By Dr.-Ing. Uli Lange

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 conditions (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 transmission system faced challenging conditions and high inclinations where it could prove its great potential, toughness and reliability while propelling a cutting continuous miner uphill, which was reassuring 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 functions 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 hydraulic motors need a constant volumetric 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 simultaneously. Once the cutter motor currents exceed the adjustable threshold, relevant proportional valves close. Accordingly the boom (See Figure 1, No. 5) decelerates 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 currents drop off peak and return to normal 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-sensing 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 continuous miner. The oil simply does not overheat because only the required amount of oil is pumped toward the actuator.

In addition to the main pump, an auxiliary gear wheel pump (See Figure 1, No. 9) serves those motors which require a constant volumetric flow rate and pressure 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 containing 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 supply 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 expected to be longer, an optional auxiliary heat exchanger integrated within the dust scrubber can cool the oil and extend the travel distance.

The hydraulic scrubber motor (See Figure 1, No. 10) can vary in size individually suiting the customers’ requirements for volumetric air flow according to any extraction heights. This type of motor is considerably less noisy than electrical comparatives and can run in silent mode while tramming the machine.

Field Proven Facts
Just like wheel loaders, continuous miners usually move slowly requiring high pulling forces while at the same time often have to change the moving direction so that they almost permanently operate under varying working conditions [Fecht 2009, p.20]. Hydrostatic transmission drives have prevailed against hydrodynamic or mechanical systems at slow and dynamically moving mining machinery because of their productivity [Renius 2003, p.1]. Hydrostatic transmission meets the demands for maneuverability and a broad range of torque-speed conversions [Rydberg 1998, pp.1-2]. The hydraulic system provides high starting torque, which is important for sumping into hard coal or sumping uphill, as well as fast tramming speeds for quick place change operations in a room-and-pillar section.

More advantages of the hydrostatic transmission are:

  • Easy overload protection: Protection of the hydraulic circuit against shocks is achieved by pressure relief valves.
  • Less shock on the gearboxes: Hydraulic motors have a very low rotating inertia compared to electric motors which delivers an advantage when it comes to external blocking.
  • Easy to measure and maintain: Measurements of wear at the hydraulic motors through optical leak oil flow meters (glasses). Generally hydraulic motors in this power class have an approximately tenfold increased power to weight ratio compared to electrical applications. In case of wear, hydraulic motors can easily be changed underground as their weight is a mere 80 kg. 
  • Lower investment: Hydraulic motors in this power class are five to 10 times less expensive than electrical motors and in addition highly priced inverters are not necessary. As long as the oil is kept clean, the hydraulic motors have a better Mean Time Between Failure than VFD drives. 
  • Less electrical issues: Especially when operating in water and muddy floor conditions a hydrostatic transmission is advantageous because there is no possibility of earth faults.

Eickhoff continuous miners are driven by two hydraulic two-stage axial piston motors. The fast tramming mode at 25 to 30 m/min still has sufficient torque for travelling through dips. Speed and torque can be adjusted to the individual mining conditions by setting the minimum displacement at the motors. In normal tramming mode at 10 m/min, the torque is sufficient to climb up very steep roadways as described later on.

Experts optimized hose cross sections, fittings’ shapes and found best hosing for minimum bending in order to reduce energy dissipation due to inner friction down to an inevitable minimum.

Uphill Mining at Halfgewonnen
Halfgewonnen Colliery is a room-and-pillar coal mine owned by Sudor Coal and the underground workings are operated by the STA Coal Mining Co. The mine is located 20 km north of Bethal in South Africa’s Witbank-Highveld coalfield east of Johannesburg in Mpumalanga. This coalfield is characterized by flat to slightly undulating seams that are penetrated by sills and dykes up to several meters thick. In some areas tilting and vertical displacements occur [Jeffrey 2005, pp.95-97]. The coal deposit comprises four seams which form a syncline in this area. The life of the mine amounts to about 15 years according to its current mining leases.

Ever since the mine started operating in October 2006 it has successfully used Eickhoff continuous miners. The two underground sections of the mine have an annual output of approximately 1.2 million metric tons run-of-mine (RoM) coal which is extracted by 107 underground and surface personnel operating two continuous miners.

Access to the No. 2 seam is done via a 50-m deep box cut. Initially the mine developed this 3- to 3.5-m seam along the bottom of the syncline in a northeasterly direction. To access further reserves lying to the northwest of the syncline required uphill mining at a gradient of about 9° over a length of 520 m or 34 pillars. In certain areas the Eickhoff continuous miner had to cut uphill on inclines of more than 10°. Typical room-and-pillar mines in steep seam mining applications advance in the dip direction of the seam [AUSIMM 2009, pp.325] instead of the uphill direction.

This performance is proof of the CM2’s engineering and hydraulic system, and how an operator can rely on it when it comes to challenging mining conditions.

References
2009 AusIMM, AustralAsian Coal Mining Practice, handbook published by AusIMM, Carlton, Australia.

2009-1/2 Fecht, N., Hydraulisch elegant statt mechanisch (“Hydraulically elegant instead of mechanical”). Paper published in FLUID 1-2/2009, Verlag Moderne Industrie GmbH, Landsberg, Germany.

2005-02 Jeffrey, L.S.,  Characterization of Coal Resources in RSA–The Journal of the Southern African Institute of Mining and Metallurgy, SIMM, Johannesburg, RSA.

2003 Renius, K.Th., Hydrostatic Transmission for Mobile Machines (“Hydrostatische Fahrantriebe für mobile Arbeitsmaschinen”). Paper at VDI conference Transmission Concepts for Off-road Purposes (“Antriebssysteme für Off-Road-Einsätze”). VDI Verlag GmbH, Düsseldorf, Germany.

1998 Rydberg, K.-E., Hydrostatic Drives in Heavy Mobile Machinery—New Concepts and Development Trends, SAE Paper No. 981989, SAE, Warrendale, USA.

Author Information
Dr. Lange is the product manager for Eickhoff’s line of continuous miners.