Fig. 28.
The crown plate of the fire-box being flat requires to be efficiently stayed, and for this purpose girder stays called fox roof stays are mostly used, as shown in the figure. The stays are now made of cast steel for locomotives. They rest at the two ends on the vertical plates of the fire-box, and sustain the pressure on the fire-box crown by a series of bolts passing through the plate and girder stay, secured by nuts and washers. [Fig. 28] is a plan and elevation of a wrought-iron roof stay.
Another method adopted in locomotive types of marine boilers for staying the flat crown of the fire-box to the circular upper plate is shown in [fig. 29]—namely, by wrought-iron vertical bar stays secured by nuts and washers to the fire-box with a fork end and pin to angle-iron pieces riveted to the boiler shell.
Fig. 29.
The letters in this figure refer to the same parts of the boiler as do those in [fig. 27], i.e., F B to the fire-box, etc., etc.
It was formerly the custom to make the tubes much longer than shown in the fig., with the object of gaining heating surface; but modern experience has shown that the last three or four feet next the smoke box were of little or no use, because, by the time the products of combustion reached this part of the heating surface, their temperature was so reduced that but little additional heat could be abstracted from them. The tubes, in addition to acting as flues and heating surface, fulfil also the function of stays to the flat end of the barrel of the boiler, and the portion of the fire box opposite to it.
In addition to the staying power derived from the tubes, the smoke box, tube plate and the front shell plate are stayed together by several long rods.