Precursors and the 'Java bogie' The SBB Ae 4/6 design originates with four express passenger locomotives, built by
Swiss Locomotive and Machine Works (SLM) and
Brown, Boveri & Cie. (BBC) in 1924 in Switzerland for the
Electrische Staats Spoorwegen of
Java. They were a development of the
rigid-framed 1′Do1′ arrangement, but
Jakob Buchli articulated this at each end, giving rise to their name of the '
Java bogie' for this
(1A)Bo(A1) form. Only a few examples of the (1A)Bo(A1) were ever built. The bogie was arranged so that the pivot axis was just behind the pivoted driven axle. The axles were driven by
Buchli drives, to permit suspension movement, and as the pivot was so close to the axle this linkage could also absorb the bogie's movement, as the driven axle twisted in place but did not move sideways by much.
Gotthard 'double locomotives' A derivative design of this layout was used for the Swiss
Ae 8/14 'double locomotives' of 1931, built for heavy freight service on the steep gradients of the
Gotthard Railway. These consisted of two articulated units as (1A)A1A(A1)+(1A)A1A(A1). A further unpowered carrying axle was also provided, splitting the central Bo group into A1A, which was needed by the extra weight of the transformer for the Swiss low frequency AC system. Again this was only a small class of three locomotives classed as
SBB Ae 8/14, although each of the three was different. The first used the same Buchli drives, but from the second they introduced the
Winterthur universal drive, with paired traction motors driving each axle through a single central gear. This could be adapted more easily to the articulation. A drawback to the sheer size of these locomotives is that there were few trains heavy enough to require them, and when used to the full they were at risk of over-straining their couplings. The third of these was built as the '
LandiLok' and exhibited at the . The SBB, however, saw disadvantages in huge locomotives and concluded their needs could best be served by smaller locomotives operating with
multiple control. This prompted the tender for the Ae 4/6.
SBB Ae 4/6 The Ae 4/6 was derived from half of the 'double locomotive', with a more modern flat-fronted cab at each end. Weight saving in the traction motors allowed a return to the
(1A)Bo(A1) layout, with the Java bogie and the Winterthur drive, and avoiding the central carrying axle. They were also intended for use on the Gotthard route, but more flexibly as they could be used as individual units for lighter trains, or run in multiple as pairs for heavier trains.
Multiple control equipment was fitted from the outset, although not much used in service as both it, and the locos, were considered unreliable. This was also the first class to be driven from the left of the cab, rather than the right. Both these and the Ae 8/14 had used
regenerative braking, useful for descending the Gotthard's steep gradients without overheating and also returning electrical power to the network. The Ae 4/6 had a simplified and lighter system, where one traction motor could serve as the
exciter for the others during braking. They were also built with
aluminium windings in the transformer and motors, rather than copper, owing to wartime shortages. Aluminium was also used for some parts of the frame and bodyshell. A problem with the Ae 8/14 was that it had a large number of transformer tappings, and could only change slowly between them. This limited their best acceleration, no matter how light the train, to a minimum of a minute to reach full speed. The Ae 4/6 avoided this by using fewer tappings, with faster actuation between them. An air-blast main high voltage circuit breaker was also used. The
Winterthur drive is a geared drive on the centreline of the locomotive, giving room for a traction motor each side, two to an axle. The two motors were geared by
spur gears to a central
layshaft carrying a third gear which drove a drive wheel on the axle. This drive wheel was not fixed rigidly, but was connected to the axle by four pivoted links in a square arrangement. The large number of gears used, and that these were straight-cut spur gears, led to high noise levels. When combined with some issues from wartime construction, the drive transmissions were not perfectly reliable. In service, the Ae 4/6 performed well in some aspects for measured power, but had problems with a lack of adhesion and mechanical unreliability. Some aspects of their wartime construction may have reduced their mechanical build quality, leading to high noise levels in the final drives, and a susceptibility to overheated bearings and gear failures, particularly after wheelslip. == Service ==