A bucket-wheel excavator (BWE) is a large

heavy equipment Heavy equipment or heavy machinery refers to heavy-duty vehicles specially designed to execute construction tasks, most frequently involving earthwork operations or other large construction tasks. ''Heavy equipment'' usually comprises five e ...

machine used in

surface mining Surface mining, including strip mining, open-pit mining and mountaintop removal mining, is a broad category of mining in which soil and rock overlying the mineral deposit (the overburden) are removed, in contrast to underground mining, in whic ...

. The primary function of BWEs is to act as a continuous digging machine in large-scale

open-pit mining Open-pit mining, also known as open-cast or open-cut mining and in larger contexts mega-mining, is a surface mining technique of extracting rock or minerals from the earth from an open-air pit, sometimes known as a borrow. This form of mini ...

operations, removing thousands of tons of

overburden In mining, overburden (also called waste or spoil) is the material that lies above an area that lends itself to economical exploitation, such as the rock, soil, and ecosystem that lies above a coal seam or ore body. Overburden is distinct from tai ...

a day. What sets BWEs 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

bucket A bucket is typically a watertight, vertical cylinder or truncated cone or square, with an open top and a flat bottom, attached to a semicircular carrying handle called the ''bail''. A bucket is usually an open-top container. In contrast, a ...

s used to scoop material as the wheel turns. They rank among the largest vehicles (land or sea) ever produced, and the largest of the bucket-wheel excavators (the 14,200 ton

Bagger 293 Bagger 293, previously known as the MAN TAKRAF RB293, is a giant bucket-wheel excavator made by the German industrial company TAKRAF, formerly an East German Kombinat. It owns or shares some records for terrestrial vehicle size in the ''Guinn ...

still holds the

Guinness World Record ''Guinness World Records'', known from its inception in 1955 until 1999 as ''The Guinness Book of Records'' and in previous United States editions as ''The Guinness Book of World Records'', is a reference book published annually, listing world ...

for the heaviest land-based vehicle ever constructed).

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 belt A conveyor belt is the carrying medium of a belt conveyor system (often shortened to belt conveyor). A belt conveyor system is one of many types of conveyor systems. A belt conveyor system consists of two or more pulleys (sometimes referred t ...

s, spreaders, crushing stations, heap-leach systems, etc.) to move and mine massive amounts of

(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 Lignite, often referred to as brown coal, is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35%, and is considered the lowest rank of coal due to its relatively low heat ...

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 "Earth Eating Monster" ''Popular Mechanics'', June 1956, pp. 114-115
/ref> 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 A superstructure is an upward extension of an existing structure above a baseline. This term is applied to various kinds of physical structures such as buildings, bridges, or ships. Aboard ships and large boats On water craft, the superstruct ...

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.Grathof, H. (1986). Design (Constructional Characteristics) of Large Bucket Wheel Excavators. Journal of Mines, Metals, and Fuels, 34(4), 204-213. 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 A counterweight is a weight that, by applying an opposite force, provides balance and stability of a mechanical system. The purpose of a counterweight is to make lifting the load faster and more efficient, which saves energy and causes less we ...

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.Chironis, N. (1984). Bucket Wheel Excavators of Compact Design Growing Popular. Coal Age, 84(10), 84-91. 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 ThyssenKrupp AG (, ; stylized as thyssenkrupp) is a German industrial engineering and steel production multinational conglomerate. It is the result of the 1999 merger of Thyssen AG and Krupp and has its operational headquarters in Duisburg a ...

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.ThyssenKrupp Fördertechnik. (2005). Business Unit: Mining
The largest BWE ever constructed is

TAKRAF TAKRAF Group (“TAKRAF”), is a global German industrial company. Through its brands, TAKRAF and DELKOR, the Group provides equipment, systems and services to the mining and associated industries. Foundation and History While the official foun ...

, which is 93 meters tall and 225 meters long, weighs 14,200

tonnes The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States c ...

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 Slewing is the rotation of an object around an axis, usually the z axis. An example is a radar scanning 360 degrees by slewing around the z axis. This is also common terminology in astronomy. The process of rotating a telescope to observe a diffe ...

