The main difficulty in designing entry drifting machines for hard rocks is to achieve a compromise between a design to meet the high loads on the mechanical rock-cutting tool, necessary for effective rock breaking, and the requirement for stability of the tool, which decreases as the load increases. At present this problem is resolved by making a large tool (usually of the roller-blt type) from the strongest grades of alloy steel, reinforced with hard alloys. However, even in this case the stability of the bit is insufficient for effective cutting of hard rocks. The high cost of replacing the rock-cutting bit is crucial in an economic comparison between entry drifting machines and drilling and blastln E. It would seem that in the immediate future entry drifting machines with mechanical rock cutting will not be able to compete in productivity and cost with drilling and blasting in hard rocks.In recent years there have been extensive developments in research and development on new methods of rock breaking [I, 2].Comparing the electrothermal and mechanical methods of breaking, we see [I, 3] that the curves for their efficiencles vs the hardness of the rock are of opposite natures [2], i.e., the region of most favorable application of the electrothermal method corresponds to the least effective utilization of the mechanical method, and vice versa. Therefore a combination of these methods in an entry drifting machine will result in a capability to cut rock of any hardness with adequate efficiency, i.e., with comparatively low relative loads on the mechanical rock-breaking tool at the face we can obtain high rates of advance. This is supported by theoretlcal calculations and laboratory investigations [4].The State Scientlfic-Research Institute of Minlng-Chemical Raw Materials (GIGKhS), the Moscow Mining Institute (MGI), and the production union "Apatit" have built an experimental electrothermomechanlcal entry drifting machine, the PK-8~TM. The general appearance of its cutting head is shown in Fig. i. It is based on the PK-8 production-model entry drifting machine, designed for driving arch-shaped workings measuring 8-9 m a in nonabrasive rocks with f ~4" (mainly in potassium and sodium chlorides), in which certain units were modified: the drill bit of the cutting head was prevented from rotating and rigidly fixed to its cross. To the cross is attached a rock-breaking tool, and the remaining space in the cutting head is fitted with infrared generators (IRG). The berm cutters and cutting drums are removed. To get the machine into the cross section of the working, the arms with the scoops on the cutting head were set at a diameter of 3.6 m. The machine was advanced and maneuvered over the ore infill or along crosstles laid under the caterpillar tracks. It was powered from two TKShVP-320/0.4 substations; energy was supplied to the IRG via a ring-shaped brush fixed between the reducing gear and the cutting head.After the modifications, the PK-8~TM machine has the following characteristics: shape of cut working, circular; ...

Workers at the "Apatit" combine, the State Imtitut~ of Mining-Chemical Resources, and MoscowMining Institutehavebeen cooperating in research on thedevelopment of drifting machines forunderground workings In the "Apatit" combine. The workings were drtven through ore. The rocks Include apatite, nepheline, urtite, and other minerals In small amounts. The rocks are maculose, banded-maculose, or banded in structure. The physical properties of the ore depend on its composition, which varies considerably. The hardness of the ore on the Protod'yakonov scale varies from 8 to 16. The tensile strength is 40-90 kg/cm z. Young's modulus is (5-9) 9 l0 s kg/cm z, the linear thermal expansion c06ffictent Is (0.7-2) 9 10 -5 deg-', and the specific heat Is 0.8-1.2 J/g 9 deg.The hardness of the ores and rocks (above eight) prevented the use of cutter-loaders with mechanical rock breaking; therefore, electrothermal breaking was used. First a careful Investigation under operating conditions was carried out; the main laws of electmthermaI breakage and the economic factors were determined, various electromagnetic energy sources were investigated, and the most viable design was chosen and served as a basis for the "business ends" (cutting heads) of the drifting machines.To determine the economic requirements imposed on drifting machines we drove short workings of small cross sectibn with hand rock loading and electrothermal breaking. The data obtained were used in designing experimental models of the drifting machines. We designed two types of cutter-loader for cutting horizontal workings and a machine for driving vertical shafts.The first experimental cutter-loader was based on the PNf3-3K loading machine. In front of the loader we attached a rectangularelectrothermalinstmment: its dimensions and hence those of the driven working, coutd be varied between 5 and 7 m z. The power consumption was 136 kW; it was supplied from the 0.4 kV ac pit power lines. Energy was transmitted to the rock without contact, and the instrument did not exert any mechanical action on the face; as a result it did not weigh more than 120 kg. Breakage began when the instrument-face distance was 0.8-1 m. When this distance was reduced,the energy consumption of breakage decreased, and the rate of advance increased. The rock broken from the face fell onto the floor of the working and was picked up by the loader. For a working cross section of 5 m 2 this cutter-loader gave a rave of advance of 0.3 m/h with an energy consumption ofg0 kWh/m s and a speci~c power at the face of 2.7 W/cm 2. With a working 7 m z in cross section we obtained a rate of advance of 0.28 m/h, an energy consumption of 70 kWh/m s, and a specific power of 1.9 watts per cm 2 of face. The energy consumption was defined as the ratio of the energy taken from the lines to the total rock broken down in the same time. The chief disadvantage of the machine was that the broken rock fell onto the floor of the working, and the loader could not pick it all up, so that the floor was badly cut and the working ...

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