The Locomotive - Lighthouse Survival Blog

1893.] THE LOCOMOTIVE. -163
nearly so. The shell (if rolled from one sheet) then looks something like Fig. 2, one
end of it "standing off" from the rest of tlie shell. (This feature has been somewhat
exaggerated in the cut, in order to show more distinctly what is meant.) In order to
bring the outer lap to the proper curvature, it is customary for one^jian to hold a sledge
against the projecting edge of tlie lap, while another workman strikes ti)e shell on tlie
inside. In this way the lap is bent down into place, and after the shell has been
brought to conform with the "sweep," or templet, in every part, it is ready for riveting.
Now it will be seen that the treatment required for bringing the laps together
in this manner is rather violent; and it follows that nothing but the best of materials
will stand it without being greatly distressed and permanently weakened. Under
the sledging operation the material is likely to be strained beyond its elastic limit,
unless it possesses great ductility. The greatest strain on it conies on the outer lap,
at or near the line where it touches the inner one in Fig. 2. We have no doubt
that the cracks shown in Fig. 1, were started in this way, and that they afterwards
crept into the plate gradually, as the boiler yielded slightly under varying pressures,
until they reached the highly dangerous state described above.
If the sledging were done while the sheet is hot, it would not be so objectionable;
but the great majority of boiler makers will not attempt to heat the plate before
sledging the lap down, because when the sheets arc hot they are apt to buckle
out of shape, and give great trouble.
In the early days of steel boilers, before the manufacture of that material
was understood as well as now, plates were much more apt to be injured by sledging
than they are at present. Steel having a high tensile strengtli is almost certain to be
deficient in ductility; and for this reason it is customary, in the specifications sent out
from this office, to make the maximum allowable strength of plate 65,000 pounds to the
square inch, when such plate is to be exposed to the fire. We also specify that the steel
used shall show an elongation of twentj'-five per cent, in a length of eight inches, that
it shall show a reduction of area of not less than 56 per cent., and that its elastic limit
shall be at least fifty per cent, of its ultimate strength. The plate should also be capa-
ble of being bent double and hammered, when either hot or cold, without showinor
cracks; and it is also desirable that it should stand this same test after being heated
and quenched in water. Steel that possesses these qualities makes excellent boilers,
and it will stand a great deal of abuse, in the boiler shop, without developing defects
in after service.
In conclusion, we may say that cracked plates are not so uncommon as the
average reader might suppose. This may be seen by glancing at our inspectors" reports,
as published from mouth to month in The Locomotive. Thus we find that during
the year 1892, our inspectors discovered no less than 2,646 plates that were cracked
in one way or another, of which 658 were considered to be dangerous.
There was an explosion in Minneapolis a short time ago, and in connection with it
the Minneapolis Trihune tells the following story: ''There is a little mother in northeast
Minneapolis who has a 16-mouths infant son who is the apple of her eye, and whom she
evidently regards as something of a heavy weight. She lives only a few squares from
the scene of the explosion, and her house was one that felt the shock. A friend after-
wards expressed his sympathy. 'And did you really feel the explosion ? ' he asked.
* Yes, indeed,' she replied; ' it shook the house from garret to cellar.' ' What
did you
think was the matter ? ' he curiously questioned. ' I thought baby had fallen out of
bed,' was the unexpected answer."