Experimental And Numerical Research On Rock Cutting By Roadheader Picks

The excavation of coal-rock tunnel by roadheader is an efficient, continuous, economic method, and it has the superiority in the fields of mining and geotechnical engineering. For a long time, mechanical hard rock fragmentation is studied deeply by domestic and foreign scholars in terms of theory, numerical simulation and experiment, and obtained many achievements. However, there are still many problems to be improved. Such as, failure mode of the rock breakage mechanism had different opinions; The application of roadheader’s performance prediction model had limitations; A variety of numerical methods had been applied to rock cutting simulation, in which the particle flow discrete element method had great advantages, but the rock-tool model is too simple; Previous experimental studies mainly carried out on the linear cutting machine, and it is inconsistent with the actual working conditions. Aimed at the above problems, this paper will further research on the rock breakage mechanism by conical pick, rotary cutting tests by single pick and the corresponding numerical simulations, the establishment of roadheader’s performance prediction model.A method is presented to find the optimal cutting condition. According to the design parameters and motion parameters of roadheader in the actual working conditions, the test scheme is determined under the conditions of different cutter spacing and cutting depth. The effects of the ratio of cutter spacing and cutting depth(s/d) on the specific energy, forces and coarseness index are analyzed, and the optimum s/d is 3~4; When the cutting depth is given, the pick forces first increased and then tend to be stabilized with the cutter spacing increased; Specific energy and coarseness index showed an inverse proportional exponential relationship and has a good correlation; The rock breakage mechanism by conical pick has been discussed, and the rock breaking process is divided into five phases, i.e. the generation of fine chips in a primary crushed zone, the formation of nucleus and its energy accumulation, the extension and connection of cracks, the falling of fracture and the regeneration of fine chips in a secondary crushed zone, besides, the fracture was dominated by tensile failure and accompanied by the squeeze and shear failure.Based on the experimental research of the single pick rotatary breaking, the wear characteristics of the conical picks with different pick tip materials are discussed. The high speed camera and the infrared thermal imager are used to monitor the cutting process, and found that the alloy steel pick and alloy steel pick coated on wear resistance material appeared significant fire, high temperature caused the hardness of pick reduced, it became more prone to wear, so thermal fatigue wear is an important factor which affects the pick failure. Calculation results show that the mass loss of cemented carbide pick, alloy steel pick and alloy steel pick coated on wear resistance material were 0.07%, 0.28% and 0.22%, there are prominent advantage using cemented carbide as pick tip material, and surfacing welding the wear-resistant coating on pick has protective effect.According to the linear cutting tests performed by previous researchers and rotating-cutting tests in this paper, the particle flow discrete element software PFC3 D was used to simulate the rock cutting tests. At first, the cutting processes are simulated in the unrelieved and relieved cutting mode. The results showed that the interaction of adjacent picks can be effectively reduced the forces during the rock breaking. In addition, the number of fractures by tensile failure is 3 times by shear failure, it was proved that tensile fracture failure of rock was in a dominant position from the microscopic view. Secondly, the particle assembly model represents the sandstone witch used in rotary cutting test is established, and the simulated cutting tests are performed in different cutter spacings and by the picks before and after wear. The destructive effects of rock specimen under the conditions of different cutter spacings are reproduced. It was concluded that the results obtained in numerical simulation are agree well with the experimental results, and the specific energy can be predicted accurately.According to the prediction formulas of pick forces proposed by N.Bilgin, a new model for predicting the instantaneous cutting rate(ICR) has been developed. In accordance to the design parameters of the cutterhead of EBZ260 W axial-type roadheader, the sequence of the picks in contact with rock and the variation law of cutting depth, a calculation procedure has been programmed. The simulation of each operating mode was carried out in accordance with the artificial rock cutting tests. The results indicated that the predicted ICRs were consistent with the average ICR obtained in artificial rock cutting test and field test respectively. In addition, the new ICR prediction model is also supported by the empirical prediction formulas developed by other researchers.Aimed at the difficult problem of the cutterhead worn seriously when excavating the hard rocks, experimental and numerical studies were researched on rock breakage mechanism by conical pick, the effect of rock breaking by the different ratio of cutter spacing and cutting depth and the wear characteristics of pick, and a new performance prediction model for roadheader has been presented. The method which combines theory, experiment and numerical simulation can be used for the optimized design of cutterhead economically and effectively, which has a certain value in engineering applications.