A coal shearer is an electrically driven hydraulic cutting machine also referred to as a power loader, stage loader or shear loader used in the longwall mining of coal. The average coal shearing machine is 15 meters long, weighs in excess of 90 tons and can travel up to speeds of 10 to 14 meters a minute. Other coal shearers such as those used in the Twentymile Coal Mine have the ability to travel at speeds of 45 meters per minute.
Coal shearers of different models are designed with varying dimensions and power output depending on the manufacturer. The parameters for selecting a coal shearer for a longwall mining operation is contingent on the type of shearer, dimensions of the shearer as well as the haulage speed and power output of the machine.
The mechanization of coal mining with cutting technology occurred as early as 1860. The first cutting machines were simple devices comprised of circular saws with picks positioned around edge instead of teeth. It took nearly an entire century however for coal cutting equipment to be developed to a level sophisticated enough that miner’s could abandon hand tools. One of the first coal shearing machines was manufactured in 1960 by Anderson Boyes.
The shearer travels along the face of the coal seam, slicing large slabs of coal with a large drum or cutting head mounted on a large number of picks. The drums featured on shearers consist of two types. One type is the conventional pick-flushing drum that sprays water through spray jets mounted on top of the cutter drum vanes which varies from two to six vanes.[4] Spray jets located in the front of the cutter pick are known as pick face flushing (PFF) and in the back as pick back flushing (PBF). Wet cutting allows for dust particles to become wet as soon as the picks on the drum impact the coal and cut it. The water gets mechanically mixed with the cut coal. With an extraction drum, water is mixed with the cut coal as it gets discharged from the drum.
The shearers are designed to cut the coal seam in two different patterns—bidirectional and unidirectional. Undirectional cutting is the most common type of cutting pattern used in U.S. longwall mining in which the shearer travels tail entry to head entry. A leading drum is raised and cuts the coal while the trailing drum remains free from cutting or cuts the floor coal depending on the height of the coal seam. On the return trip back along the face the cleaning of any loose remaining is achieved. With bidirectional cutting, a web width of coal is cut in both directions of travel with each cutting trip requiring involving two-faced end operations to turn the machine around.[5] The machine is usually operated by remote control from a safe working position inside the underground mine.
Coal shearers also come equipped with self-haulage unit usually comprised of a conveyor belt system that transports the coal away from the face after it is cut. The most common conveyor belt system, called an armored face conveyor, travels under the shearers collecting the coal as it is cut.
When using a shearer in a longwall mining operation a temporary mechanical hydraulic roof support system is needed to stablize the mine’s roof. The roof supports, positioned behind the shearers, advance along the entire length of the coal seam face. As the coal shearer moves over the face, the roof supports are lowered and get push hydraulically up to the newly positioned shearer.
Today three types of coal shearers are used in longwall mining:
Double-ended ranging drum shearer (DERS) – are capable of extracting coal seams from 58 to 156 inches in thickness. They are used predominantly when the seam thickness exceeds 60 inches because the entire coal seam can be cut in one pass of the machine and in either direction of travel enabling a higher level of productivity and shorter roof exposure time.
Single-ended ranging drum shearer (SERS) – two of the machines are used simultaneously on longwall faces with a 60 inch mining height.
Single-ended fixed drum shearer (SEFS) – two of the machines are used simultaneously on longwall faces to extract thin coal seams of a 48 inch thickness.