Experimental study on dynamic characteristics of axial-torsional coupled percussive drilling

Axial-torsional coupled percussive drilling is a novel technology to enhance the rate of penetration (ROP), which takes advantage of both axial and torsional percussive drilling technologies. Our paper focuses on experimental investigations on coupled percussive drilling. A novel experiment setup, using a rock breaking tester, was developed to carry out dynamic drilling tests on granite specimens under different impact modes, weight on bit (WOB), rotation rates, and lithologies. Based on the optimization analysis of window functions, by combining the fast Fourier transform (FFT) and short-time Fourier transform (STFT) processing methods, the spectrum and 3D time-frequency-amplitude evolution characteristics of coupling impact response signals were demonstrated. According to experimental results, a high-frequency and high-amplitude rotational rate and lower torque are produced in coupling impact. The agglomeration of the cutting force is alleviated and the occurrence of “stick-slip effect” is slowed down. Compared with axial impact, the penetration depth and ROP are increased by 173.38% and 182.74% in coupling impact, respectively. As the WOB increases, a stable-high frequency and high amplitude rock breaking torque is generated, which increases the cutting force and ROP. There is an optimal ratio between the rotation rate controlled by the axial rotator and the torsional impact frequency, which takes advantage of axial and torsional percussion for a bit and obtains the optimal speed-raising effect. The variation of rock mechanical properties has a significant influence on the amplitude of time domain signals in coupling impact. Our results in this paper are of great value for future studies of axial-torsional coupled percussive drilling.