The full story
What is a dragline excavator?
A dragline is an excavator that digs with a bucket suspended on ropes. The bucket hangs from a hoist rope running over the tip of a long boom, and a drag rope pulls it towards the machine. The operator casts the bucket out, drags it back so its teeth bite and it fills, hoists it, swings the machine and dumps the load. Large draglines are used in strip mining to remove overburden, the soil and rock above a flat seam of coal or phosphate, and cast it into the space left by the previous cut.
How a dragline walks
Large draglines rest on a circular base, the tub, which carries the weight while digging. To move, the machine uses two large shoes driven by cams or hydraulics to lift itself, step forward and lower itself again. Walking is slow, but it lets machines of thousands of tonnes move along a pit without tracks.
Big Muskie
Big Muskie, a Bucyrus-Erie 4250-W, was the largest dragline ever built and the only one of its model. It worked for American Electric Power's Central Ohio Coal Company from 1969 to 1991. Its bucket held 220 cubic yards (about 170 m³), its boom was 310 ft long, and the machine weighed about 13,500 short tons. AEP says it moved more than 483 million cubic yards of material. It was dismantled and scrapped in 1999; the bucket is displayed at Miners' Memorial Park near McConnelsville, Ohio, and part of the land it mined is now The Wilds conservation centre.
Shovels and draglines
From the 1950s to the 1970s US coal operators also built giant stripping shovels. The Marion 6360, The Captain, started work in Illinois in 1965 with a 180 cubic yard dipper and is listed by Guinness World Records as the largest shovel; it was scrapped after a fire in 1991. Big Brutus, a Bucyrus-Erie 1850-B in Kansas, worked from 1963 to 1974 and is preserved as a museum. Draglines, which can stand on top of the bank and dig below themselves, proved more flexible and outlasted the shovels.
Draglines today
Draglines still strip overburden in surface coal mines in the United States, Australia, India and South Africa, and in Florida's phosphate mines. Ursa Major, a Bucyrus-Erie 2570WS with a 160 cubic yard bucket on a 360 ft boom, works at Black Thunder in Wyoming. The dragline makers have consolidated: Bucyrus bought Marion in 1997 and Caterpillar bought Bucyrus in 2011.
What draglines teach
- The cheapest tonne is the one you never put in a truck. Casting overburden straight into the last cut is why draglines survive.
- Giant single machines are a single point of failure: a fire ended The Captain overnight.
- Reclamation lasts longer than mining. Big Muskie worked 22 years; the reclaimed land will be there for centuries.
More machines: all mining equipment, including bucket wheel excavators and haul trucks. Coal and phosphate country: mining in the United States, Australia and phosphate.
Questions people ask
How does a dragline excavator work?
The bucket hangs on a hoist rope from the end of a long boom and is pulled by a second rope, the drag rope. The operator casts the empty bucket out, drags it back towards the machine so it fills, lifts it with the hoist rope, swings the whole machine round and opens the bucket over the spoil pile on the other side of the cut.
What was the biggest dragline ever built?
Big Muskie, a Bucyrus-Erie 4250-W built for American Electric Power's Central Ohio Coal Company. Its bucket held 220 cubic yards (about 170 m³), its boom was 310 ft long and it weighed about 13,500 short tons. It worked from 1969 to 1991 and was scrapped in 1999; only the bucket survives.
How does a dragline walk?
A dragline sits on a large round base called a tub. To move, it lifts itself on two big shoes, one on each side, pushes forward a short step and sets down again. It is slow, but it means a machine weighing thousands of tonnes needs no tracks or wheels.
What is the difference between a dragline and a shovel?
A stripping shovel pushes a dipper up through the bank in front of it, so it must sit at the face. A dragline throws its bucket out and pulls it back on ropes, so it can stand on top of the bank and dig below itself, and reach further. That reach is why draglines took over strip mining from the giant shovels.
Where are draglines used today?
Mainly in surface coal mines in the United States, Australia, India and South Africa, and in phosphate mining in Florida, wherever a flat seam lies under tens of metres of overburden that can be dug and cast aside.
