MIL-HDBK-1038
helical and herringbone), spiral bevel, or worm. The output (low speed) gear set
may be of the straight gear tooth form spur or plain bevel. Gear shafts may be
arranged in any orientation with respect to each other and either horizontally or
vertically. There is a wide variety of options for shaft connections and housing
mountings in any position. The input and output shaft lengths and other details
can be customized by the gear reducer manufacturer per the crane designer's
request. In such situations it is important to ensure that the magnitude of the
"overhung" moment on the output shaft and the associated bearing capacity/life are
satisfactory. For vertical shaft designs, oil pumps are available to ensure
adequate lubrication of the last gear set above the drywell. Since such designs
are most common on rotate drives, which have relatively little motion but heavy
starting loads, electric pumps are used to discharge oil on the last gear set as
soon as the rotate drive is energized.
Base/foot mounted housings normally have four or six feet with holes for
through bolts. The foot-foundation interface requires a close fit, which is
obtained by accurate shimming. (The shimming also serves to align the shafts with
the adjoining components.) Base/flange mounted gear reducers mate with a machined
surface which ensures a solid seating on the foundation or an adaptor. Flange
connections on the output end of gear reducers should be made with through bolts.
A commonly available option on gear reducers is an input shaft and flange design
for connection to standard industry flanges of electric motors and disc brakes.
The flange connections are normally made with screws and threaded holes in one of
the flanges. Shaft mounted gear reducers are often used on the travel drives of
various cranes. They have a hollow output shaft for mating with the driven axle
or squaring shaft. The shaft-to-shaft connection is made with a light press fit
and key, a spline with a sliding fit, or a compression sleeve. With the last two
connections, the gear reducers (and all other attached drive components) are
easily removable for servicing. The housing is mounted by means of a torque arm
which is subjected to the full torque of the drive. Since the torque arm reaction
is reversed whenever the drive direction is changed, the torque arm should be a
sturdy structural-type of a link. The torque arm connection to the foundation
should be made with the pin in double shear. (Threaded rods are prohibited for
this purpose.) In lieu of the torque arm, these gear reducers may be base
mounted; however, such mounting must be done very accurately to preclude the
introduction of any built-in stresses due to misalignment.
Gear reducer housings are of intricate shapes, with numerous stiffeners
and provisions for mounting and disassembly. The smaller the size, the more
likely that the housing will be a casting (iron or aluminum); and the larger the
size, the more likely that it will be a steel casting or weldment. For built-up
hoists and rotate drives of portal and floating cranes, the housings must be steel
or ductile cast iron. On other drives, the housings must be gray cast iron or
cast aluminum. Some of the smaller size gear reducers have blind tapped mounting
holes in their walls. This mounting method does not provide a visual confirmation
of adequate thread engagement and should not be used without approval of NCC.
4.3.6.3
Planetary Gear and Cycloidal Speed Reducers. These speed reducing
devices are more compact than the gear reducers for comparable reduction ratios.
Their internal components are totally enclosed and otherwise similar to gear
reducers. Planetary gear speed reducers are normally used on packaged underhung
hoists. The cycloidal speed reducers have been used successfully on rotate
drives. These devices may be used only with the approval of NCC.
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