Wire Ropes Under Bending and Tensile Stresses - Wire Ropes: Tension, Endurance, Reliability - page 211

3.2 Rope Bending Tests

When wire ropes run over sheaves, the wires are loaded by constant and fluctuating
stresses and pressure, Sect. 3.1 . However, it is not possible to derive the endurance
of a particular wire rope even if all these stresses and the wire endurance under these
stresses are known. This is the case partly because the relative motion causes the
wires to become worn and also because of irregularities which result in neigh-
bouring wires being loaded by very different tensile stresses. Therefore, wire ropes
running over sheaves always have a finite life and the number of bending cycles can
be only evaluated by wire rope bending fatigue tests.

3.2.1 Bending-Fatigue-Machines, Test Procedures

3.2.1.1 Test Principle

The predominant test principle used today for bending-fatigue machines is shown
in Fig. 3.27 . The wire rope to be tested is reeved in a loop over the traction sheave
and the test sheave. The traction sheave that moves the rope has a much bigger
diameter than the test sheave so that it is always the rope piece running over the
test sheave that will break. Therefore the distance between the sheaves is larger
than the rope stroke so that the rope test piece does not move over the traction
sheave.

Fig. 3.27 Arrangement of
sheaves for testing the
number of simple bending
cycles

traction
sheave

rope

test
sheave