Wire Ropes Under Bending and Tensile Stresses - Wire Ropes: Tension, Endurance, Reliability - page 192

The running rope normally used today is one with parallel lay strands as there
are no wires crossing. Nevertheless, in these ropes there is still a form of secondary
bending stress, i.e. the bending stress of the wires due to the bending of the strands
in wire ropes with independent steel cores. In these ropes, the outer strands are
supported by the inner strands at single points as shown for wires in Fig. 3.11 .This
bending stress arising from the strands bending also could be called tertiary
bending stress.
Wolf ( 1987 ) found that the endurance of wire ropes increases with the number
of supporting points [smaller distance between these points, Apel ( 1981 )] for the
strands of the independent steel wire core IWRC. This is beside the pressure due to
the tertiary bending stress. Of course, this tertiary bending stress can be avoided
by using fibre cores, steel cores with parallel-closed ropes PWRC or steel cores
enveloped with solid polymer ESWRC. Running ropes with steel cores such as
PWRC or ESWRC have a much higher endurance than wire ropes with inde-
pendent wire rope cores IWRC.

3.1.5 Sum of the Stresses

The secondary and the tertiary bending stresses can be avoided with right rope
construction. From the other stresses the tensile stress, the bending stress, the
secondary tensile stress and the rope ovalisation stress sum up to a longitudinal
stress in every fibre of the rope wires. Together with the pressure and the small
torsion stress this total longitudinal stress has the main influence to the endurance
of running wire ropes. It still has to be considered just what the maximum and
minimum total longitudinal stress should be over the whole course.
In Fig. 3.12 , Wiek ( 1973 ) has shown the typical course of the longitudinal
stress in a wire of a rope running over a sheave. For the rope's endurance, the
important stresses are the stress range

2 r a ¼ r 2 þ r 3 þ r 4 ð 3 : 12 Þ

Fig. 3.12 Course of the
longitudinal wire stress in
rope running over a sheave,
Wiek ( 1973 )

3

2

4

1

route of rope