Efficient Manufacturing Solutions for Steering Gear Casings

This article examines the machining of automotive steering gear housings and highlights the challenges posed by U-shaped production lines. These lines face difficulties in accommodating the accumulated errors from secondary clamping, making them ill-suited for small batch and high-variety production demands. The paper proposes a flexible production cell process, along with the design of specific fixtures and manipulators. This approach enhances product quality, lowers costs, and facilitates the efficient production of high-quality, small batches of diverse products.

 

01 Introduction

The housing is a crucial component of automotive steering gears and is a specialized part, as illustrated in Figures 1 and 2. It not only supports the piston in the steering system but also exceeds the functionality of a conventional hydraulic cylinder. In addition to its hydraulic role, the housing integrates a rocker shaft output branch. The cavity within the housing accommodates both the piston and the rocker shaft, converting the linear motion of the piston into oscillation of the rocker shaft.

Key characteristics of the housing include a low Ra internal surface roughness in the cylinder bore, high positional accuracy for the two main holes, and wear resistance. Typically, the housing is made from QT500-7, a type of ductile cast iron. The rocker shaft bore incorporates features such as bearing holes and sealing grooves.

Due to varying vehicle loads and connection dimensions, the shape of the housing can differ, with a mass ranging from 9 to 25 kg. Therefore, it is essential to analyze and design the production process for these irregular shell components.

 

02 Processing Technology Analysis

When shell parts are mass-produced, they are typically processed on a U-shaped production line. This design allows for the input of raw materials and the output of finished products at both ends, facilitating the feeding of blanks and the transfer of completed items to the warehouse. In terms of structural analysis of shell parts, there are several key processing challenges to consider.

(1) Cylindricity and surface roughness of the shell cylinder hole
The shell cylinder hole is designed to fit the piston from another component, requiring a tight clearance fit. To ensure smooth operation during reciprocating motion, a high level of cylindricity of f0.022 mm is necessary. Additionally, because of the prolonged reciprocating motion, a low surface roughness value of Ra=0.4 μm is required. These two precision requirements are especially critical for the shell.

 

(2) Coaxiality between the bearing hole and the sealing groove

The sealing groove of the shell is designed to accommodate the oil seal, primarily to prevent high-pressure oil from leaking out. Achieving a high level of coaxiality with the bearing hole is crucial for this function. The ability of the steering wheel to smoothly return to the center while driving and the longevity of the oil seal are closely linked to this coaxiality. The required coaxiality tolerance is f0.05mm, and the surface roughness specification is Ra=1.6μm, which poses a significant challenge during processing.

 

(3) Eccentricity accuracy of the bearing hole and the side cover surface

To meet the requirements for compact and lightweight steering gear products, the housing is designed with an eccentric structure. However, this design increases the difficulty of processing. The acceptable eccentricity error between the bearing hole and the side cover surface is ±0.025 mm. Even a slight deviation can impact the steering force characteristics of the steering gear assembly, and in severe cases, could render assembly impossible.

 

The machining process on the U-shaped production line is as follows:
- Housing Blank: Receiving material.
- Machining Mounting Surfaces: Drilling and reaming positioning holes on a vertical machining center.
- Rough Machining of the Arm Shaft Bore: Conducted on a vertical lathe.
- Rough Machining of the Cylinder Bore: Performed on an assembly machine.
- Fine Machining of the Arm Shaft Bore: Includes milling the seal groove on a vertical machining center, ensuring seal groove coaxiality of f0.05 mm and eccentricity accuracy of ±0.025 mm.
- Fine Boring of the Cylinder Bore: Carried out on a horizontal machining center to ensure cylindricity of f0.022 mm.
- Deep Hole Drilling.
- Machining of the Side Cover and End Faces: Completed on a tapping center.
- Machining of the Housing Tail Hole and Deep Oil Transverse Holes: Also performed on a tapping center.
- Rolling Extrusion: Done on a dedicated machine to achieve a surface roughness of Ra = 0.4 μm.
- Threading: Executed on a tapping machine.
- Deburring: Final process before storage.

 

The U-shaped production line is ideal for high-volume production of a single product. However, with company expansion and increased production capacity, over 100 varieties of steering gears have been developed. The repeated positioning and clamping of workpieces on U-shaped production lines can lead to cumulative errors, limiting the potential for quality improvements in part processing. To enhance precision and meet future demands for small-batch, high-variety production, a flexible production cell is more suitable.

