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MIL-HDBK-1038
Although less noticeable than the difference between the booms and gantry
structures, the rotate mechanism of the newer cranes differs even more
dramatically from the old roller path design.  Unlike the older cranes, which
rotate on a simple assembly of exposed rollers on bent rail or cast segments with
a king pin for centering, the newer cranes rotate on a precision sealed roller
bearing.
2.4.2
Distinctive Features.  Portal cranes have the ability to travel anywhere
within their rail network (circuit) and provide heavy lift capability at a reach
unmatched by any other outdoor traveling crane.  The wide and high portal opening
with narrow travel trucks and gantry legs of the portal cranes minimize
obstruction in the busy work areas where they operate.
Corrosion is always a major maintenance concern with any outdoor crane,
but this problem is particularly acute with the intricate cage-like construction
of the old gantries and booms with crevices and water pockets throughout.  The
mainly welded construction of newer cranes presents a smooth exterior which
minimizes this maintenance burden.
2.4.3
Industry Standards.  There are no industry design standards for portal
cranes.  Structurally these cranes are designed in compliance with the applicable
requirements of the Manual of Steel Construction, published by The American
Institute of Steel Construction, Inc.
ASME B30.4, Portal, Tower, and Pillar Cranes, published by the American
Society of Mechanical Engineers.  This standard focuses primarily on the safety
aspects of the design and operation of the cranes.
API Specification 2C, Specification for Offshore Cranes, published by
American Petroleum Institute, prescribes a method for calculation of loads on the
rotate bearing.  However, since rotate bearings and their mounting are a specialty
of a limited number of manufacturers, their guidance for bearing selection and
installation should be followed when it is available.
2.5
Floating Cranes.  Floating cranes are comprised of the upperworks of
portal cranes mounted on barges.  These cranes are intended for operation in
sheltered waters but are designed for towing in the open sea.  They are not self-
propelled, except that they are equipped with capstans which pull on mooring lines
to reposition themselves over short distances.  Floating cranes are versatile
they can be positioned between the shore and the vessel for rapid loading or off-
loading, inside a drydock in close proximity to the work area, or anywhere within
the basin.  As with the portal cranes, the older designs (shown in figure 7)
feature deep triangular truss booms, while the newer cranes (shown in figure 8)
have slender lattice booms.  Older floating cranes are "straight-line" rated, the
newer designs are variably rated through most of the operating range.
2.5.1
General Description.  The upperworks of the floating crane is supported
on a tub (structural column) built into the barge near the stern.  As a
counterbalance to the crane, the bow end of the barge deck is provided with a
thick (usually 1.5 to 2.0 inches) steel cargo laydown area.  The upperworks,
including the rotate bearing, is similar in design to that of the portal crane,
with only a few exceptions:
a)
Boom design must consider side pulls due to list and trim of the
barge.
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