service classes from "standby" to "continuous severe" service. (NCC policy is
to specify Class C "moderate" service as a minimum.) In addition to the crane
design requirements, this specification provides the runway design and condition
criteria for straightness, levelness, span, and deflection tolerances.
CMAA Specification #70 is not entirely adequate for meeting Navy OET
design requirements, especially for the mechanical and electrical design features.
These additional Navy requirements are addressed in detail in Sections 4 and 5 of
ANSI/ASME B30.2, Overhead and Gantry Cranes (Top Running Bridge, Single
or Multiple Girder, Top Running Trolley Hoist) published by the American Society
of Mechanical Engineers. This standard focuses primarily on the safety aspects of
the design and operation of OET and gantry cranes.
The Whiting Crane Handbook, published by the Whiting Corporation, is a
comprehensive crane design reference textbook. It is especially useful for
estimating weights, dimensions, required runway and building clearances, and
maximum wheel loads of OET cranes.
Underrunning Cranes. These cranes, also called "underhung cranes",
always feature an underrunning hoist/trolley unit or a trolley with a separate
hoist; however, the bridge end trucks may be overrunning (on runway rails) or
underrunning (on the lower flanges of runway beams). The underrunning runway
beams are secured to the roof support structure of the building. The crane
configurations are either in the form of a single girder (with a combination
hoist/trolley unit mounted on its lower flange) or twin girders (with a trolley
mounted on their lower flanges). The end trucks may be of the rigid type, with
wheels on fixed axles protruding from the end truck frame; or of flexible type, on
swivel connections supported by two-wheel carrier yokes. Crane operator's
controls are usually on a suspended control station near the floor level. The
design and condition of the runway, whether for overrunning or underrunning
trucks, must comply with industry standards to ensure satisfactory crane
Suspension Systems. Underrunning cranes with underrunning bridge end
trucks are often procured with runway beams and suspension systems. The
suspension systems may be rigid or flexible, depending on the building
construction and the available headroom, and are designed to support the bridge
end trucks on the lower flanges of the runway beams. In the rigid systems the
runway beams are fastened directly to the building structure. In the flexible
systems the runway beams are suspended on tie rods which can move laterally (like
a pendulum) about their individual suspension points. Lateral and longitudinal
bracing must be installed to limit the horizontal motion (sway) of such systems.
Standard commercial tie rods, clamps, and various fittings and hardware are
available from manufacturers of patented track.
(Runways for cranes with overrunning bridge end trucks are usually
constructed as an integral part of the building.)
General Description. There is a wide selection of arrangements and
design features available for underrunning cranes. The runway beams and the
bridge girders may be in the form of standard structural shapes (wide flange or
I-beam sections) or patented (monorail type) track. (NCC policy is to specify