Section 2: MAIN CRANE TYPES
Overhead Electric Traveling (OET) Cranes. These cranes, also called
"bridge cranes", are installed on overhead runway rails to provide hoisting
(lifting) coverage throughout the entire length and width (span) of the runway.
The trolley is equipped with one or two hoists. OET cranes are ideal for heavy
duty service in warehouses, machine shops, maintenance bays, and similar work
areas. They function equally well indoors and outdoors. The controls for the
cranes may be located in an operator's cab, on a suspended pendent pushbutton
station near the floor level, or at a remote control station. The design and
condition of the runway must comply with the crane industry standards to ensure
satisfactory crane operation.
General Description. The main structure of OETs is a pair of parallel
bridge girders, which span the runway and rest on end trucks. In the common four-
wheel configuration, the end trucks also function as end ties for the bridge
girders. The hoists are mounted on a trolley frame, which travels on rails
fastened to the bridge girders. Crane motions are driven by electric motors. The
electric power is transferred from a fixed location near the runway to the
traveling bridge by means of collector shoes sliding along rigid conductors
parallel to the runway or through extendable loops of flexible conductors
festooned along the runway. Figure 1 shows a typical OET with two hoists.
The majority of OET's have two hoists (main and auxiliary) on the trolley
for maximum operational utility. The main hoist is capable of lifting the rated
capacity of the crane and is relatively slow; the auxiliary hoist has a lower
lifting capacity (from 10 percent to 30 percent of the main hoist) but is
correspondingly faster. In normal service it is the auxiliary hoist that is used
for most lifts since the need for the rated capacity of the main hoist is
The bridge girders are always custom designed to fit the runway span.
The end trucks/end ties, trolley frame, bridge and trolley drives, and hoists may
be manufacturer's standard commercial or custom designed (built-up) assemblies,
depending on the application. In either case, many of the subassemblies and
components are standard off-the-shelf components. Usually cranes intended for
general purpose service (GPS) are in large measure manufacturers' standard
designs, those for special purpose service (SPS) or handling of ordnance are often
built-up custom designs.
Distinctive Features. The stable platform of the bridge structure of OET
cranes permits a wide range of design options to suit virtually any operational
requirement. The rated capacities of these cranes vary from 5 tons to 500 tons,
and their spans may range up to 200 feet. With the hoists centered between the
bridge girders, crane lifting capacity is never limited by stability. The high
overhead location of these cranes avoids interference with the floor activity and
equipment by lifting the loads over the floor obstructions. Walkways along the
bridge girders and the floor on the trolley structure, provide excellent
accessibility to all mechanical and electrical components.
Industry Standards. The governing industry standard for OET cranes is
CMAA Specification #70, Specifications for Top Running Bridge & Gantry Type
Multiple Girder Electric Overhead Traveling Cranes, published by the Crane
Manufacturers Association of America, Inc. This specification defines crane