Open Cut » Overburden Removal
ACIRL Ltd was commissioned by ACARP to investigate and research aspects of dragline bucket rigging, (Project C4003). This project commenced in May 1995 and was completed in October 1996. The total project value was $195,000 of which ACARP provided funding for $150,000. The following organisations contributed a total of $45,000 to the project:
- Collinsvale Coal Company Pty Ltd
- Tarong Coal
- Ulan Coal Mines Ltd
- Drayton Coal Pty Ltd
- Warkworth Mining Ltd
- Mt Thorley Operations
- Barlows equipment Manufacturing Co Ltd
The project was broken into three stages:
Stage 1 was an audit of Australian dragline rigging usage;
Stage 2 was to quantify effects on bucket performance of basic changes to specific aspects of conventional rigging;
Stage 3 was to investigate modifications to existing rigging and new rigging arrangements.
There are no simple global answers to optimising the performance of a dragline bucket. Many factors, (often site-specific), can affect different aspects of behaviour to varying degrees. For example, there are completely independent factors that affect digging, disengaging, carrying, dumping and chopping. These factors may also be in conflict with each other e.g. maintain satisfactory carry angle, minimise dump rope load.
It is important to understand the inherently complex and dynamic nature of dragline bucket and rigging performance. A small change in one aspect, eg. dump rope length, may radically affect aspects of behaviour in ways that were completely unexpected. In addition a change in one variable often requires change in another to compensate and/or enhance the ultimate performance.
With this in mind, two approaches were taken to achieve the aims of this project:
A specific approach, whereby the individual parts of the system were examined and their effects analysed.
A global viewpoint, which considered the entire system as a whole.
It should be noted that where conventional rigging is referred to, the BE HPS system of connecting hoist chains on the inside of the bucket without a spreader bar is precluded.
This project had the following as its key objectives:
- provide a better understanding of what others are doing with dragline rigging;
- provide a better understanding of the potential productivity gains/losses which may be achieved/incurred through correct/incorrect rigging and the best methods of achieving gains;
- identify and evaluate novel changes to the conventional rigging arrangements.
Conclusions and Recommendations
The typical drag assembly is either 25 drag links plus two pear links or 24 drag links plus two pear links plus a hitch link. In each case a total of 27 links. There are some operations which use as low as 21 links. The typical lower hoist assembly is 9 or 11 links plus 2 stud or pear links. Unlike the drag assembly there is not a large capacity to vary the length of the hoist chain; shortening the hoist chain causes the lug on the arch to run into the block while lengthening them reduces the dumping height and could increase rehandle. The typical upper hoist chain is very similar to the lower hoist chain having 9 or 11 links plus 2 stud or pear links.
Many mines use old hoist rope for the dump rope. There does not appear to be any negative impact on rope life by doing this relative to using new rope. Generally the BE1370W and M8050 class machines are using 83mm rope.
The smaller class machines are using 64mm or 76mm while the BE1570W / M8200 class machines are using 92mm rope. The majority of the BE1370W and M8050 class machines are using 1040mm diameter dump block sheaves although some use 1220mm sheaves. The standard case of 83mm rope going over a 1040mm sheave gives a D/d ratio of 12.5. The discussions with minesite personnel indicated a strong belief that the thicker rope provided more strength and improved life.
In AS1418.1, the action of the dump rope corresponds to a mechanism classification of M1. Dump ropes could be assumed to be classified as under an L4 (Very Heavy - Mechanisms subjected regularly to the maximum load) category. These two together indicate that there is no appropriate class of utilisation for the item (with D-d ratio of 12.5) and a total duration of use of less than 200 operating hours is predicted. Assuming the dragline actually cycles (not including non-operating time and walking) for 70% of the time the dump rope life will be less than 11.9 days.
Over time each mine should successively try 76mm, 70mm and 64mm as each of these has sufficient strength to withstand the peak loads and the increased D-d ratio should provide improved life. Alternatively this report has recommended further industry funded work on improving dump rope life and these trials could be included in this work.
There is no production related reason for using double dump ropes rather than a single dump rope. Where feasible a single dump rope should always be used.
The rigging components which have the lowest reliability (determined qualitatively from minesite visits) are:
- Dump Ropes
- Drag Shackles
- Lower Hoist Chain - third link
- Locking Pins
There is no discernible trend in optimum carry angle between conventional buckets (hoist chains on outside of bucket with spreader bar) with respect to spoil and bucket geometry. The BH 3 records a significantly lower optimum carry angle than the conventional buckets. The average value is 35.5° with standard deviation 1.1°. It is suggested that the carry angle of the bucket be set slightly below the average as there is a tendency for the carry angle of the bucket to increase as components wear. The conclusion is to set the rigging (chain and dump rope lengths) so that the angle of the bottom of the bucket is approximately 35° to the horizontal at the point where the drag hitch is 27.5 metres away from the fairleads and 10 metres below them. Due to operational factors this angle will change over time. Once it falls below 33.5°(-2) or exceeds 37.5° (+2) the dump rope length should be varied to bring the angle back to 35°. Under normal statistical process control techniques, the limits are set at plus and minus three standard deviations. However, in this work, two standard deviations are suggested, as the value of production is very high.
Neither the average payload for a specific bucket nor dump rope loads are impacted positively or negatively by varying chain lengths (provided interactions in the rigging are avoided) so long as the bucket is set to the optimum carry angle at the specified point (bucket equilibrium
The 'rigging modification rules' are similar fro all conventional buckets (all except the BE HPS bucket). There do not appear to be any trends with respect to bucket geometry. The average values are:
- Optimum Bucket Carry Angle 35.5°;
- Reduce dump rope length by 0.25m for every one degree increase in carry angle required;
- Increase dump rope length by 0.70 metre for every 2 additional links added to drag chains (converse is also true);
- Increase dump rope length by 1.15 metres for every 2 additional links added to lower hoist chains (converse is also true);
- Increase dump rope length by 1.80 metres for every 2 additional links added to upper hoist chains (converse is also true).