Nowadays, the double ranging-drum shearer is used predominantly. The shearer cuts the whole seam height in one trip. The two drums can be positioned to any required height��within the designed range��during cutting and lowered well below the floor level. The arrangement of the drums enables the whole seam to be cut in either direction of travel, thereby ensuring rapid face advance and shortening roof exposure time. There are various types of double ranging-drum shearers. Based on the location of the drums, there are two types: one with one drum mounted on each side of the shearer��s body and the other with both drums mounted on one side of the machine. The former type is the most widely used. Its advantage is that the shearer with one drum on each side can sump [29] in either direction. During the cutting trip, the leading drum cuts the upper 70% of the seam height while the rear drum cuts the lower 30% and cleans up the broken coal on the floor. The two drums are approximately 7��10m apart. When the shearer is traveling in the opposite direction to that of the face conveyor , the coal cut by the leading drum has to pass under the shearer��s body, which increases the moving resistance [30] of the shearer and the face conveyor and could cause a ��crowding�� [31] condition. If the broken coal is too large, it may block the shearer and stop the production. In general, when the shearer and face conveyor are traveling in the opposite directions, approximately 70% of coal taken by the leading drum will pass under the shearer. But when they are traveling in the same direction, the coal taken down by the rear drum together with the float coal from the floor constitutes approximately 30% of the coal that has to pass under the shearer. The former case consumes 25% more power than latter. As compared to the single-ended shearer, the underframe of the double-ended shearer is higher, thereby ensuring a sufficient cross section for coal passage.

Based on the method of adjusting the height of the cutting drum, there are also two types of shearers: ranging-arm shearer and gearhead shearer [32]. The former one is commonly used, whereas the latter one is a recent development. The advantage of the gearhead shearer is that the haulage unit is located at the center of the shearer��s body and amounted on the underframe. On both sides of the haulage unit, there is a gearhead. Each gearhead contains an electric motor and speed-reduction unit [33]. The gearhead is raised and lowered by an adjustable hydraulic ram. The adjustable range of cutting height is large. It can reach up to 1.4m.

Based on the mounting relation between the shearer and the face conveyor, there are also two types of shearers: the regular type which rides on the conveyor and in-web shearer [34] which moves on the floor in front of the conveyor. The in-web shearer is used mainly for the thin seams. As it moves along the face, the leading drum c

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