In recent years, the electric heavy-duty truck market has experienced rapid growth, with sales reaching 462,000 units in 2024, representing a 31.1% year-over-year increase and a market penetration rate exceeding 17%. Projections indicate that by 2030, annual sales of electric heavy-duty trucks will reach approximately 2 million units, with a penetration rate of 50%. Electric heavy-duty trucks, due to their zero emissions and low noise advantages, have become a critical direction in the logistics and transportation sector. The spiral bevel gears in their transmission systems bear the responsibility of high-torque transmission and power functions. However, the torque characteristics of electric heavy-duty trucks differ significantly from those of traditional fuel vehicles. The power system outputs higher torque, requiring the spiral bevel gears to withstand stronger instantaneous impacts and sustained high loads. During frequent start-stop and acceleration/braking condition switches, the spiral bevel gears must adapt to dynamic load changes, gradually increasing the demands on their endurance.
The role of spiral bevel gears in the axle is primarily twofold: first, to change the direction of power transmission, converting the longitudinal output from the motor to the lateral rotation of the wheels, enabling normal vehicle movement and steering; second, to achieve speed reduction and torque multiplication through the design of the drive and driven spiral bevel gears’ tooth ratio, meeting the vehicle’s torque and speed requirements under different driving conditions. Tooth breakage failure can lead to serious safety hazards, making the analysis of failure causes crucial for industry development.
As the proportion of our company’s axles matched with electric heavy-duty trucks increases, tooth breakage failures have gradually appeared in the after-sales market. In a certain mining area, our company’s axles matched with electric heavy-duty traction vehicles frequently experienced gear tooth breakage failures. The operating conditions involve temporary roads with gravel and soil, carrying stone materials, heavy loads downhill, and empty loads uphill, primarily focusing on heavy-load downhill conditions. The single trip distance ranges from 60 to 100 km. When the mileage reaches 10,000 to 20,000 km, spiral bevel gear tooth breakage failures occur, which could impact our company’s reputation. Therefore, we conducted a failure analysis to identify the root causes.
We analyzed a set of failed spiral bevel gears from the mining area, with a failure mileage of 12,000 km and an operating time of three months. The customer reported abnormal noise during heavy-load downhill operation. Disassembly revealed that the primary failure was tooth breakage on the driven spiral bevel gear. The macro failure morphology of the spiral bevel gear pair is shown below.
The drive spiral bevel gear exhibited scoring on the tooth tips, particularly more pronounced on the coast side, but no tooth breakage was observed. Approximately ten teeth on the driven spiral bevel gear were broken, primarily located at the toe end along the tooth length direction and near the tooth tip. Analysis of the tooth surfaces and fracture surfaces revealed a fatigue pitting and scuffing band along the lower edge of the coast side contact pattern on the driven gear, extending from the toe end to about 10 mm from the heel end, forming a distinct groove approximately 3–4 mm wide. The contact pattern was biased towards the toe end, with tooth breakage occurring from the mid-width to the toe end tooth tip, and crushing observed at the toe end. From the failure morphology analysis, adhesive wear accompanied by material transfer was present near the lower edge of the coast side contact pattern, and crushing and tooth breakage occurred near the toe end tooth tip.
The macro fracture morphology of the broken driven spiral bevel gear teeth showed distinct bright striations, typical of contact fatigue fracture. Adjacent broken teeth lacked obvious fatigue fracture characteristics, exhibiting a dark gray impact toughness fracture. The presence of two different types of fracture characteristics in the same component system suggests identifying the initial fracture site first. The general principle is that when both contact fatigue fracture and impact toughness fracture are present, fatigue fracture usually occurs first. The macro analysis conclusion is that microcracks initially appeared in the subsurface near the tooth root on the concave tooth face. With increasing fatigue stress, crushing first occurred in the surface carburized layer, and the cracks further propagated towards the core, leading to unstable fracture at the transition shear lip. The broken fragments then caused other teeth to break.
