1. Introduction
As a critical component in the transmission systems of new energy vehicles, worm gears play a pivotal role in enabling efficient power transmission, speed reduction, and torque enhancement. In particular, the ZN-type worm gear, characterized by its normal straight-tooth profile, is widely adopted in electric vehicle (EV) transmissions for its compact structure, high transmission ratio, and smooth operation. My team and I have conducted an in-depth analysis of the manufacturing processes for ZN-type worm gears used in the EP21 transmission system of a new energy vehicle, focusing on design parameters, material selection, machining techniques, and quality control. This article outlines the detailed process flow, technical specifications, and practical insights gained from real-world production, aiming to provide a comprehensive reference for similar worm gear manufacturing applications.
2. Structural Characteristics of ZN-Type Worm Gears
Worm gear mechanisms are essential for transmitting motion between intersecting axes, typically at a 90° angle. The ZN-type worm gear, also known as the normal straight-tooth worm, is distinguished by its tooth profile: the tooth groove in the normal section features a straight line, while the end face exhibits an extended involute profile. This geometry allows for easier machining with straight cutting edges and ensures stable meshing with the worm, minimizing noise and vibration during operation.
Key Geometric Features:
- Tooth Surface: A ruled surface generated by a straight line in the normal plane moving helically around the worm axis.
- Axial Section: Convex curve profile, providing strength and wear resistance.
- Normal Section: Straight-tooth profile, simplifying tool design for machining.
3. Design Parameters of Worm Gears
3.1 Worm Design Specifications
The worm in the EP21 transmission is a double-threaded (head number \(Z_1 = 2\)) component with the following key parameters (Table 1):
| Parameter | Value | Description |
|---|---|---|
| Module (m) | 1.25 mm | Defines tooth size and pitch |
| Normal Pressure Angle (\(\alpha\)) | 20° | Angle between tooth profile and radial line |
| Diametral Quotient (q) | 10 | Ratio of worm 分度圆直径 to module (\(d_1 = mq = 12.5\) mm) |
| Lead Angle (\(\gamma\)) | 11.3° | Angle between helix and worm axis, calculated as \(\gamma = \arctan(Z_1/q)\) |
| Helix Direction | Right-hand | Direction of the helical teeth |
| Tooth Tip Height Coefficient | 1 | Standard height for tooth tip |
The axial pitch \(P_x\) and lead \(P_z\) are derived as:\(P_x = \pi m = 3.927 \, \text{mm}\)\(P_z = P_x Z_1 = 7.854 \, \text{mm}\)
3.2 Worm Gear Design Specifications
The mating worm gear has 60 teeth (\(Z_2 = 60\)) and is designed with the same module as the worm to ensure compatibility (Table 2):
| Parameter | Value | Description |
|---|---|---|
| Module (m) | 1.25 mm | Matches worm module for proper meshing |
| Number of Teeth (\(Z_2\)) | 60 | Determines gear ratio (\(i = Z_2/Z_1 = 30\)) |
| Pitch Diameter (\(d_2\)) | 75 mm | Calculated as \(d_2 = m(Z_2 + 2x_2)\), where 变位系数 \(x_2 = 0.2\) |
| Helix Direction | Right-hand | Matches worm helix direction for smooth meshing |
| Normal Pressure Angle (\(\alpha\)) | 20° | Same as worm for consistent tooth profile |
4. Material Selection
4.1 Worm Material
The worm is manufactured from 40Cr alloy steel, chosen for its high strength, good hardenability, and wear resistance. After machining, the tooth surface undergoes carburizing and quenching to achieve a hardness of 45–55 HRC, enhancing its durability under high-load conditions. The core remains relatively tough to withstand dynamic loads, balancing strength and flexibility.
4.2 Worm Gear Material
The worm gear is made from ZCuSn10Pb1 tin bronze, a material renowned for its excellent anti-friction properties, high load-carrying capacity, and good compatibility with steel worms. Bronze’s softness allows for rapid running-in, forming a smooth contact surface that reduces wear and noise over time. This material is ideal for high-precision applications where minimal backlash and long service life are critical.
