The Locomotive - Lighthouse Survival Blog

-[34 THE LOCOMOTIVE. [September,
Figs. 2 and 3, the projecting end is flared, or further expanded. Figs. 4 and 5 show
these tools as seen endwise. Little explanation of the manner of using them is necessary.
After the tubes are rolled in, either of the above tools can be used for expanding the
ends. Some prefer the tool with two points of contact, and others use the one with
three. The tool is inserted into the end of the tube and driven with a hammer until the-
end of the tube is brought solid against the tube sheet. Only light blows are required,
and the workman can readily tell when the expanding is sufficiently done. Fig. &
shows a tube which has been expanded by this method. In order to ascertain what the
holding power of tubes set in this manner would be, we arranged with Messrs. H. B.
Beach & Sons to prepare for us, in this manner, two test specimens, which were then
submitted to Prof. Richards as before, and tested by him in substantially the same
manner as those described above. The ends of the tubes projected /g of an inch beyond
the plate, and were flared so that the external diameter of the extreme end was 3.2^
inches, the diameter of the tube where it entered the plate being 3.1 inches. (The
original external diameter of these tubes was 3.0 inches, and they were 0.109 of an inch
thick. The plates into which they were rolled were § of an inch thick and 6 inches
square.) Prof. Richards' report says: "The stress which was sustained without the
tube yielding in the plate was 20,000 lbs. in specimen No. 1078, and 18,500 lbs. in
specimen No. 1079; the observed stress which flrst produced yielding was 20,500'
lbs. in specimen No. 1078, and 19,000 lbs. in specimen No. 1079; and the observed stresswhich
occasioned failure was 21,000 lbs. in specimen No. 1078, and 19,500 lbs. ia
specimen No. 1079."
It will be seen that there was an enormous increase in strength from the additional
expansion, the lowest strain that occasioned yielding in the present case being 19,000'
lbs., against 5,000 lbs. in the previous experiments, where the tubes were simply rolled
out, and not afterwards expanded.
To show that the holding power of tubes is sufficient to stay the head on the part
to which the tubes are secured, let us take a numerical example, choosing for the pur-
pose a head 66 inches in diameter, with 86 3-inch tubes, and a steam pressure of 100 lbs.
to the square inch. Assuming the lowest result of the experiments in each case — that
is, 19,000 lbs. in the flared tubes and 5,000 lbs. in those not flared — we have, as the
holding power of the 86 tubes, 86x5, 000 ==430, 000 lbs. with the plain tubes, and 86 X
19,000=1,634,000 lbs. with the flared ones. The tubes being confined to the lower half
of the boiler (the upper part being supposed to be substantially braced by methods to be
explained later), this combined holding power has to resist only the pressure that acts-
against the lower half of the head. The area of a 66-inch head is 3,421 square inches,
so that the area of the lower half of such a head is 3,421-^2 = 1,711 sq. in. From this,
area, however, we have to subtract the combined sectional area of the 86 3-inch tubes;
for the steam pressure acts only on the spaces hetween the tubes. The area of a 3-inch hole
is 7.069 sq. in., and the combined area of 86 of them would therefore be 7.069x86 =
608 sq. in. Deducting this from the area of the half-head under consideration, we have
left 1,711-608=1,103 sq. in., as the area against which the steam pressure acts. With
steam at 100 lbs. pressure per square inch, the total pressure against this area would be
1,103x100=110,300 lbs. Comparing this with the combined holding power of the
tubes, we find that the plain tubes (not flared) have a factor of safety of nearly 4, and
that the flared tubes have a factor of safety of almost 15. It is doubtful if the plain
tubes would give perfect satisfaction under these circumstances, for although we have
assumed the tubes to be confined to the lower half of the head, it is quite common, and,
indeed usual, in boilers of this diameter, to place one of the tube-rows above the center