Steam locomotive

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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Steam locomotives were the dominant form of rail traction until superseded in the mid 20th century by diesel and electric locomotives.

Origins

The earliest railways employed teams of horses to draw carts over the track. As steam engines were developed in the 1700s, various attempts were made to apply these to railroad use; for example, Richard Trevithick built a working locomotive in 1804. These early efforts culminated in Stephenson's Rocket, which proved the be the prototype for almost all successive efforts.

In the United States, steam locomotive development was initiated somewhat independently from that in Britain, though locomotives (such as the Stourbridge Lion and the John Bull) were also imported. Among the earliest engines built in the United States were the Best Friend of Charleston and the Tom Thumb. Early engines were often unconventional in arrangement compared to their successors, often employing vertical boilers and/or complicated linkages to transmit power to the wheels.

Basic form

The typical steam locomotive employs a horizontal water tube boiler with the firebox at the rear, partly inside the cab which shields the locomotive operators from the weather. At the front of the boiler a smokebox is attached, with chimney (US: "smoke stack") protruding from the top. On the top surface of the boiler is a dome, where the steam is collected. The main steam pipe starts in the dome and passes through a throttle (known as the "regulator") and thence to the cylinders of a reciprocating engine. The pistons of this engine drive the driving wheels directly through a connecting rod and a crankpin on the main driver. The valves of the engine are controlled through a set of rods and linkages called the valve gear; this gear is adjustable to control the direction and power application of the locomotive. (The reversing lever which controls this is therefore roughly analogous to the gearshift of an automobile.) Exhaust steam is directed through the chimney via a nozzle called a blastpipe, thus roughly regulating the draft of the engine according to the steam consumed. The blast of exhaust produces the familiar "chugging" sound of the steam locomotive. The pistons are double acting; a two-cylinder locomotive having a set on either side of the locomotive, set 90 degrees out of phase which each other, giving four power strokes per revolution of the wheels. The driving wheels are connected on each side by coupling rods to transmit power from the main driver to the other wheels.

The boiler rests on a frame to which the cylinders are mounted and which in turn rest on the axles. The driving axles are mounted in bearings which can move up and down in the frame; connected to the frame by leaf springs and linkages which allow axles some degree of independent movement in order to handle bumps in the track. Many locomotives have leading and/or trailing bogies to support the ends of the locomotive and to guide it.

Many locomotives are permanently coupled to a tender, which holds the water and fuel for the locomotive (See tender locomotive). Some locomotives carry the fuel and water directly on the (called "tank engines" after the prominent tanks atop or alongside the boiler. From the beginning the predominant fuel was coal, though wood-burning engines were used in rural and logging enterprises. When petroleum came into wide use, oil-burning locomotives were used in some areas. Almost anything that burns can be used as fuel, however.

A steam locomotive is operated by a crew of at least two people. One, the driver or engineer, is responsible for controlling the locomotive (and thus the train as a whole); the other, the fireman, is responsible for the fire and the water supply to the boiler.

see also

Appliances

Almost all locomotives are fitted with a variety of appliances. Some of these related directly to the operation of the steam engine; while others are for signalling, train control, or other purposes. In the United States the Federal Railroad Administration mandated the use of some appliances over the years in response to safety issues. The most typical appliances were as follows:

Steam Pumps and Injectors

Some accessory was required to force water into the boiler. Early engines used pumps driven by the motion of the pistons; later on steam injectors took their place, and some engines use turbopumps. Standard practice was to have two independent systems for feeding water to the boiler.

Superheating

In the 1800s most engines used saturated steam. By 1900 superheated steam began to be applied. The normal method for achieving this was to route the steam from the dome to a header in the smokebox. The steam was then directed through a set of small tubes in enlarged boiler tubes, and then into a second header from whence it proceeded through the cylinders. Superheating produced enormous increases in efficiency and was standard on 20th century locomotives.

Stokers

One limiting factor on the power produced was the rate at which fuel could be added to the fire. In the early 20th century locomotives began to be large enough to where a fireman could not shovel fuel in fast enough. Various steam-powered automatic stokers were introduced and became standard equipment by the end of steam operations.

Feedwater heaters

Introducing cold water into a boiler reduced power, and towards the end of steam service a variety of heaters were employed to extract waste heat from the exhaust and raise the temperature of the feed water.

Condensers and track pans

Steam locomotives consumed vast quantities of water, and supplying this was a constant logistical problem. In some desert areas condensing engines were used. These had huge radiators in the tenders to which the exhaust steam was routed; steam condensed here and was used to replenish the tender. These engines particularly saw use in the Karoo Desert.

At the other extreme, tenders on some British and American locomotives were equipped with scoops to allow them to collect water from track pans while in motion, thus obviating stops for water.

