When rock is broken up by the cutting head of a dynamic plow, a part of the impact energy is expended on the generation of elastic waves which are attenuated as they travel through the rock. The energy carried by the elastic wave is lost unproductively in the rock, and the magnitude of the loss depends mainly on the strength parameters of the rock, together with its fracture parameters and the total energy of the impact.in bench tests of a powerful dynamic plow on a coal-cement block, we made special investigations in order to estimate the amount of energy expended on generation of an elastic wave for various impact energy values. For this purpose, in the coal-ce~nent block at various distances from the cutting edge of the head of the plow, we measured the intensity of the elastic wave emitted at impact.The stresses in the wave were rectxded with specially developed three-component temometric gages fixed in the block. Each gage was a cube, 60x 60x 60 mm in size, made of strong cement. In the cube was embedded a shaped steel p.late, to which wire strain gages were glued in three planes and carefttlly insulated. Before being fixed in the block, each cube was calibrated in a press. The signals from tlm strain gages were amplified by a UTSI-VT-12 rensometric amplifier (transmission band 0-7 kI-Iz) and recorded on photographic paper by a K-10fi oscillograph.The fracture parameters of the rock at the time of the measurements were as follows: the thickness of the plowed slice varied between 15 and 20 cmi the angle of attack of the blade was 10"; and the total impact energy for one blade varied between 200 and 700 kg-rn. The resistance of the coal-cement block to fracture Ok) was found with the aid of an SDM dynamometer drill, and averaged 240 kglcm. The scheme of the measurements is t ~,~Y ~ ~=2o cm l~s;l-VZ-19 ~=={KsI~

In impulsive rock breaking, the question of the best shape of the active force pulse on the bit is very ~mpor-rant. It is involved in the design of powerful impulse planes.The problem of the optimum relation between the duration of the active force pulse and the natural period of the bit-rock system was discussed by Dokukin [1]. The optimum ratio is considered to be that corresponding to the maximum dynamicity coefficient 15. The dynamicity coefficient is defined as the ratio of the static force to the amplitude of a force pulse of finite duration with an equivalent action [2]. Optimization of the dynamicity coefficient involves trying to get the maximum breaking effect per unit force pulse amplitude. The system bit-rock is regarded as a linear one-mass vibrating system with rigidity (quasielasticity) k and damping H.The frequency properties of such a system depend on two constants: the natural frequency u) 0 = 2rr f0 = 4 k/M and the reduced damping factor g = H/2M w0, where M is the mass of the bit. According to determinations made in tests of impulse planes on a coal--cement block [1], g = 0.6-0.7.If the active force pulse is a half-period of a sine wave, with duration At, the dynamicity coefficient of the bit-rock system depends on the ratio of the duration of the active force pulse to the natural period of the system (see Fig. 1). Let us consider the curve for g = 0.6 in more detail. For small values of the argument, for 0 _< 2At/T 0 = 2f0At _< 0.7 the curve 8 =/3 (2At/T 0) coincides with a straight line making an angle of 45" to theaxis. In this case the dynamicity coefficient is equal to the ratio of twice the force pulse duration to the natural period, B = 2At/To = 2:oAt.The static force equivalent to the pulse is To the right of the value At = 0.35T 0 or 2At/T 0 = 0.7, the curve for the dynamicity coefficient begins to flatten out, and the static force equivalent to the pulse p becomes less than 4f0 p. In this range the active energy for fracture of the rock is not fully utilized, although the coefficient of dynamicity continues to increase with the pulse duration up to 2At/T 0 = 2. Finally, for pulse durations greater than T 0, the impulsive action becomes equivalent to a static value which is equal to the force pulse amplitude.The right-hand boundary of the region of complete utilization of the energy on fracture, AEop t = 0.35T 0' is the optimum duration, because it permits fracture with a dynamicity coefficient 15 which is maximal forthe region in which a fall in static force equivalent to the pulse is still not observed. The optimum duration depends on the natural frequency of the bit-rock system, which is in turn governed by the strength properties of the rock, the A. A. Skochinskii Institute of Mining, Moscow.

In connection with the construction of powerful activated ploughs with impact energies of 700-1000 kg 9 m [1-3], it is becoming important to investigate the optimum conditions of force interaction between the blade and the cut rock. It is known [4] that the effectiveness of the impact method of cutting largely depends on the duration of the rising front of the power pulse, its shape, and the physical properties of the cut medium. The aim ofthemethod suggested in this article is to estimate the elastic and viscous properties of the blade-medium system and to determine the parameters of the power pulse which will give the maximum breaking effect.In static breaking, which is realized in the case of a continuous power action, the loading characteristics for driving the blade into the coal can be fairly accurately approximated by a linear relation. With this characteristic, the resistive forces increase in proportion to the depth of penetration; the mechanical resistance is quasielastic and can be represented by a stiffness K [5] which is equal to the angular coefficient (slope) of the load characteristic.

The external load on a mining machine, determined by the forces of resistance, is one of the principal conditions for establishing the main parameters of the machine, i.e., the strengths of its elements and the operating routine (depth of cut, speed of cut, width of web, etc.).As shown in [1], during operation the initial external loads acting on the cutting tool are transformed in the various elements of the transmission according to definite laws. The loads on these elements can thus be either greater or less than the initial external loads.The type and magnitude O f the load on any element of the drive (in the electric motor, on the transmission elements, etc.) can be determined if we know the external forces and dynamic (operative) characteristics of the drive. In this case the load on any element in the drive can be found from the symbolic expressionwhere A t is the "system operator," which indicates which operation must be performed on the external load function x(t) i so as to obtain the load acting in the i-th element of the drive.It is assumed here that we know the external resistance forces and that these are given as functions of time.The magnitudes and types of the loads in the drive elements can thus be affected by changes in the dynamic parameters of the machine and also in the operating conditions corresponding to various external loads.In this connection the following problems may arise in the construction of modern high-productivity mining machines with high reliability indexes: a)for the most representative characteristics of the external load, to choose optimal parameters for the drive so as to obtain a permissible level of dynamic loading on the elements or on the weakest link of the drive, together with the required level of reliability; b)given the permitted level of dynamic loading of the drive, to choose the operating conditions so that they shall correspond to the best technical indices (productivity, coal grade, power capacity, etc.) for the given properties of the rock in the given conditions; c) in the construction of automatic machines, to arrange that the power expended by the motor is constant, and to introduce control corrections for the dynamic loading of the drive which shall enable the equipment to operate with high reliability in fulfilling its main technological requirements.Owing to the effect of various random factors which are decisive in the process of forming the resistance of the rock to the cutting tool of a cutter-loader, digger, etc., a complete analytical solution of these problems is possible using the theory of random functions, with the use of electronic computers.In connection with experiments on new mining techniques, various oscillograms are used by researchers to characterize the formation of the external loads on the cutting tools. 37

No abstract