The automotive transmission is a critical component of the powertrain system, directly influencing vehicle performance and durability. As noise regulations become stricter and consumer demand for comfort increases, managing vibration and noise in transmissions has become increasingly urgent. A transmission consists of multiple parts such as gears, shafts, bearings, housing, shift forks, and synchronizers. During operation, it generates various noises—related to shaft frequency, gear meshing frequency, and natural frequencies of components. These noises are superimposed, making it difficult to identify the exact source using basic power spectrum analysis.
In the case of a self-owned brand manual transmission (SC30M5C) from Shanghai Auto Passenger Vehicle Company, a whining noise was reported during third-speed taxiing at 1400–2400 RPM. The project team conducted vibration and noise tests to pinpoint the main sources. After analyzing the mechanisms behind the noise generation, they optimized and matched transmission components, resulting in improved performance.
Currently, our factory detected NVH issues with the SC30M5C transmission, specifically 13th and 20th order vibration noises. Vibration sensors were placed as shown in Figure 1. The F80# test vehicle exhibited a "wooo" sound, with data showing that at 2400 RPM, 13th-order meshing noise appeared first, followed by 20th-order noise, peaking at 2000 RPM before disappearing at 1400 RPM. The anti-drag spectrum (Figure 2) showed bright bands in both orders, confirming the presence of howling.
Testing revealed that some transmissions had slight howling, while others had more severe issues. Adjustments to drag torque helped reduce the problem in some cases, but not all. It became clear that testing on the calibration bench couldn't fully capture the real-world conditions, so vehicle-based NVH measurements were essential to confirm the 20th-order issue.
Through order tracking and spectral analysis, it was determined that the 20th order was the primary contributor to the noise. Structural analysis of the SC30M5B transmission revealed that the one-axis and two-axis tapered bearings, along with the third gear pair, were likely the main sources. Improving gear tooth surface finish and adjusting bearing preloads were key steps in reducing the howling.
To address these issues, the intermediate shaft third gear underwent honing to improve surface smoothness. Key components like the one-axis assembly, three-gear pair, and intermediate shaft were tested extensively. Assembly interference was reduced, and new processes were implemented to ensure better meshing and alignment. After these improvements, the noise was significantly reduced, with only minor highlights remaining in the anti-drag spectrum.
The final results showed that the modified transmissions received positive feedback from users. Moving forward, we plan to integrate advanced Discom-Noise Analysis Systems into the production line, enabling real-time detection and control of potential whining issues.
Howling remains a common NVH challenge in vehicle development. Early-stage testing and continuous monitoring are crucial for identifying and resolving such issues. By combining manual checks with advanced analytical tools, we can ensure higher quality and customer satisfaction.
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