Nonlinear dissipative spring mass model for a percussive drill rod joint of the coupling sleeve type

A nonlinear dissipative spring mass (NDSM) model for a percussive drill rod joint of the coupling sleeve (CS) type is established. Such a joint consists of a cylindrical coupling sleeve with internal thread which connects two drill rods with external threads at their ends. The model disregards wave motion in the coupling sleeve but accounts for axial mobility of the sleeve relative to the rods. This mobility is due to local deformation and slip of the threads. The model is characterized by the mass of the sleeve and by three parameters which represent the coupling between the sleeve and the drill rods through the threads. A static and a dynamic test have been developed for determining the coupling parameters. The model is validated by simulating previous impact tests with a commercial CS joint. Good agreement is obtained between theoretical and experimental results for waves and energies by using values for the two stiffness parameters from the static tests and for the friction parameter from the dynamic tests. Then, the discrepancies for transmitted and dissipated energy are only of the magnitude of the estimated error in the impact tests.