Figure 1. The RB293 bucket-wheel excavator en route to a mining site.The largest mobile land machine to move under its own power is manufactured by MAN Takraf of Leipzig, Germany (Figure 1). Bucket-wheel excavators are massive earth-moving machines that are used in a variety of mining and civil engineering activities.  This machine is used in the surface mining of coal in the German state of North Rhine Westphalia.

 The dimensions of the RB293 are staggering:

• 14,196 metric tons (31.3 million lb)
• 722 ft (220 m) long
• 310 ft (94.5 m) tall at its highest point
• capable of moving 8.475 million ft³ (240,000 m³) of earth daily. That means that it can dig a hole the length of a football field to over 25 meters deep in a single day.

Its maximum speed is less than one kilometer per hour. It took over three weeks to make the 22 kilometer (14 mile) trip to the mine, traveling across Autobahn 61, the Erft River, a railroad line and several roads.

The principle behind bucket-wheel excavators is simple. A series of scoops or buckets are attached to a large rotating wheel mounted on an arm or boom. As the wheel rotates, the buckets excavate soil or rock from the target area and carry it around to the backside of the wheel, where it falls onto a conveyor, which in turn carries it up the arm toward the main body of the excavator. Additional conveyors then may carry the material further; in some cases, several long conveyors are placed end-to-end, each supported by a large vehicular base.

Figure 2. Surface mining in Germany. Credit: NASA.

Figure 2 is a simulated natural color image taken by a NASA satellite in the German state of North Rhine Westphalia, where the RB293 operates. The image covers an area of 30 by 36 km, and was acquired on August 26, 2000. The rectangular patches are agricultural fields; light green hues show where crops are growing and grey hues show bare soil. Darker green hues show forested areas. The various blue-grey clusters of pixels seemingly linked together by dark thin lines are towns and villages connected by roads.

On the right side of the image are three enormous opencast (surface) coal mines that are developed by the RB293 and other large earthmoving machines. In the middle-right portion of the image is the massive Hambach surface lignite mine (50º54'36.14"N, 6º30'8.48"E). This 85 km² (33 mi²) area holds lignite reserves of some 2.5 Gt (2.8 billion st), the mining of which involves removal of 15.4 km3 (20 billion yd3) of overburden, yielding an overall stripping ratio of 6.2:1 m³/t (199 ft³/st). Stripping ratio describes the ration of overburden or waste rock that must be removed relative to thickness of coal that lies beneath it.

Sources

• TAKRAF GmbH, Bucket-wheel excavators, Accessed 23 August 2008.
• MAN Takraf: Machinery in Motion, Engineering Journal, September 2005, Accessed 23 August 2008.
• National Aeronautics and Space Administration, Earth Observatory, Coal Mines in Germany, Accessed 28 August 2008.