Trains and Railroads
The First Locomotives
The first self-propelled steam vehicle was built by Frenchman Nicolas Cugnot in 1769, followed by William Murdoch's model experimental locomotive in 1784. In 1802, Richard Trevithick built the first full-size locomotive to run on rails, thus winning a wager for his employer. A horizontal cylinder sat in the boiler and drove a piston, which drove a connecting rod that connected to a crank/flywheel. A set of gears transferred energy from the crankshaft to the drive wheels that moved the engine. To meet the terms of the bet, the locomotive successfully pulled a series of cars loaded with ten tons of iron and 70 people.
Trevithick used an artificial draft through the firebox to fan the flames of the coals, an important innovation. This increased the heat of the fire, generating larger amounts of high pressure steam. He dispensed with any additional traction mechanism to keep the engine from slipping, convinced that the friction between the iron rails and the wheels was enough to drive the vehicle forward. His invention worked admirably. At several tons in weight, however, it was far too heavy for the brittle iron plateway and left a string of broken rails in its wake.
Traction, or wheel to rail adhesion, is fundamental to the operation of a locomotive. In order to move a string of cars, the locomotive drive wheels must grip the track. If traction is insufficient, the wheels simply spin without pulling the train forward, just as car wheels can spin uselessly in mud or on ice. This was a special concern for early locomotive designers who, unlike Trevithick, were not convinced that wheel-to-rail adhesion was sufficient to move the train down the track. Because frictional force between wheels and rail is proportional to the downward force or weight on the driving wheels, lighter engines were more likely to encounter adhesion problems. Heavier engines had better adhesion but their weight tended to break or split the brittle cast iron tracks, and locomotive builders were under continual pressure to reduce engine weight.
A variety of solutions that balanced the issues were proposed and built. In 1812, John Blenkinsop built a substantially lighter engine than Trevithick's, compensating for any loss of adhesion by using a rack and pinion drive. The drive wheels were cogged and rails were toothed on the outside face. The teeth on the drive wheels meshed with the teeth on the rails, driving the locomotive forward with no chance for slippage. Costly and complicated, the rack and pinion drive soon proved
to be unnecessary for conventional railroads and never became popular. Other high traction methods such as chain drive or external pushing legs were simply impractical, and most of the locomotives that followed reverted to adhesion drive.
A second Blenkinsop innovation became a standard feature of almost all subsequent steam locomotives. Blenkinsop designed a two cylinder engine, with a cylinder to power the drive wheel on each side. This eliminated the use of an overhead flywheel to transfer mechanical energy from the cylinder to the drive wheels. Unlike Trevithick's design, however, the cylinders on Blenkinsop's engine were vertical. At high speeds, the rapid bounce of the pistons added a great deal of chop to the engine movement, exacerbated by the fact that the engine had no springs to absorb the motion. Given the weight of the engines, the chop placed a significant amount of stress on the rails, resulting in more splits and fractures.
The next groundbreaking design was an adhesion drive, two cylinder locomotive called the Puffing Billy. It was the first steam locomotive to feature cylinders outside of the boiler where they were easily accessible. Designed by William Hedley, the Puffing Billy distributed its weight over eight drive wheels, putting less concentrated load on the track and causing less wear.
One of the major locomotive companies during the nineteenth centuries was run by the father-son team of George and Robert Stephenson. The Stephensons were responsible for some of the most important technical innovations in locomotive operation. George Stephenson replaced the cylinder-to-wheel gear interface by coupling and connecting rods, streamlining the design and bringing it closer to the style of locomotive that we know. He also introduced the locomotive steam spring, which cushioned the action of the engine. The spring consisted of a vertical cylinder with a piston that carried the engine weight. Steam forced the piston to the upper end of the cylinder, applying upward force to counter the downward force of the weight of the engine. As a result of the shock-absorbing effect of the spring, the heavy engine rode more easily on its wheels and caused fewer cracks in the iron rails.
Locomotive wheels also cracked frequently, requiring costly replacements and engine down-time. Robert Stephenson and Timothy Hackworth replaced the solid cast-iron wheels of early engines with a combination design that featured durable, replaceable wrought-iron tires mounted on cast-iron hubs. In his 1827 locomotive the Rocket, Robert Stephenson also introduced the multitube boiler. Frenchman Marc Seguin developed a similar design at around the same time. In the multitube boiler, hot gases from the firebox move through tubes that run the length of the boiler. Heat is exchanged over a much greater surface area, making the design far more efficient than the single chamber type. Around this same time, locomotive designers abandoned the vertical cylinder for the smoother horizontally mounted type, though the cylinders on the Rocket compromised with a slanted orientation.
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