Appendix E
rope strands.
But
even rotation resistant ropes twist due to the greater torque
applied by the outer strands over the core strands.
With rotation resistant ropes, the torque factors vary
accdtding to the number
and lay length
of
outer strands, the construction and lay length
of
the core and
the lay type (Regular
or
Lang)
of
the rope and core.
Bands are used to cover the rotational properties
of
the various ropes. The
bands
on
the graph in figure
El
display the approximate limitations
of
the four
rope types in a multi-part system. Four independent variables are used as para-
meters and are used in pairs to locate a reference point
on
the graph. They are
grouped as follows:
LlS= Length of fall
per
unit rope spacing
D/d= Average pitch diameter of block and crown sheaves
per unit rope diameter.
(For 2-fall system, with parallel falls, the ratio is the pitch diameter
of
the sheave
divided by the nominal rope diameter.)
Various constructions
of
rope shown in the graph indicate the limited condi-
tions for torsional stability with the angular displacement
of
the hoist block to
maximum
of
90 degrees. When the operating conditions for a particular installa-
tion lie above the appropriate wire rope construction band, then cabling
of
the
falls most likely will occur.
If
the operating conditions lie below any particular
band, then cabling
of
the falls will most likely not occur.
If
the operating condi-
tions for any particular installation fall within the band, cabling is unpredictable.
EXAMPLE
A 2-fall crane uses 3/4" diameter hoist rope and a block with a pitch diameter
of
18
inches. The rope spacing is assumed to
be
parallel and the height
of
lift required
is
100 feet. Based upon these conditions we would have the following values:
d=.75 inches
D=18 inches
S=l.5
feet
L=100 feet
LlS=66.7
D/d=24
Using these numbers and entering the graph, we find that it would be a border-
line condition for six-stranded, regular lay rope; however, there should be
no
trouble using one
of
the Rotation Resistant ropes.
"L"
is the length
of
fall (feet) and is measured from the centerline
of
point sheave
to the centerline
of
the sheave in the traveling block. "s" is the spacing
of
the outer
142
• Wire Rope Technical Board - Wire Rope Users Manual, Fourth Edition
To Next Page
