The Locomotive - Lighthouse Survival Blog

70 THE LOCOMOTIVE. [May,
by tirlce the thickness of the head, and divide tlie product by the pressure that is to be
carried. The (juotient is the greatest allowable radius of the head. If it is bumped to
a longer radius than this, it is not safe : but there is no objection to the use of a shorter
one.
The foregoing rule, wlien cf)inpiued with the rule for cylinders, shows that if a
cylinder and a sphere aie to be made of the same material, and to have the same burst-
ino- point, the radius of the S2)here must be equal to the dinmetei' of tlie cylinder.
Heuce it is important to bump the head of the dome to a radius equal to the diameter of
the dome; for this makes the head and the sides equal in strength. Fig. 5 represents
a dome-head bumped in this manner.
We do not recommend the use of bumped heads for higli pressures. If it is possible
to avoid them; the braced head, presently to be considered, njipears to us to be far
better. Leaving the bumped head, therefore, we pass to the consideration of (3) the
braced head. This form of head is shown in Fig. 1, and is the one most commonly met
with in practice. The riveted joint, by which the head is attached to the shell of the
dome, is strong enough to stand the entire pressure on the liead, provided the head is
kept flat, so that the strain on the rivets is a simple shear. The object of bracing is to
stiffen the head so that it will not bulge out and produce radial strains that would tear
it out around tlie flanges. In the head of a 30 inch dome, allowing 3 inches all around
as sufficiently stiffened by the flange, there are 707 square inches to be braced. Under
a pressure of 90 pounds to the square inch this would correspond to a total pressure on
the head of G3,630 lbs. ; and this, allowing 7,000 lbs. to each brace, would call for nine
braces. In practice only eight are used, as the nozzle for the steam-pipe contributes
materially toward the stiftening of the head. In general, si.x braces are used on a 24-
inch dome, eight braces on a 30-inch dome, and ten braces on a 36-inch dome; though
this cannot be considered an absolute rule, for with high pressures the number of braces
may have to be increased. Crow-foot braces should be used, as shown in the cut, and
the rivets by which tlicy are held must not be less than f of an inch in diameter. The
braces should run to the sides of the dome, as shown in the cut. It might seem unnec-
essary to say this, but we have occasionally seen the braces in domes secured to the shell
of the loiler, close to the steam ojiening; and of course they are of practically no use in
such cases.
The fourth method of stiffening the heads of domes, is to rivet tee-irons or angle-
irons across them. This method is sometimes used, but we believe it to be inferior to
the bracing, because defects are more readily detected by the inspector where braces
are used than where the stiffening is obtained by angle-irons only; and also because the
strength of Itraced heads can be calculated more readily, and with greater accuracy.
In conclusion, let us say (1) that we believe that if a boiler is properly designed and
properly set, it does not need a dome; but if a dome is to be used, we believe (2) that
the shell should not be cut away to the full diameter of the dome, but that an elliptical
hole, say 11" X 15", is much better, and that a man-hole frame should be securely riveted
to this opening; (3) that the dome should be secured to the shell by a double-riveted
joint, at least; (4) that the head should be of wrought-iron or steel and that it should
be braced to the sides of the shell with a sufficient number of crow-foot braces, secured
with rivets at least f of an inch in diameter; (5) that braced heads are superior to cast-
iron heads, bumped heads, or heads stiffened only by tee-iron; (6) that if the circum-
stances are such that bumped heads become necessary, or highly desira1)le, they should
be biunped to a radius not greater than the diameter of the dome; (7) that the curve of
the flansre of the dome should fit the curve of the boiler-shell exactly, so that the boiler