coalface. An individual chock can weigh 30–40 tonnes, extend to a maximum cutting height of up to and have yield rating of 1000–1250 tonnes each, and hydraulically advance itself at a time. The coal is cut from the coalface by a machine called the shearer (power loader). This machine can weigh 75–120 tonnes typically and comprises a main body, housing the electrical functions, the tractive motive units to move the shearer along the coalface and pumping units (to power both hydraulic and water functions). At either end of the main body are fitted the ranging arms which can be ranged vertically up down by means of hydraulic rams, and on to which are mounted the shearer cutting drums which are fitted with 40–60 cutting picks. Within the ranging arms are housed very powerful electric motors (typically up to 850 kW) which transfer their power through a series of lay gears within the body and through the arms to the drum mounting locations at the extreme ends of the ranging arms where the cutting drums are. The cutting drums are rotated at a speed of 20–50 revs/min to cut the mineral from coal seam. The shearer is carried along the length of the face on the armoured face conveyor (AFC); using a chain-less haulage system, which resembles a ruggedised rack and pinion system specially developed for mining. Prior to the chainless haulage systems, haulage systems with chain were popular, where a heavy duty chain was run along the length of the coal face for the shearer to pull itself along. The shearer moves at a speed of depending on cutting conditions. The AFC is placed in front of the powered roof supports, and the shearing action of the rotating drums cutting into the coal seam disintegrates the coal, this being loaded on to the AFC. The coal is removed from the coal face by a scraper chain conveyor to the main gate. Here it is loaded on to a network of conveyor belts for transport to the surface. At the main gate the coal is usually reduced in size in a crusher, and loaded on to the first conveyor belt by the beam stage loader (BSL). As the shearer removes the coal, the AFC is snaked over behind the shearer and the powered roof supports move forward into the newly created cavity. As mining progresses and the entire longwall progresses through