Gears are pivotal components in mechanical systems, renowned for their compact structure, high transmission efficiency, reliability, accurate transmission ratio, and longevity. As core elements in numerous electromechanical products, they find extensive applications in critical sectors such as aerospace, automotive, instrumentation, and more. The quality of gears directly influences the performance of related products. Traditional machining methods, primarily cutting, dominate gear production but suffer from significant drawbacks, including severed metal fibers that reduce strength and material utilization rates as low as 40%–60% due to chip formation. Precision forming technology, also known as near-net shaping, involves obtaining refined blanks or final parts through精密塑性加工 methods. Precision forging of gears offers superior comprehensive mechanical properties, uniform fiber distribution, and enhanced fatigue strength, noise reduction, corrosion resistance, and wear resistance compared to cut gears, thereby extending gear life and performance. Moreover, precision forging boasts higher production efficiency and material utilization, reducing costs by over 20% compared to cutting methods. Consequently, adopting precision forging to replace cutting processes has become a global trend, with ongoing research into advanced gear forging technologies.
Existing forming processes for straight cylindrical internal gears face challenges such as incomplete filling of tooth corners, complex die structures, and high forming forces. To address these issues, we propose a novel bidirectional upsetting-extrusion closed-die forging process. This method leverages the “effective friction” principle, where opposing frictional forces on the billet’s side surfaces facilitate metal flow, reduce forming loads, and improve corner filling. This study focuses on a 20-tooth straight cylindrical internal gear, analyzing the process through 3D modeling and numerical simulation using Deform-3D software. The deformation process is categorized into three stages: free deformation, large deformation, and filling. Comparative analysis with unidirectional upsetting-extrusion reveals a 19.5% reduction in maximum load and lower effective stress, ensuring normal deformation without failure. Physical experiments using lead billets confirm the production of defect-free gears with full tooth filling, clear contours, and no flash, folding, or cracking. These findings provide valuable insights for near-net shaping of internal gears, benefiting internal gear manufacturers seeking efficient production methods.
The limitations of conventional internal gear forming techniques, such as unidirectional upsetting-extrusion in closed-die forging, include inadequate corner filling, high forming forces, and difficulties in part ejection. In unidirectional processes, the billet experiences upward frictional forces from the die and core rod side surfaces, impeding metal flow and increasing loads as deformation progresses. This results in poor die life and impractical industrial application. In contrast, the bidirectional upsetting-extrusion process introduces simultaneous movement of upper and lower punches, creating opposing frictional forces that enhance material flow into the tooth cavities. This approach reduces overall forming pressure and improves filling efficiency, particularly in critical corner regions. For internal gear manufacturers, this translates to lower equipment tonnage requirements, extended tool life, and higher-quality components. The schematic of the bidirectional process illustrates how the billet is compressed from both ends, promoting uniform deformation and minimizing defects.
To evaluate the bidirectional upsetting-extrusion process, we conducted numerical simulations for a straight cylindrical internal gear with module m = 2.5, tooth count z = 20, pressure angle α = 20°, height H = 30 mm, and pitch diameter d = 50 mm. The billet material was AISI-1045 steel, modeled as a ring with dimensions Ø80 mm × Ø52 mm × 30 mm. Simulation parameters included a billet temperature of 1100°C, die temperature of 350°C, friction coefficient of 0.3, and punch speeds of 4.8 mm/s for both directions. The mesh consisted of 300,000 tetrahedral elements, with a step size of 0.05 mm to ensure accuracy. Volume compensation was applied to account for material flow. The simulation captured key stages of deformation, highlighting metal filling sequences and potential defects. For comparison, unidirectional upsetting-extrusion was simulated by setting the lower punch velocity to zero.
The filling process in bidirectional upsetting-extrusion unfolds in distinct phases. Initially, the ring billet undergoes upsetting at both ends, with metal flowing rapidly toward the central tooth region. As deformation progresses, the tooth cavities near the middle section fill first, contacting the die, while the end corners remain underfilled. Continued compression leads to near-complete filling of the central cavity, but corner regions require additional steps to achieve full saturation. Finally, the corners fill entirely, resulting in a well-formed gear with smooth contours and no defects like folding or cracking. In contrast, unidirectional upsetting-extrusion shows uneven filling, with upper sections filling faster than lower ones, exacerbating corner defects and increasing loads during the final stages. The bidirectional method ensures uniform distribution and efficient utilization of material, crucial for internal gear manufacturers aiming for high precision and reduced waste.
