The Locomotive - Lighthouse Survival Blog

132 THE LOCOMOTIVE. [September,.
iug in use for a sufficient length of time. Certain facts observed by Mr. Nicholls in
connection with the heads described above indicate, however, that this is not the case.
For instance, in the third experiment, when the pressure reached 10 lbs. the end had
bulged -^^ of an inch; at 20 lbs., it had bulged jig inch; at 40 lbs., ^ inch; at 60 lbs.,
T^g inch; at 80 lbs., ^ inch; at 100 lbs., ^ inch; at 120 lbs., | inch; at 140 lbs., ^ inch;
at 155 lbs., -/g inch; at 170 lbs., | inch; at 185 lbs., ii inch; at 200 lbs., the bulge-A\-as.
exactly | of an inch. "The pressure was now reduced to zero," says Mr. Nicholls,
" and the end sjjrang back ^^ inch, leaving it with a permanent set of -f^ inch. The
pressure of 200 pounds was again applied on 36 separate occasions during an interval of
five days, the bulging and permanent set being noted on each occasion, but without any
appreciable difference from that noted above." That is, as we understand it, no
tendency to increase in the set could be detected.
It must be remarked, however, that the experiments we have described were con-
fined to plates not very widely diff'erent in their dimensions, so that even Mr. Nicholls'
rule cannot be relied upon for heads that depart very much from the proportions given
in the example above.
It will be seen that, considering the discrepancy in the various rules, and the insufiiciency
of the data available, our knowledge
of the strength of flat unstayed heads is
very meager; and the moral is, as in all other
such cases in engineering practice, that
we should avoid such heads whenever it is
possible to do so, and should use in the place
of them something that we know more about.
Bumped up heads are therefore recommended
'
in cases where braces cannot be used. ^ 'i \ v_J
Fig. 4. Fig. 5.
In horizontal tubular boilers a considerable part of the head is held by the tubes;
and as it is of the highest importance to know to what extent we can rely upon the
holding power of tubes, numerous experiments have been made, with tubes set in various-
ways, to determine this point. In 1880 we instituted two series of tests on tubes rolled
with the Dudgeon expander, as there was some question among engineers, at that time,
about the effectiveness of this tool. We arranged with Messrs. H. B. Beach & Son,
boiler-makers in this city, to prepare for us three specimens composed of tubes three-
inches in external diameter, rolled into f-inch plate in the ordinary way, without any
expanding other than that produced by the apparatus. Figure 1 shows one of these
tubes as it appears in section, the cut being one-half the original size. (It should be
stated that f-inch plate is thinner than is ordinarily used for boilers of usual dimensions,
and that thicker heads would give more frictional surface, and consequently more hold-
ing power.) These specimens were delivered to Prof. Charles B. Richards, then consulting
engineer at Colt's Armory in this city, with the request that he would subject them
to the required test. In his report Mr. Richards says: "The external diameter of the
tubes was three inches, and the thickness .109 of an inch. One end of each tube was-
fasteued in an iron plate 6 inches square, being simply expanded into the plate without