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Commercial Underhung Hoists.  Underhung hoists may be integral with the
trolleys (hoist/trolley units) or separate hook, clevis, or lug suspended.  (For
fixed installations, hoist designs are available for ceiling, wall, or base/deck
mounting.)  All underhung hoists and hoist/trolley units are standard commercial
items which are mass produced by established manufacturers of the crane industry.
For many applications these hoists and hoist/trolley units are the only practical
choice; therefore, the specified features and options must be limited to those
offered by the manufacturers.  The only feasible modifications are the replacement
of the wire rope, hook block, and the suspension hardware.  The hoists may be
electrically powered or manually operated by means of a hanging hand chain.  The
integral trolleys may be electrically powered, manually operated through a hand
chain and gears, or plain (hand pulled on the load hook or suspended load).  The
reeving may utilize wire rope or welded link load chain.
All underhung hoists are equipped with an electro-mechanical disc brake
and optionally, a mechanical load brake.  The mechanical load brake is a device
which prevents the load from lowering unless the drive motor is energized to turn
in the lowering direction.  When the mechanical load brake is not provided, the
entire hoist assembly is arranged in the shape of a long cylinder with the wire
rope drum in the center, motor on one end, and the planetary gear reducer with the
disc brake on the other.  When the mechanical load brake is provided, it is
installed on the wire rope drum shaft and the other hoist components are arranged
in line on a parallel axis.  The hoists may be single or double reeved.  With
single part reeving, the hoist must be suspended with the wire rope drum in line
with its supporting girder because the hook block shifts its position horizontally
during vertical travel.  With double reeving, its parts being mirror images of
each other and thus providing balance and true vertical travel of the hook block,
the hoist can be suspended in line or crosswise under its support girder.  Chain
hoists, although "single reeved", are arranged so that the hook block does not
shift horizontally during hoisting and can be suspended in either orientation.
Underhung hoists and their trolleys are intended for operation on flanged
beams (structural shapes or patented track) and can negotiate curves.  ASTM HST-1M
is the industry standard for electric chain hoists and hoist/trolley units, and
ASTM HST-2M is the industry standard for hand operated chain hoists and
hoist/trolley units.  ASTM HST-4M is the industry standard for electric wire rope
hoists and hoist/trolley units.  ASTM HST-5M and HST-6M are the industry standards
for pneumatic chain hoists and hoist/trolley units, and pneumatic wire rope hoists
and hoist/trolley units, respectively.  These items are usually specified for
hazardous (explosive) environments and require special features to protect against
sparking; they should always be procured, or have the procurement specifications
approved, by the NCC.
Travel Drives.  Travel drives on all cranes (except mobile, which are of
the automotive type) are either of the center drive type (with outward extending
drive shafts to wheels on both rails) or individual type (driving the wheel or
wheels on one rail directly or through a short drive shaft).  The center drive
types are mechanically interlocked; the individual types require electrical
synchronization of their drive motor speeds when the cranes operate on straight
rails; and electrical de-coupling to permit them to adjust their speeds as
required when they travel through curves.


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