Improvement of the pressure riveting process of water temperature sensor housing

A few years ago, during the manufacturing of the water temperature sensor, a recurring issue of casing cracking and deformation was observed. Despite multiple attempts, this problem remained unresolved. The primary cause of the cracking was found to be the low elongation of the material used for the casing. When the elongation fell below the standard limit, cracks would form in various sizes and proportions during the extrusion process. To address the cracking issue, the conventional approach was to improve the material properties or apply annealing to the casing with poor elongation. However, these methods had limitations, especially in terms of surface quality and dimensional accuracy. In this paper, through several rounds of testing and verification, an innovative solution based on the riveting process is proposed. This method involves modifying the flange structure and the shape of the sensor button, thereby reducing or eliminating the cracking problem. This new approach opens up a fresh direction for solving the casing cracking issue in water temperature sensors. Currently, the water temperature sensor housing is made from Y15 free-cutting structural steel. Analysis revealed that the main reason for cracking was the low elongation of Y15. To maintain normal mass production and resolve the cracking issue, an annealing process is typically added to reduce hardness and increase material elongation. However, this approach brings two major challenges: 1) During annealing, the oil layer on the surface of the casing tends to char, making it difficult to remove after pickling and passivation. This results in a patterned appearance, which compromises the visual quality of the casing. 2) After annealing, the material becomes too soft, leading to excessive thickening of the hexagonal section during the riveting process. This often causes the hexagonal size to exceed tolerance, making it impossible for standard sleeves to fit properly. As a result, installation issues arise, affecting the performance of the sensor. Clearly, using annealing as a solution is not the most effective method for addressing the casing cracking problem. Through detailed analysis of the deformation process during riveting, three key regions were identified: the force transmission zone, the deformation zone, and the deformed zone. Stress concentration occurs at the interface between the force transmission and deformation zones, often leading to cracks. Additionally, the outer layer of the casing experiences greater strain than the inner layer, making it more prone to cracking. Based on these findings, several improvements were implemented: 1) The height of the flange was increased by 0.3 mm to shift the interface between the deformation and force transmission zones away from the abrupt thickness change, reducing stress concentration. 2) The right-angle support on the end button was replaced with a 30° chamfer to prevent crack propagation. 3) The rivet clamp angle was adjusted to better suit the modified casing design. These changes significantly reduced the occurrence of cracking in the actual production process, proving to be a more effective and practical solution. In conclusion, by modifying the sensor housing and end button structure, the stress distribution during riveting can be improved, effectively solving the cracking problem. Annealing, while commonly used, is not the optimal method. A thorough theoretical analysis of deformation, stress, and strain during the riveting process provides strong support for implementing such improvements, ensuring long-term reliability and quality.

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