Continuous miners for Underground Coal

Though there are many variations in design, continuous miners mostly consist of five main elements:

• A central body to carry all other components mounted on some type of drive mechanism to provide mobility (most commonly caterpillar tracks).
• A "cutting head" usually rotating drum(s) and/or chains with cutting picks attached
• A loading mechanism to pick up cut coal and deliver it into the central part of the machine
• A conveying system, usually a chain conveyor running in a steel trough from front to rear of the miner
• A rear jib section capable of a degree of vertical and horizontal movement to enable the coal to be delivered into a transport or loaded at a desired point.

Some continuous miners (at one time almost all) could not cut the full roadway width in one pass but had to be moved backwards and forwards and from side to side in order to cut the full profile. This often results in a very rough rib line (bad for stability and ventilation flow) and delays the ability to install support into/under freshly exposed roof for a period. The advantages of the ability to cut the full profile in one pass was recognized early, but was not easy to achieve. Cutting forward in a straight line could be readily accommodated, but it is necessary to be able to turn corners, mostly at right angles, and to be able to retreat the cutting machine from one roadway to relocate at frequent intervals. These factors have proved major stumbling blocks to many developments. In machines which covered the full face, steering in the vertical plane could also be a major difficulty.
The term "continuous" as applied to development machines has been one of the biggest misnomers used for mining equipment because, on development, they are usually anything but continuously cutting. The main delays mostly occur while roof support is installed and often waiting for shuttle cars to return from their discharge point for reloading. When actually cutting, cutting rates are usually more than adequate, but when averaged over a shift cutting rates are often poor and this is one of the reasons many mines have difficulty developing at rates adequate to prevent long delays on longwall production. As a result, development is an area receiving major attention in recent times. Many of the difficulties have been overcome and most modern continuous miners are "full face" machines. They also have roof bolting equipment mounted on the miner in locations allowing roof bolts to be installed reasonably close to the face.
The "ideal" continuous miner would:

• Be able to cut the full face in one pass
• Be easily moveable between locations without dismantling parts
• Be able to excavate right angle turns with a minimum radius
• Have roof and rib bolters fixed to the machine in a location where each row of the designed support pattern can be installed without moving the miner and be installed close to the cut face if necessary
• Have adequate space alongside to allow good ventilation of the face area for efficient removal of gas and dust.
• Allow strata supports to be safely installed while coal cutting continues

Many of these ideals have been achieved to varying degrees of satisfaction (the last being an area of minimal success), but matching development rates with longwall retreat rates is still a major problem for many mines. Increasing numbers of development units to attain longwall continuity is expensive and provides extra strain on all other service functions (personnel, ventilation, materials handling, power and water supplies, gas drainage, etc, etc) and is not usually a satisfactory solution.