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 MIL-HDBK-1038
There is a striking visual difference between the older portal cranes
(shown in figure 5) and those of newer design and manufacture (shown in figure 6).
The older cranes have a deep triangular truss boom and a cage-like gantry
consisting of hundreds of riveted structural members. The newer cranes have a
slender lattice boom and smooth, streamlined, gantry structure consisting of a few
large weldments bolted together. The rated capacities and hook reaches range up
to 170 tons and 120 feet, respectively. The elevation of the machinery deck, boom
hinge, and operator's cab are of major importance to the utility of the crane
the boom hinge position controls the crane's reach over nearby obstructions and
the position of the operator's cab determines his field of vision. The elevations
of these elements vary from 48 feet to 81 feet.
There are significant differences between Navy and commercial portal
cranes.
a)
Navy cranes are normally "straight-line" rated up to the maximum
reach of the hook; for example, 50-tons from 55-foot radius to 95-foot radius.
Commercial cranes are variably rated; for example, 75 tons at 55-foot radius and
decreasing to 12 tons at 95-foot radius.
b)
Navy cranes are required to travel around tight curves at the head
of drydocks, which necessitates complex travel truck designs to compensate for
increase in the effective rail gauge. Commercial cranes travel on straight or
gently curving tracks with a fixed rail gauge. The geometry of this effect is
explained in paragraph 5.2.12.
c)
Navy cranes are always self-powered with an onboard diesel engine-
generator set. Commercial cranes are often shore-powered by means of a gantry
mounted cable reel connected to a fixed electric terminal near one rail. The
cable reel pays out or takes up the cable as the crane travels back and forth
along the track.
2.4.1
General Description. The main structural components of portal cranes are
the rotating upperworks (including the machinery deck, A-frame, boom, and strut)
and the traveling portal base. Each of the four corners of the gantry is
supported by a complement of travel wheels. A system of equalizers (rocking
beams) under each corner distributes the corner load equally to all wheels. Pairs
of wheels are mounted in the travel trucks, which are free to swivel (steer) to
follow the curvature of the track. Typically, 1/3 or 1/2 of the wheels are
driven.
The upperworks is comprised of the machinery deck on which are mounted
two or three hook hoists, luffing hoists, rotate drive, electrical controls and
resistors, diesel engine-generator set, fuel tank, A-frame, boom, counterweight,
and operator's cab. (In the case of cranes with machinery decks at 80-foot
elevation, the diesel engine-generator set and fuel tank are installed on the
portal base cap to make them more accessible for servicing.) The entire
upperworks is supported by roller path and king pin assemblies or a rotate
bearing. With only rare exceptions, all drives on Navy portal cranes are
electric. Commercial portal cranes, however, are increasingly converting to
hydraulic (hydrostatic) drives, which have been refined to offer some tangible
advantages over the electric drives compactness, continuously variable speed
range, and lower costs. This view is confirmed by the fact that mobile cranes are
exclusively hydrostatic or mechanical/hydrokinetic.
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