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 MIL-HDBK-1038
Container cranes, instead of a hook block, have a large rectangular frame
with a sheave in each corner. The reeving system is intentionally not equalized
between the four corners, so that the lifted container does not tilt even when it
is unbalanced. The frame has a head block for a rigid connection to the spreader
or a cargo beam.
4.3.6.19 Hooks. Load hooks of any capacity are available from commercial sources.
In the smaller capacities, the hooks are the manufacturers' standard steel
forgings of the single-barb design. Large capacity hooks (50 tons and higher) are
often double-barbed and specially fabricated to the customer's specifications.
Hooks must be forged by a process that ensures material ductility of at least 18
percent elongation in two inches. Carbon steel and alloy steel are the most
common materials for forged hooks. Except for the whip hoist hooks and hooks on
small packaged hoists, the hook shanks are always threaded for engagement with a
matched nut. The whip hoist hook shanks may be threaded or drilled for a pinned
connection; packaged hoist hook shanks may terminate with a ring for pinning, or
with a large groove for loose clamping between the halves of the hook block
casting. Hooks with the pinned ring mounting have no provisions for swiveling.
Hooks may be fabricated from bronze or corrosion resistant steel for use
in hazardous (explosive) or highly corrosive environments. These hooks are
usually cast and their mechanical properties are significantly different from
forged steel.
Hooks often have some means of
securing the slings or other attachments
being lifted. Spring loaded devices are
available from the hook manufacturers to
bridge the hook opening. Alternatively,
the hook tip may be drilled and a ring
installed for tying a lanyard across the
hook opening.
4.3.6.20 Shaft Seals. Seals on shafts of commercial oil lubricated assemblies,
such as gear reducers or wet clutches, should be of the spring-loaded synthetic
dual lip type whenever this option is available. The selection of oil lubricated
assemblies should give preference to those in which the shaft oil seals are not
submerged in oil. Oil seals should weep oil slightly to ensure lubrication of the
seal lip. If a seal is too tight to weep, its dry lip will damage the sealing
surfaces and begin to leak oil prematurely.
Grease seals are used for bearing lubrication on commercial pillow blocks
and built-up assemblies, such as travel trucks. These seals usually have a single
lip and are intended to allow the old grease to be easily purged outward through
them. In certain clean environments, the seals are installed with the lips turned
inward (so that grease cannot pass under them) and a separate grease outlet path
is provided. (The outlet path should be relatively short so that the purging
pressure is not so high that it might blow out the seal lips.) Purged grease
should never be permitted to mix with the adjacent lubricating oil.
For sealing shafts between pressed on components or in locations where
seal replacement is difficult, a split type of lip seal or a packing gland should
be considered. Split lip seals are seal strips that are accurately pre-cut by the
manufacturer and may include a garter spring for sealing pressure or may be press
fitted into their seats. The size selection of split seals is limited, and if
their use is expected, then the original seal size should be made to match an
available
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