Wire Ropes Under Tensile Load - Wire Ropes: Tension, Endurance, Reliability - page 141

Fig. 2.35 Fluctuating tensile

S

force

S a

S a

S upper

S m

S lower

O t

that time was that there was no convincing regression formula. To overcome the
first problem OIPEEC ( 1991 ) passed OIPEEC-Recommendation No. 7 laying
down that the specifications for wire ropes and test conditions should at least be
described. The solution for the other problem will be described in the next chapter.

2.6.2 Evaluating Methods

2.6.2.1 Goodman Line

Wire rope endurance under fluctuating tension depends on the amplitude force S a
and the middle force S m or the lower force S lower . These forces are defined for a
sinus course in Fig. 2.35 . For the evaluation of a group of wire ropes with varying
rope diameters, all these forces have to be replaced by the wire rope stresses or by
the specific forces S/d 2 .
The first proposals to evaluate the results of tension-tension fatigue tests with
wire ropes came from Yeung and Walton ( 1985 ) and at the same time from
Matsukawa and others ( 1985 ). For spiral ropes, they proposed to combine the force
range 2S a and the middle force S m to produce an equivalent force S q on the basis of
the Goodman line. According to their proposal, the equivalent force is

F
F þ S a S m

F
F S lower

S q ¼ 2 S a or S q ¼ 2 S a ð 2 : 101 Þ

F is the wire rope breaking force, for this Yeung and Walton, and Matsukawa and
others differ in their definitions. The lower force is S lower = S m - S a .
The use of the equivalent force seems very attractive, because this means that the
number of variables is reduced. The endurance of a wire rope can be described with
the single variable S q by the very simple equation for the number of the load cycles

N ¼ a S q :

This equation has been used to evaluate the results of different tension-tension
fatigue tests and it discloses a profound difficulty. For the same equivalent force,
the number of load cycles is much smaller with a small lower force than with large