All About Engines

when leave some outer Fig. 34.—Girder stays on Loco firebox the great difference in temperature 68 All About Engines striking feature of locomotives of the “ Great Bear ” type. The larger diameter lies behind the driving wheels. It enables a bigger firebox to be used, and increases the proportion of water at the hottest end of the boiler. Another advantage is that the surging backwards and forwards of the water the engine varies its speed—which is liable to the crown of the firebox uncovered—is to extent prevented by the conical shape. The firebox of a locomotive is fixed to the shell by a ring at the bottom, by another ring round the fire door, and by numerous stays, about 4 inches apart, to the sides. Owing to between the fire- box and the outside shell, great stresses are thrown on these stays, and failure is common. Some fire- boxes have curved crowns, and others have flat crowns like the Belpaire firebox—a type growing in popu- larity. The curved crowns are merely suspended by stays from the boiler shell; the flat crowns are stiffened by small girders as shown in Fig. 34. The number and dimensions of these stays or girders are largely a matter of experience ; the tendency of the firebox to change its shape renders it almost impossible to make exact calculations of the stresses which have to be borne.