An evaluation of magnetic rope testing instrument defect detection capabilities, particularly in respect of low rotation, multi-layer rope constructions
The South African Minerals Act and Regulations- section 16.33 (1991) states that "A winding rope, balance rope or tail rope shall not be used if the breaking force at ANY POINT in the rope is less than nine tenths of the original breaking load". The aim of such legislation is obviously to enhance safety and to minimize the risk of catastrophic rope failures. The question of establishing the remaining strength accurately at any point along a rope, using non-destructive methods, has been a point of debate for many decades. ln the 1980's, mining houses in South Africa started assessing ways of hoisting greater tonnages from existing facilities as well as hoisting economical payloads from very deep shafts. One of the issues raised during these investigations was the efficacy of rope condition assessment. Further investigation indicated that no common standard existed in South Africa for rope condition assessment and that the remaining strength of discarded ropes varied greatly. This indicated the need for a South African standard for rope condition assessment. Numerous projects were funded, initially by the Chamber of Mines of South Africa, later by the Government, which addressed mine hoisting ropes in general and the condition assessment and discard criteria of ropes, particularly six stranded ropes. Same of this work culminated in the publication of the SABS 0293:1996 Code of Practice: Condition Assessment of Steel Wire Ropes on Mine Winders. This code is now applied throughout the South African mining industry by rope inspectors certificated in terms of the code by the South African Qualification and Certification Committee (SAQCC). Application of the code has led to enhanced safety and a reduction in operating costs. Parallel research has indicated that six stranded rope constructions, so commonly used in South Africa, may not be suitable for hoisting from great depths and that multi-layer, low rotation ropes may be required for this application. Rope trails, funded by the Anglo American Corporation of South Africa Limited, Gold Division, employing four, 48 mm diameter, low rotation, 15-strand fishback ropes were conducted at Vaal Reets 9# during the period December 1990 to April 1993. These ropes were non-destructively tested at monthly intervals. ln the final report relating to this rope trial (Kuun, 1993) the following was stated: "Interna/ deterioration was suggested by anomaly indications appearing where no rope outer wire fractures were visible. Since the nature of this deterioration was unknown the condition of the ropes could not be assessed accurately on the basis of the non-destructive, magnetic rope tests"
Numerous samples were cut from the ropes after discard. Three sections were destranded
to ascertain the actual condition of the ropes. It is interesting to note that no broken wires had occurred on the 9 outer strands of any of the rope samples.
Author(s): Dohm, M.