Wire Ropes Under Bending and Tensile Stresses - Wire Ropes: Tension, Endurance, Reliability - page 213

Fig. 3.29 Reverse bending

U

a

U

h

cycles Z, the number of simple rope bending cycles is . In this

case the rope bending length is

ð 3 : 39 Þ

On both sides of this bending length l = h - u with the number of bending
cycles there are two bending zones of the length u with the

number of bending cycles . Of course these bending zones

have no influence on the endurance of the rope.
Reverse bending cycles without additional simple bending cycles on the same
bending length are only possible on a relatively small rope bending length. Fig-
ure 3.29 makes this clear. If the rope stroke is just h = u + a, then on a bending
length l = u for the machine cycle Z there are two reverse bending cycles
N rev = 2Z. On other zones, the rope is only bent by simple bending cycles.
To evaluate the number of reverse bending cycles for each bending length, it is
practical to use a sheave arrangement as shown for example in Fig. 3.30 . Then the
most stressed rope zone is bent for one machine cycle with several reverse bending
cycles and only two simple bending cycles. This can be seen in Fig. 3.30 where the
bending sequence for the sheave arrangement is shown. The number of reverse
bending cycles can be separated from the machine cycles Z with the help of the
Palmgren-Miner-Rule. For the sheave arrangement in Fig. 3.30 the number of
reverse bending cycles is

6 Z
1 2 Z
N sim

N rev ¼ : ð 3 : 40 Þ

To evaluate the number of reverse bending cycles N rev (up to rope breakage or
discarding) bending tests have to be carried out with the standard test sheave
arrangement in Fig. 3.27 to find N sim and with a test sheave arrangement as shown
in Fig. 3.30 to find Z and then with ( 3.40 ) the number of reverse bendings N rev .