Triangle drive

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 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.

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 '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."

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Wikipedia