Braking

Locomotives have their own braking system, independent from that of the rest of the train. Locomotive brakes employ large shoes which press up against the driving wheel treads. With the advent of air brakes, a separate system also allowed the driver to control the brakes on all cars. These systems required steam-powered pumps, which were mounted on the side of the boiler or on the smokebox front.

Pilots and Buffers

In British practice buffers were used in the connection between the locomotive and other cars. In the United States the focus at the front of the locomotive turned to dealing with obstructions on the track, and the "pilot" was developed. This was a plow-shaped apparatus which was originally quite large, but which even in vestigal form was retained on mainline locomotives to the very end. Its intent to throw livestock clear of the train led to it being nicknamed the "cowcatcher".

Headlight

As soon as night operations began, locomotives needed lights to allow the driver to see what lay ahead of the train. Originally headlights were oil lamps, but when electric lights became available they quickly replaced the oil lamps.

Bells and Whistles

Locomotives were given bells and steam whistles from very early on. Bells were used to warn of train motion; whistles were used to signal personnel and give warnings to trackside persons.

Booster engines

In the United States the trailing truck was often equipped with an auxiliary steam engine which provided extra power for starting. This booster engine was set to cut out automatically at a certain speed.

see also steam locomotive nomenclature

Variations

Numerous variations to this basic arrangement appeared over the years. Some locomotives added extra cylinders and sometimes essentially combined two locomotives in one (e.g. the Mallet locomotive). Some locomotives drove the wheels through a system of shafts and gears (e.g. the Shay locomotive; see "geared steam locomotive"). In the United States on the Southern Pacific Railroad a series of cab forward locomotives were build in which the entire machine was turned around, putting the cab and the firebox and the front of the locomotive. At the end of steam practice some attempts were made to employ steam turbines instead of reciprocating engines, both through direct drive and through electrical transmissions. these were not notably successful.

Considerable improvements were applied to the steam engine arrangement over the years. Early locomotives had very simple valve gear that allowed no more than full power applied in either forward or reverse. Soon Stephenson valve gear was applied to allow the driver to control cutoff; this was largely superseded by Walschaert valve gear and similar patterns. Early practice used slide valves and outside admission, which was easy to construct but inefficient and prone to wear. Eventually slide valves were superseded by inside admission piston valves, though there were attempts to apply poppet valves (common by then on stationary engines) in the 20th century. Stephenson valve gear was generally placed within the frame, where it was difficult to access for maintenance; later patterns were applied outside the frame, where they were readily visible and maintained.

Around 1900 compound engines were introduced. Some attempts were made to apply this to a single piston (e.g. the Vauclain compound) but the predominant form was the Mallet locomotive, which used two separate engines in one articulated frame. The high pressure stage was attached directly to the boiler frame; in front of this was a separate low pressure engine on its own frame, powered by the exhaust from the rear engine. Articulation itself proved very popular, and there were numerous variations, both compound and simple. Duplex locomotives with two engines in one rigid frame were also tried, but were not notably successful.

Locomotive manufacture

With the notable exception of the USRA standard locomotives in the United States, steam locomotive manufacture was always somewhat customized. Railroads ordered locomotives tailored to their specific requirements, though basic similarities were always present. Certain railroads were noted for certain characteristics; for example, the Pennsylvania Railroad was known for its preference for the Belpaire firebox, and the Delaware and Hudson Railroad was famous for its elaborately flanged smokestacks. In the United States locomotives were ordinarily built by specialized manufacturers, but all railroads had shops which were capable of massive repairs, and some railroads (for instance the Norfolk and Western Railway) built many locomotives in their own shops. It was common for a group of locomotives to be sold from one railroad to another.

Steam locomotives required regular maintenance and overhaul (often at government-regulated intervals), and locomotives were frequently altered in the course of these. New appliances could be added, unsatisfactory features removed, and even new boilers, cylinders, and in fact almost any part of the locomotive replaced. In one case on the Baltimore and Ohio Railroad two 2-10-2 locomotives were taken apart; the boilers were used to make 4-8-2 locomotives with new machinery, and the machinery was used to make a pair of 0-10-0 switchers with new boilers.

Steam locomotive categorization

Steam locomotives are categorized by their wheel arrangement. The predominant system for this was Whyte notation, which represented each set of wheels with a number. Different arrangements were given names which usually reflected the first usage of the arrangement; for instance the "Santa Fe" type (2-10-2) was so called because the first examples were built for the Atchison, Topeka and Santa Fe Railroad. These names were informally given and varied according to region and even politics.

On each railroad locomotives were organized into classes. These broadly represented locomotives which could be substituted for each other in service, but most commonly a class represented a single design. Classes also commonly acquired nicknames.

Locomotive performance

In the steam locomotive era two measure of locomotive performance were generally applied. At first, locomotives were rated by tractive effort: the maximum force exerted by the locomotive in pulling the train. This can be roughly calculated by multiplying the total piston area by the boiler pressure and dividing by the ratio of the driver diameter over the piston stroke. Tractive effort is the fundamental factor in rating a locomotive in terms of how heavy a train it can pull over a given territory. As pressure grew to run faster freights and heavier passenger consists, however, tractive effort was seen to be an inadequate measure of performance, because it did not take into account speed.

