Abstract: To address the issue of easy petal tooth fracture and short service life of the cardan universal joint for screw drilling tools under complex downhole load conditions, optimization schemes for petal tooth cross-sectional structures in the form of trapezoidal, rectangular, or fan-shaped were proposed. Based on fracture theory and numerical simulation methods for crack propagation, a virtual crack propagation simulation physical model was established. The distribution patterns of equivalent stress and crack stress intensity factors in the crack area were studied for the original and optimized structures when cracks were located on the left (zone Ⅰ), middle (zone Ⅱ), and right (zone Ⅲ) of the petal tooth root. Furthermore, field tests were conducted to verify the effectiveness of the optimized structure. The study shows that when cracks were distributed in zones Ⅰ,Ⅱ, or Ⅲ, the optimized structure could reduce the equivalent stress and stress intensity factor at the crack tip, with the average crack propagation capability decreasing by 18.6%, 16.2%, and 12.1% respectively. Among them, the rectangular cross-section reinforcement structure was the optimal solution. When cracks were located in thezone Ⅱpetal tooth root, the stress at the crack tip was the highest, and the propagation capability was the strongest; when cracks were located in the zone Ⅰ petal tooth root, the stress at the crack tip was the lowest, and the crack propagation capability was the weakest; when cracks were located in the zone Ⅱ petal tooth root, the propagation capability was between the two. Field test results verified the reliability of the optimized structure, with an average service life increase of 18.77%. This study can provide a reference for the life prediction and structural optimization of cardan universal joint petal teeth.

Key words: screw drilling tools, cardan universal joint, petal tooth optimization, fracture, crack pattern, stress intensity factor