In the rapidly evolving electric vehicle industry, the reliability of battery systems is paramount for safety and performance. As a key component, the battery cover must be securely fastened to the enclosure using bolts, but static torque attenuation after assembly and under vibration poses a significant challenge. This issue can lead to potential failures, compromising the integrity of electric vehicle batteries. In this study, we investigate the factors contributing to static torque attenuation in fastening bolts for battery covers in China EV applications, employing Design of Experiment (DOE) and Fault Tree Analysis (FTA) methodologies. Through systematic testing and optimization, we identify critical parameters and propose effective solutions to mitigate torque decay, thereby enhancing the durability and safety of electric vehicle systems.
Static torque attenuation refers to the reduction in residual torque after initial tightening, which can occur due to various factors such as material relaxation, vibration, and assembly processes. For electric vehicle batteries, this is particularly critical as it affects the sealing and structural stability. Our initial observations on a specific China EV model revealed substantial torque decay after vibration tests, with attenuation rates averaging around 64%. This prompted a detailed analysis to understand the underlying causes and develop countermeasures. The electric vehicle sector in China is growing exponentially, and addressing such issues is essential for maintaining competitive advantage and ensuring consumer safety.
The phenomenon of static torque attenuation was first observed through standardized vibration tests based on GB 38031-2020, which simulates real-world conditions for electric vehicle batteries. After 36 hours of vibration, the static residual torque of M6x20 bolts fastening the battery cover was measured at multiple points, as illustrated in sampling diagrams. The results indicated significant decay, with average residual torques falling below the control limits specified in industry standards like Q/JLY J7111129B-2021. For instance, with an initial assembly torque of 10 N·m, the static torque control lower limit (LCL) is calculated as 7.125 N·m, but post-vibration measurements showed values as low as 3-4 N·m, representing an attenuation rate exceeding 60%. This underscores the urgency for intervention in electric vehicle battery design and assembly processes.
To quantify the attenuation, we define the attenuation rate using the formula: $$ \text{Attenuation Rate} = \frac{T_{\text{initial}} – T_{\text{residual}}}{T_{\text{initial}}} \times 100\% $$ where \( T_{\text{initial}} \) is the initial tightening torque and \( T_{\text{residual}} \) is the measured residual torque. For electric vehicle applications, maintaining a low attenuation rate is crucial to prevent loosening under dynamic loads. Our preliminary data, collected over different resting periods after assembly, showed that torque decay stabilizes over time, but initial rates were consistently high, averaging 35% within 30 minutes of assembly. This early decay highlights the need for optimized assembly strategies in China EV manufacturing.
| Location | Sample 1 | Sample 2 | Sample 3 | Sample 4 | Sample 5 | Average | Attenuation Rate (%) |
|---|---|---|---|---|---|---|---|
| Front | 3.19 | 3.88 | 3.81 | 3.39 | 3.19 | 3.49 | 65.1 |
| Rear | 3.07 | 4.32 | 3.39 | 4.26 | 3.37 | 3.68 | 63.2 |
| Left | 3.42 | 3.23 | 4.07 | 3.49 | 3.22 | 3.49 | 65.1 |
| Right | 3.51 | 4.20 | 3.22 | 3.01 | 4.07 | 3.60 | 64.0 |
In our DOE approach, we applied a full factorial design to evaluate multiple factors simultaneously. The FTA helped visualize potential failure modes, such as issues with rivet nut installation, tightening strategies, and component design. For electric vehicle batteries, factors like rivet nut pull force, tightening torque, sequence, and step-wise tightening were considered. We also examined product structure elements, including the use of seals and adhesive backings on cover strips. The goal was to identify interactions and main effects that influence static torque attenuation, with a focus on applications in China EV markets. By randomizing trials and using Minitab for analysis, we ensured robust results that account for variability in electric vehicle production environments.
One key area of investigation was the installation process of rivet nuts, which are critical for securing bolts in electric vehicle battery enclosures. We tested different pull forces and sheet thicknesses to assess their impact on torque decay. The rivet nut pull force was varied between 13,000 N, 14,500 N, and 16,000 N, while sheet thickness was set at 2.0 mm and 2.5 mm. After tightening bolts to 10 N·m in a sequential order, we measured the static torque attenuation after 30 minutes. The results, analyzed through standardized effect plots, showed that higher pull forces significantly reduce attenuation, with interactions between pull force and sheet thickness being statistically significant at α=0.05. This emphasizes the importance of precise rivet nut installation in electric vehicle assembly lines.
