Theoretical Analysis and Optimal Design of a New Rotary Percussion Drilling Method

To overcome some shortcomings in conventional rotary percussion drilling, such as low impact frequency and enabling not to make the most of impact energy, this paper proposed a new rotary percussion drilling method. Based on this method, the corresponding drilling device was designed. At first, the inner structure and working principle of this device were illustrated in detail. This device has two bits, namely, internal bit and external bit. The internal bit can generate high-frequency impact on rocks and concentrate impact energy near central areas of the bottom hole. The coupled action of these two bits can produce an alternating shear stress zone, which can exacerbate fatigue damage to rocks. And then the mechanics models for analyzing alternating shear stress and impact energy were built. Finally, the effects of some key parameters on alternating shear stress and impact energy were analyzed. The key findings of this study are as follows. As blades number or dip angle of the internal bit decreases, the amplitude of the alternating shear stress increases, which is beneficial for rock fatigue damage. Moreover, as the loading and unloading speed of impact force increases, the impact energy increases, which will promote damage and fragmentation of rocks. Compared with other wave shapes, the square wave has the highest loading and unloading speed, which means its rock-breaking efficiency is the highest. The new rotary percussion drilling method proposed in this study provides a new idea to make full use of impact energy and improve rock-breaking efficiency.