Editing Longwall mining (section)

==Equipment==
[[File:Schildausbau.jpg|thumb|Hydraulic chocks]]
A number of [[hydraulic jack]]s, called powered roof supports, chocks or shields, which are typically {{cvt|1.75|m}} wide and placed in a long line, side by side for up to {{cvt|400|m}} in length in order to support the roof of the [[coal face]]. An individual chock can weigh 30–40&nbsp;tonnes, extend to a maximum cutting height of up to {{cvt|6|m}} and have yield rating of 1000–1250&nbsp;tonnes each, and hydraulically advance itself {{cvt|1|m}} at a time.

[[File:Longwall with hydraulic chocks, conveyor and shearer.jpg|thumb|right|Hydraulic chocks, conveyor and shearer]]
The coal is cut from the coal face by a machine called the shearer (power loader). This machine can weigh 75–120&nbsp;tonnes typically and comprises a main body, housing the electrical functions, the tractive motive units to move the shearer along the coal face 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&nbsp;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&nbsp;revs/min to cut the mineral from coal seam.

[[File:Longwall2.png|thumb|Chocks providing support to allow shearer to work]]
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 {{cvt|10|–|30|m/min}} 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 [[Conveyor chain|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 the seam, the goaf increases. This goaf collapses under the weight of the overlying strata. The strata approximately 2.5 times the thickness of the coal seam removed collapses and the beds above settle on to the collapsed goaf. This collapsing can lower surface height, causing problems such as changing the course of rivers and severely damaging building foundations.<ref name="undergroundcoal">[http://www.undergroundcoal.com.au/pdf/Longwall_Mitchell.pdf "LONGWALL MINING"] BHP Billiton Mitsubishi Alliance (2005) accessed 19 December 2011</ref>