Rope condition assessment and discard criteria

Various SIMRAC projects have been carried out over the past seven years to verify and refine the discard criteria of SABS0293: The South African Bureau of Standards Code of Practice for the Condition Assessment of Steel Wire Ropes on Mine Winders.
 
The objectives of the investigation described in this report (GAP502) were the same as those of the previous SIMRAC projects, i.e. to verify, and possibly refine, the discard criteria of SABS0293. For this investigation special emphasis had to be placed on non-spin ropes, because the discard criteria for non-spin ropes were not yet well defined in SABS0293, and the possible use of non-spin ropes in future deep shaft operations required the situation to be addressed. The majority of (drum) winder ropes are discarded because of broken wires. The discard criteria for broken wires were therefore the focus of this investigation.
 
The samples collected and tested from discarded ropes included samples with broken wires, corrosion, substantial plastic deformation, and ropes that had sustained abnormal damage. However, the most important finding of this report ensued from a thorough analysis of "cut-wire" tests.
 
Very few rope samples from discarded non-spin ropes were, or could be, obtained for the establishment and verification of the discard criteria for non-spin ropes. The effects of broken wires in non-spin ropes were therefore simulated by testing laboratory prepared specimens with selected wires cut in the outer and inner strands. These tests were a continuation of work carried in two previous SIMRAC projects: GAP324 and GAP439. Although not part of the scope of GAP502 (this investigation), the author of this report decided that all cut-wire tests on non-spin ropes of the previous investigations had to be evaluated together with those of GAP502 to be able to establish and propose proper discard criteria for broken wires in non-spin ropes.
The discard criteria for broken wires in SABS0293 were based on a 10% reduction in strength of a rope. An expectation was therefore created that by complying with these discard criteria, a rope would not fail as long as the rope loads did not exceed 90% of the new rope breaking strength.
 
However, it is shown in this report that rope strands with "allowable" broken wires could fail at loads considerably lower than 90% of new rope breaking strengths. An example: A rope sample from a discarded fishback rope that operated on a drum winder had four broken wires in one outer rope strand. Although the broken wires only made up 1,5% of the total cross-sectional steel area of the rope, that rope strand failed at a load that was 16% lower than the new rope strength, while the remainder of the rope broke at more than 95% of the new rope strength. A thorough analysis of the results of the cut-wire tests of GAP439 further substantiated that weakened rope strands fail at rope loads far lower than originally anticipated.
A theoretical failure analysis of stranded ropes was then performed to explain how and why weakened strands could fail at relatively low rope loads. The failure analysis was not part of the scope of GAP502 but was considered essential, otherwise the apparent anomalies in the results of cut-wire tests would have remained unexplained.
 
The failure analysis further showed that the load at which a weakened strand in a rope specimen would fail depended on the length of the rope specimen tested. It was also shown that the length of a strand affected by broken or cut wires was the most probable reason for relatively large scatter in breaking strengths observed in the past for identical specimens.
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