Published in the December 2005 American Longwall Magazine

This has been achieved through technical developments including new shearer downdrive designs and the BROADBAND Low Profile Chain armored face conveyor (AFC).

Ed Niederriter, engineering director for shearers, noted that as challenging as the low-seam mining environment can be, it is an inevitability the industry is facing. In recent years, lower-seam longwall mining has become increasingly necessary but it’s the mine’s perspective on that seam’s potential that is the key to keeping productivity up, according to Niederriter.

As a result, there have been a couple of gradual shifts that have been occurring in longwall mining in the past two decades in the United States. In particular, the number of longwalls has decreased from 92 to 52 in the past 17 years and at the same time the range of mining heights has narrowed somewhat. In the prior period, there were 12 longwalls operating in less than 5 feet. Five of these faces used plows while the other seven utilized shearers. Presently, there is just one plow longwall in the US and there are no shearer faces operating in less than 5 feet.

Several factors have brought about this change. A major issue was operating cost, since the lower longwalls tended to be less productive and therefore more costly to operate. As the realized price of coal declined in the previous period, these ventures simply were not economically viable.

In recent years, another factor preventing pursuit of the lower seams is that increasingly more robust equipment has become the standard in longwall mining. As operators demanded more powerful and longer lasting shearers and AFCs, these machines became taller and less suited to the thinner seams.

In 2005, Appalachian longwall reserves above 5.4ft become increasingly hard to find. The situation has resulted in a cluster of about 20 longwalls mining in the range 5.4-6ft. Many of these operations are forced to mine an increasingly large percentage of dirt and rock to achieve this extraction. Joy believes that a number of these operators would mine lower if suitable equipment were available.

As the available reserves become thinner, the equipment required to mine them efficiently becomes a necessity. Joy is meeting that demand with new machinery advances, including lower operating height shearers that continue to deliver the production levels operators demand.

Another development is Joy’s BROADBAND Low Profile Chain AFC Pan, which preserves a 42mm chain strength while lowering the sigma 1.26 inches. At the same time the AFC rack mountings are lowered, resulting in an additional .79-inch reduction in height.

Joy has also introduced a new downdrive, allowing the 7LS and 4LS shearers to be lowered on its popular Ultratrac 2000 haulage system. This change brings the body of the machine an additional 3in closer to the AFC.

The concept is simple: if the minimum shearer height on the pan is 5in lower, the mining height can be lower by the same amount (assuming the same required running clearance under the shield canopies).

While a number of US mines operate very successfully mining some stone in addition to the coal, there are compelling reasons to minimize the amount of stone that must be cut. These can include:

Lower cleaning cost.

Reduced bit cost.

Reduced abrasive wear on shearer drums and AFC components.

Improved reliability of shearer.

Less refuse to dispose of.

Cleaning plant capacity is limited; mining less rock would allow a mine to process more saleable product.

Mining rock consumes more horsepower per unit than coal; mining less rock usually allows faster shearing speeds.

A major factor that determines the magnitude of benefit is the make-up of the stone. If the adjacent material is soft, the operator may see little advantage in mining lower. However, when a hard and abrasive material borders the coal seam, the mine will find it much better to avoid extracting it altogether.

Joy now has a complete set of components allowing operators the luxury of reducing their operating height to 4.75ft. This is all being done with a conventional shearer and AFC configuration, giving customers a familiar, low-risk option.