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 MIL-HDBK-1038
On container cranes there are several trolley drives available. The
trolley may be driven by means of an axle with two wheels, individual drives (as
on newer cranes described in paragraph 4.3.2.3 above) on two wheels, or by a wire
rope hoist reeved to pull the trolley in either direction along the boom and main
beams. There is no clear advantage with any of these drive options; however, the
hoist type may produce some degree of jerking as the slack of the loose wire ropes
is taken up at the start of the motion.
4.3.4
Portal and Floating Crane Rotate Drives. Rotate drives are normally
installed on the machinery deck of the rotating upperworks and are unique in that
their gear reducers must have vertical (downward projecting) output shafts. These
gear reducers include an internal drywell around the output shaft to avoid
submerging the shaft bearing seal under oil, which might necessitate periodic seal
replacements involving major disassembly. However, the presence of the drywell
requires that the lower output shaft bearing be lubricated separately with grease.
The last gear set, which in most cases is located above the lubricating oil level,
must be lubricated by means of an electrically driven oil pump discharging oil
directly on the gear teeth. The output shaft is normally coupled to an
intermediate shaft with a pinion which engages the bull gear teeth. The gear
reducer input shaft, when horizontal, allows the normal installation of a shoe
brake. When the gear reducer input shaft is vertical, the standard shoe brakes
must be modified to prevent their linkage from sagging and applying the brake
shoes unevenly to the brake drum. Electric motors should also be ordered for
vertical operating orientation.
Planetary gear reducers and cycloidal speed reducers (both with vertical
input shafts) driven by electric or hydraulic motors, have been used successfully
on portal cranes of 25-ton capacity. These units include disc brakes designed for
mounting on vertical shafts.
Good contact pattern between the pinion and bull gear teeth is critical
to the proper and long-term operation of the rotate drive. Therefore, the rotate
drive installation must include provisions for accurate adjustment of the gear
tooth mesh. The intermediate pinion shaft, connected through a full-flexible
coupling, provides the means for such adjustment. The pinion end, which is
supported by a self-aligning bearing, can be brought into accurate gear tooth
engagement by mounting the bearing housing on the structural extension of the
machinery deck. The bearing housing can then be permanently secured in that
position. Alternatively, the output shaft may be supported by a rigid tubular
extension from the base of the gear reducer and the entire rotate drive assembly
is then positioned and permanently secured in the desired position. With either
arrangement, the pinion should have its teeth crowned to compensate for likely
non-parallel condition of the two gear axes due to rocking of the upperworks under
varying loading conditions. Lack of crowning in this situation will cause severe
edge loading of the gear teeth and premature failure.
4.3.4.1
Special Considerations. Wind load on the boom is a major, often the
dominant, force on the rotate drive. On floating cranes, the compound list/trim
angle determines the slope against which the rotate drive must advance the
upperworks with rated load at maximum radius or no load and boom at minimum
radius. It is also important to consider the overhauling (backdriving) condition
which can be imposed on the rotate drive by the wind load on the boom. A
dangerous "runaway" situation, where the rotation starts when the brake is
released or is faster than intended by the operator, may be created if the rotate
drive train is easily backdriven.
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