5. Machining Processes for Worms
5.1 Process Flow
The worm machining sequence is carefully optimized to ensure dimensional accuracy and surface finish, consisting of the following key stages (Table 3):
| Process Step | Operation Details | Machine Tool | Fixtures/Tools | Key Parameters |
|---|---|---|---|---|
| 1. Blanking | Cut 40Cr round bar to rough dimensions | Band Saw | N/A | Diameter: \(\phi 16 \, \text{mm}\), Length: 80 mm |
| 2. Rough Turning | Remove excess material from outer diameter and ends | Common Lathe (CA6140) | Self-centering chuck | Cutting speed: 80 m/min, Feed: 0.3 mm/rev |
| 3. Center Hole Drilling | Machine center holes at both ends for subsequent machining | Drill Press | Center drill (60° tip) | Depth: 5 mm, Coaxiality ≤ 0.05 mm |
| 4. Finish Turning | Precision turn to intermediate dimensions, including gear blank | CNC Lathe (Mazak Integrex) | Spring collet chuck + two-center alignment | Cutting speed: 120 m/min, Feed: 0.1 mm/rev |
| 5. Thread Milling | Machine double-thread helical profile using form tools | CNC Milling Machine (Mazak VCN) | Custom fixture for axial positioning | Spindle speed: 500 rpm, Axial feed: 7.854 mm/rev (lead) |
| 6. Heat Treatment | Carburize and quench tooth surface to 45–55 HRC | Vacuum Furnace | N/A | Heating temperature: 850°C, Cooling: oil quenching |
| 7. Outer Diameter Grinding | Grind shaft sections to final dimensions | External Grinder (Mägerle MGB) | Center tips + magnetic chuck | Grinding wheel: Al₂O₃, Grit 80, Speed: 30 m/s |
| 8. Tooth Grinding | Precision grind tooth profile for final accuracy | Thread Grinding Machine (Hölscher HP 400) | Diamond dressing tool + two-center support | Grinding pressure: 5–10 N/mm, Feed: 0.01 mm/rev |
5.2 Critical Machining Techniques
- Thread Milling: The tool is aligned with the normal plane of the worm to cut the straight-tooth profile in the normal section. The double-thread design requires precise indexing to ensure uniform lead and thread spacing.
- Tooth Grinding: A diamond dresser shapes the grinding wheel to the inverse tooth profile, allowing for accurate replication of the ZN-type geometry. The workpiece is supported by two centers to maintain high rotational accuracy, minimizing runout errors.
6. Machining Processes for Worm Gears
6.1 Process Flow
The worm gear manufacturing process focuses on achieving precise tooth alignment and concentricity, with key steps outlined in Table 4:
| Process Step | Operation Details | Machine Tool | Fixtures/Tools | Key Parameters |
|---|---|---|---|---|
| 1. Blanking | Cast ZCuSn10Pb1 into cylindrical blank | Gravity Casting Machine | N/A | Diameter: \(\phi 85 \, \text{mm}\), Height: 30 mm |
| 2. Rough Turning | Machine end faces and inner bore to rough dimensions | CNC Lathe (Haas SL-20) | Self-centering chuck | Cutting speed: 60 m/min, Feed: 0.2 mm/rev |
| 3. Finish Turning | Precision turn bore and end faces to final dimensions | CNC Lathe | Custom 定心 chuck for concentricity | Cutting speed: 100 m/min, Feed: 0.08 mm/rev |
| 4. Drilling | Machine through-holes for auxiliary fixtures | Radial Drill Press | Custom positioning fixture | Hole diameter: \(\phi 10 \, \text{mm}\), Position tolerance: ±0.02 mm |
| 5. Gear Hobbing | Generate involute teeth using a custom hob | Gleason Hobbing Machine (600H) | Hobbing fixture with centering mandrel | Hob speed: 200 rpm, Feed: 0.05 mm/rev (radial), Depth: 1.25 mm (full tooth height) |
| 6. Slot Milling | Machine circumferential slots for structural features | CNC Milling Machine (Mazak FJV) | Vacuum fixture for thin-walled stability | End mill: \(\phi 12 \, \text{mm}\), Spindle speed: 800 rpm, Depth: 10 mm |
| 7. Deburring & Cleaning | Remove burrs and degrease for inspection | Ultrasonic Cleaner | N/A | N/A |
6.2 Critical Machining Techniques
- Gear Hobbing: The hob is designed to match the worm’s geometric parameters, ensuring correct tooth profile and backlash. The radial feed method is used, with the hob advancing from the tooth tip to full depth at a rate of 0.03–0.05 mm per revolution, balancing efficiency and accuracy.
