The Locomotive - Lighthouse Survival Blog

162 THE LOCOMOTIVE. [November,
that have been turned to too short a radius. Careless flanging is apt to start small cracks
through the skin of the iron, and these frequently extend inward and eventually become
dangerous. Incipient cracks on the inside of a boiler sometimes develop into deep
grooves, the slight yielding of the shell, under varying pressure, opeuing up the
interior of the metal to the corrosive action of the water. Defects of this kind usually
occur along the edge of lai^-joints, or near stay-bolts, where the shell is partially stiffened,
and the buckling action of the plates more pronounced.
The accompanying wood-cut (Fig. 1) shows a crack due to a different cause, and it
ought to carry with it a useful lesson. It represents a piece of plate that was cut from
a boiler in active service, and which was believed to be in good condition. The boiler
from which it was taken was 48 inches in diameter, with tubes 15 feet long; and the
plates were of steel, ^ of an inch thick. The piece of plate shown in the cut formed the
edge of one of the sheets, where two sections of the shell were united by a longitudinal,
double -riveted lap-joint. It was taken from the upper part of the boiler, and was not
exposed to the fire. It contained one well-marked crack extending completely through
the plate, besides many
Iliastkating the "Off-set" of the Lap.
other shorter ones, run-
ning into one another in
all sorts of ways, some of
them extending through
the plate, and others not
quite through it. All
these cracks were entirely
covered by the inside lap
of the joint, so that they
could not be .seen from
the interior of the boiler;
and on the outside, the
boiler was covered at this
point by a thick layer of
non-conducting asbestos
covering. We mention
these points in order that
the reader may understand how easy it would be to overlook this defect. Yet it would
not be putting the case too strongly to say that although the boiler appeared to be
in good condition, it was actually on the verge of explosion. For a considerable dis-
tance along the joint the strength of the plate was entirely destroyed; and at other
places it was held together by the merest skin of metal, as was afterwards shown by
breaking the plate across along the line of the cracks. The fractured area was almost
entirely black, though bright spots were noticeable at intervals of two or three inches
•or so.
The cause of this defect will be sufficiently obvious to those who are familiar with
the processes of boiler making. In rolling plates into the cylindrical form, preparatory
to riveting them up into shells, it is customary to bend one end of the plate to what is
judged to be the proper radius, by the use of the sledge-hammer. The plate is then run
through the rolls and rolled into shape, the end that was previously bent being introduced
first. When the plate has been rolled all but the last five or six inches, the last
end slips off of the first roll, and the rolls can no longer "grip" the sheet. The result
is, that the last end of the sheet is not bent to the proper radius, but remains straight or