The articulated locomotive type that was first proposed, and patented, by Anatole Mallet (1837 – 1919) may well be regarded as one of the high points of locomotive design and even mechanical engineering in general.
This paper focuses on two interesting aspects of Mallet-type locomotives. The evolution of the different types of hinges that connect the front truck with the rear one. And one often overlooked and little understood detail of the rear truck’s frame structure of numerous (but far from all) Mallet locomotive designs: Two cantilever beams, forward of and rigidly attached to the rear truck, that appear to transfer about half of the weight of the boiler into said rear truck and, yet, whose front ends are in turn vertically supported by the front truck.
The hinges are discussed in chapter 3. The paper explains and discusses the evolution from a hinge configuration with two bolts that allows only one degree of freedom (pivoting about the vertical axis) as originally patented by Mallet to a one-bolt hinge configuration, including ball bushing bearings, that allows rotations about all three axes as well as some displacement in the vertical direction. Fritz Rimrott (1849 – 1923) is identified as the inventor of this one-bolt bearing configuration which, over time, became prevalent, especially in the case of large U.S. American Mallet locomotives. Some other, less widely used hinge configurations are also, briefly, discussed.
The structural arrangement of two cantilever beams mounted on the front end of the rear truck (see, amongst others, figures 22, 24 and 30) is discussed in chapter 4: The paper notes that no rationale for its existence has been documented and that extremely little information or data on it is available in the literature. To attempt to get at least some insight – however limited – into the thinking of the designers, some reverse engineering was applied. The author had the opportunity to take some measurements of and obtain information on a 1898-vintage German narrow gage Mallet locomotive, featuring such two cantilever beams, while it was undergoing heavy maintenance and, thus, in a disassembled state. Based on the measurements taken and the combined area moment of inertia of the two cantilever beams computed as well as taking into account typical stress allowables, a basic static analysis as it might have been performed by the designers in the 1890s led to the insight that the beams were dimensioned for the load case where about half of the weight of the boiler can be carried by the cantilever beams.
Yet, as also discussed in chapter 4, convincing qualitative engineering reasoning leads to the conclusion that these cantilever beams were unnecessary and would not even work as a fail safe secondary load path. Which is probably also the reason, why these kinds of beams were abandoned again by many later Mallet locomotive designs, especially in the United States.
As an addition to the more technical discussions of chapters 3 and 4, chapter 5 provides some biographical background on both Anatole Mallet and the undeservedly little-known Fritz Rimrott and offers some appreciation of both of these men’s contributions to articulated locomotives.