Exploring dynamic parameters and tooth structure of self-propelled borehole repairing drill bits

Self-propelled borehole repairing drill bits provide an effective technique for repairing long boreholes that have failed. In order to further improve the borehole repair speed and gas drainage efficiency, based on the theory of jet recoil, the relationship between the jet recoil force and nozzle parameters was analyzed, and a theoretical model was established for rotational mechanics of self-propelled borehole repairing drill bits. Subsequently, the coal-breaking processes of drill bits with different tooth structures were simulated using LS-DYNA, the failure modes in coal mass broken by cutting teeth and the rules of variation in the volume of coal broken by cutting teeth (coal-breaking volume) were analyzed under different drilling force and torque conditions, and coal-breaking experiments were conducted to verify the numerical simulation results. In addition, a method for determining the parameters of self-propelled borehole repairing drill bits was established, and a self-propelled borehole repairing drill bit was designed and used in the methane drainage roadway at the working face of No.22 Coal Pillar in Chaohua Coal Mine (Zhengzhou Coal Industry (Group) Co., Ltd.) for experimental purposes. The results show that (1) the nozzle opening angle (α) and eccentric angle (β) are key parameters that determine the distribution of drilling force and torque, and the optimal distribution of drilling force and torque can therefore be achieved by adjusting these two parameters. (2) Under the same drilling conditions/parameters, tooth structure has a great impact on the coal-breaking volume, stepped teeth have the best coal-breaking performance, and when the drilling force is 120 N and the torque is 0.6 N·m, a drill bit with stepped teeth can achieve the optimal distribution of drilling force and torque. (3) The optimal parameters determined for borehole repair drill bits are as follows: drill bit OD (outer diameter): 28 mm; opening angle of rear nozzles: 20°; eccentric angle of rear nozzles: 90°; opening angle of front nozzles: 90°; eccentric angle of front nozzles: 0°. (4) The results of engineering applications show that the scalar drainage rate, gas concentration and borehole repair speed have been increased by 1.96 times, 3.98 times and 1.2 times, respectively. By improving the dynamic parameters of the drill bit and optimizing the structure of the drill teeth, The self-propelled borehole repair drill bit designed with optimized dynamic parameters and tooth structure has improved the borehole repair speed and gas extraction efficiency in engineering practice.