Overburden Conveyor Bridge

Overview

An overburden conveyor bridge spans an open pit and carries stripped waste rock straight across it, from the side being mined to the spoil side, without trucks. In soft-rock lignite mining the layer of soil and rock over the coal seam, the overburden, is enormous in volume but worthless, so the cheapest way to move it is to lift it onto a belt and fly it across the pit in one continuous span. A conveyor bridge is among the largest mobile machines ever built, with a span of hundreds of metres riding on crawlers and weighing many thousands of tonnes, advancing slowly along the working face as the mine progresses.

The machine is a long Bridge Truss carrying a Conveyor Line, supported at each end by a Receiving Car and a Discharge Car that ride on Crawler Travel Set travel sets. A Discharge Boom throws the spoil off the far end, while a Electrical System system and a Operator Cab coordinate the whole machine.

How it works

Excavators or a spreader feed overburden onto the bridge at the Receiving Car. The Feed Hopper and Transfer Chute guide the stream onto the belt, which runs the length of the truss at several metres per second and discharges over the Discharge Car onto the spoil side. Because the two ends sit on opposite walls of a moving pit, each end travels on its own crawlers and the truss is free to pivot relative to each car through a Pivot Bearing, so the span can follow the curving advance of the face without binding. The whole bridge inches forward continuously as the digging side advances, a few metres per minute at most.

The conveyor line

The Conveyor Line is the working heart of the bridge. A single Conveyor Belt of steel-cord rubber, up to three metres wide, runs the full span. It is supported on its loaded run by hundreds of Carrying Idler trough idlers, each a set of three Idler Roll rolls on a Idler Frame, and on its empty return by Return Idler flat idlers. The belt is driven by Drive Pulley pulleys at one or more Drive Station stations, each with a Belt Drive Motor motor, a Gearbox Housing reduction, a Fluid Coupling fluid coupling to ease starting, and a Backstop to stop a loaded incline running backward. Bend Pulley pulleys turn the belt path.

Belt tension

A belt this long stretches under load and with temperature, so it needs constant tensioning. The Belt Take-Up does this with a movable Take-Up Pulley on a Take-Up Carriage, pulled by a Take-Up Winch. Too little tension and the belt slips at the drive; too much and the splices and idlers suffer. The take-up keeps tension in the right window automatically as conditions change over a shift.

Travelling on crawlers

The bridge cannot stand on fixed footings because the pit it spans is always moving, so it rides on Crawler Travel Set crawler sets. Each set runs Crawler Track tracks driven by Travel Motor motors through Gearbox Housing gearboxes, with a Steering Cylinder to steer it along the bench and Leveling Cylinder cylinders to level the structure on uneven ground. Steering and speed of every set must be coordinated so the long span is never twisted or dragged sideways, which is the main control problem of running the machine.

Discharging the spoil

At the far end the Discharge Boom discharge boom reaches out over the spoil side. The Boom Truss cantilever, kept in balance by the Counterweight on the Discharge Car, carries a short Boom Belt and Tripper Pulley pulleys that lift the belt to throw the material through a Discharge Chute onto the growing pile. The boom can slew on its Boom Slew Ring to spread the spoil evenly rather than dumping it all in one heap.

Power, control, and operation

The bridge runs on grid power through a Cable Reel that pays out a trailing cable as the machine advances. A Transformer steps the voltage down, a Drive Cabinet feeds the belt and travel motors with variable-speed drives, and a Control System synchronizes the crawler sets and protects the belt against overload, misalignment, and slip. The operator works from the Operator Cab at a Control Desk with LCD Panel displays of belt load, crawler steering, and boom position. A conveyor bridge replaces a large fleet of haul trucks for cross-pit waste movement, runs on cheaper grid power, and works continuously, which is why these machines remain central to large lignite operations despite their cost and the engineering needed to keep a span of that size moving as a single coordinated structure.