Suspension ball hinge optimization design_Hinge knowledge_Tallsen 1

The suspension ball hinge is the key product of ZF Chassis Technology Components Division, and its structural design is the core technology of the department. As the automobile industry continues to evolve, the demand for ball hinge products is also increasing. In the past, certain product designs were no longer able to meet the market's current needs. Customers now require more stringent simulation environments, more complex working loads, and compliance with new regulatory requirements such as pedestrian protection and post-collision failure criteria. Given these circumstances, it is imperative to optimize the technical aspects of the ball joint.

The ball joint is primarily used in the front suspension, facilitating the connection between the rod and the steering knuckle. This connection provides the second degree of freedom required for steering. To meet higher customer expectations, the focus of research and optimization shifts towards improving sealing performance and fatigue wear resistance.

This article is based on ZF's actual mass production of the Dongfeng Liuzhou B20 project for a domestic Original Equipment Manufacturer (OEM), with the intention of optimizing the structure of the suspension ball hinge. Initially, the plan was to continue using parts from the current mass-produced project. However, after the first round of Design Validation (DV) tests, it was identified that there were still potential risks, mainly in the form of water leakage and premature wear. Upon analysis, it was decided that design improvements were necessary to meet the current test requirements.

Further analysis of other new domestic OEM projects revealed that many OEMs have established specific specifications for ball hinge performance, with design requirements having significantly increased. Similarly, global OEMs are continuously updating their specifications for ball hinges. ZF products need to withstand harsher environmental conditions, more complex and variable operating conditions, as well as more detailed collision protection requirements. In light of these developments, this article aims to propose a reasonable optimization scheme based on research and analysis of the new specifications, in order to obtain products that meet performance standards at a lower cost.

to Ball Hinge:

Ball hinges ensure the connection of mechanism chains by maintaining continuous contact and relative movement. The points of connection for these movements are known as joints. Ball hinges can be categorized as radially loaded hinges (guided ball hinges) or axially loaded hinges (loaded ball joints). Each joint consists of two connecting elements, such as shafts, plain bearings, gear teeth, etc., that cooperate with each other and have a suitable geometry for their function. The main connecting elements of the ball joint are the ball stud and the ball socket. Apart from the performance of the ball joint itself, other characteristics such as material, size, surface quality, load carrying capacity, and lubrication are also important.

Function and Technical Requirements of the Ball Hinge:

The function of the ball hinge is to connect the rod with the steering knuckle, thereby providing three degrees of freedom. Two of these degrees of freedom are used for wheel beating and steering, while the third allows for an elastokinematic variation for the wheel. The ball joint can only introduce tensile, compressive, and radial forces due to its three rotational degrees of freedom. Ideally, ball joints should not have any free play to avoid unnecessary noise. The elastic displacement should be minimized to prevent discomfort while driving and to maintain the driver's subjective evaluation. Additionally, the working torque of the ball hinge is an important evaluation index and should not be lower than the allowable value to avoid premature wear and noise.

Original Design Failure Mode Analysis:

1. Failure of Sealing Performance Test:

During the initial stage of the B20 project, it was requested by the customer to continue using the existing project products to reduce research and development costs and cycle time. However, during the DV test, failure modes such as water leakage and rust were observed in the sealing performance of the ball hinge. Upon inspection, it was discovered that the ball hinge and the steering knuckle had poor fitment, resulting in a 2.5mm free gap between them. This gap could potentially lead to water leakage, indicating that the sealing system did not meet the test requirements. Further disassembly of the ball hinge revealed severe corrosion on the mating surface with the steering knuckle. This confirmed that the current product's sealing performance did not meet the design requirements for the B20 project. Notably, visible water stains and severe corrosion were observed on the ball pins in the area of the dust cover. This indicated that the current dust-proof system was insufficient and required improvement.

2. Analysis of Test Results:

The test results indicated that the water ingress during testing fell under the W3 level, where water stains were visually observed. This highlighted the severity of water ingress conditions in the sealing system after the test. The water ingress area mainly affected the collars at both ends of the ball hinge. Possible reasons for the failure were as follows:

- Assembly quality and size selection of the collar: The collar had a maximum size definition after being stretched, which was aimed at ensuring that the clamping force met the design requirements after the elastic deformation of the collar. However, if the actual assembly did not strictly follow the specifications, it could result in inadequate clamping force and a loose collar.

- Design failure of the dust cover: A comparative analysis of the dust cover design revealed a deviation in the cone angle of the labyrinth area. The current design had a cone angle of 20°, while the standard design had a cone angle of 12°. This deviation increased the risk of leakage.

- Design failure of the ball pin sealing area: The ball pin design had a stepped structure at a specific area, with a diameter 1mm larger than the ball pin shaft. This structure aimed to prevent the dust cover from being pressed into the neck position of the ball pin. However, under extreme working conditions of the ball joint, such as at the limit position, the contact area between the dust cover and the step was too small, resulting in the possibility of failure. Additionally, low temperatures could also lead to small contact areas, creating gaps and water leakage.

Ball Hinge Optimization Design Scheme:

1. Collar Assembly Optimization:

The failure of the collar end primarily resulted from issues with production assembly. To address this, it was deemed effective to define the collar's installation size in the Internal Process Specification (IPS), which becomes a part of the production operation instruction. The IPS would define the installation direction, the maximum diameter of the tooling fixture, and the diameter range of the collar opening. Furthermore, it would also include the Finite Element Analysis (FEA) report and layout report of the dust cover. This method would improve the assembly process and ensure that it meets the design requirements.

2. Optimal Design of the Ball Pin:

The analysis of failure modes revealed that the unreasonable design of the dust cover's labyrinth area and the small contact area of the ball pin step were the main factors contributing to the sealing test failure. Considering cost and project development constraints, optimizing the ball pin structure was deemed the most cost-effective solution. The optimized design aimed to provide a larger contact area between the ball pin step and the dust cover when the ball hinge was at its maximum working angle. The original design featured a semicircular cross-sectional shape for the step, while the new design introduced a rectangular cross-sectional structure and increased the outer diameter of the step. This resulted in a larger contact area and provided a greater reaction force under extreme working conditions, reducing the risk of gaps and dust covers being pressed into the neck.

3. Optimal Design Test Verification:

Samples based on the optimized design were produced and subjected to sealing performance tests. The results showed that the water content at the end of the ball pin and the end of the ball shell was only 0.1% to 0.2