Analysis on working characteristics of cam-roller impact mechanism of rotary percussion drilling tool

Rotary percussion drilling tools can effectively improve their rock breaking efficiency in the deep and hard strata, thus having broad application prospects. The cam-roller is the key mechanism to realize the impact action, but the complex and changeable downhole environment leads to serious damage to the mechanism, which in turn affects the efficiency of rock breaking. Based on the test drilling data of rotary impact drilling tools in several wells, the cam-roller dynamics model was established, the working characteristics under different strokes, speeds and drilling pressures were analyzed by Autodyn (explicit dynamics), and the deformation characteristics of the mechanism were summarized to propose an optimization method. Besides, the analysis results were compared with the test data and the theoretical values to verify the accuracy of the calculation model. The results show that increasing the stroke can improve the impact effect, but the actual motion trajectory of rollers with a stroke greater than 10 mm will become smoother due to the influence of drilling speed, roller size, and cam contour curvature. When the rotating speed is 9‒18 rad/s, the fluctuation of axial acceleration is large, which has a certain influence on the stability of drilling tools. The deformation characteristics at the inner edge of the cam are obvious when the drilling pressure increases, and its deformation is directly proportional to the drilling pressure. The deformation of the cam under 25 kN drilling pressure is about 2.6 times of 10 kN. As the top of the cam tooth and the inner edge of the roller are easily deformed points, a replaceable roller was installed at the top of the tooth and the roller was changed to the drum configuration. Thus, the characteristics of four types of cam contour lines: parabolic-straight-parabolic, quadratic polynomial, cycloid and seventh degree polynomial, were analyzed. The results indicate that the use of cycloid and seventh degree polynomial cam seat profiles and drum roller could improve the impact effect, and the calculation results could provide a reference basis for the design of rotary percussion drilling tools.