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Automatic adjustment arm overshoot failure analysis
As a critical 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 commonly used as a standard part in the market. In recent years, with the improvement of domestic highway infrastructure and the increasing load capacities and harsher operating environments of vehicles, the automotive industry—along with its parts sector—has seen rapid development. As a result, end users have raised their expectations for vehicle braking performance, making the automatic adjustment arm, a key element in the brake system, increasingly popular.
**Overview of Automatic Adjustment Arms**
The primary distinction between an automatic and a manual adjustment arm lies in the former’s ability to automatically compensate for brake gap wear caused by vehicle operation through an internal mechanism. This ensures that the gap remains within a reasonable design range, preventing dangerous situations like brake drag or uneven braking due to excessive wear. Currently, the structure of automatic adjustment arms can be categorized into three types: Hadlex, Rockwell, and Bendix. The principle of automatic gap adjustment mainly falls into two categories: air chamber stroke sensing and gap sensing.
From a market perspective, Hadlex and Rockwell structures are less commonly adopted in China due to their complex installation requirements, high environmental demands, and intricate designs. They are primarily found on medium-to-high-end commercial vehicles or export models. For example, Dongfeng Commercial Vehicle Co., Ltd. introduced the automatic adjustment arm as a standard feature in 2007. Based on cost efficiency and maintenance convenience, the R&D center of Dongfeng recommended using the Bendix-type automatic adjustment arm produced by Suzhou Renhe Company for their 4.5t and 6t series front axle assemblies.
**How the Bendix Automatic Adjustment Arm Works**
The Bendix structure, which our company currently utilizes, is illustrated below:
The working principle of the Bendix structure is based on air chamber stroke sensing. During braking, the angular travel of the adjusting arm is divided into three components:
1. The normal clearance angle (ω), representing the set gap between the shoe and drum.
2. The excess gap angle (Δω), which occurs when the gap exceeds the normal setting due to wear.
3. The elastic deformation angle (ωe), resulting from the deformation of friction plates, brake drums, and transmission components.
To achieve automatic gap compensation, the Bendix mechanism incorporates a one-way clutch system consisting of a clutch spring and a brake spring mounted on the adjustment worm shaft. This system plays a crucial role in maintaining proper brake clearance.
**Failure Analysis**
Since 2008, we have extensively used this product. By 2010, approximately 80% of after-sales service requests were related to quality issues. Most failures involved excessive brake clearance. Investigations revealed that improper user understanding and imprecise assembly of the one-way clutch were major contributing factors.
The main function failures of the Bendix automatic adjustment arm are summarized in the attached table:
From a structural standpoint, the automatic adjustment function relies on a one-way clutch and small racks formed by a clutch spring and brake spring mounted on the adjustment worm shaft. The one-way clutch acts as an elastic system, where the amount of deformation is inversely proportional to the stiffness of the system. During assembly, the stiffness of the system is directly related to the pre-tightening force applied.
Therefore, the design of spring preload torque and on-site quality control are critical to the effectiveness of the automatic adjustment. Although no theoretical issues were found during fatigue testing, failure analysis focused on the system stiffness. Field sampling showed that 25% of the products exceeded the upper limit of the design torque, leading to increased system stiffness and reduced sensitivity in detecting abnormal gaps.
This resulted in a gradual decrease in brake drum clearance over time, causing “grinding†and overheating, ultimately reducing braking performance. This phenomenon is known as "self-adjusting arm tuning."
**Solutions and Improvements**
In response to these challenges, Suzhou Renhe implemented stricter control measures. By ensuring 100% qualified torque values and incorporating feedback from host factories and end-users, the rate of overshoot failures dropped to below 10% of the total market failure rate. This demonstrates that controlling the preload torque is an effective way to reduce field failures.
**Conclusion**
This paper identifies the influencing factors behind the overshoot fault in automatic adjustment arm assemblies by analyzing their structure, working principles, production process, and subsequent solutions. It provides a comprehensive approach to addressing quality issues in the braking system.