Coal cutting by a ring-type jet head

Abstract

Research on hydraulic cutting of hard materials by high-and superhigh-pressure water jets [1][2][3][4][5] shows that the cutting efficiency is determined by the hydraulic parameters of the jets and their relative velocities.Jets issuing with high initial velocity from a nozzle undergo breakup (atomization) which increases with distance from tile nozzle [6][7][8][9][10]. The axial dynamic pressure and mean pressure gradually decrease.In coal cutting, as the depth of cut increases, the distance between the nozzle and file face also increases, so that the stresses arising in the plane of contact of the jet with the rock gradually decrease. This unvoidably affects the cutting efficiency (Fig. 1). From Fig. 1 we see that the water jet has greatest cutting capacity when the nozzle is very near the face (at all the velocities used).As a criterion of jet quality we take the length of the initial section [7,9,10], which gets longer with increasing nozzle diameter, retaining its high dynamic indices to greater distances. This, in particular, explains why the decrease in the depth of cut with distance is less rapid for larger nozzles (Fig. 1).Research on jet dynamics [7,9,10] has shown that a criterion of similarity of jets with different initial parameters is the length of file initial part within which the dynamic characteristics of the jet remain constant or change very little.This leads us to conclude that cutting should be conducted in such a way that the distance between file nozzle and the plane of contact of the jet with the cut material will not exceed the length of the initial section. In this connection it is of interest to consider the use of multijet hydraulic cutters with small nozzles mutually inclined at some angle ce.To test this method of coal cutting, with two-jet hydraulic cutters, we carriedout experiments under pit and laboratory conditions. We aimed at determining the optimum cutter constructions, the maximum distance (caving step) between adjacent slots, the basic laws of the process, and the optimum cutting conditions.The experimental apparatus included a ring-type cutting head and a group of rotary drives and feed drives. The cutting head was a comb-type manifold attached at its center to the shaft. A two-jet hydraulic cutter could be fixed on the manifold at various distances from the center, enabling it to be rotated with various radii (of 10to 50 cm). The drive for rotating the cutting head was hydraulic, consisting of a hydromotor and a hydropump of regulable throughput (type 5). The speed of the hydraulic cutter (the speed of jet translation) was given by vj --~cns/30,where vj is the ram of translation in m/sec, R c is the radius at which the cutter is set on file cutting head, in meters, and n s is the angular velocity of the shaft in rpm.The angular velocity of the shaft was linked by the transfer ratio of the drive system to the angular velocity of file hydromotor:where nhm is the angular velocity of the hydromotor in rpm, and t t is the transfer ratio of the drive s)~mm which rotates the c...