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MIL-HDBK-1038
Older cranes with roller path and king pin rotate mechanisms are normally
equipped with a single rotate drive.  Its pinion-bull gear forces affect the
sizing of the king pin bushing.  Newer cranes with the rotate bearing have two
rotate drives, one on each side of the machinery deck.  Depending on their
relative positions, the pinion-rotate gear separating forces can be neutralized to
a large extent.  The additional advantage of two independent drives is the
capability of limited rotate operation with the loss of one drive.
4.3.5
Micro-Drives.  Sustained slow speed operation of a hoist or travel motion
can be provided by a micro-drive.  This drive is comprised of an electric motor,
gear reducer, brake (on the motor shaft), and a clutching mechanism to the motor
shaft of the main drive.  The clutching mechanism is either a toothed coupling
(two opposed discs with interlocking radial teeth) or a friction clutch.  The
clutching mechanism is electrically operated and sized for continuous energization
in the disengaged position.  The toothed coupling additionally requires a sensor
to confirm its engagement (via axial position) prior to release of the main drive
brake.  When engaged, the micro-drive delivers the same torque to the main drive
train as would the main motor (which at this time acts merely as a connecting
shaft to the main drive gear reducer) but at much slower speed.
On main drives with a single brake, the micro-drive brake substitutes for
the main drive brake, which is deactivated when the
micro-drive is engaged.  On main drives equipped with two brakes (usually on a
hoist), the micro-drive brake substitutes for the primary (instantaneous) brake;
the time delayed brake is not deactivated when the micro-drive is engaged, and its
function is not changed.
Micro-drives are usually built-up and added-on assemblies.  They
complicate the electrical control system and expand the drive train.  They should
be used only when normal production process includes prolonged slow speed
operation or precise positioning.  Micro-drives are also called inching or pony
drives.
4.3.6
Assemblies and Components.  Mechanical assemblies and individual
components include those which are always standard commercial off-the-shelf items,
those which are always custom made, and those which may be either.
4.3.6.1
Definition of Standard Commercial Assemblies and Items.  Standard
commercial assemblies and items are defined as products which are advertised for
sale in current commercial literature and are being sold in substantial quantities
on the open market in the course of normal business operations.  (Nominal
quantities, as normally associated with models, samples, prototypes, or
experimental units are not within the limits of this definition.)
The choice of standard commercial assemblies and items should be based on
the particular manufacturer's published ratings, selection method, or prorating
(if required for the specific application).  Industry standards that
govern the design or quality of commercial assemblies and items should always be
invoked as the acceptance criteria.
4.3.6.2
Gear Reducers.  Gear reducers (also called enclosed gearing) are sets of
mating gears within housings which totally enclose them, provide rigid structural
support, and bathe them with lubricating oil.  The input (high speed) gear set
should always be some form of angled gear tooth form helical (including double
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