Page 63 - Failure Analysis Case Studies II

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0 the construction is considered generally unsuitable for operation at pulley and winch diameter
ratios less than about 35 : 1;
0 the construction is inherently expensive to manufacture because of the number of separate
operations needed to build up the layers, and the high capital cost of the large scale equipment
needed;
0 this construction is mt tolerant to imposed twist, which is quite different to being torque balanced,
as discussed below.

3.2, Six strand

This is the wire rope construction that is most widely used for general engineering purposes. Six
(or eight strand) ropes (Fig. l(b)) are manufactured in essentially two stages: wires are twisted
together to form the strands (which have an equal lay length to allow different wire diameters to
nest together), then six (or eight) strands are twisted together over a core to make the rope. Two
main categories of core can be used: a fibre core, as in general engineering rope (easier to splice
and cheaper to make), and wire rope core which provides higher strength, and higher axial, as well
as transverse, stiffness, the latter being especially important in resisting crushing when wound at
high load onto multi-layer winches. Another major feature of the construction of six strand ropes
is the helical sense of the strands in the rope relative to the wires in the strand: where these are
different the outer wires appear to align with the rope axis and the construction is termed ordinary
(or regular) lay; where the helical sense is opposite, along the strand crowns the wires are at a
greater angle to the rope axis and the construction is termed Lang’s lay. For offshore use in
connection with moorings, when a stranded rope is used, ordinary lay constructions are used
almost exclusively. The relevant characteristics of such ropes are as follows:

0 the rope has a high degree of damage tolerance and can be used at high loads on multi-layer
winches;
0 for any given ultimate breaking load this construction tends to be the cheapest of all;
0 because of the problems of sheathing, and thinner wires, these ropes are not generally selected
for very long term exposure to seawater;
0 six strand ropes are not torque balanced (though that is possible-three strand ropes are manu-
factured for oceanographic use at very high tensions in very deep water where a high degree of
torque balance is essential);
0 ordinary lay ropes have better torque balance than Lang’s lay ropes;
0 this construction is tolerant to torsion, though perhaps not to the same degree as Lang’s lay
ropes.

3.3. Multi-strand
Multi-strand ropes (Fig. l(c)) have two or more layers of strands, the sense and lay of which are
selected to achieve a maximum degree of torque balance. The strands of these ropes are sometimes
compacted (either by drawing or rolling) to improve the outer profile of the rope, and the strand
to strand contact stresses. These ropes are used offshore for applications requiring flexibility and
torque balance, such as single fall crane ropes or diving bell hoist ropes. The characteristics of
multi-strand ropes are as follows: