MIL-HDBK-1038
only patented track for all new crane procurements.) The cranes are most often
electrically powered, and with standard commercial wire rope hoists or
hoist/trolley units. However, several other power and design options are
available pneumatic or manual power and chain hoist. (See paragraph 4.3.1.4 for
standard commercial design variations.) The electric power is transferred from a
fixed location near the runway to the traveling bridge through expandable loops of
flexible conductors festooned along the runway or rigid conductors and sliding
collector shoes.
The single-girder bridge with a hoist/trolley unit is the most prevalent
configuration. It is comprised of the main girder, which carries the
hoist/trolley unit, and an outrigger beam braced to the upper flange of the main
girder. The outrigger beam improves the lateral stability of the main girder,
supports the hoist/trolley electrification system, and may support the bridge
travel drive. The main girder and the outrigger beam are connected to an end
truck at each end. Bridge travel drives on flexible end trucks are in the form of
two powered yokes (drive heads), one on each end truck. On cranes with rigid end
trucks, as depicted in figure 2, the bridge drive is centrally mounted and drives
both end trucks. The hoist/trolley unit has an integral travel drive.
The twin-girder bridge configuration has the upper flanges of the girders
braced to each other for lateral stability and carries a trolley frame with an
independent hoist on the lower flanges. The end trucks may be of either the rigid
or flexible design, with bridge drives as described above. The trolley frame is
usually a custom designed, built-up assembly.
2.2.3
Distinctive Features. The unique feature available with these cranes is
the ability of the hoist/trolley unit or the trolley to cross over between bridge
girders of adjoining cranes or travel onto a spur track. Such cross-overs may be
direct (girder-to-girder) or across an intermediate transfer section. The
hoist/trolley units are standard commercial designs, which are selected from among
the catalog models advertised by the major manufacturers of such equipment; it is
impractical to have them custom designed. The trolleys of twin-girder bridge
cranes, can mount either a standard model or a built-up, custom designed hoist.
2.2.4
Industry Standards. Due to the wide variety of configurations and design
options available with this category of cranes, there are several applicable
industry standards. The governing standard for the design of single girder cranes
with underrunning hoist/trolley units is CMAA Specification #74, Specifications
for Top Running & Under Running Single Girder Electric Overhead Traveling Cranes
Utilizing Under Running Trolley Hoist, published by the Crane Manufacturers
Association of America, Inc. It prescribes design requirements for bridge girders
in the form of all types of single web structural beams, including specially
reinforced, and of box section designs. The bridge end trucks are either of the
top running type as in OET's (and running on runway rails) or underrunning type
with wheels on fixed axles. This specification also provides design and condition
requirements for straightness, levelness, span, and deflection tolerances for
runways for top running and underrunning cranes.
Being limited in scope, however, and specifically excluding the patented
track and swivel carrier yoke end trucks, the NCC references CMAA Specification
#74 only for the evaluation of existing structural shape/section bridge girders
and runway beams.
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