Wire Ropes Under Bending and Tensile Stresses - Wire Ropes: Tension, Endurance, Reliability - page 260

Fig. 3.70 Number of wire
breaks B 360 on a bending
length l = 8 9 2 9 22.5
d = 360 d, Feyrer ( 1983b )

500

300

200

100

50

30

20

Warrington 8 19 − SFC

30kN

10

ordinary lay, bright
d = 16 mm, R o = 1570 N/mm 2
D/d = 25, r = 0.53 d

D

5

steel, hardened, rope lubricated
before and during test, mineral oil

3

2

1

10 5 10 5

10 5

2 − 3 −

0

number of bending cycles N

takes the load more or less completely. Different critical rope lengths occur
depending on the kind of load and the construction of the wire rope, Woernle
( 1929 ), Herbst ( 1934 ), Costello ( 1997 ) and Raoof ( 1992 ). Based on these findings,
the reference rope lengths where the number of wire breaks is important have been
standardised. For stranded ropes, the reference lengths are normally L = 30 d and
6 d and for spiral ropes L = 500 d, 200 d and 6 d.
Figure 3.69 shows the numbers of wire breaks B 6max and B 60 in relation to the rope
reference length L = 6 d and the bending length l = 2 9 30 d. The numbers of wire
breaks B 6max are the maximum number of wire breaks to be found on the rope
bending length l = 2 9 30 d or l = 60 d. For a big diameter ratio D/d = 63, the wire
breaks occur at very early stage, i.e. 10 or 20 % of the rope endurance (relative
number of bending cycles N 0 = 10 or 20 %). In contrast, for a small diameter ratio
such as D/d = 10, the first wire breaks only occur just before the end of the rope life.
For greater bending lengths l, the increase in the number of wire breaks
B produces a smoother curve. Such a curve is given in Fig. 3.70 for the growth of
the total number of wire breaks on a bending length l = 360 d. Up to about the
total number of wire breaks B 360 = 15 ( wh ereby, on the reference length L = 30
d, the mean number of wire breaks is B 30 ¼ 1 : 25) the number of wire breaks
increase exponentially with the number

of bending cycles

B ¼ a 0 e a 1 N :

ð 3 : 75 Þ

This function of the increase in the number of wire breaks has also been found
during the inspection of mining shaft hoists by Daeves and Linz ( 1941 ) and Ulrich
( 1980 ) and in ropeways by Beck ( 1992 ). The fact that this corresponds to ( 3.75 )is
possible as for safety reasons the wire ropes in these installation s h ave to be replaced
at an early stage with a mean number of wire breaks of about N 30 ¼ 1to2.