The Locomotive - Lighthouse Survival Blog

1893.1 THE LOCOMOTIVE 136
lino. 'I'lie total steam- pressure to be resisted would therefore be a little larger than we
have assumed, aud the factor of safety would be correspondingly smaller. On the other
hand, we have taken no account of the stiffening action of the flange of the head; and
as there can be no d()ul)t that this flange stays the head sufliciently for a distance of at
least three inches from the shell, perhaps we could fairly say that theunflared tubes have
a factor of safety of 4, and the flared ones a factor of 15, notwithstanding the one row
that projects above the center, line of the boiler. As a general rule, we consider 5 to be
tiie proper factor of safety for boilers, except in braces and flat heads; the exception
being made in the ca.se of braces partly because of the uncertain quality of iron that is
used for them, and in the case of flat heads, on account of the general nebulosity sur-
rounding all the formuhv yet proposed for calculating their strength. Hence we should
not be inclined to recommend the use of plain, unflared tubes in a 66-inch boiler
that is to carry 100 pounds pres-
sure, especially when such an
enormous gain in strength may
be had by a little extra work.
Moreover, all the foregoing calcu-
lation has been based upon the
assumption, which, as indicated
above, is too often erroneous, that
the tuhe-rollw(j has heen icell done.
It is far better to use the flared
tubes; and when these are used,
there is not the least chance for
doubt about the sufficiency of their
holding power. We come, now,
to the consideration of that part
of the boiler-head which lies above
the tubes. The tubes, it has been
shown, possess sufficient holding
power to amply stay the part of
the head to which they are at-
tached, and we may safely consider
that they will also possess suffi-
cient staying power to take care
of the heads for, say, two inches
'
Fig. 7. — Illustrating the ' Area
Braced."
to be
above their upper surfaces. The flanges of the heads being securely united to
the shell, and being also curved or dished, it may likewise be safely assumed that no
braces need be provided for that part of the head which lies within three inches of the
shell. The part of the head that requires bracing, therefore, consists of a segment of
a circle whose circumference lies three inches within the circle of the shell, and whose
base is two inches above the upper row of tubes. Thus in a 66-inch boiler, whose upper
row of tubes is 26 inches below the top of the shell, the part of the head that recjuires
bracing consists in a segment of a circle the diameter of which is 60 inches, and the
height of which is 21 inches: 21 inches being the measured height (26 in.) minus the 3
inches that lies between the shell and the segment to be braced, and minus the two
inches that lies between this segment and the top of the tubes. (This segment is indicated
by the dotted lines in Fig. 7.) The area of such a segment is easily found by
means of the table given in The Locomotive for June, 1891, on p. 94. Thus 21^60 =