534
DOCK ENGINEERING.
gaining space on narrow quays. It embodies all the features of a pedestal
crane minus the back legs.
3. Cranes (fig. 374) which are carried entirely upon the shed structure,
either at some floor level or upon the roof. This arrangement is inevitable
when there is not sufficient space to accommodate the crane upon the quay,
and, in other cases, it avoids the obstruction caused by the front legs of the
semi-pedestal crane, but it involves a corresponding increase in the amount
of outreach.
The hydraulic roof-crane at Liverpool, shown in fig. 371, has luffing gear
capable of altering the outreach or rake from 18 to 33 feet beyond the line
of coping. The total height of lift is 76 feet, and the rate of lifting the full
load of 30 cwts. is 150 feet per minute.
Fixed cranes have the advantage of greater stability, and are employed
for lifting heavy loads. One at Malta, capable of raising a weight of
160 tons, is described at p. 536, post. Others, of various types, are illus-
trated in figs. 542 to 546.
The difficulty of employing large cranes with long outreach is the
revolution of the jib amid the intricacies of masts, yards, stays, &c., of
shipping. In many cases a pair of sheer legs, or oscillating derrick crane, is
to be preferred. In form, the apparatus is a tripod witli two legs pivoted
horizontally at the edge of the quay and the third adjustable to the amount
of outward projection. The movement of the load is entirely in one plane,
at right angles to the direction of the quay, by which arrangement any
interference with objects on either side is avoided.
Floating Cranes.—A floating crane, or sheers, is a valuable adjunct to
the equipment of a dock system, as apart from its availability for shipping
Fig. 547. —Floating Crane. Elevation.
and commercial purposes, it is of great utility in lifting dock gates for
repairs, in berthing temporary dams, and in many other cases. Such cranes
are constructed up to 100 tons lifting power. One of 25 tons is shown in
figs. 547 and 548.