Abstract: Fracture occurred at the joint between the bracket and the rod during assembly of the binaural plate screw. The fracture morphologies and characteristics of the broken screw were analyzed and identified as overload fracture. According to the metallographical analysis, hardness test, tensile test, wedge load test, and simulated installation test performed on the same batch of the screws, the results show that the cause of the screw fracture is mainly related to the intensive streamline at the fracture position, while the mismatch of heat treatment system and the being cut off of the metal streamline promote the fracture. Improvement measures such as improving the cold heading process, adjusting the heat treatment process and optimizing the processing of the head (supporting plate) rod were put forward for the screw fracture reasons. The improvement has been verified as effective, and no same fracture occurs during the screw assembly manufactured by the new process.

Key words: binaural plate screw, overload fracture, flow line, process improvement