Objective Constrained by complex stratigraphic conditions in underground coal mines, directional drilling faces challenges such as low efficiency when encountering medium-hard and hard formations. Positive displacement motors (PDMs) with a diameter of 89 mm are commonly used for directional drilling. Their efficiency is primarily influenced by the sealing performance of their volumetric cavities, which further significantly affects the drilling efficiency.

Methods Based on the structural characteristics of the three-dimensional (3D) volumetric cavities of PDMs and the simulation method of two-way fluid-structure interactions, this study proposed the criteria for quantitatively evaluating the efficiency and volumetric cavity performance of PDMs, followed by the analysis of the dynamic sealing performance of the 3D volumetric cavities during motor operation. Targeting the sealing characteristics of the 3D volumetric cavities of hypocycloidal PDMs, this study proposed two improvement schemes: an optimized hypocycloidal profile and an irregular stator profile. The feasibility of the improvement schemes was verified through numerical simulations and whole-machine tests.

Results and Conclusions  During the operation of a PDM, its volumetric cavities underwent severe leakage in the suction stage, when the rotor meshed with the upper side of the stator inner diameter, but exhibited high sealing performance in the extruding stage, when the rotor meshed with the lower side of the stator inner diameter. Compared to the PDM with the original structure, those with the optimized hypocycloidal profile and irregular stator profile demonstrated significantly improved sealing performance of volumetric cavities, with total efficiency increasing by 7.9% and 15.15%, respectively. The whole-machine tests confirm the rationality of the improvement schemes. Overall, the results of this study provide a reference for enhancing the efficiency and equipment performance of directional drilling in underground coal mines.