Depth limitations in the use of triangular strand ropes for mine hoisting+

Mine hoisting plant in South Africa are predominantly clutched, double drum winders having one or two Lang's lay triangular strand ropes per drum for hoisting in vertical shafts. Over the last hundred years the trend has been for ever increasing shaft depths with single lift shafts currently being sunk to 3200 m. The preferred construction for hoisting ropes has been the single layer triangular strand design as its properties include very good wear and abrasion resistance, ability to tolerate high crushing forces arising from multi-layer coiling on the drums, robustness and relatively low costs when compared to more sophisticated constructions. The rope design also allows for very efficient conversion of wire strength to rope strength. It is generally agreed that the performance versus cost ratio of these ropes is acceptable and a large amount of experience in the their manufacture, operation and inspection exists in the South African industry. The down side of the single layer Lang's lay construction is that it generates high torque under tensile load which results in considerable changes to the as-manufactured lay length of the rope. The extent of shortening of lay length at the splice end and lengthening at the drum end increases with depth and is also compounded by maintenance practices. It is accepted that at a certain depth these changes in lay length will eventually be so great as to prejudice the performance and safety of such ropes in service. Using previously developed and verified modelling procedures this paper investigates the depth limits of triangular strand ropes as a function of both manufacturing and operational parameters. Analysis supported by site observations has shown that seemingly small changes in rope design and the manner in which ropes are maintained or handled in service can have significant advantages in terms of the maximum depth of operation. The analysis indicates that, using current available technology, torsional deformations in triangular strand hoisting ropes can be kept within acceptable limits for winding depths of at least 3200 m.