The Locomotive Of Today

The Boiler: Stays. 13 The crown of the firebox is supported either with direct stays to the outer shell or with girder stays. When direct stays are employed they are passed through the crown to the outer plate, the end inside the firebox having a square to fit a spanner, whilst the other is hammerecl over to form a head after the stay is screwed home, the holes, of course, being previously tapped to receive them. When the fire is first lighted, the gases passing through the tubes heat the tube plate more than any other part of the box, and it naturally follows that this expands more than any other portion ; to allow for this expansion the first two rows of stays are made of special form, which will permit of an upward movement of the box, but will resist collapsing when subjected to pressure; this construction is shown in sectional views at C in the plate. When girder stays or roof bars are used, the crown plate is fixed to them by means of bolts, so that it is independent of the outer box, though some of the roof bars are usually connected to the outer shell by sling stays as a precaution. These girders may be built up of stout plates, fixed together at the ends.and about i|-in. apart, having stay bolts passing up through the crown plate and between them, the heads being inside the box, and the nuts on the top of the girder ; but more generally now, they are cast in Steel with lugs provided along their length for the reception of short set bolts, which are screwed into them up through the crown plate. D shows in part elevation and in section, one of these girders the ends of which are made to fit and take a bearing on the top corners of the box sides or ends, according to whether they are arranged transversely or longitudinally. Another form of crown staying has been introduced with half girders, the rear half of the crown plate being fitted with transverse stays, each of which extends to about the centre of the box where it is supported by a sling stay ; the front half of the crown is also supported in a similar manner by longitudinal girders, with their front ends testing on the tube plate and the back ends hung from the firebox wrapper plate by a sling stay. This method has been adopted to avoid too great rigidity and allow for the expansion of the plates. Direct crown stays are to be preferred when the water used is very dirty, as they permit greater freedom for it to circulate and for steam bubbles to rise, and thus to an extent, minimise the formation of scale; they also allow the scale and dirt that does accumulate on top of the box to be more easily removed. They do not, however, let the firebox have the same amount of freedom for expansion as the girder stays.