In the development of new energy vehicle transmissions, the worm gear mechanism plays a critical role in achieving efficient power transmission and motion control. As an engineer specializing in automotive components, I have extensively analyzed the manufacturing processes for ZN-type worm gears, which are integral to the gearshift and clutch systems in electric vehicles. These worm gear systems offer advantages such as compact structure, high reduction ratios, and smooth operation due to their multiple meshing pairs. However, their complex geometry demands precise machining techniques. This article delves into the structural characteristics, design parameters, and step-by-step processing methods for ZN worm gears, incorporating tables and formulas to summarize key aspects. The focus is on ensuring that all parameters meet technical specifications through rigorous manufacturing and inspection.
The ZN-type worm gear, also known as the normal straight-sided worm, features a tooth surface generated by a straight line in the normal plane undergoing helical motion relative to the worm axis. This results in an extended involute profile in the transverse section and a convex curve in the axial section. The worm gear pair typically operates at a 90° shaft angle, providing high torque transmission in confined spaces. Understanding these geometric properties is essential for designing accurate machining processes. Below, I present a detailed exploration of the design and manufacturing workflow, emphasizing the importance of tooling, fixtures, and inspection methods to achieve optimal performance in electric vehicle applications.
The design parameters for the ZN worm gear are derived from the requirements of electric vehicle transmissions, where precision and durability are paramount. The worm has a module of $$ m = 1.25 \, \text{mm} $$, number of starts $$ Z_1 = 2 $$, normal pressure angle $$ \alpha_n = 20^\circ $$, diameter quotient $$ q = 10 $$, and lead angle $$ \gamma = \tan^{-1}\left(\frac{Z_1}{q}\right) = \tan^{-1}(0.2) \approx 11.3^\circ $$ with a right-hand helix. The axial pitch is calculated as $$ P_x = \pi m = 3.927 \, \text{mm} $$, and the lead is $$ P_z = P_x Z_1 = 7.854 \, \text{mm} $$. For the worm gear, the module remains $$ m = 1.25 \, \text{mm} $$, with a tooth count of $$ Z_2 = 60 $$, reference diameter $$ d_2 = m Z_2 = 75 \, \text{mm} $$, and profile shift coefficient $$ x_2 = 0.2 $$. These parameters ensure proper meshing and load distribution in the worm gear assembly. To summarize the key design aspects, the following table provides a concise overview of the worm and worm gear specifications:
| Parameter | Worm | Worm Gear |
|---|---|---|
| Module (mm) | $$ m = 1.25 $$ | $$ m = 1.25 $$ |
| Number of Starts/Teeth | $$ Z_1 = 2 $$ | $$ Z_2 = 60 $$ |
| Normal Pressure Angle | $$ \alpha_n = 20^\circ $$ | $$ \alpha_n = 20^\circ $$ |
| Diameter Quotient | $$ q = 10 $$ | — |
| Lead Angle | $$ \gamma \approx 11.3^\circ $$ | — |
| Helix Direction | Right-hand | Right-hand |
| Axial Pitch (mm) | $$ P_x = \pi m = 3.927 $$ | — |
| Lead (mm) | $$ P_z = P_x Z_1 = 7.854 $$ | — |
| Reference Diameter (mm) | $$ d_1 = m q = 12.5 $$ | $$ d_2 = m Z_2 = 75 $$ |
| Profile Shift Coefficient | — | $$ x_2 = 0.2 $$ |
The manufacturing process for the worm gear components involves a series of meticulously planned steps to achieve the desired geometry and surface quality. For the worm, made from 40Cr alloy steel, the process begins with material selection and progresses through turning, milling, heat treatment, grinding, and final gear grinding. The worm gear, fabricated from ZCuSn10Pb1 tin bronze, undergoes similar stages but includes rolling and additional milling operations. Each step is designed to maintain dimensional accuracy and enhance the worm gear’s performance. Below, I outline the detailed process plans for both components using tables to highlight the sequence of operations, equipment, and tooling requirements.
