In modern automotive systems, the steering mechanism is indispensable, with the steering gear serving as the sole means to deflect wheels and alter vehicle direction. As demands for enhanced steering performance grow, concepts like four-wheel steering, initially proposed by Japanese engineers and commercialized in the early 1990s, have gained prominence. Steering systems are categorized based on drive mechanisms, including mechanical, hydraulic, electro-hydraulic, and Electric Power Steering (EPS). EPS, comprising components such as motors, control units, sensors, and mechanical parts, offers numerous advantages and is widely adopted in passenger vehicles, while commercial vehicles often utilize hydraulic or electro-hydraulic systems. Despite its benefits, EPS faces challenges like loss of assistance, uneven steering effort, and noise issues. This article delves into the failure analysis of an EPS assistance gear shaft, employing finite element analysis, bench testing, and production line inspections to identify root causes and propose solutions.
The assistance gear shaft is a critical component in EPS systems, transmitting torque to facilitate steering. Failures in this gear shaft can lead to catastrophic outcomes, such as fracture during operation. Through preliminary finite element analysis, the fatigue life of the gear shaft was validated. However, bench tests revealed unexpected failures, prompting a comprehensive investigation. The results indicate that heat treatment processes, particularly quenching cooling media and carburizing depth, significantly influence surface hardness and overall durability. By comparing optimized samples, we demonstrate that adjustments in heat treatment can rectify these issues, ensuring reliability and performance.
Fatigue analysis is essential for automotive safety, often employing finite life design criteria. Methods include the nominal stress approach based on S-N curves, the local stress-strain method using ε-N curves, and fracture mechanics for crack propagation studies. For the EPS assistance gear shaft, we applied the Goodman criterion to estimate fatigue life under cyclic loading. The Goodman equation accounts for mean and alternating stresses, defined as:
$$ \frac{\sigma_a}{\sigma_{a(-1)}} + \frac{\sigma_m}{\sigma_b} = 1 $$
Here, $\sigma_a$ represents the stress amplitude, $\sigma_m$ the mean stress, $\sigma_b$ the material’s tensile strength, and $\sigma_{a(-1)}$ the corrected stress amplitude. Using finite element modeling, we constructed a detailed representation of the dual-pinion EPS system. The model incorporated mesh refinement in heat-treated layers to simulate stress distributions accurately. The maximum and minimum stresses calculated were 605 MPa and -720 MPa, respectively, yielding a fatigue life of 1.1 million cycles, which exceeds the design requirement of 200,000 cycles. This analysis confirmed the initial design’s adequacy, yet real-world tests indicated discrepancies, necessitating further investigation.
Bench testing was conducted on three EPS assemblies to validate durability. Each assembly underwent rigorous cyclic loading until failure. The results, summarized in Table 1, revealed that all samples failed prematurely, with the assistance gear shaft experiencing fatigue fracture and tooth pitting. For instance, Sample 1 failed at 51,350 cycles, Sample 2 at 36,670 cycles, and Sample 3 at 44,650 cycles. These failures suggested a systemic issue, potentially linked to material or processing defects. The consistent mode of failure across samples underscored the urgency to identify underlying causes, leading to a series of microscopic and material analyses.
| Sample ID | Durability Cycles (×10³) | Failure Mode | Observations |
|---|---|---|---|
| 1 | 51.35 | Gear shaft fatigue fracture | Tooth pitting and breakage |
| 2 | 36.67 | Gear shaft surface pitting | Black spots on tooth surfaces |
| 3 | 44.65 | Gear shaft fatigue fracture | Similar to Sample 1 |
Prior to failure analysis, performance tests were conducted on new samples to rule out functional defects. Parameters such as forward no-load torque, reverse force, and assistance characteristics were evaluated. As shown in Table 2, all measurements fell within specified limits, indicating that the gear shaft’s initial performance was not the culprit. For example, forward no-load torque averaged 3.7 Nm against a requirement of ≤5 Nm, while assistance torque was 0.2 Nm, below the 0.3 Nm threshold. These results directed focus toward material and heat treatment aspects.
| Test Parameter | Requirement | Sample 1 | Sample 2 | Sample 3 |
|---|---|---|---|---|
| Forward No-Load Torque (Nm) | ≤ 5 | 3.76-4.17 | 3.48-4.22 | 3.59-4.13 |
| Reverse Force (N) | 300-500 | 375-443 | 416-462 | 366-395 |
| Assistance Torque (Nm) | ≤ 0.3 | 0.20 | 0.22 | 0.13 |
Production line tests further corroborated these findings, with forward no-load torque and assistance characteristics meeting standards. This eliminated assembly or operational errors, pointing to inherent material properties. Consequently, microscopic examinations were initiated to assess the gear shaft’s internal structure. Metallographic analysis revealed a surface microstructure of martensite (Grade 2), retained austenite (Grade 2), and carbides (Grade 2), while the core consisted of low-carbon martensite and ferrite (Grade 2). All grades complied with the 1-4 level requirement, indicating no abnormalities in phase composition. Fracture surface analysis, however, showed pitting morphology on tooth sides and roots, characteristic of fatigue-induced damage. This suggested that despite conforming to metallurgical standards, the gear shaft succumbed to cyclic stresses due to insufficient hardness or residual stresses.
