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MIL-HDBK-1038
include ladders and platforms for access to the various sheaves, sheave frames,
equalizer beams, and other equipment.
4.1.17
Machinery Decks.  Machinery decks of portal and floating cranes are
usually constructed around two longitudinal main beams, which may be large wide
flange sections or built-up box girders.  They are positioned in line with the
boom hinges and extend aft to the end of the machinery house.  A circular
transition section to match the diameter of the roller path or the rotate bearing
is welded to the main beams.  In case of the roller path design, the center
steadiment is centered between the main beams in a heavy structural frame.
Normally pin housings for the A-frame connections and boom hinges are welded to
the upper flanges of the main beams.  Alternatively, rigid intermediate pedestals
or structural frames may be bolted or welded to the main beams to support the A-
frame and boom hinge pins.
The machinery deck is extended sideways with cantilevered structural
supports to provide additional mounting foundations for the operator's cab,
electrical equipment, fuel tank, and other miscellaneous items.  On new cranes the
entire machinery deck should be a single weldment.  If practical considerations
preclude this, then the bolted connections should be designed to ensure an
accurate fit/alignment for field assembly.  Interiors of major closed sections of
the structure are accessible through hatches in the floor.  On the older cranes of
riveted construction, most areas of the machinery deck are accessible from the
open bottom of the structure.
The main beams support the hoists on their independent foundations and
the counterweight.  This arrangement aligns all the hoist wire ropes nearly
parallel with the sheaves on the A-frame and the boom.  The rotate drives should
also be mounted adjacent to the main beams to take advantage of their inherent
strength and rigidity.  The diesel engine-generator set, depending on the
machinery house space constraints, may also be mounted on the main beams, but the
newer cranes locate it on the side support on the opposite side from the
operator's cab.  On cranes with rotate bearings, the main beams should include
reinforced jacking points for raising the crane upperworks to permit rotate
bearing replacement.
4.1.18
Portal Bases.  Portal bases (gantries) of the older cranes are four-
legged structures constructed from plates and standard structural sections with
riveted or bolted joints.  These structures are susceptible to corrosion at the
joints and on exposed but inaccessible surfaces.  They are slightly flexible, and
this characteristic allows them to deflect sufficiently to keep all the travel
wheels in full contact with uneven rails.  The upper section (gantry cap) includes
a near circular (usually octagonal) girder arrangement to support the roller path.
The roller path is usually 26 to 30 feet in diameter, regardless of the rail
gauge.  The legs are connected by heavy sill beams at their lower ends.
On newer cranes with rotate bearings, the portal bases are one-piece
weldments or bolted assemblies of several major weldments.  The weldments are
closed sections made up of flat plates and present a smooth, low maintenance
exterior.  All structural reinforcements, normally in the form of open diaphragms,
are confined to the interior.  All interior sections which are large enough for
entry (approximately 2-foot by 3-foot cross section) are accessible for inspection
and repair; smaller sections are seal welded.  The circular column, of
approximately the same diameter as the rotate bearing, is installed on top of the
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