| Tweet |
Custom Search
|
|
|
||
 MIL-HDBK-1038
When the hook block of a bridge crane has four or more sheaves, the lead
sheaves (those receiving the wire ropes directly from the drum) must be larger
than the adjacent sheaves to allow these ropes, as they wind/unwind on the drum,
to pass/clear the other wire ropes. The design and arrangement of the lower block
must be such that the wire ropes will not be pinched or cut in case of two-
blocking.
Reeving systems of container cranes are different from all others because
they are intentionally non-equalized. The four sheaves of the headblock frame are
supported by two-part, single reeved quarters of the reeving system with the wire
rope dead ends terminating in adjustable fittings that are used to precisely match
the lengths of the individual quarters. This arrangement maintains the level
orientation of the lifted container even when its center of gravity is off the
geometric center. In these situations, one of the reeving system quarters is
loaded more than the others. In calculating the wire rope safety factors, the
center of gravity of the lifted container is assumed to be at the geometric center
and that all wire ropes are loaded equally.
5.3.14.1 Overhauling Weight. On portal and floating cranes the lower (hook)
blocks must be sufficiently heavy to overhaul the reeving system and keep the wire
rope coming off the drum taut with an unloaded hook in the upper limit switch
position. For double reeved systems, the basic weight of the hook block is
usually adequate. However, single reeved whip hoists, with only the weight of the
hook and its wire rope fitting, must have a sufficiently heavy overhaul weight
installed above the hook to keep the wire rope taut as it comes off the drum.
The following approximation may be used to determine the minimum block
weight required to overhaul the reeving systems:
EQUATION
B = RNT/E
(12)
Where
B
=
block weight, in pounds
N
=
number of parts of line supporting the load
R
=
weight of the unbalanced wire rope, in pounds
T
=
tautness factor (a value of 2.00 is recommended to ensure proper
spooling on the drum)
E = reeving system efficiency, see paragraph 5.3.13.1.
5.3.14.2 Fleet Angles. Fleet angle is defined as the angle that the wire rope, at
its point of tangency, forms with the groove of the sheave or the drum. (The
helix angle of the wire rope grooves is very small and may be omitted in
calculating the fleet angle.) The angle of the sheave groove and the depth of the
drum groove, as prescribed in the preceding paragraphs, prevent rubbing contact
between wire rope and the outer edges of the grooves if the fleet angles are
limited to 3.00 degrees. This fleet angle may be exceeded in some cases, but only
with NCC approval. On older cranes, a small amount of cusp edge shaving on drums
is sometimes evident in locations of largest fleet angles, but this worn-in
condition is not considered damaging to the wire rope.
Reeving on bridge cranes is arranged so the fleet angles at the high and
low positions of the hook block are equal but opposite. On portal and floating
155
|
 
|
|
|
||
