| The dominant position of China’s energy system is occupied by coal,but the inefficiency of traditional mechanical cutting methods of complex coal seams has always been a difficult problem.As the reserves of high-quality coal seams have been greatly reduced in recent years,how to find a mining method with high applicability to complex coal seams has become a scientific issue in the intelligent construction of coal mines.Aiming at the hard and brittle characteristics of most deep coal seams,a combination of theoretical analysis,numerical simulation and field test was adopted based on the intersection of mechanical design and mining engineering disciplines.The constitutive model of brittlely hard coal was analyzed.The reformed structure of the shearer drum for the coordinated coal crushing by impact combined cutting was designed.On the premise that the temporal and spatial relationship of the impact sequence of the picks was considered,the movement trajectories of the drum under different impact sequences and their pre-splitting and crushing characteristics for brittlely hard coal were analyzed.The coal crushing criterion under the synergistic effect of multi-pick impact combined cutting was revealed.A multi-objective optimization model for the cooperating coal crushing performance of impact combined cutting was established.The field test of coal crushing by impact combined cutting was carried out,and the theoretical system of coal crushing by impact combined cutting was constructed.The main results obtained in this paper are as follows:The limitations of existing brittleness indices were pointed out according to the step-drop-type stress-strain curve of coal.The energy evolution law of the coal before and after the stress peak of loading conditions was analyzed.The brittleness calculation method of brittlely hard coal was obtained,and the brittleness grading of coal was clarified in the calculation method of the obtained brittleness.The damage model of brittlely hard coal was established,and the concept of damage transition point was proposed according to the deformation stage characteristics of brittlely hard coal.Which was,the effective pre-fracture pressure range of brittlely hard coal should be between the damage transition stress and the uniaxial ultimate compressive stress.Uniaxial compression experiments of conventional coal specimens were simulated by EDEM.Five coal specimen models with different shear modulus and bonding stiffness were established,and the influence characteristics of discrete element constitutive parameters on coal brittleness were analyzed.Finally,the discrete element constitutive model of brittlely hard coal was established according to the quantitative relationship between coal brittleness,shear modulus and hardness.The structure of the drum with impact combined cutting was designed.The kinematics models based on the parameter equation of the space coordinate of the drum were established under the conditions that the synchronous and asynchronous impact motion of the picks and the rotation and traction motion of drum were comprehensively considered.The coal with brittleness value of 0.7 and hardness of f4,f5,and f6 were taken as the scheme,and the discrete element models of different impact space-time relationships of picks were established.The average and peak value of the force on the drum are lower than those of the asynchronous impact,and they are more stable than the asynchronous impact.The coal crushing simulation of traditional drum(without picks impact)was carried out under the same hardness condition.Based on the Evans coal-crushing mechanics model,an impact combined cutting coal-crushing mechanics model was established by single pick.The stress wave model of coal crushing by impact pick was established with reference to the rock crushing by water jet.The analytical solutions to the radial displacement and radial velocity of the stress wave particle in the joint cutting mode were obtained.The development law of coal fissures under the impact on the pick was analyzed.The pre-splitting action range of the impact on single pick was determined.And the theoretical models of coal crushing by single pick were verified based on DEM.A spatial model of stress wave energy transfer from multiple pick impacts was analyzed.The triaxial stress components of the stress wave of each stage were calculated,and the coal crushing criterion under the synergistic effect of impact combined cutting was revealed based on the established spatial stress wave model.In addition,the difference between pre-splitting effect of synchronous and asynchronous impact sequences was explained.Which was,the stress waves of different sizes are induced due to the impact time difference of each pick in the asynchronous impact mode.Conflicts from various energy propagation processes are generated,and the stronger stress waves are difficult to form.Therefore,the pre-splitting effect of asynchronous impact is weaker than that of synchronous impact.A multi-objective optimization model based on the synergistic coal crushing performances of impact combined cutting was established with productivity,cutting specific energy consumption and load fluctuation coefficient as optimization objectives.Traction speed,drum speed,impact strength and impact frequency were used as design variables.Constraint functions as cutting force,cutting power,traction force,traction power,pick impact strength and pick impact frequency were proposed.And the simplified model of MG500/1180-WD shearer was used as the scheme.The optimized results of different cutting impedances at a brittleness value of 0.7 were obtained.The coal crushing performances of the optimized parameters and the original parameters were compared with the hardness of f4,f5 and f6.Therefore,the coal crushing performances of the drum with optimized parameters were effectively improved.The impact combined cutting test device was designed.The no-load operating characteristics of the test device were tested based on different pick impact frequencies and different pick impact strengths.The test results showed that there are no movement interference and oil leakage of each scheme.The underground coal crushing test with and without impact combined cutting of the pick was carried out.The test results showed that the phenomenon of end coal was improved by the method of impact combined cutting.In addition,it could be judged that the coal crushing performances of the impact combined cutting method were better than that of the traditional cutting method.The paper has 63 pictures,35 tables,and 194 references. |
