Wire Ropes, Elements and Definitions - Wire Ropes: Tension, Endurance, Reliability - page 45

Fig. 1.37 Strand helix
outside the rope

d s,m

d H

U W ¼ d S ; m d

d S ; H d :

ð 1 : 6d Þ

d S,H is the outside diameter of the loose wire helix taken from the strand or the
spiral rope, d S,m is the measured strand diameter and d is the wire diameter.

1.6 The Geometry of Wire Ropes

The geometry of wire rope can be demonstrated in principle by the geometry of a
strand. This is true in particular for spiral ropes. For stranded ropes, a strand is to
be considered as a wire in a strand. However for stranded ropes, the core, and here
especially the fibre core, gives additional problems.

1.6.1 Round Strand with Round Wires

The geometry of both the strand and the wire rope has a great effect on their
properties. In the strand, the clearance between the wires of a wire layer should not
be too large, but on the other hand there should be no overlapping (or negative
clearance). If the clearance is too large, the position of the wires is undefined. This
is especially true if the clearance in the inner wire layer of a parallel lay rope is too
large as that leads to an irregular structure of the strand with unequal stresses of the
outer wires. In some cases, as time passes wire loops may even occur on the
outside. In the opposite case where wires overlap (negative clearance), an arching
of the wire layer occurs with high secondary tensile stresses when the strand or the
rope is bent. Therefore, great care must be taken to dimension wire ropes as well as
possible.
There is a great deal of literature already available on strand geometry. All the
calculations presented here—with the exception of winding a tape by bending
only—have the same following presuppositions

• The cross-section of the wire remains unchanged and perpendicular to the helix
for the wire turning point (normally the centre of the wire cross-section) when
the straight wire is wound to the helix of the wire.