Heat treatment parameters, including carburizing depth and hardness, were meticulously evaluated. The gear shaft, made from 20CrMo steel, underwent carburizing to enhance surface durability. Measurements taken along the radial direction from the tooth top are summarized in Table 3. The carburized layer depth was defined as the distance from the surface to where hardness drops to 550 HV. Results indicated that both tooth-side and root depths met specifications, and surface hardness exceeded 82 HRA, within the required 79-82 HRA range. Core hardness was 302 HV, also within limits. Despite this, the gear shaft failed, hinting at issues with hardness gradient or quenching efficacy.
| Measurement Item | Requirement | Result |
|---|---|---|
| Tooth-Side Carburizing Depth (mm) | 0.4-0.8 | 0.45 |
| Root Carburizing Depth (mm) | ≥ 0.38 | 0.43 |
| Tooth Surface Hardness (HRA) | 79-82 | 82.1 |
| Core Hardness (HV) | 279-460 | 302 |
Elemental composition analysis confirmed that the 20CrMo material adhered to standards, with carbon content at 0.20%, chromium at 1.06%, and other elements within acceptable ranges. This ruled out material impurities as a failure cause. Attention then turned to hardness gradients, which play a pivotal role in fatigue resistance. Comparative tests were conducted between failed samples and a reference gear shaft from a different supplier. Hardness was measured at 0.2 mm intervals from the surface to a depth of 2 mm. The failed gear shaft, processed with a carburizing depth of 0.5 mm and quenching oil temperature of 120°C, exhibited a rapid decline in hardness. In contrast, the reference sample, with identical carburizing depth but a quenching temperature of 60°C, showed a gradual decrease. Data for both are presented in Table 4 and visualized in Figure 1, illustrating the stark difference in gradient profiles.
| Distance from Surface (mm) | Failed Gear Shaft | Reference Gear Shaft |
|---|---|---|
| 0.2 | 803 | 709 |
| 0.4 | 757 | 683 |
| 0.6 | 702 | 632 |
| 0.8 | 578 | 581 |
| 1.0 | 445 | 559 |
| 1.2 | 390 | 508 |
| 1.4 | 365 | 474 |
| 1.6 | 337 | 463 |
| 1.8 | 350 | 458 |
| 2.0 | 340 | 450 |
The nonlinear hardness drop in the failed gear shaft indicated inadequate core hardening, likely due to suboptimal quenching conditions. To address this, the supplier proposed two optimized heat treatment schemes: Optimized Sample 1 (carburizing depth 0.5 mm, quenching oil temperature 60°C) and Optimized Sample 2 (carburizing depth 0.7 mm, quenching oil temperature 60°C). Hardness gradients for these were compared against the failed and reference samples, as shown in Figure 2. Optimized samples exhibited smoother transitions, with core hardness values rising to approximately 427-428 HV, significantly higher than the failed sample’s 302 HV. This improvement underscored the importance of quenching media temperature in achieving desirable hardness profiles.
Validation tests on the optimized gear shafts included static torsion tests to assess load-bearing capacity. Results, detailed in Table 5, revealed that Optimized Sample 1 achieved a failure torque of up to 63.3 Nm, compared to 44.0 Nm for the failed sample. Optimized Sample 2 showed similar gains, but with minimal additional benefit despite increased carburizing depth. This demonstrated that enhancing core hardness through controlled quenching was more effective than merely increasing carburizing depth. Further tests on carburizing depth and hardness, summarized in Table 6, confirmed that Optimized Sample 1 met all criteria without cost-intensive modifications, making it the preferred choice.
| Sample Type | Sample ID | Loading Direction | Failure Torque (Nm) | Failure Mode | Judgment |
|---|---|---|---|---|---|
| Failed Gear Shaft | 1 | Clockwise | 44.0 | Gear shaft fracture | Qualified (≥30 Nm) |
| 2 | Counterclockwise | 38.6 | Gear shaft fracture | ||
| 3 | Clockwise | 40.5 | Gear shaft fracture | ||
| 4 | Counterclockwise | 39.3 | Gear shaft fracture | ||
| 5 | Clockwise | 42.3 | Worm gear fracture | ||
| 6 | Counterclockwise | 43.0 | Worm gear fracture | ||
| Optimized Sample 1 | 1 | Clockwise | 63.3 | Gear shaft fracture | Qualified |
| 2 | Counterclockwise | 46.6 | Gear shaft fracture | ||
| 3 | Clockwise | 54.8 | Gear shaft fracture | ||
| 4 | Counterclockwise | 56.5 | Gear shaft fracture | ||
| Optimized Sample 2 | 1 | Clockwise | 55.7 | Gear shaft fracture | Qualified |
| 2 | Clockwise | 54.5 | Gear shaft fracture | ||
| 3 | Clockwise | 55.3 | Gear shaft fracture | ||
| 4 | Clockwise | 57.8 | Gear shaft fracture |
| Sample Type | Root Carburizing Depth (mm) | Tooth-Side Carburizing Depth (mm) | Core Hardness (HV) | Tooth Surface Hardness (HRA) |
|---|---|---|---|---|
| Failed Gear Shaft | 0.427 | 0.449 | 302 | 82.1 |
| Optimized Sample 1 | 0.453 | 0.464 | 427 | 82.57 |
| Optimized Sample 2 | 0.701 | 0.707 | 428 | 82.77 |
In conclusion, the failure of the EPS assistance gear shaft was primarily attributed to heat treatment inconsistencies, specifically the quenching cooling medium and its effect on hardness gradients. Finite element analysis provided a foundational assessment of fatigue life, but real-world tests uncovered vulnerabilities. Through systematic analysis, we identified that optimizing quenching parameters—such as reducing oil temperature to 60°C—resulted in a more linear hardness profile, enhancing durability without significant cost increases. This study underscores the importance of integrating simulation, testing, and material science in automotive component design, ensuring that gear shafts meet rigorous performance standards. Future work should focus on standardizing heat treatment protocols across suppliers to prevent similar issues.