Wire Ropes Under Bending and Tensile Stresses - Wire Ropes: Tension, Endurance, Reliability - page 270

breaking point of the rope (sun blind, sliding door, etc.), the breaking number of
bending cycles is N 10 and for rope drives with safety requirements (crane, elevator,
etc.), the discarding number of bending cycles is N A 10 .
For safety reasons it is also important the rope has sufficient endurance. The
rope endurance should last long enough for the rope to be inspected several times
before the rope life comes to an end so that any dangerous rope condition can be
detected in time.
3.3.1.2 Discard Criteria
The second most important aspect is to have a rope with safe discard criteria
before a dangerous situation arises. Technical regulations list the discard criteria
required. The most important discard criterion is the number of wire breaks on
reference rope lengths. Discard numbers of wire breaks based on test results can be
calculated, see Sect. 3.4 . These numbers are more detailed and generally stricter
than those found in technical regulations.
Furthermore, the calculated discard number of wire breaks shows the degree of
safety at the time the rope is due to be discarded. If the calculated discard number
of wire breaks is smaller than B 30 \ 2, the rope drive should not be used for lifting
appliances at all and if B 30 \ 10, the rope drive should not be used in cases where
a load cannot be prevented from moving over people safely (Passenger lifting
installations, see Sect. 3.3.2 ).
It is not easy to detect discard criteria in small bending lengths. Therefore, small
zones of the rope which are bent frequently should be avoided.
It is only if there is no other subsequent severe damage to be expected when the
rope breaks (for example in a window awning or a sliding door) that the second
limiting factor ''discard criteria'' may be disregarded. In such cases, the broken
wire rope simply has to be replaced without any other consequences.
3.3.1.3 Donandt-Force
For small diameter ratios D/d, the usable tensile force can be limited by the
yielding of the rope wires. This limit is given by the Donandt force, first inves-
tigated by Schmidt ( 1965 ). To be on the safe side, the rope tensile force should be
S\S D1 ;
where S D1 is the Donandt force that at most in one percent of the cases does not
exceed the yield force.
The Donandt force S D1 can be calculated, see Sect. 3.4 .