Bridge Travel Drives (On Walkways). On cranes with twin overrunning
bridge girders, the bridge travel drive is located on the full-length walkway of
the drive girder. The drive may be any of several arrangements, but the most
common are those identified in CMAA #70 as types A-1, A-4, and A-5. All bridge
drives are designed to accommodate any relative/rocking motion between the walkway
and the end trucks.
The A-1 drive has a parallel shaft gear reducer positioned at the center
of the drive girder, with the output shaft coupled to a squaring shaft at both
ends. The squaring shafts are coupled directly to the axles of the drive wheels
mounted in the travel trucks. The input shaft of the gear reducer is coupled to
the electric drive motor. A shoe brake is installed either on the outboard end of
the drive motor or of the gear reducer, or on a brake wheel coupling between the
motor and the gear reducer. On small capacity cranes, the drive motor may include
an integral disc brake.
The A-5 drive is identical to the A-1 drive, except that intermediate
gear reducers are added between the center gear reducer and the drive wheel. A-5
drives are appropriate for cranes with long bridge spans when the single center
drive is desired but where the length of large diameter shafts becomes a
significant cost consideration.
The A-4 drive is comprised of two mechanically independent drives one
at each end of the drive girder. Each independent drive has an arrangement
similar to the A-1 drive, except that the gear reducer output shaft is coupled to
only one short drive shaft. Since there is no mechanical connection between their
two halves, the A-4 drives require electrical synchronization of the drive motor
speeds. These drives are frequently used on cranes with long bridge spans when
there is no requirement for the center drive.
Bridge Travel Drives (Suspended). On cranes with underrunning bridge
designs (that have no walkways) the bridge travel drive may be a built-up center
drive type, similar to the A-1 drive, or a set of standard commercial drive heads.
The center drive is mounted on the drive girder of a twin girder
underrunning bridge or the outrigger beam of a single girder underrunning bridge.
The center drive differs from the A-1 drive only in its drive wheels, which may be
in the form of spring loaded elastomeric traction tires pressing on the underside
of the runway beams or flanged steel wheels running on the runway beam flanges.
The flanges of the steel wheels have gear teeth cut into their circumference which
mate with the drive pinions. The drive pinions are configured to engage both
geared wheel flanges (on both sides of the runway beam web). The bridge end
trucks for these drives are often built-up custom designs.
Drive heads are standard commercial units, each equipped with its own
electric motor, a speed reducing gear set, and a pair of wheels (on opposite sides
of the runway beam web) driven in the same manner as for the center drive.
For some applications, it is also practical to use drive heads of the
tractor type. Tractor drive heads do not carry any crane weight and must depend
on a spring applied load to develop the drive traction. These drives are arranged