Influence of springy impact interface and curved drill rods on energy transfer in percussive rock drilling

Abstract: The influence of springy impact interface and of curved drill rods on energy transfer in percussive rock drilling was studied. Analytical models for pneumatic and hydraulic rock drills were introduced, in which the springy piston-rod impact interface was taken into account. A nonlinear model for the springy impact interface was also implemented in a computer simulation program for percussive rock drilling systems. Laboratory tests were performed and the experimental results were compared with the results by three theoretical models. The agreement between theoretical and experimental incident stress waves was found to be satisfactory. The factors affecting the rock drilling efficiency were analysed. For the hydraulic rock drills investigated, it was found that a springy impact interface slightly decreases the drilling efficiency. For pneumatic rock drills, however, the analysis showed that an appropriate springy impact interface might increase the drilling efficiency significantly. The reflection and transmission of harmonic stress waves at a constant curvature bend of an ilifinite uniform bar was analysed theoretically. The energy fluxes of the reflected and transmitted extensional and flexural waves were obtained for an incident wave which may be either extensional or flexural. Results are shown in polar radiation diagrams for an incident extensional wave. It was found that for bends of small angles, the radius of curvature has little effect on the transmission of extensional stress waves and a significant amount of extensional wave energy is transmitted through the bends. Furthermore, the efficiency of percussive rock drilling with a bent drill rod was investigated using a combined approach of FFT and matrix method. In percussive rock drilling with slightly bent rods (bend angle up to 100), it was found that the efficiency is practically the same as with straight rods. It was also found that the efficiency can remain high even if the rod is significantly bent (bend angle up to 30°).