ABSTRACT

Various forms of three-axle vehicle have been used widely in the past. In most of these designs the wheelsets were connected to the car body by a conventional suspension similar to that used in two-axle vehicles. Negotiation of curved track was catered for by allowing greater flexibility or clearances for the central wheelset. However, there is also a long history of inventions which attempt to ensure that wheelsets are steered so that they adopt a more or less radial position on curves. It was argued that three axles, connected by suitable linkages, would assume a radial position on curves and then re-align themselves correctly on straight track. As will be shown below, a wide range of new potential instabilities are introduced. In fact, the three-axle configuration is also important because it gives considerable insight into the dynamic behaviour of articulated vehicles discussed in Chapter 8. Three-axle vehicles were in use from an early date. According to Liechty [1] a three-axle vehicle in which the lateral displacement of the central axle steered the outer axles through a linkage was tried out on the Linz-Budweis railway in 1826. Germain patented a design in 1837 in which radial steering was provided [2], and in 1844 Themor built a similar vehicle which was operated for some time [1]. Fidler also patented a similar arrangement in 1868 [3], Figure 7.1(a). In these last three schemes the outer wheelsets were pivoted to the car body. In 1889, Robinson’s arrangement [4] introduced the refinements of guides for the central wheelset, the body pivots was placed slightly inboard of the outer wheelsets, and the central wheelset had a much smaller radius than the outer wheelsets. Faye’s 1898 patent [5] removed the guides for the central wheelset in order to avoid reported difficulties with Robinson’s design on reverse curves. There were, of course, many different ways of providing inter-wheelset steering, such as complex linkages, and this is exemplified by the variety of designs produced since. Fidler introduced direct shear connection between the outer wheelsets in 1868, [3]. The central wheelset was mounted without lateral freedom in the car body, Figure 7.1(b). A similar arrangement was invented by Grover in 1880 [6]. All these developments were based on very simple ideas about the mechanics of vehicles in curves. In this Chapter, the basic instabilities of three-axle vehicles with a single car body will be considered, and how they are related to the natural steering properties of the three-axle vehicle. Particular emphasis will be given to the various

possibilities for the connections between the wheelsets and the car body in order to meet the conflicting requirements of stability and curving. The three-axle vehicle was first examined in this context in a series of papers [7]-[14]. A similar approach, though with slightly different assumptions, has been followed by de Pater [15, 16] and Keizer [17].