A bucket-wheel excavator (BWE) is a large heavy equipment machine used in surface mining.
Their primary function is that of a continuous digging machine in large-scale open-pit mining operations, removing thousands of tons of overburden a day. What sets them apart from other large-scale mining equipment, such as bucket chain excavators, is their use of a large wheel consisting of a continuous pattern of buckets which scoop material as the wheel turns. They are among the largest land or sea vehicles ever produced. The 14,200-ton Bagger 293 holds the Guinness World Record for the heaviest land-based vehicle ever built.
History
Bucket-wheel excavators have been used in mining for the past century, with some of the first being manufactured in the 1920s. They are used in conjunction with many other pieces of mining machinery (conveyor belts, spreaders, crushing stations, heap-leach systems, etc.) to move and mine massive amounts of overburden (waste). While the overall concepts that go into a BWE have not changed much, their size has grown drastically since the end of World War II.
In the 1950s two German mining firms ordered the world's first extremely large BWEs, and had three BWEs built for mining lignite near Cologne, Germany. The German BWEs had a wheel of over in diameter, weighed and were over long, with eighteen crawler units for movement and could cut a swath of over at one time
BWEs built since the 1990s, such as the Bagger 293, have reached sizes as large as tall, long, and as heavy as . The bucket-wheel itself can be over in diameter with as many as 20 buckets, each of which can hold over of material. BWEs have also advanced with respect to the extreme conditions in which they are now capable of operating. Many BWEs have been designed to operate in climates with temperatures as low as . Developers are now moving their focus toward automation and the use of electrical power.
Structure
A bucket wheel excavator (BWE) consists of a superstructure to which several more components are fixed.
The bucket wheel from which the machines get their name is a large, round wheel with a configuration of scoops which is fixed to a boom and is capable of rotating. Material picked up by the cutting wheel is transferred back along the boom. In early cell-type bucket wheels, the material was transferred through a chute leading from each bucket, while newer cell-less and semi-cell designs use a stationary chute through which all of the buckets discharge.
A discharge boom receives material through the superstructure from the cutting boom and carries it away from the machine, frequently to an external conveyor system.
A counterweight boom balances the cutting boom and is cantilevered either on the lower part of the superstructure (in the case of compact BWEs) or the upper part (in the case of mid-size C-frame BWEs). In the larger BWEs, all three booms are supported by cables running across towers at the top of the superstructure.
Beneath the superstructure lay the movement systems. On older models these would be rails for the machine to travel along, but newer BWEs are frequently equipped with crawlers, which grant them increased flexibility of motion.
To allow it to complete its duties, the superstructure of a BWE is capable of rotating about a vertical axis (slewing). The cutting boom can be tilted up and down (hoisting). The speeds of these operations are on the orders of 30 m/min and 5 m/min, respectively. Slewing is driven by large gears, while hoisting generally makes use of a cable system.
Size
The scale of BWEs varies significantly and is dependent on the intended application. Compact BWEs designed by ThyssenKrupp may have boom lengths as small as , weigh 50 tons, and move of earth per hour. Their larger models reach boom lengths of , weigh 13,000 tons, and move per hour. The largest BWE ever constructed is TAKRAF's Bagger 293, which is 93 meters tall and 225 meters long, weighs 14,200 tonnes and is capable of moving of overburden every day. Excavations of per day have been recorded. The BWEs used in the United States tend to be smaller than those constructed in Germany.
Operation
BWEs are used for continuous overburden removal in surface mining applications. They use their cutting wheels to strip away a section of earth (the working block) dictated by the size of the excavator. Through hoisting, the working block can include area both above and below the level of the machine (the bench level). By slewing, the excavator can reach through a horizontal range.