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a percentage of the core mass as was required by the former German Rule for Rope
Ways BOSeil. The breaking number N and the discarding number N A of bending
cycles increase in all cases with the mass of the fibre core. The wire ropes with
cores made of natural fibres and polypropylene fibres reach nearly the same
endurance. With polyamide fibre cores, the numbers of bending cycles increased
remarkably, probably as a result of the higher durability of the polyamide.
Unlike the case of ropes with fibre cores, the number of bending cycles
decreases with an increasing clearance between the strands for ropes with a steel
core IWRC. This was discovered by Müller during a large series of tests, but was
not published until after his death by one of his employees, Greis ( 1979 ). Wolf
( 1987 ) confirmed this result with his own series of bending fatigue tests. The
reason for the loss of the number of bending cycles when there is greater strand
clearance is that the strands are free to move laterally during bending which
induces stresses and wear.
For wire ropes with steel cores enveloped with solid polymer ESWRC or wire
ropes with parallel steel core with the outer strands PWRC, the position of the
strands is well defined. As for ropes with fibre cores, a lateral movement of the
strands is practically impossible. Therefore these special steel cores achieve a
greater number of bending cycles. Figure 3.42 shows the breaking number of
bending cycles for 6 and 8 strand ropes with different steel cores in relation to
those for 8 strand ropes with independent wire rope cores. This relation is also
nearly valid for the discarding number of bending cycles.
Fig. 3.42 Number of
bending cycles N for six and
eight strand ropes with
different steel cores from
Wolf ( 1987 ) and others
2,0
1,0
8 strands
6 strands
IWRC
PWRC
EFWRC
ESWRC
0
3.2.2.6 Extended Equation for the Number of Bending Cycles
The number of bending cycles (simple bendings) is given in Eq. ( 3.49 ) as function
of the diameter related tensile force S/d 2
and of the diameter ratio D/d.
 
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