02/23/2012 | Autor / Editor: Brett Wilson, Steve J. Davis, Mark Wolton / Marcel Dröttboom
The conventional overland conveyor is limited in ability with respect to gradient and alignment curvature. However, there are two less-traditional conveying systems worthy of evaluation against conventional conveyors for long-distance, large-capacity installations, the Cable Belt and the Ropecon System.
The conventional troughed overland conveyor belt is a proven technology that has been used to transport bulk materials for nearly a century. The invention of the three-roll idler belt configuration for material transport dates back to the 1930s.  Since then, the requirements for higher capacities and longer runs have led to innovation and new technologies used in conventional overland conveyors. Although a proven and widely used technology, the conventional overland conveyor is limited in its ability to traverse natural terrain. The effort to overcome this limitation has led to new technologies that offer more flexibility with respect to gradient and alignment curvature.
One such technology is the cable-hauled system. Although this technology has been around since the 1950s, it has not been widely used compared to the conventional overland conveyor. To date, all large cable-hauled systems are Cable Belt systems, as currently manufactured by Metso Corporation. The largest cable-hauled system holds the world record for the longest single-flight conveyor, with a centre-to-centre distance of 31 kilometres.
Another technology, which is relatively early in its development compared to the conventional conveyor, is Ropecon, a proprietary system developed by Doppelmayr Transport Technology. Doppelmayr has combined aerial ropeway technology and proven conveyor belt technology to create this unique system. The first system was installed in 2000, and the largest system of this kind to date – installed in 2007 – has a capacity of 1200 tonnes per hour and a horizontal length of 3.4 kilometres.
Options for overland conveying vary according to numerous factors, including capacity, length, terrain, geotechnical properties, and the environment. Overland conveying options include the conventional conveyor, Cable Belt, and Ropecon systems. This section provides an overview of these technologies, with an emphasis on the less traditional Metso and Doppelmayr systems.
Conventional overland conveyors are predominantly ground or truss mounted (Fig. 1). The technology has been developing over the past 20 years and continues to do so. The technology and components are readily available from many suppliers. The baseline is a solid foundation of design and operation experience. However, technical risks are still associated with these systems at the ultimate limits of existing knowledge and experience.
Belt: Conventional large-capacity, long-distance overland conveyors use a steel cord belt with a single layer of parallel steel cords to carry the belt tension. Vulcanised splices are used to connect belt segments, and are the limiting factors in a belt's carrying capacity. Current findings have led to the acceptance that splice safety factors can be reduced from previous empirical numbers, which had a significant impact on carrying capacity.
Idlers: The number of idlers installed directly affects the cost of the conveyor, and idler spacing has increased significantly for overland conveyors. The selection of the idler bearing, seal, and lubricant also has a significant effect on conveyor life, noise emission, and energy consumption.
Tensioning System: There are numerous options for tensioning the belt on a conventional overland conveyor, which allow the system to be very flexible and cater to numerous applications and loading conditions. Current knowledge of dynamic conveyor behaviour and take-up control has allowed overland conveyors to be designed with sophisticated take-up management that reduces belt tension and overall costs.
Conveyor Drives: Conventional conveyors are typically driven and braked at one or more locations along the conveyor. The designs, locations, and controls of these drives, combined with take-up and splice technology, have allowed these conveyors to perform at capacities and lengths inconceivable 25 years ago. Several types of intermediate drives are available, from the tripper style through the booster style to the power-strip style, which does not need trippers.
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