| Process Name | Process Content | Machine Tool | Fixture | Cutting Tool |
|---|---|---|---|---|
| Material Selection | 40Cr alloy steel | — | — | — |
| Rough Turning | Blank rough turning | Conventional Lathe | Self-centering Chuck | Rough Turning Insert |
| Center Hole Machining | Drill center hole | Conventional Lathe | Self-centering Chuck | Center Drill |
| Finish Turning | Precision turning of profiles | CNC Lathe | Spring Collet (Custom) | CAPTO Toolholder with Finish Turning and Grooving Inserts |
| Thread Turning | Machine double-start thread | CNC Lathe | Custom Fixture | Standard Milling Cutter |
| Milling | Mill square features | Milling Machine | Custom Fixture | General Milling Cutter |
| Heat Treatment | Surface hardening to 45-55 HRC | Heat Treatment Furnace | — | — |
| Grinding | Grind outer diameter | Cylindrical Grinder | None | General Grinding Wheel |
| Gear Grinding | Precision grind tooth profile | Thread Grinder | None | Form Grinding Wheel with Diamond Dresser |
| Process Name | Process Content | Machine Tool | Fixture | Cutting Tool |
|---|---|---|---|---|
| Material Selection | ZCuSn10Pb1 tin bronze | — | — | — |
| Rough Turning | Rough turn face and bore | CNC Lathe | Self-centering Chuck | Rough Turning Insert |
| Finish Turning | Finish turn face and bore | CNC Lathe | Self-centering Chuck | Finish Turning Insert |
| Drilling | Drill holes | Drilling Machine | Custom Locating Fixture | General Drill Bit |
| Turning | Rough and finish turn outer diameter | CNC Lathe | Custom Centering Mandrel | Finish Turning Insert |
| Milling | Slot milling | Conventional Milling Machine | Custom Locating Fixture | General Slotting Cutter |
| Gear Rolling | Roll teeth | Gleason Gear Hobbing Machine | Rolling Fixture | Custom Hob (Designed and Ordered) |
| Milling | Mill excess material | CNC Milling Machine | Custom Locating Fixture | Milling Cutter |
One of the most critical steps in worm gear manufacturing is the turning of the worm tooth profile. For the ZN-type worm, the tooth槽 in the normal section has a straight-sided profile, with the tooth width and clearance at the pitch circle both equal to $$ 0.5 P_x = 1.9635 \, \text{mm} $$. The geometric dimensions of the normal tooth profile are defined by the relationship $$ b_n = s_n = 0.5 P_x $$, where $$ b_n $$ is the tooth width and $$ s_n $$ is the tooth space. To machine this, I use a custom-ground turning tool with specific geometry: the tool’s cutting edge is shaped to match the normal profile, ensuring accurate generation of the helical surfaces. During turning, the tool is aligned with the normal plane of the worm axis, and the worm is rotated to cut the double-start thread. The cutting process involves first machining one helix to near-final dimensions, leaving a small allowance, and then indexing to cut the opposing helix. This method ensures symmetry and precision in the worm gear geometry.
The mathematical foundation for the worm gear tooth profile can be expressed using the normal module and pressure angle. The normal tooth thickness $$ t_n $$ at the pitch circle is given by $$ t_n = \frac{P_x}{2} = \frac{\pi m}{2} $$, and the tooth depth $$ h $$ is calculated as $$ h = 2.25 m $$ for standard full-depth teeth. In practice, the tool geometry is derived from these parameters to achieve the required profile. For instance, the tool’s included angle $$ \theta $$ for the ZN worm is set to $$ 2 \alpha_n = 40^\circ $$ in the normal plane. During turning, the feed rate and depth of cut are controlled to maintain the lead accuracy, which is crucial for proper meshing with the worm gear. The relationship between axial feed $$ f_a $$ and rotational speed $$ N $$ is given by $$ f_a = \frac{P_z}{N} $$ for a single pass, but in multi-start worms, this is adjusted for each thread.
After turning, the worm undergoes heat treatment to enhance surface hardness, followed by grinding operations to achieve final dimensions and surface finish. The grinding process uses a formed grinding wheel dressed with a diamond tool to match the worm profile. The wheel profile is corrected based on the worm’s lead and pressure angle, using the formula for axial profile curvature. For the ZN worm, the axial profile is convex, so the wheel must be dressed to a corresponding curve. The grinding is performed between centers to ensure alignment, and the worm is rotated at a constant speed while the wheel traverses axially. This process removes minimal material but achieves high accuracy, with tolerances within $$ \pm 0.01 \, \text{mm} $$ for critical dimensions. The final worm gear inspection includes checks for lead error, profile deviation, and surface roughness, all of which are vital for the worm gear’s performance in electric vehicle transmissions.
For the worm gear, the rolling process is essential for generating the tooth profile that meshes with the worm. The custom hob is designed based on the worm’s parameters, and the rolling machine is set up with the correct center distance $$ a $$, calculated as $$ a = \frac{m (q + Z_2 + 2 x_2)}{2} = \frac{1.25 (10 + 60 + 2 \times 0.2)}{2} = 44.0 \, \text{mm} $$. During rolling, the hob performs a radial feed into the worm gear blank, with a feed rate of 0.03 to 0.05 mm per revolution, until the full tooth depth is reached. This method ensures efficient material removal and high productivity while maintaining the accuracy of the worm gear teeth. Post-rolling, the worm gear is inspected for tooth spacing, cumulative error, and profile deviations using coordinate measuring machines and gear analyzers. The results show that all parameters, such as tooth flank form and lead, fall within specified limits, confirming the effectiveness of the manufacturing process for the worm gear assembly.
In conclusion, the manufacturing technology for ZN-type worm gears in electric vehicles involves a comprehensive approach from design to inspection. By carefully selecting materials, optimizing tool geometry, and implementing precise machining sequences, I have ensured that the worm gear components meet all technical requirements. The use of custom fixtures and cutting tools, combined with rigorous quality checks, results in worm gears that offer reliable performance, high efficiency, and long service life. This methodology not only enhances the production of worm gears for current applications but also serves as a reference for future developments in automotive transmission systems. The success of this process underscores the importance of integrating theoretical design with practical manufacturing to achieve excellence in worm gear technology for the evolving electric vehicle industry.