Articulated Steam Locomotives
An articulated or jointed locomotive is one in which there are two or more independent groups of wheels, at least one of which can turn relatively to the boiler, in a manner similar to a bogie or to a bissel.
The purpose of the articulated design is to provide an engine of considerable power but with comparatively low axle loading. The difficulty of restricted axle load can be overcome by the coupling together of several pairs of wheels. The curvature of the tracks is still a denominator, especially to the length of the rigid wheel base, and therefore to the number of pairs of wheels of any given size which can be coupled together. To overcome the problem of providing sufficient coupled wheels to transmit the required power, the wheels are seperated into two groups; each group driven by an independent pair of cylinders. This arrangement also lowers the axle load. This is the principle behind the articulated locomotive, flexible enough to run on severely curved track, yet powerful.
The four types of articulated locomotives used by the South African Railways are the Mallet, the Garratt, the Modified Fairlie and the Union-Garratt.
The Mallet
The locomotive has two groups of coupled wheels with the rear group mounted on the main frame of the engine, and thus rigid. The front group of wheels is mounted on a separate frame, and this group is coupled to the rear group by means of a hinged joint. The firebox of the boiler is carried on the main frame, and the smoke box rests on a flat slide on the front frame. The tender is coupled the usual manner. The front end buffer and draw gear are mounted on the frame of the front unit.
The four cylinders of the engine works on the compound expansion principle. Two high pressure cylinders are fitted on the main frame and drive the rear group of coupled wheels. The exhaust steam from these cylinders is taken through a system of pipework to the low pressure cylinders, which is mounted on the front unit, thus driving the front group of coupled wheels. Thus the steam is used, economically, twice before emitted into the atmosphere.
The Garratt
The Garratt locomotive differs completely from the Mallet. It consists of two units, each driven by a pair of cylinders at its outer end, and these are connected together by the cradle, which carries the boiler. A water tank is attached to the frame of the front unit, while a combined coal bunker and water tank is carried on the frame of the back unit. No tender is used because the locomotive is entirely self contained.
To ensure maximum rotation of each unit on curved tracks, the units are coupled to the cradle by means of pivots. The four cylinders all take high pressure superheated steam direct from the superheater header. The buffer and draw gear at each end are carried on the main frame. An electric headlamp is mounted on the water tank in front and on the coal bunker at the back.
The South African Railways Handbook on the Steam Locomotive commments: “If it is essential to use two Garratts on a train, it is better where possible to place one of them in the middle or at the rear of the train, rather than to marshal both at the front.
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The Modified Fairlie
It is very similar to the Garratt, the main difference being that the front water tank and the combined coal bunker and water tank at the rear are carried on extensions of the boiler cradle and not on the unit frames.
The Union-Garratt
This is a combination of the Garratt and the Modified Fairlie designs. The front unit carries a water tank on the unit frame, as thus the Garratt do, while the boiler cradle is extended over the back unit to carry the coal bunker. No water tank is fitted at the back, but a tank is fitted to the cradle underneath the boiler barrel.