Our newly built flexible production cell primarily employs three Henglun 5000 horizontal machining centers, equipped with KUKA robots for automated loading and unloading. A conveyor belt sensor automatically transfers workpieces and finished products, resulting in a fully automated production line.

The process flow in the flexible production cell is as follows:
- Process 1: The horizontal machining center drills mounting holes, taps threads, and reams positioning holes.
- Process 2: The horizontal machining center rough-machines cylinder bores, arm shaft bores, and deep holes.
- Process 3: The horizontal machining center fine-bores cylinder bores and arm shaft bores, while also drilling and tapping threaded holes on all sides.

 

This new process combines the fine boring of cylinder and arm shaft bores into a single fixture, which reduces errors caused by repeated positioning. Therefore, the key to process optimization is to design and produce a set of automatic rough positioning fixtures for machining positioning benchmarks, as well as two sets of high-precision automatic clamping fixtures for the horizontal machining centers. This will enable unified positioning through a transmission mechanism, allowing for rapid production changes for multiple varieties, thereby achieving the requirements for small-batch, high-variety, and high-precision production.

 

03 Design of shell fixture scheme

Following the production process requirements, a rough positioning automatic fixture for machining positioning reference and a high-precision automatic clamping horizontal machining center fixture were designed and manufactured. Additionally, a Kuka manipulator was utilized for automatic loading and unloading.

 

3.1 Process 1

The setup for the rough positioning automatic fixture used in the machining process for positioning holes is illustrated in Figure 3 [3]. The rough positioning automatic fixture is installed in the following manner: a fixture base (12) is mounted on the machine tool worktable (13). On this fixture base, three sequence valves (11) and a self-centering shaft (10) are installed. Additionally, an adjustable positioning bolt (9) is positioned on the end face of the self-centering shaft (10) to adjust the height of the part’s end face.

The fixture base is also equipped with a fixed positioning support (7) and a movable positioning support (3) to facilitate Z-axis rotation positioning. To clamp the shell part (6), the setup includes side clamping cylinder 1 (8), side clamping cylinder 2 (1), and end face clamping cylinder (5). For center positioning, a self-centering upper pressure block (4) and a self-centering lower pressure block (2) are installed on the self-centering shaft.

 

The installation of the coarse positioning automatic fixture is carried out as follows: The fixture base (12) is mounted on the worktable (13) of the machine tool. Three sequence valves (11) and a self-centering shaft (10) are attached to the fixture base. Adjustable locating bolts (9) are installed on the end face of the self-centering shaft (10) to allow for height adjustment of the part’s end face.

Additionally, the fixture base includes fixed locating supports (7) and movable locating supports (3) to facilitate Z-axis rotational positioning. Side clamping cylinders (18, 21, and 5) are utilized to secure the housing part (6). For centering purposes, the self-centering upper clamp (4) and lower clamp (2) are mounted on the self-centering shaft.

During automated machining, the system initiates a command that prompts the robot to grasp the housing part from the conveyor and position its inner bore onto the fixture’s self-centering shaft. Oil is then introduced into the fixture’s hydraulic ports, which tightens the hydraulic cylinders within the self-centering shaft. The self-centering upper clamp and lower clamp fully extend to accurately locate the inner bore of the housing blank. Meanwhile, the movable locating support stabilizes and positions the workpiece, achieving proper alignment in the Z-axis rotational direction.

The side clamping cylinders secure the workpiece, completing the positioning and clamping of the housing parts. Once this process is finished, oil is directed to the return port, causing all hydraulic cylinders to release simultaneously, allowing the robot to remove the CNC machinery parts.

This coarse positioning automatic fixture utilizes internal positioning within the blank, ensuring a consistent machining appearance. The self-centering axis represents an innovative solution, effectively addressing challenges such as large variations in blank size and the misalignment and deviation typically seen in conventional fixtures during hole positioning. Overall, this design offers a compact structure, reliable performance, and high centering accuracy.

 

3.2 Processes 2 and 3

Processes 2 and 3 utilize the same fixture design for the horizontal machining center, as illustrated in Figure 4. In both processes, the automatic fixture for the horizontal machining center is set up as follows: a fixture base (10) is mounted on the machine table (11), and a sequence valve (9) is attached to the fixture base (10).

