valve gear includes devices that allow reversing the engine, adjusting valve travel and the timing of the admission and exhaust events. The cut-off point determines the moment when the valve blocks a steam port, "cutting off" admission steam and thus determining the proportion of the strok
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