Wire Ropes Under Bending and Tensile Stresses - Wire Ropes: Tension, Endurance, Reliability - page 273

Table 3.7 Safety methods preventing car plunge in passenger lifting installation
safety method elevator

hoisting rope way

mining shaft

visual and

tactual
inspection
magnetic

yes yes yes

partly yes yes

inspection
permanent

yes yes

supervision
redundant

partly

yes

bearing ropes
safety gear

partly

yes

The technical regulations for lifting installations for passenger transportation
always require that a combination of several methods be used to prevent a car from
plunging downwards. The required methods are shown in Table 3.7 . The most
important method of protection involves the detection—both visually and mag-
netically; Table 3.5 —of wire breaks, of cross-section loss as well as other failures
so that the wire rope can be replaced before a dangerous situation occurs.
Installations should be designed to facilitate the inspection of the wire ropes.
Ropeways and mine hoistings must be permanently supervised by a responsible
manager so that any change in the state of the installation will be noticed in time.
It is advisable to have redundant bearing ropes, installed as they make it much
easier to inspect the state of the wire rope. However, the main function of these
redundant ropes—if this is at all possible for the given safety factor—is that in
case of rope breakage there is another surviving rope or ropes. The probability of
one of these other ropes breaking depends on the arrangement of the ropes. The
effect on the failure probability of different arrangements of two ropes is shown for
example in Fig. 3.78 .
The beneficial effect of having redundant ropes can be demonstrated with
elevator ropes. Firstly, it is much easier to inspect the ropes with the result that
only one of the redundant bearing wire ropes breaks in about one million elevator
years. The second effect of the redundant rope is that even if one of these very rare
rope breakages occurs, as far I know there has never been another rope breakage
found. The failure probability of having a further rope breakage for multi-bearing
wire ropes in elevators therefore lies theoretically between about 3 and 5 %,
Feyrer ( 1991b ).
For installations where the cars have to overcome a very great difference in
height, special rotation-resistant ropes should be used. If the difference in height is
not too great, for many years now wire ropes with fibre cores have been regarded
as acceptable. Wire ropes, with steel cores in normal construction with relatively