CRANES.
It appears that Mr. Stevenson was much perplexed as to what sort
of cranes he should use in building the Bell Rock Lighthouse. His
difficulties were twofold:--
_First_, In consequence of the dovetailed form of the stones he
required a crane that would drop them as nearly as possible on the beds
on which they were permanently to rest.
_Second_, Supposing he devised a _guy crane_ that overcame this
difficulty, what was to be done as the building rose in height, and the
guys became too nearly perpendicular to admit of such a crane being
used?
In his private notes Mr. Stevenson regrets that he could get no advice
from anybody he consulted, all of whom recommended him to employ common
sheer poles, such as had been used by Smeaton at the Eddystone; and he
adds, “I may say, morning, noon, and night, these difficulties have
haunted me.” But thrown back on his own resources, and appreciating the
difficulty as no one else could so well do, he found, as is often the
case, that he was his own best counsellor, and he succeeded in solving
the problem that had given him so much concern, by inventing what he
called the “moveable beam crane,” and also the “balance crane,” which
are shown in Plate XI. The former, as modified to suit particular
cases, is now in universal use for building purposes, and the latter
has been employed in rearing most of our Rock Lighthouses, so that I
think professional readers will not object to my giving Mr. Stevenson’s
description of these cranes, as designed by him at the beginning of
this century. He says:--
“In cranes of the common construction the beam is a fixture, and
is placed at right angles to the upright shaft: but in the machine
represented in the Plate (Fig. XI-1), its attachment is at the lower
extremity of the crane, where it is moveable up and down upon a journal
or bolt. This crane is therefore termed a moveable beam crane. The
moveable property of the beam, in so far as the writer knows, is new,
and possesses the advantage of laying any stone within its range
perpendicularly on its site. This, from the dovetailed form of the
stones at the Bell Rock, rendered it particularly fitted for this work,
to which a crane of the ordinary construction could hardly be said to
be applicable. At the Eddystone Lighthouse this operation was performed
by means of triangular sheers; but, from the greater extent of the Bell
Rock works, and their greater depth in the water, such means must have
rendered the process of building extremely tedious. These cranes were
necessarily immersed at high water, and were retained in their places
by four guys fixed at the top of the upright shaft, and the moveable
jib or beam being lowered down, was secured to an eyebolt batted into
the rock.”
[Illustration:
_PLATE XI._
_MOVEABLE JIB AND BALANCE CRANES._
_W. & A. K. Johnston, Edinburgh._
]
“The ‘balance crane’ (Fig. XI-2) was constructed on a new principle
for building the upper part of the Bell Rock Lighthouse, when the guy
ropes of the moveable beam crane became ‘too taut,’ as sailors express
it, or were too near the perpendicular, thereby rendering the beam
cranes unstable. To remedy this, the balance crane was so arranged as
to be kept in equilibrium by a back weight of cast iron, so adapted
as to counteract the varying load upon the working arm or beam. The
elevation here represented is the same in principle with that used at
the Bell Rock, but differs somewhat in form, agreeably to improvements
made in order to adapt it to the erection of the Carr Rock Beacon.
The upright central column is a tube of cast iron put together in
convenient lengths with flush joints, after the manner of spigot and
faucet, fitted by turning and boring. The centre column of this machine
might have been carried to any suitable or convenient height, by adding
length to length, as the building advanced, without once moving the
foot on which it rested, but at the Bell Rock not more than three
lengths of from six to nine feet were generally in use. A malleable
iron cross head was stepped into the void of the central shaft or
column when the body of the crane was to be elevated. This operation
was accomplished simply by hooking the main ‘purchase’ and ‘traveller’
chains into the eyes of the crosshead, when the machinery of the crane
was employed with great facility as a locomotive power for lifting
itself as each new length of central column was added. The weight of
this crane as used at the Carr Rock did not exceed two tons.”