Load-stroke curves from the simulations delineate three deformation stages: free deformation, large deformation, and filling. The bidirectional process exhibits a lower maximum load of 4.87 MN compared to 6.05 MN for unidirectional upsetting-extrusion, representing a 19.5% reduction. This reduction underscores the efficiency of bidirectional metal flow in minimizing resistance and enhancing formability. The stress distribution analysis further supports this, with maximum effective stress concentrations at the tooth root and tip corners. In bidirectional forming, the peak stress is 837 MPa, whereas unidirectional forming reaches 960 MPa, indicating higher risk of material failure in the latter. The lower stress levels in bidirectional processing contribute to improved die longevity and part integrity, factors critical for internal gears used in demanding applications.
Physical validation involved designing and manufacturing a bidirectional upsetting-extrusion die assembly, tested on a 5 MN hydraulic press with lead billets of identical dimensions to the simulation. The resulting internal gears demonstrated full tooth filling, sharp轮廓, and absence of defects such as flash or folds, confirming the numerical predictions. This successful试验 underscores the practicality of the bidirectional method for producing high-quality internal gears, offering a viable alternative for internal gear manufacturers seeking to adopt near-net shaping technologies. The process not only enhances mechanical properties but also aligns with sustainable manufacturing by reducing material waste and energy consumption.
In conclusion, the bidirectional upsetting-extrusion process presents a significant advancement in the precision forming of straight cylindrical internal gears. By mitigating issues like high forming forces and incomplete filling, it enables the production of superior gears with consistent quality. The 19.5% load reduction and lower stress levels highlight its efficiency, while experimental results validate its feasibility. For internal gear manufacturers, this approach promises cost savings, improved performance, and broader adoption of precision forging in industries reliant on high-strength, durable components. Future work could explore optimization of process parameters for varied gear geometries and materials, further solidifying the role of bidirectional forming in advanced manufacturing.
| Parameter | Value |
|---|---|
| Billet Material | AISI-1045 Steel |
| Billet Dimensions | Ø80 mm × Ø52 mm × 30 mm |
| Billet Temperature | 1100°C |
| Die Temperature | 350°C |
| Friction Coefficient | 0.3 |
| Punch Speed | 4.8 mm/s (bidirectional) |
| Mesh Elements | 300,000 tetrahedra |
| Step Size | 0.05 mm |
The deformation mechanics can be modeled using the following equation for effective strain rate in metal forming:
$$ \dot{\epsilon} = \sqrt{\frac{2}{3} \dot{\epsilon}_{ij} \dot{\epsilon}_{ij}} $$
where $\dot{\epsilon}_{ij}$ is the strain rate tensor. For internal gears, the material flow stress $\sigma$ under hot working conditions is described by:
$$ \sigma = K \epsilon^n \dot{\epsilon}^m $$
Here, $K$ is the strength coefficient, $\epsilon$ is the strain, $n$ is the strain hardening exponent, and $m$ is the strain rate sensitivity. In bidirectional upsetting-extrusion, the reduced friction alters the strain distribution, enhancing formability. The forming force $F$ can be approximated as:
$$ F = A \sigma Y $$
where $A$ is the contact area and $Y$ is a yield factor. For internal gear manufacturers, optimizing these parameters ensures efficient production of high-precision internal gears with minimal defects.
| Process Type | Maximum Load (MN) | Peak Effective Stress (MPa) |
|---|---|---|
| Bidirectional Upsetting-Extrusion | 4.87 | 837 |
| Unidirectional Upsetting-Extrusion | 6.05 | 960 |
The advantages of bidirectional forming extend to material savings and enhanced mechanical properties. For instance, the volume of material used in precision forging can be calculated as:
$$ V = \pi (R_o^2 – R_i^2) H $$
where $R_o$ is the outer radius, $R_i$ is the inner radius, and $H$ is the height. In traditional cutting, material loss is significant, but forging minimizes waste. Internal gear manufacturers can achieve near-net shapes, reducing subsequent machining. The fiber orientation in forged internal gears improves fatigue life, as expressed by:
$$ N_f = C \sigma_a^{-b} $$
where $N_f$ is cycles to failure, $\sigma_a$ is stress amplitude, and $C$ and $b$ are constants. This makes bidirectional formed internal gears ideal for high-cycle applications.
In summary, the bidirectional upsetting-extrusion process represents a transformative approach for internal gear manufacturing, addressing key challenges in precision forming. Through numerical and experimental validation, it demonstrates superior performance in load reduction, stress management, and defect elimination. As industries increasingly demand efficient and reliable components, this method positions internal gear manufacturers to meet these needs with advanced, sustainable solutions. Further research could integrate real-time monitoring and adaptive control to refine the process for diverse gear types, solidifying its role in the future of manufacturing.