We cut samples from the failed gear teeth and performed chemical composition analysis using a direct-reading spectrometer. The test results for both the drive and driven spiral bevel gears met the requirements of the standard GB/T 5216-2014 for 22CrMoH steel, as shown in Table 1.
| Component | C | S | Si | Mn | P | Cr | Mo |
|---|---|---|---|---|---|---|---|
| 22CrMoH Requirements | 0.19–0.25 | ≤0.035 | 0.17–0.37 | 0.55–0.90 | ≤0.030 | 0.85–1.25 | 0.35–0.45 |
| Drive Spiral Bevel Gear | 0.21 | 0.018 | 0.25 | 0.83 | 0.011 | 1.11 | 0.45 |
| Driven Spiral Bevel Gear | 0.22 | 0.014 | 0.23 | 0.83 | 0.011 | 1.22 | 0.45 |
We polished the cut gear samples, cleaned them with anhydrous ethanol, and observed the microstructure at 500x magnification using an optical microscope. According to the standard GB/T 25744-2010, the carburized, quenched, and tempered microstructure of the gears met the drawing technical requirements. The carburized layer microstructure consisted of tempered martensite and retained austenite, while the core comprised low-carbon martensite, with no coarse grains or abnormal structures observed. The working area on the coast side tooth surface exhibited secondary quenching and tempering characteristics, primarily caused by tooth surface scuffing. The effective case depth was measured according to GB/T 9450-2005, and hardness was tested per GB/T 230.1-2018. The results complied with technical requirements, as detailed in Table 2.
| Item | Surface Martensite | Retained Austenite | Carbides | Effective Case Depth (mm) | Non-Martensite Layer Depth (mm) | Surface Hardness (HRC) | Core Hardness (HRC) |
|---|---|---|---|---|---|---|---|
| Drawing Requirements | 1–4 | 1–4 | 1–2 | 1.3–1.8 | ≤0.025 | 58–62 | 33–42 |
| Drive Spiral Bevel Gear | 4 | 3 | 1 | 1.68 | 0.025 | 62 | 35 |
| Driven Spiral Bevel Gear | 4 | 3 | 1 | 1.66 | 0.024 | 61.5 | 36 |
We conducted scanning electron microscopy (SEM) analysis on the lower edge of the coast side contact pattern of the driven spiral bevel gear, specifically at the crack initiation site of the broken teeth. The analysis revealed that the coast side tooth surface formed pits and streaks due to instantaneous high-temperature welding and subsequent tearing, with local areas showing plastic flow traces. Based on these characteristics and comparison with the tooth surface wear morphology in the international standard ISO 10825-1:2022(EN), these are typical scuffing features. Scuffing occurs when two meshing tooth surfaces, under dry friction or mixed lubrication conditions, experience material loss due to welding and tearing caused by frictional temperature rise. During meshing, excessively high contact pressure and relative sliding speed cause instantaneous temperature rise at the tooth contact. At this point, the lubricating oil film separating the two meshing tooth surfaces ruptures, and metal activity increases. Under high contact load, microscopic metal welding forms between the two contact tooth surfaces. With continued relative rolling and sliding motion, the locally welded metal is irregularly torn, resulting in tooth surface material removal.
The actual collected vehicle message data (see Table 3) showed that the maximum reverse input torque reached 8,478 Nm, while the axle’s rated input torque is 7,567 Nm, reaching 112% of the rated torque. The technical protocol specified the motor drag torque as 40% of the rated torque, i.e., 3,026 Nm, but the actual motor braking drag torque was as high as 72%. Traditional fuel vehicles use reverse gear less frequently, so historical gear design schemes for oil vehicles only focused on loaded contact patterns up to 60% load. After matching with electric vehicles, the actual load for reverse gear increased from 60% to 100%, and the drag input torque increased from 40% to 70%, indicating increased demands on the spiral bevel gear pair.
| Name | Rated Full Load | Actual Maximum Input Torque |
|---|---|---|
| Forward Input Torque | 7,567 | 9,580.5 |
| Reverse Input Torque | 7,567 | 8,478.1 |
| Drag Input Torque | 3,030 | 5,448 |
In traditional fuel vehicles, the loaded contact pattern for the spiral bevel gear reverse operation is only conducted up to 60% load, where the contact pattern does not run out at either the toe or heel end. After matching with electric vehicles, the loaded contact pattern can no longer stop at 60% as in traditional fuel vehicles but should be conducted up to 100%. According to Kimos calculation results, under 100% load on the coast side, the contact pattern runs out at the toe end, and the maximum contact stress reaches 2,490 MPa. The greater the load, the closer the maximum contact area is to the tooth tip. Combined with the high drag torque, this caused a scuffing band to appear near the lower edge of the contact pattern. Prolonged high temperature near the tooth root more easily leads to lubrication failure at the spiral bevel gear meshing interface, causing initial scuffing to occur near the tooth root and extend along the tooth height direction to the entire tooth surface.
