Open Cut » Maintenance & Equipment
A detailed analysis conducted by MTI in 2002 on dragline maintenance costs identified dragline ropes, including suspension, IBS, drag, hoist and dump ropes, to be a major driver of maintenance costs. These account for 10-15% of the total maintenance costs, and amounts to ~$300K per annum per machine. The project was to explore the possibilities of reducing the overall cost of ropes for the operators.
The project as outlined in the original proposal was not completed as the research team and the industry monitors were of the opinion that due to the rope failure data limitations, it was unlikely that new ground would be achieved. This report documents the study up to that stage, including material collected during the project and the conclusions that could be derived from the completed work. Despite this limitation the understanding of key factors contributing to the performance of ropes has been significantly advanced and useful conclusions have been made.
The average/expected service life for dump ropes is only about 10 days, for drag ropes about 5 weeks, hoist ropes 10 weeks and suspension and IBS ropes 4-10 years. Significant differences were also found in the performance of ropes in different types of draglines, with the most striking differences being the hoist and suspension rope life between M8050 and BE1370 draglines. The average M8050 hoist and suspension rope life is only about 50% of the corresponding BE1370 rope life. The average hoist rope life for both types of draglines has reduced by about 12-13% since the mid-eighties. Despite the significant expenditure on ropes and the large scatter in their life, there has been very little effort made either to understand the main reasons for short rope life or to develop appropriate improvement strategies.
The following are the conclusions and recommendations that can be made from the completed work.
Dragline rope data for the major part of the Australian fleet is being collected by OneSteel, which is also the main supplier of dragline ropes in Australia. The main purpose of collecting this data may be to determine the rope life information and other data relevant for the commercial agreements between the parties. The data mainly provide average life of last five ropes and the reasons for discarding the rope. However, more detailed information is needed if this data is to be used for technical and research purposes. For example, in the case of drag and hoist ropes it is very important to understand the wear patterns in the ropes and at which locations the ropes fail that lead to them being discarded. Different parts of the rope interact with different sheaves etc and if the locations of failures are know, the reasons for the failures can be identified more easily.
The drag and hoist rope life (in terms of BCM) has gradually fallen over last 10-15 years. Although it is difficult to pinpoint the reason for this, the following are some of the possibilities:
- quality issues related to rope construction;
- increase in rope loads due to increased RSL;
- influence of maintenance practice;
- increase in dynamic loading due to introduction of digital control systems.
The type of rope constructions, manufacturing methods, etc have not changed over the last 10-15 years. Although the wire rope technology has remained stagnant, there have been new developments that are noteworthy. One of these is the Cus hion TM 8 ropes that have been tested in draglines overseas. These ropes are made with a plastic coated core that can cushion the forces on the crown wires and also reduce fatigue due to bending. The trials of Cushion TM 8 ropes in Marion 8050, Marion 8200, BE1350 and BE 1370 machines have shown an increase of 150-450% in life. It is strongly recommended that these ropes are trialed in Australian draglines.
The Marion hoist rope life was found to be generally 40-50% of that of similar size BE1370 ropes, in machines with similar RSL's. It is difficult to definitively conclude the reason for this striking variation using the available data. The inspection of discarded ropes from Marion 8050 and BE 1370 machines indicates that the BE hoist ropes wear reasonably uniformly around their perimeter, whereas the wear in Marion ropes generally is concentrated over one quarter of the perimeter. In some cases it was found that all the wire breaks are concentrated over a small segment of the perimeter (~25% of perimeter) but the crown wires on the opposite side have hardly worn. Eventually the rope is discarded due to uneven wear and wire breaks. It is very likely that the main reasons for this localised wear can be attributed to the gantry sheave. That is, the out of plane rope tracking at the gantry sheave imposes excessive side rubbing against the sheave groove. Due to the same reason the rope is not allowed to rotate when the load is applied. It is recommended that the reasons for uneven wear are confirmed and necessary structural modifications are undertaken to modify the gantry sheave arrangement. This improvement has the potential of improving the Marion hoist rope life significantly.
Drag rope life is limited for two main reasons:
- shortening of rope due to wire breaks at sockets and cropping;
- wearing of crown wires and wire break due to dragging on hard rocky material on the ground. The improvements can be derived through improved operational practice. Another opportunity for improvement is the interaction between the rope and the socket i.e. the performance of the sockets. Consideration may be given to redesigning socket geometry and achievement of gradual change in stiffness at the socket mouth. In general, the life of drag and hoist ropes can be improved by improving:
- rope sub-systems configuration, i.e. sheaves, drums etc.;
- rope quality;
- rope maintenance and inspection; and
- rational and improved discard criteria.
The recent upgrade of dragline control systems from analogue to digital systems has made the control systems more responsive and lively. Although this is desirable from operations point of view, this can induce higher levels of loading and vibrations in ropes and consequently can increase the high frequency load cycles in ropes, particularly in main suspension ropes.
The weekly rope inspection procedure used at present for drag and hoist ropes is very rudimentary. There is evidence that important parts of the ropes are not inspected at all and consequently the risk of failure is present. More comprehensive inspections are difficult to justify, considering the cost of a new pair of ropes and the opportunity cost of dragline downtime. It is recommended that consideration is given to more reliable inspections using magnetic inspection methods on scheduled maintenance down days. Such inspections can determine all wire breaks (internal and externa l) and the loss of cross sectional area and as such can enhance maintenance decision making.