A jackshaft is an intermediate shaft used to transfer power from a powered shaft such as the output shaft of an engine or motor to driven shafts such as the drive axles of a locomotive. As applied to railroad locomotives in the 19th and 20th centuries, jackshafts were typically in line with the drive axles of locomotives and connected to them by side rods. In general, each drive axle on a locomotive is free to move about one inch (2.5 cm) vertically relative to the frame, with the locomotive weight carried on springs. This means that if the engine, motor or transmission is rigidly attached to the locomotive frame, it cannot be rigidly connected to the axle. This problem can be solved by mounting the jackshaft on unsprung bearings and using side-rods or (in some early examples) chain drives. Jackshafts were first used in early steam locomotives, although the designers did not yet call them by that name. In the early 20th century, large numbers of jackshaft-driven electric locomotives were built for heavy mainline service. Jackshaft drives were also used in many early gasoline and diesel locomotives that used mechanical transmissions.
Steam locomotives
The Baltimore and Ohio Railroad was a pioneer in the use of jackshaft driven locomotives. While the drive axle of the first Grasshopper locomotive was directly driven by spur gears from the crankshaft, the Traveler delivered in 1833, used a jackshaft, as did all the later Grasshopper and Crab locomotives. These locomotives used step-up gearing to achieve a reasonable running speed using small diameter driving wheels. It is notable that the term jackshaft was not used by the designers of these machines. Instead, they referred to what would later be called a jackshaft as "a separate axle, about three feet forward of the front axle, and carrying cranks coupled by connecting rods to cranks on the two road axles." In his 1837 patent for what became known as the crab class of locomotives, Ross Winans referred to his jackshaft as "a pinion wheel shaft", or "third axle." In a conventional steam locomotive, the crankshaft is one of the driving axles. In a jackshaft-driven steam locomotive, the crankshaft turns a jackshaft which, in turn, turns the driver. Some steam locomotives have had designs intermediate between these extremes, with crankshafts distinct from the driving axle. Phineas Davis's first B&O Grasshopper tested on the B&O in 1831 was in this class, as was the Stockton and Darlington Railway's Swift from 1836, where the crankshaft was directly between the driving axles. Both of these examples used vertical cylinders, with the crankshaft in the plane of the driving axles. The former used a geared drive to the first driving axle, the latter used side rods for this linkage. In the latter case, the reason inferred for using a crankshaft distinct from the driven axles was "to take the shocks of working away from the power shaft." Several locomotives have been built with horizontal cylinders driving a crankshaft directly above the rear driving axle, with a common spring supporting both the shaft and axle so that they could move vertically together. Ross Winans designed a series of 0-8-0 locomotives starting in 1842, launching what became the B&O Mud Digger class of engines. Like the Grasshopper locomotives before them, the crank shafts on these engines were geared to the driven shafts. In his 1843 patent, Winas referred to the crankshaft as a fifth shaft, or axle. In 1880, the Fowler Steam Plough Works of Leeds England received a patent on a similar 0-4-0 locomotive design with vertical side rods between the crankshaft and rear axle. Here, the motivation was to get the cylinders and piston rods up away from dust and dirt on an engine with diminutive drive wheels. One such Fowler locomotive survives, a very small narrow-gauge 0-4-2T. Early designers of steam turbine locomotives did not understand the need for reduction gearing or sprung suspensions. Once these problems were understood, jackshafts emerged as one alternative for linking the output gearbox of the turbine to the driving wheels. Giuseppe Belluzzo, of Italy, was granted several US patents on variations of this idea. Alternatives to jackshaft drives included use of a quill drive with the turbine above the drive axle, or a combination of a quill drive with a gearbox suspended horizontally between a locomotive driving axle and the turbine shaft.
Electric locomotives
Many early electric locomotives were also equipped with jackshafts. A general survey of electric locomotive design from 1915 shows 15 distinct jackshaft-drive arrangements out of 24 distinct locomotive designs. Some early locomotives used small diameter DC traction motors mounted on individual axles, but the majority, especially for AC powered locomotives, had only one or two large diameter motors. These large diameter motors were larger than most driving wheels and so were mounted well above the level of the driving axles. The motor or motors drove the jackshaft or jackshafts through gears or side rods, and then the jackshaft turned the wheels through side rods. In Europe, Oerlikon and Brown, Boveri pioneered a variety of jackshaft designs, while in the United States, Westinghouse was dominant. The early surveys of electric locomotive designs cited here all use the term jackshaft or jack-shaft. Examples include the PRR DD1 and FF1 electric locomotives, as well as the Swiss Class Ce 6/8 Crocodile and its narrow-gauge cousin, the Rhaetian Railway Ge 6/6 I. Continuing development of electric motors made them smaller, and by World War II, most new and made jackshafts obsolete.
Internal combustion locomotives
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