MIL-HDBK-1038
cranes, the length of drum grooving and the proximity of the deflector sheaves on
the A-frame often make it impossible to limit fleet angles to 3.00 degrees. In
those situations, the deflector (fleeting) sheaves are installed so that they can
slide on their pins when the fleet angle is sufficient to cause a side force from
the wire rope large enough to slide them on their pins. The wire rope should bend
at least 45 degrees to develop the lateral force required to slide the fleeting
sheave without itself developing an excessive fleet angle. Fleeting sheaves must
be designed with great care to function properly and require frequent lubrication.
It is best to avoid the need for them.
5.3.15
Hook Blocks. The hook (lower) hoist block weight must be sufficient to
overhaul the reeving systems and accelerate all parts of the wire rope with the
block in any position. Except for wire rope slots and drain holes, the block
design must fully enclose the sheaves and wire ropes. The internal arrangement
and clearances must prevent the wire ropes from leaving the sheave grooves under
any conditions. Blocks on hoists that have capacities greater than 5 tons must
have a steel trunnion for mounting the load hook on a thrust bearing. The
trunnion must be a separate component from the sheave pin and must swivel in the
block side plates. The maximum bearing pressure of the trunnion on the side plate
bores should not exceed 6000 psi, based on the projected area. The ends of the
trunnion must be retained with
keeper bars and their fasteners must be lock wired. The blocks must be designed
to permit easy disassembly without unreeving. Standard commercial hook blocks may
be used at their published ratings on custom designed cranes.
Whip hoist hook blocks (headache or overhaul balls) are usually standard
commercial items. They include a fitting with an integral bearing for hook
rotation. Standard commercial whip hoist blocks may be used at their published
ratings.
5.3.16
Hooks. Standard commercial hooks are available in all capacities. They
are normally carbon or alloy steel forgings with a minimum elongation of
18 percent in 2.00 inches. Hooks are also available in other materials, such as
corrosion resistant steel or cast bronze; however, their use must be approved by
NCC. The hook shanks may be obtained threaded and otherwise machined or in as
forged (unmachined) condition. The shank and nut threads are required to have a
Class 2 fit.
Standard commercial hooks are rated on the basis of maximum working load
(at which yielding starts) and ultimate load (50 percent of the load at which the
hook deforms to the point where the attached sling would slip off). (These
ratings are confirmed by manufacturers' periodic destructive testing.) Except
where expressly prohibited, standard commercial hooks may be used at their
published ratings; however, the shank and the mating nut threads must be analyzed
for the minimum required safety factors.
Custom forged load hooks are required to be analyzed for minimum design
factors of 4.00 and 5.00, based on material yield and ultimate strengths,
respectively. Double barbed (sister) hooks should be analyzed with two maximum
loads (50 percent of rated load each) applied 30 degrees from the shank
centerline. The stress increasing effect of the hook curvature may be omitted.
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