Invention of smart tightening tool for directly controlling the preload of bolted joints

Bolted joints are widely used in industries to connect and fasten two separated components. During the assembly of bolted joints, torque wrench is commonly employed to control preload indirectly. Much scatter in preload will occur and decrease the reliability of mechanical product. This study invented a novel tightening tool, which could directly control preload with high accuracy in the process of tightening bolts. The novel tightening tool is composed of an ameliorative intelligent wrench and signal processing system. There are two characteristics or innovations in the intelligent wrench. First, an electric tripping structure with multi-level small-size lever is designed to ensure that the target preload can be controlled effectively when the threshold value is reached. Second, an intelligent socket integrated with PZT sensor is designed, which can avoid many PZT sensors being sticked on bolt heads and greatly reduce cost. In the signal processing system, the waveform tracking strategy based on phase-locked loop is proposed to obtain the stable phase difference, which improves the robustness of the calculation of echo time. Three types of bolts with different sizes were tested. The average preload error was approximatively 2%, validating the accuracy and effectiveness of our tightening tool.