Therefore in the 20th century, locomotives began to be rated by power output. A variety of calculations and formulas were applied, but in general railroads turned to the use of dynamometer cars to measure tractive force at speed in actual road testing. This measure was termed drawbar horsepower in the United States and remained the standard measure of performance to the end of mainline usage.

Whyte classification is connected to locomotive performance, but through a somewhat circuitous path. Given adequate proportions of the rest of the locomotive, power output is determined by the size of the fire, and for a bitumeous coal-fueled locomotive, this is determined by the grate area. Modern non-compound locomotives were typically able to produce about 40 drawbar horsepower per square foot of grate. Tractive force, as noted earlier, is largely determined by the boiler pressure, the cylinder proportions, and the size of the drivers. However, it is also limited by the weight on the drivers (termed adhesive weight), which needs to be at least four times the tractive effort.

The weight of the locomotive is roughly proportional to the power output; the number of axles required is determined by this weight divided by the axleload limit for the trackage where the locomotive is to be used. The number of drivers is derived from the adhesive weight in the same manner, leaving the remaining axles to be accounted for by the leading and trailing bogies. Passenger locomotives conventionally had two axle leading bogies for better guidance at speed; on the other hand, the vast increase of the grate and firebox in the 20th century meant that trailing bogie was called upon to provide its support.

As a rule, switching engines omitted leading and trailing trucks, both to maximise tractive effort available and to reduce wheelbase. Speed was unimportant; making the smallest engine (and therefore smallest fuel consumption) for the tractive effort was paramount. Drivers were small and usually supported the firebox as well as the main section of the boiler. Helper engines tended to follow the principles of switchers, except that the wheelbase limitation did not obtain. Therefore helpers tended to multiply the number of drivers, leading eventually to the Mallet type with its many driven wheels. These tended to acquire leading and then trailing trucks as guidance of the engine became more of an issue.

As locomotive types began to diverge in the late 1800s, freight engines at first emphasized tractive effort, whereas passenger engines emphasized speed. Freight locomotives multiplied axles, kept the leading truck to a single axle, and grew a trailing truck as firebox expanded and could no longer fit between or above the drivers. Passenger locomotives had two axle leading trucks, fewer axles, and very large drivers in order to limit the speed at which the reciprocating parts had to move.

In the 1920s the focus in the United States turned to horsepower, epitomized by the "super power" concept promoted by the Lima Locomotive Works. Freight trains were to be driven faster; passenger trains needed to pull heavier loads at speed. In essence, the grate and firebox expanded without changes to the remainder of the locomotive, forcing the trailing bogie to grow a second axle. In the United States this led to a convergence on the 4-8-4 configuration, which was used for both freight and passenger service. Mallet locomotives went through a similar transformation and were upgraded from helpers into huge road engines with gargantuan fireboxes; their drivers increased in size in order to allow faster running.

The end of steam

The introduction of diesel-electric locomotives in the first part of the 20th century spelled the end of steam locomotives, though they were used in the North American and Europe to mid-century, and continued in use in other countries to the end of the century. Steam locomotives can be fairly simple machines, maintainable under fairly primitive conditions and amenable to a wide variety of fuels. They are also relatively inefficient and require constant maintenance, and need substantial labor to do so. Water must be supplied at many points throughout the system, a problem in desert areas or where the local water is unsuitable. The reciprocating mechanism pounds on the rails (see "hammer blow"), thus requiring more maintenance of way. Finally, the smoke from steam locomotives is frequently objectionable; in fact, the first electric and diesel locomotives were developed to meet smoke abatement requirements.

Mainline diesel-electric locomotives were first introduced on the Baltimore and Ohio Railroad, and were soon found to reduce maintenance costs dramatically, while increasing locomotive availability. World War II delayed dieselization, but the pace picked up in the 1950s, and by 1960 the last American Class I holdout, the Norfolk and Western Railway, had discontinued steam operations. Some shortlines continued steam operations into the 1960s, and one steel mill continued to switch with steam up to 1980.

In other countries steam remained in wide use and continued to the end of the century. Diesel locomotives were relatively expensive and far more complicated, and the pressure of labor costs was not as great. The expense of oil gave other fuels a cost advantage. An oil embargo combined with an abundance of cheap local coal led South Africa to continue using steam locomotives into the 1990s. China continued to build mainline steam locomotives until late in the century, even building a few examples for American tourist operations.

Dramatic increases in the cost of diesel fuel prompted several initiatives to revive steam power. None of these progressed to the point of production, however, and in the early 21st century the steam locomotive reigns only in isolated regions and in tourist operations.

See also

• List of heritage railways

 

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