| Run Order | Center Point | Block | Pull Force (N) | Sheet Thickness (mm) | Attenuation Rate (%) |
|---|---|---|---|---|---|
| 1 | 0 | 1 | 14500 | 2.5 | 35.2 |
| 2 | 0 | 1 | 14500 | 2.0 | 33.8 |
| 3 | 0 | 1 | 14500 | 2.5 | 34.8 |
| 4 | 0 | 1 | 14500 | 2.0 | 33.2 |
| 5 | 1 | 1 | 13000 | 2.0 | 34.7 |
| 6 | 1 | 1 | 13000 | 2.5 | 36.3 |
| 7 | 0 | 1 | 16000 | 2.0 | 30.8 |
| 8 | 0 | 1 | 14500 | 2.0 | 33.3 |
| 9 | 0 | 1 | 14500 | 2.5 | 34.5 |
| 10 | 1 | 1 | 16000 | 2.5 | 31.4 |
The tightening strategy for bolts was another focal point, as it directly affects preload distribution and relaxation in electric vehicle batteries. We evaluated tightening torque (10 N·m vs. 12 N·m), sequence (sequential vs. star-shaped), and step-wise tightening (one-step vs. two-step). Using an orthogonal array, we conducted trials and measured attenuation rates after 30 minutes. Main effects and interaction plots from Minitab revealed that a higher tightening torque of 12 N·m combined with two-step tightening and a star-shaped sequence minimized torque decay. For instance, attenuation rates dropped to as low as 21.8% under optimal conditions, compared to over 35% with suboptimal settings. This highlights the synergy between these factors in reducing static torque attenuation for China EV applications.
| Run Order | Center Point | Block | Tightening Torque (N·m) | Tightening Sequence | Step-wise Tightening | Attenuation Rate (%) |
|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 10 | Star | One-step | 35.2 |
| 2 | 1 | 1 | 12 | Star | Two-step | 21.8 |
| 3 | 1 | 1 | 10 | Sequential | One-step | 34.8 |
| 4 | 1 | 1 | 10 | Sequential | Two-step | 30.2 |
| 5 | 1 | 1 | 12 | Sequential | Two-step | 23.7 |
| 6 | 1 | 1 | 12 | Sequential | One-step | 31.3 |
| 7 | 1 | 1 | 10 | Star | Two-step | 29.8 |
| 8 | 1 | 1 | 12 | Star | One-step | 31.3 |
Product structure elements were also scrutinized to isolate their effects on static torque attenuation. For example, we compared the normal assembly with configurations where seals or adhesive backings on cover strips were removed. In tests where bolts were directly fastened into rivet nuts without other components, torque attenuation was negligible, indicating that the rivet nuts themselves were not the primary cause. However, removing the seal resulted in a reduced attenuation rate of 27.7%, compared to 36.0% with the seal, suggesting that soft seals contribute to creep and torque loss. Most notably, eliminating the adhesive backing on cover strips nearly eliminated attenuation, with residual torques close to the initial 10 N·m. This finding is crucial for electric vehicle design, as it points to the adhesive as a major factor in torque decay.
To balance production efficiency and performance in China EV manufacturing, we proposed a modified design with localized adhesive application on cover strips, avoiding bolt areas. Comparative tests showed that this approach performed similarly to the no-adhesive condition, with minimal impact on attenuation. The static torque after 30 minutes remained high, demonstrating that localized adhesive can effectively mitigate decay without increasing assembly labor. This optimization is particularly relevant for electric vehicle batteries, where automation and speed are essential in high-volume production.
Validation of the optimized parameters involved applying a combination of high rivet nut pull force (16,000 N), increased tightening torque (12 N·m), star-shaped sequence, two-step tightening, and localized adhesive on cover strips. We tested 100 electric vehicle battery samples, measuring static torque at multiple points after 30 minutes. The average residual torque improved significantly to 10.19 N·m, with a process capability index (Cpk) of 2.18, indicating high reliability and compliance with control limits. After 36 hours of vibration, the torque remained above 8 N·m, well above the LCL of 7.125 N·m, and attenuation rates were reduced to around 17-18%. This confirms the effectiveness of our approach in enhancing the robustness of electric vehicle battery systems.
The relationship between tightening parameters and torque attenuation can be modeled using a simplified formula for preload loss: $$ \Delta T = k \cdot \frac{F_{\text{relaxation}}}{A} $$ where \( \Delta T \) is the torque loss, \( k \) is a constant related to material properties, \( F_{\text{relaxation}} \) is the relaxation force, and \( A \) is the cross-sectional area. In electric vehicle contexts, minimizing \( F_{\text{relaxation}} \) through optimized assembly is key. Our DOE results align with this, showing that higher pull forces and strategic tightening reduce relaxation effects.
| Parameter | Optimized Value | Effect on Attenuation |
|---|---|---|
| Rivet Nut Pull Force | 16,000 N | Reduces attenuation by minimizing installation stress |
| Tightening Torque | 12 N·m | Increases initial preload, lowering decay rate |
| Tightening Sequence | Star-shaped | Ensures even load distribution, reducing localized relaxation |
| Step-wise Tightening | Two-step | Allows for stress redistribution, decreasing creep |
| Cover Strip Adhesive | Localized application | Eliminates interference with bolt areas, preventing torque loss |
In conclusion, our analysis demonstrates that static torque attenuation in fastening bolts for electric vehicle battery covers is influenced by a combination of工艺 and product design factors. By increasing rivet nut pull force, optimizing tightening strategies, and modifying adhesive applications, we achieved substantial improvements in torque retention. These findings are vital for the China EV industry, as they provide practical solutions to enhance battery safety and reliability. Future work could explore additional factors, such as environmental conditions and long-term aging effects, to further advance electric vehicle technology. As the demand for electric vehicles grows, addressing such engineering challenges will be crucial for sustaining innovation and market leadership.