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walkway or platform at the back end for servicing the festoon system.
The two
walkways must be connected by a crossover walk.
Mobile Crane Fixed Length Booms.  These booms are lattice type standard
components of various manufacturers and may be of either tubular or open
structural sections.  The individual sections are all-welded and normally the
grade of steel is of higher strength than on other cranes.  The main chords and
lacing are most often thin-wall tubular sections.  Boom sections may be bolted or
pinned for quick disassembly and assembly.  The assembled boom lengths vary,
depending on the required reach, and the crane capacity is adjusted
correspondingly.  The boom length is changed by addition or removal of standard
boom sections.
The booms are normally raised by a boom hoist and wire ropes.  Walkways
or platforms are not provided because their weight detracts directly from the
crane's capacity. Flying Jibs.  Boom extensions in the form of lattice type pinned flying
jibs are common on mobile cranes.  The installation of flying jibs includes
additional standard components such as a mast and pendant cables.  The common
practice is to align (angle) the flying jib below the axis of the boom.
Mobile Crane Telescopic Booms.  Telescopic booms have closed rectangular
or trapezoidal cross sections.  They may be standard structural tubular shapes or
built-up box sections.  They are extended (telescoped out) by means of internal
hydraulic cylinders.  Each section slides on non-metallic low-friction pads
installed at the open end of the surrounding section.
The booms are normally raised by hydraulic cylinders installed between
the machinery deck and reinforced connections on the boom.  As in the case of
fixed length booms, walkways and platforms are not provided so as not to detract
from the crane's capacity.
A-Frames.  A-frames, so called because of their profile, serve to elevate
the boom luffing sheaves to a favorable position for raising the boom.  The two
back legs, which are subjected only to tension, are standard open or closed
structural shapes.  The two forward legs, which are subjected to compressive loads
and bending loads from the reeving systems' sheaves mounted on them, are usually
relatively heavy closed sections and are extensively braced.  The A-frames also
support the ladders and platforms that provide access to the sheaves and other
service points.
The forward legs are connected to the machinery deck main beams or their
rigid vertical extensions in the form of pedestals or structural frames; the back
legs are connected at the back ends of the main beams directly above the
counterweight.  The connections between A-frames members and with the machinery
deck may be rigid (common practice on older cranes) or pinned.  Pinned connections
make assembly and disassembly (which is sometimes required for repairs or access
to the machinery house interior) easy to perform.  It is NCC policy to require
pinned connections for new cranes.
A-frames are distinct structural assemblies and do not differ between
those on portal and floating cranes.  On container cranes, however, the A-frames
are integral vertical extensions of the gantry structure.  The A-frames must


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