Coal Yard Stacker

Overview

A coal yard stacker is a continuous bulk material handling machine that builds coal stockpiles in an open or covered storage yard. Coal arrives by an incoming yard conveyor, transfers onto a belt conveyor mounted along a long boom, and is thrown off the boom head into a growing pile. The machine moves on rails laid along the length of the yard, and the boom both luffs (raises and lowers) and slews (rotates) so the discharge point can be placed anywhere across the stockpile footprint. By combining travel, slew and luffing motion the operator lays coal in controlled layers rather than dumping it at a single point, which keeps the pile dense, stable and free of large voids.

Stackers of this class handle boom belt widths from 1,200 to 2,000 mm running at 3 to 5 m/s, giving throughputs of 500 to 6,000 t/h. Boom length runs from 25 m on a small power-plant yard to 50 m on a port or large mine stockyard. The whole machine, often 120 to 450 t, rides on a rail-mounted undercarriage at 15 to 30 m/min and is powered through a trailing cable managed by a cable reel.

Material flow

Coal reaches the machine on the yard conveyor running between the rails. A tripper car lifts that belt and discharges into the feed chute, which directs the stream through a receiving hopper and onto the tail end of the boom belt. The transfer point is where most wear and dust occur, so it carries impact idlers, a skirted loading zone and a dust suppression spray.

The boom conveyor then carries the coal up the inclined boom. Troughing carry idlers hold the loaded belt in a deep trough to maximise cross-sectional load, while return idlers support the empty belt underneath. At the boom head a rubber-lagged drive pulley turns the belt, and coal leaves through the discharge chute into the pile. Belt scrapers on the head pulley clean off carryback before the belt returns to the tail.

Motion systems

Three powered motions place the discharge point. Slewing rotates the whole superstructure on a large slew bearing, a triple-row roller ring with integral gear teeth. Two slew drive units, each a brake motor and planetary gearbox driving a pinion into the ring, give a typical swing of ±110°. A slip ring column passes power and control signals across the rotating joint.

Luffing sets the height and reach of the discharge point. Two double-acting hydraulic cylinders raise and lower the boom between about +12° and -12°. A counterbalance valve block holds the boom against gravity and lowers it under control, and pressure sensors feed boom load and angle back to the controller. Oil comes from the hydraulic power unit, a variable-displacement pump and reservoir with filtration and an accumulator.

Travel moves the machine along the yard. Four rail bogies carry double-flanged wheels, and two geared travel drives move the machine at 15 to 30 m/min. When parked, spring-applied rail clamps grip the rail so wind cannot push the machine, and buffers limit overtravel at the rail ends.

Stacking methods

Two pile shapes are common. In cone stacking the machine stands still and slews while luffing down as the cone grows, building one conical pile, then travels a boom diameter and starts the next, producing a row of overlapping cones. In chevron or windrow stacking the machine travels back and forth along a fixed line while the boom discharges at the ridge, laying coal in thin V-shaped layers along a long prism. Chevron piles blend coal grades layer by layer, which matters when a power plant or coke oven needs a consistent feed. The counterweight, a 40 to 150 t concrete and steel mass on the tail of the platform, balances the boom moment about the slew centre across the full luffing range so the slew bearing load stays within limits.

Control and electrical

The control system coordinates all motion and protects the machine. A PLC reads a boom angle sensor, a slew encoder and a belt speed sensor, then drives the travel and slew motors through variable-frequency drives in the motor control centre. Limit switches stop each motion at its end of travel, and interlocks prevent stacking while the boom is above safe load or the cable reel is at its limit. The operator works from an air-conditioned cab with an LCD display, though modern machines run most stacking sequences automatically.

Maintenance and common failures

Idlers are the highest-count wear item: seized carry idlers cut belt grooves and add drag, so they are inspected on a route and changed in bulk. The boom belt itself is a major spare, replaced on a multi-year cycle or after a rip; steel-cord construction resists the impact at the loading zone. Scraper blades wear fast in abrasive coal and are swapped on a short interval to control carryback. The slew bearing needs scheduled greasing and periodic bolt-tension checks, since a neglected ring is expensive to replace. On the hydraulics, cylinder rod seals weep with age and the return filter is changed on condition. Rail clamps and travel wheel flanges are checked for wear because a stacker that drifts in a storm or derails on a worn rail is a serious hazard. Dust suppression nozzles clog and are cleaned routinely to keep airborne coal dust within limits.