Suzhou FUJI Precision Elevator Co.,Ltd , https://www.profuji.com
Automatic adjustment arm overshoot failure analysis
As an essential component of heavy-duty vehicle braking systems, the automatic adjustment arm has gained widespread adoption in developed industrial regions such as Europe and North America. It is now a standard part in many markets. In recent years, with improvements in domestic highway infrastructure and changes in vehicle load and usage environments, along with the rapid development of the automotive industry and its supply chain, users have increasingly demanded better braking performance. This has led to a growing preference for key components like the automatic adjustment arm, which plays a crucial role in maintaining consistent brake gap and enhancing overall safety.
**Overview of Automatic Adjustment Arms**
The primary distinction between automatic and manual adjustment arms lies in their ability to automatically compensate for brake gap wear caused by vehicle operation. This is achieved through an internal mechanism that keeps the gap within a safe and designed range, preventing issues such as brake imbalance due to excessive wear. Currently, automatic adjustment arms are categorized into three main types: Hadlex, Rockwell, and Bendix. Their working principles mainly fall into two categories: air chamber stroke perception and gap perception.
In terms of market adoption, Hadlex and Rockwell structures are less commonly used in China due to their complex installation requirements, high environmental and user demands, and intricate design. These models are mostly found on mid-to-high-end commercial vehicles or export models. For example, Dongfeng Commercial Vehicle Company, which introduced this technology in 2007, now includes it as a standard feature. Based on cost-effectiveness and ease of maintenance, the company's R&D center recommended using Bendix-type automatic adjustment arms from Suzhou Renhe for its 4.5t and 6t series front axle assemblies.
**How Bendix Automatic Adjustment Arms Work**
The Bendix structure automatic adjustment arm, as shown in the figure below, operates based on air chamber stroke sensing. During braking, the angular movement of the adjustment arm is divided into three phases:
1. The normal clearance angle (ω) corresponds to the standard gap between the brake shoe and drum.
2. The excess gap angle (Δω) represents the additional gap caused by wear, such as from friction plates.
3. The elastic deformation angle (ωe) reflects the angular change due to the elasticity of the friction plate, brake drum, and transmission components.
To enable automatic gap compensation, a one-way clutch system—composed of a clutch spring and a brake spring—is added to the traditional manual adjustment mechanism. This allows the system to adjust itself during each braking cycle.
**Failure Analysis**
Since 2008, our company has widely adopted this product. However, by 2010, the number of after-sales service requests related to the product reached approximately 80% of all service cases. Most failures were attributed to excessive brake clearance. Investigations revealed that improper user understanding of the device and imprecise control of the one-way clutch assembly were major causes. Common failures and their solutions are summarized in the attached table.
From a structural perspective, the automatic adjustment function relies on a set of one-way clutches and small racks mounted on the adjustment worm shaft. The one-way clutch is central to this process. Its elastic deformation directly affects the amount of gap that can be adjusted. According to the principle of elasticity, the deformation is inversely proportional to the stiffness of the system. Additionally, during assembly, the stiffness of the system is linearly related to the pre-tightening force applied.
Therefore, the design of the spring preload torque and on-site quality control are critical to ensuring proper brake gap adjustment. Although the product design theory passed fatigue testing, the failure analysis focused on the stiffness of the system. Field sampling showed that 25% of the products exceeded the specified torque limits, leading to increased system stiffness and reduced sensitivity in detecting abnormal gaps. This resulted in decreased adjustment efficiency and eventually led to "grinding" phenomena and reduced braking performance.
To address these issues, Renhe implemented stricter torque control measures, achieving 100% compliance in sampling. Combined with feedback from original equipment manufacturers and end-users, the rate of overshoot failures dropped to under 10% of the total market failure rate. This demonstrates that controlling the preload torque is an effective way to reduce market failures.
**Conclusion**
This paper analyzed the factors contributing to overshoot faults in automatic adjustment arm assemblies by examining their structure, working principles, production processes, and post-treatment strategies. It provides a practical solution to quality issues and highlights the importance of precise control in ensuring reliable performance.