The contact stress calculation for spiral bevel gears can be expressed using the Hertzian contact stress formula. The maximum contact stress $\sigma_H$ is given by:
$$ \sigma_H = \sqrt{\frac{F_n}{\pi L} \cdot \frac{1}{\frac{1-\nu_1^2}{E_1} + \frac{1-\nu_2^2}{E_2}} \cdot \frac{1}{\rho}} $$
where $F_n$ is the normal load, $L$ is the contact length, $\nu_1$ and $\nu_2$ are Poisson’s ratios, $E_1$ and $E_2$ are Young’s moduli, and $\rho$ is the equivalent radius of curvature. For spiral bevel gears under high torque, the load distribution affects the contact pattern. The safety factor against scuffing $S_S$ can be estimated as:
$$ S_S = \frac{\sigma_{H,\text{lim}}}{\sigma_H} $$
where $\sigma_{H,\text{lim}}$ is the limiting contact stress for the material. In our analysis, the actual torque conditions lead to $\sigma_H$ values exceeding design limits, reducing $S_S$.
The torque transmission in spiral bevel gears involves complex dynamics. The input torque $T_{\text{input}}$ relates to the gear ratio $i$ and output torque $T_{\text{output}}$ by:
$$ T_{\text{output}} = i \cdot T_{\text{input}} \cdot \eta $$
where $\eta$ is the efficiency. For electric heavy-duty trucks, the instantaneous torque during acceleration and braking can be modeled as:
$$ T_{\text{instant}} = T_{\text{rated}} \cdot k_{\text{dynamic}} $$
where $k_{\text{dynamic}}$ is a dynamic factor, often exceeding 1.2 in harsh conditions. Our data shows $k_{\text{dynamic}}$ reached 1.12 for reverse and 1.8 for drag torque, highlighting the need for design adjustments.
To address the tooth breakage in spiral bevel gears for electric heavy-duty trucks, we concluded through fracture morphology analysis and tooth surface microanalysis that: first, the motor manufacturer’s drag torque is too high, and according to actual conditions, the contact pattern should be loaded up to 100%; second, the higher the drag torque, the closer the contact pattern is to the toe end. After loading 100% load, the toe end runs out. As torque increases, the maximum stress zone shifts towards the tooth tip, which is the main reason for crushing and tooth breakage near the toe end tooth tip; third, due to high torque and high sliding speed, scuffing and pitting fatigue bands first appear near the tooth root, reducing gear thickness and stiffness. Under high torque, the stress at the toe end tooth tip is large, causing tooth breakage on the coast side tooth tip, originating from the toe end.
As the stress domain for commercial electric heavy-duty trucks gradually expands, traditional spiral bevel gear design schemes can no longer match the application conditions of electric heavy-duty trucks. To address this, motor manufacturers need to optimize the torque switching and torque design strategies of the power system. Simultaneously, for spiral bevel gears, design enhancements should be implemented, primarily including: optimizing the contact pattern through macro design and micro-modification of the spiral bevel gear, reducing the scuffing safety coefficient; improving the anti-scuffing ability of gear oil. The modified contact pattern should ensure even stress distribution under 100% load, preventing run-out and stress concentration. The scuffing safety coefficient can be improved by adjusting the tooth profile and lead modifications, which influence the load distribution factor $K_H$ in the contact stress calculation. Additionally, selecting gear oils with higher extreme pressure (EP) additives can increase $\sigma_{H,\text{lim}}$, thereby enhancing $S_S$.
Further, we recommend implementing real-time monitoring of torque and temperature in spiral bevel gear systems to prevent overload conditions. The use of advanced materials with higher fatigue strength and better thermal stability can also mitigate failure risks. For instance, exploring alloys with improved hardenability and resistance to thermal softening could benefit spiral bevel gear performance in electric heavy-duty trucks. Regular maintenance and oil analysis should be enforced to detect early signs of wear or scuffing in spiral bevel gears, ensuring timely intervention before catastrophic failures occur.
In summary, the failure analysis of spiral bevel gears in electric heavy-duty truck axles reveals that operational torque exceeding design limits, combined with inadequate contact pattern optimization, leads to scuffing and subsequent tooth breakage. By addressing these factors through coordinated efforts between gear designers and vehicle manufacturers, the reliability of spiral bevel gears in electric heavy-duty trucks can be significantly improved, supporting the sustainable growth of this emerging market segment.