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room-and-pillar mining Hydraulic Traction Drive Prevails at Steep Inclines Tough deposits need tough cutting machines BY DR.-ING. ULI LANGE currents drop off peak and return to nor- mal level. The main hydraulic pump (See Figure 1, No. 8) is a variable displacement pump driven by a 125-kW AC motor (See Figure 1, No. 7). Both are part of the load-sens- ing circuit which automatically adjusts the volumetric flow according to the load on the respective cylinders and hydraulic motors. Instead of using a simple bypass system, the variable displacement pump avoids significant heat losses—saving energy which has a positive impact on the availability and lifetime of the con- tinuous miner. The oil simply does not overheat because only the required amount of oil is pumped toward the actuator. A new Eickhoff CM2 is being offloaded in the box cut at Halfgewonnen. About six years ago, Eickhoff introduced its CM2 series continuous miner into South African mines. More recently the machines have been cutting coal in low seam condi- tions (1.5 m) as well as high seams (4.5 m). All of them are equipped with modern hydraulic traction drives. In South Africa, this hydrostatic trans- mission system faced challenging condi- tions and high inclinations where it could prove its great potential, toughness and reliability while propelling a cutting con- tinuous miner uphill, which was reassur- ing in such undesirable geology. The tramming and turning functions of a CM2 are shown in Table 1. The designers put a special emphasis on hydraulic func- tions to come up with both simple and robust solutions that are smart enough to save energy and oil, but at the same time, meet the operators' demands for more power as well as faster tramming speed and proportional sumping and cutting. The unit's hydraulic circuit consists of two requirements. Some cylinders and 186 www.coalage.com hydraulic motors need a constant volu- metric flow rate and pressure, while others vary in speeds or have varying loads and therefore require a continuous adjustment of volumetric flow and pressure. The central valve block (See Figure 1, No. 1) includes conventional on-off- valves for manipulating the conveyor, spade and stabilizer cylinders (See Figure 1, Nos. 1, 2, 3 and 4). Proportional valves in the valve block are part of sumping and shearing control loops protecting the cutter motors and cutter gearboxes. These control loops measure the cutter motor currents while the machine sumps into the face and shears down simultane- ously. Once the cutter motor currents exceed the adjustable threshold, relevant proportional valves close. Accordingly the boom (See Figure 1, No. 5) deceler- ates its movement and the traction motors (See Figure 1, No. 6) push the machine more gently forward to relieve the cutter motors of load just until their 100th Anniversary Special Issue In addition to the main pump, an aux- iliary gear wheel pump (See Figure 1, No. 9) serves those motors which require a constant volumetric flow rate and pres- sure as is the case with the dust scrubber unit (See Figure 1, No. 10). Several tanks (See Figure 1, No. 11) in a serial setup with the main tank con- taining a pressure relief valve, a level indicator and a temperature sensor are part of the protection system. Filters (See Figure 1, No. 12) continuously clean the oil while differential pressure gauges indicate filter condition. The refilling unit (Figure 1, No. 13) consists of a filling filter and small hydraulic motor driving a pump that can easily be used by the operators to recharge the machine with new clean oil. Even though the traction motors are hydraulically driven, the entire hydraulic circuit only contains a relatively small amount of hydraulic oil (600 l). Heat exchangers (See Figure 1, No. 14) cool the oil as long as the water sup- ply is maintained at the required levels. In the case of tramming the machine without water, the continuous miner is designed to drive through the section for at least 20 minutes at moderate ambient temperatures. If tramming distances are August 2012