All About Engines

27° All About Engines “ coupled,” it is known as a four-coupled ” ; and if three pairs are connected it is termed a “ six- coupled ” engine. All these are “driving wheels” because the power is applied to them, and their “ grip ” on the rails determines the motion of the train. In some of the smaller engines there is a single pair of wheels in front, but generally the weight of this end is supported on a “ bogie ” truck with two pairs of wheels. The truck is pivoted to the engine frame so that it can turn slightly when the engine goes round a curve. Usually, but not always, there is a pair of trailing wheels or a bogie truck at the back. The chief types are shown in Fig. 154, which will indicate the meaning of the terms “ 2-2-2,” “ 4-6-4,” etc., used in describing them. The names are those of the first engine of the type constructed, and they serve to describe the class to which this and all similar engines belong. A great many pas- senger express engines are of the 4-4-2 and 4-6-0 types, which, for this purpose, have displaced the types 2-4-0 and 4-4-0. On the G.W.R. the 2-6-0 type is used, but the “ Great Bear ” is a 4-6-2 engine. Heavy goods engines are often 0-6-0 and 0-8-0, which gives a large grip on the rails, but owing to the absence of leading and trailing bogies are not good for taking curves at high speeds. Heavy shunting engines, which need a good grip on the rails and have to negotiate points, are often of the 4-8-0 and 0-8-4 types. The “ Decapod,” 2-10-0, and “ Santa Fé,”