The automotive transmission is a critical component of the powertrain system, directly influencing vehicle performance and durability. With stricter noise regulations and growing consumer demand for smoother driving experiences, managing vibration and noise in domestic transmissions has become increasingly important. A car's transmission is a complex system composed of gears, shafts, bearings, housings, shift forks, fork shafts, and synchronizers. During operation, it generates noise related to shaft frequencies, gear meshing frequencies, and the natural frequencies of components. As a result, the collected noise signal is a combination of these various sources, making it challenging to identify the root cause using basic power spectrum analysis.
In response to feedback about a self-owned brand manual transmission (SC30M5C) from Shanghai Auto Passenger Vehicle Company, engineers investigated the third-speed taxiing whistle problem occurring between 1400–2400 rpm. Through systematic vibration and noise testing, they identified the main contributors and optimized the relevant parts, significantly improving the issue.
Currently, our factory’s project team detected 13th and 20th order vibration noise problems in the NVH (Noise, Vibration, and Harshness) of the SC30M5C transmission. The sensor layout for the vibration noise meter is shown in Figure 1. The F80# test sample car exhibited a “wooshing†howling noise, with data showing that at 2400 rpm, 13th-order meshing noise appeared first, followed by 20th-order superimposed noise, peaking at 2000 rpm before disappearing at 1400 rpm. The anti-drag spectrum diagram (Figure 2) showed clear bands at both 13th and 20th orders, confirming the presence of noticeable howling.
Testing on the F80# sample revealed that the existing gears met technical requirements, but some transmissions still exhibited slight or obvious howling. Adjustments to drag torque improved the situation, but not completely. Some transmissions could not be distinguished on the calibration station, leading to further vehicle-based NVH measurements that confirmed the 20th-order noise as the primary source.
Through detailed analysis, the team found that the main causes of howling were related to the assembly fit of the one-axis and two-axis tapered bearings and the meshing precision of the third gear pair. While increasing the accuracy of the one-axis gear helped, structural constraints limited the process. Additionally, gear meshing spots and drag torque settings played significant roles in the noise generation.
To address the issue, improvements were made, including honing the intermediate shaft third gear to enhance tooth surface smoothness. Key components such as the one-axis assembly, three-gear gear assembly, and intermediate shaft were tested under real-world conditions. Assembly interference was reduced, and strict adherence to work instructions ensured consistent quality.
After implementing these measures, user feedback was positive, and the howling issue was significantly reduced. The final results showed only weak highlights in the anti-drag spectrum, with no subjective complaints.
In conclusion, howling remains a common NVH challenge in vehicle development. Early-stage vibration and noise testing, combined with manual monitoring, helps identify and resolve issues. Once in production, installing advanced noise analysis systems can enable efficient online detection and control, ensuring better product quality and customer satisfaction.
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