Positioning pin 1 (8), positioning pin 2 (3), and the fixed support surface (4) are installed at the center of the fixture base, creating a stable three-point support system. The two positioning holes in the housing component (7) align with the two pins for accurate positioning on a single surface. Additionally, a floating support surface (1) serves as auxiliary support. Clamping cylinders (1, 6), (2, 5), and (3, 2) are employed to secure the housing component during the machining process.

 

During automated machining, the system first issues a command that instructs the robot to mount housing part 7 onto the fixture’s locating pins. Oil is then supplied to the hydraulic ports, activating the three main clamping hydraulic cylinders: clamping cylinder 1 (6), clamping cylinder 2 (5), and clamping cylinder 3 (2). These cylinders work together to securely clamp the workpiece.

Next, floating support surface 1 clamps housing part 7, completing both the floating support and all clamping actions. Once machining is finished, the fixture’s inlet and return oil ports reverse, simultaneously releasing all hydraulic cylinders. This allows the robot to grab and replace the housing part.

The design of this fixture employs a unified positioning reference, which facilitates rapid production changeovers. Automatic clamping eliminates the need for manual operation, setting the conditions for fully automated production.

 

3.3 KUKA Robot Automatic Clamping Jaw Design

The design of the KUKA robot’s automatic clamping jaw is illustrated in Figure 5. Two workstations have been created for different clamping processes.

Clamping Station 1 is designated for securing housing parts on a Henglun 5000 horizontal machining center. This is achieved using a rough positioning automatic fixture, as shown in Figure 3. In Processing Step 1, the machining of locating holes, mounting holes, and threads is completed. This fixture utilizes the blank face for positioning.

Clamping Station 2 is used to secure parts for installation on a second Henglun 5000 horizontal machining center. The fixture for this station, depicted in Figure 4, is employed for clamping to complete the rough CNC shaft machining of the shell cylinder hole and the arm shaft hole system in Processing Step 2.

In Processing Step 3, Clamping Station 2 is also utilized, and parts are installed on a third Henglun 5000 horizontal machining center. The fixture, shown in Figure 4, is used again for clamping to accomplish the fine machining of the shell cylinder hole and the arm shaft hole system.

Both fixtures in Processes 2 and 3 employ a one-sided, two-pin precise positioning system. The robot’s automatic clamping jaws are mounted on the robot arm. The shell parts are accurately positioned by the conveyor mechanism to transfer processed parts from each stage. The robot handles the clamping and unloading of parts for the next process (as seen in Figure 6), thus forming a flexible unit for fully automated production.

04 New Process Benefit Analysis

The new housing processing method, which utilizes an automated flexible production cell along with robotic loading and unloading, has significantly enhanced product quality, particularly in terms of geometric tolerances. By analyzing data from housing parts inspected on a three-dimensional coordinate measuring machine, we have improved coaxiality and perpendicularity from 0.05mm on the original U-shaped production line to within 0.02mm. This increase in processing precision has greatly enhanced the steering force characteristics and functional test results of the power steering assembly, resulting in a smoother and more continuous feel in the test curves.

The previous U-shaped production line required at least 10 machines and four operators per shift, with a cycle time of 769 seconds. In contrast, the new flexible production cell uses only three machine tools, eliminates the need for operators, employs robotic clamping, and facilitates continuous production around the clock, with a cycle time of 1,183 seconds.

For full-load production, running two shifts of 8 hours each, the monthly processing capacity of the old system can be calculated as follows: 8 × 3,600 seconds ÷ 769 seconds × 2 shifts × 28 days = 2,097 pieces. In comparison, the monthly processing capacity of the new system is: 8 × 3,600 seconds ÷ 1,183 seconds × 3 shifts × 28 days = 2,044 pieces.

Remarkably, the monthly production capacity of the new flexible production unit is equivalent to that of the old U-shaped production line running two shifts. With the ongoing rise in labor costs due to economic developments, the new production line is projected to save over 5 million yuan in labor costs over a period of ten years, based on current operator salaries.

 

05 Conclusion

The new flexible production cell has established an innovative process for processing shell parts, significantly enhancing product quality. The analysis indicates that the production capacity of the new process is on par with that of the old one. In terms of equipment investment, the old production line utilized over ten horizontal and vertical machining centers, along with specialized machines. In contrast, the new line relies on just three horizontal machining centers and an automatic loading and unloading system, resulting in comparable costs. Additionally, the new process has delivered considerable economic benefits, effectively meeting the market demand for small-batch, high-variety, and high-quality production.

 

 

 

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