, the excavator can reach through a horizontal range. The overburden is then delivered to the discharge boom, which transfers the cut earth to another machine for transfer to a spreader. This may be a fixed belt conveyor system or a mobile conveyor with crawlers similar to those found on the BWE. Mobile conveyors permanently attached to the excavator take the burden of directing the material off of the operator. The overburden can also be transferred directly to a cross-pit Spreader, which reaches across the pit and scatters overburden at the dumping ground.

Automation

Automation of the BWEs requires integrating many sensors and electrical components such as GPS,

data acquisition Data acquisition is the process of sampling signals that measure real-world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems, abbreviated by the acro ...

systems, and online monitoring capabilities. The goal of these systems is to take away some of the work from the operators in order to achieve higher mining speeds. Project managers and operators are now able to track crucial data regarding the BWEs and other machinery in the mining operations via the Internet. Sensors can detect how much material is being scooped onto the conveyor belt, and the automation system can then vary the speed on the conveyor belts in order to feed a continuous amount of material.

Applications

Bucket wheel excavators and bucket chain excavators take jobs that were previously accomplished by rope shovels and

dragline A dragline excavator is a piece of heavy equipment used in civil engineering and surface mining. Draglines fall into two broad categories: those that are based on standard, lifting cranes, and the heavy units which have to be built on-site. ...

s. They have been replaced in most applications by

hydraulic excavator Excavators are heavy construction equipment consisting of a boom, dipper (or stick), bucket and cab on a rotating platform known as the "house". The house sits atop an undercarriage with tracks or wheels. They are a natural progression fro ...

s, but still remain in use for very large-scale operations, where they can be used for the transfer of loose materials or the excavation of soft to semi-hard overburden.

Lignite mining

The primary application of BWEs is in

(brown coal) mining, where they are used for soft rock overburden removal in the absence of blasting. They are useful in this capacity for their ability to continuously deliver large volumes of materials to processors, which is especially important given the continuous demand for lignite. Because of the great demand for lignite, lignite mining has also been one of the areas of greatest development for BWEs. The additions of automated systems and greater manoeuvrability, as well as components designed for the specific application, have increased the reliability and efficiency with which BWEs deliver materials.

Materials handling

Bucket wheel technology is used extensively in bulk materials handling. Bucket wheel reclaimers are used to pick up material that has been positioned by a stacker for transport to a processing plant. Stacker/reclaimers, which combine tasks to reduce the number of required machines, also use bucket wheels to carry out their tasks. In shipyards, bucket wheels are used for the continuous loading and unloading of ships, where they pick up material from the yard for transfer to the delivery system. Bucket chains can be used to unload material from a ship's hold. TAKRAF's continuous ship unloader is capable of removing up to 95% of the material from a ship's hold, owing to a flexibly-configured digging attachment.

Heap leaching

An extension of their other uses, BWEs are used in heap leaching processes. Heap leaching entails constructing stacks of crushed ore, through which a solvent is passed to extract valuable materials. The construction and removal of the heaps are an obvious application of stacking and reclaiming technology.

Manufacturers and market

Few companies are willing or able to manufacture the massive, expensive gears required for BWEs. Unex, Czech Republic, still has the original casting forms, and is still able to manufacture BWEs. However, these machines were built to last indefinitely under continuous heavy use and strip mining is now less popular, so there is little demand for new machines. The manufacturers of BWEs and similar mining systems now receive some revenue from maintenance and refurbishing projects, but also produce large steel parts for other purposes. Current use of bucket-wheel excavators is mainly focused in the area of lignite (brown coal) mining for the production of electricity, mostly in Germany and East/Southeastern Europe. Unex has also made a BWE for extraction of diamonds from the Siberian permafrost.

References

External links

Economic and technologic aspects of Bucketwheel Excavators - and Crusher/Conveyor-Systems

Bucket Wheel Excavators World wide Installations

* * ttp://www.womp-int.com/story/2008vol4/story024.htm "Big Wheels Keep on Turning" - Information about the development of bucket-wheel excavators and similar vehicles.
"Shovels Attached to Wheel in Big Evcavator" ''Popular Mechanics'', February 1935

Sandvik Mining and Construction Products > Bulk materials handling equipment > Bucket wheel excavators >PE200-1400/2x30

Structural Analysis of Continuous Ship Unloader, Society of Mechanical Engineers, 2012 Proceedings, 2012.11, 2075-2078 (4 pages)
{{DEFAULTSORT:Bucket-Wheel Excavator Articles containing video clips Engineering vehicles * Mining equipment Surface mining