Objective The impact and vibration loads often accompany the working environment of infrared imager, which may cause the optical axis to shift and affect its directionality and imaging quality. Vibration environment leads to a decrease or even complete loosening of the pretension of screwed joints, leading to irreversible deformation of the connected components, especially the optical components (such as lens mounting bases and mirror mounting bases), resulting in the drift of the optical axis. Therefore, the pretension of screwed joints is an important factor affecting the optical axis vibration stability of infrared imagers. This work aims to explore the mechanism by which the pretension of screwed joints affects the optical axis drift of infrared imagers. It will provide practical guidance for improving the reliability of screwed joints to ensure the optical axis vibration stability of infrared imagers.

Methods To explore the mechanism of the influence of screwed joint reliability on the optical axis vibration stability of infrared imagers. This work presents a detailed modeling of screwed joints using Ansys Workbench finite element analysis software (Fig.1). Using transient dynamic analysis methods, the mechanism of the influence of vibration on the pretension of screwed joints and measures to improve the reliability of screwed joints are analyzed. Furthermore, taking the folding reflector assembly as the research object, an equivalent finite element model of the screwed joints was established (Fig.4), and a testing system is established (Fig.8). The simulation results are consistent with the experimental data, with an error of around 10%.

Results and Discussions This work establishes a detailed finite element model of screwed joint based on Workbench, and uses transient dynamics method to analyze the influence mechanism of vibration on the pretension of screwed joints. The simulation results show that the overall stress level of screwed joints decreases under vibration load, and the pretension and rotation angle of screws have an almost synchronous trend of change (Fig.3). The loosening of screw rotation is the main reason for the reduction in pretension. Furthermore, taking the optical axis sensitive component, the folding reflector assembly, as the object, an equivalent finite element model of the screwed joint was established. Simulation analysis and experimental verification were conducted on the effect of pretension changes on the optical axis drift. The simulation results show that the change in pretension of the mounting screws on the mirror base has a significant impact on the optical axis drift, while the change in pretension of the mounting screws on the mirror has a relatively minor impact(Fig.6 and Fig.7); The simulation results and experimental results of the effect of the change in pretension of the mounting screw on the deviation of the first mirror are within 10% (Fig.9), which verifies that the change in pretension of the screw will have an impact on the optical axis drift. Finally, based on a detailed finite element model of screwed joint, measures to improve the reliability of screwed joint were analyzed. The results showed that fine-pitch screws have better anti loosening performance compared to coarse-pitch screws (Fig.10), increasing the friction coefficient of the thread contact surface and the friction resistance of the support surface can improve the vibration anti loosening performance of screwed joint (Fig.11 and Fig.12), and the thread glue has a significant effect on improving the vibration anti loosening performance of the thread connection (Fig.14). The research results have practical guiding significance for improving the reliability of screwed joint to ensure the optical axis vibration stability in infrared imagers.

Conclusions This article establishes a detailed finite element model of screwed joints based on Ansys Workbench, and uses transient dynamic methods to analyze the mechanism of the influence of vibration on the pretension of screwed joints. Furthermore, taking the optical axis sensitive component of a certain type of infrared imager, the folding reflector assembly, as the object, an equivalent finite element model of the threaded connection was established. Simulation analysis and experimental verification were conducted on the effect of pretension changes on the optical axis drift. Finally, based on a detailed finite element model of screwed joints, measures to improve the reliability of screwed joints were analyzed, and the following conclusions were drawn: 1) Under vibration loads, the pretension and rotation angle of screws in screwed joints have a nearly synchronous trend, therefore, the loosening of screw rotation is the main reason for the reduction of pretension. 2) The simulation results show that the change in pretension of the mounting screws on the mirror base has a significant impact on the optical axis drift, while the change in pretension of the mounting screws on the mirror has a relatively minor impact on the optical axis drift. Therefore, in the actual production process, special attention should be paid to the anti loosening of the mounting screws on the mirror base; The simulation results and experimental results of the effect of the change in pretension of the mirror base’s mounting screw on the drift of the first mirror are within 10%, which verifies that the change in pretension of the screw will have an impact on the optical axis drift. 3) The simulation model has verified that fine-pitch screws have better anti loosening performance compared to coarse-pitch screws、increasing the friction coefficient of the thread contact surface and the friction resistance of the support surface can improve the vibration anti loosening performance of screwed joints、the thread glue has a significant effect on improving the vibration anti loosening performance of the thread connection, these further reflect the accuracy and practicality of the simulation model. The research results reveal the mechanism by which the pretension of screwed joints affects the stability of the optical axis of infrared imagers, and have practical guiding significance for improving the reliability of screwed joints to ensure the optical axis vibration stability of infrared imagers.