- Concentricity Control: A custom 定心 chuck is used during turning to maintain coaxiality between the bore and outer diameter, critical for proper meshing with the worm.
7. Quality Control and Inspection
7.1 Worm Inspection
Key parameters checked for the worm include:
- Dimensional Accuracy: Diameter, length, and thread lead using coordinate measuring machine (CMM).
- Tooth Profile: Using a gear measuring center to verify the normal straight-tooth profile and lead angle, ensuring compliance with \(\gamma = 11.3° \pm 0.1°\).
- Hardness: Rockwell hardness test (HRC) on the tooth surface, targeting 45–55 HRC.
- Runout: Axial and radial runout measured with a dial indicator, limited to ≤0.01 mm for smooth rotation.
7.2 Worm Gear Inspection
Critical checks for the worm gear include:
- Tooth Spacing: 相邻齿距误差 and cumulative pitch error measured with a gear checker, ensuring \(\Delta f_p \leq 0.02 \, \text{mm}\) and \(\Delta F_p \leq 0.05 \, \text{mm}\).
- Tooth Profile Accuracy: Deviation in the involute profile checked using a form tester, with allowable error ≤0.015 mm.
- Concentricity: Between bore and outer diameter, measured with a CMM to ensure ≤0.01 mm deviation.
- Surface Finish: Ra 1.6 μm on tooth surfaces and Ra 3.2 μm on non-critical surfaces, verified with a surface roughness tester.
7.3 Meshing Test
A final meshing test is conducted to evaluate backlash, noise, and load distribution. The worm and gear are assembled in a test fixture, and a torque meter measures the frictional resistance and backlash, which should be ≤0.05 mm for optimal performance.
8. Challenges and Solutions
8.1 Tooling Complexity
The ZN-type tooth profile requires custom tools, such as modified lathe cutters for worms and specially designed hobs for gears. To address this, we collaborated with tool manufacturers to optimize tool geometries, using finite element analysis (FEA) to simulate cutting forces and prevent tool wear.
8.2 Heat Treatment Distortion
Quenching the worm can cause minor dimensional changes. To mitigate this, we implemented a controlled cooling process in a vacuum furnace and performed stress relief annealing before grinding, reducing distortion to within acceptable limits (≤0.02 mm radial deviation).
8.3 Thin-Wall Deformation in Worm Gears
The lightweight design of the worm gear leads to thin walls, prone to deformation during clamping. We resolved this by using a vacuum fixture during milling, which provides uniform clamping force without inducing mechanical stress.
9. Conclusion
Through systematic analysis and optimization, we have developed a robust manufacturing process for ZN-type worm gears in new energy vehicle transmissions. The combination of precise design parameters, material selection, and advanced machining techniques ensures that the worm gears meet stringent performance requirements, including high load capacity, low noise, and long service life. The detailed process flow, tooling strategies, and inspection protocols outlined in this article serve as a valuable reference for manufacturers aiming to produce high-precision worm gears for similar applications. As the demand for efficient and reliable EV transmissions continues to grow, the insights gained here will contribute to enhancing the manufacturing efficiency and quality of critical powertrain components.