A body-on-frame chassis gives an off-road vehicle its torsional rigidity, but it also removes the packaging freedom that a transverse electric drive normally relies on. The frame rails and the space needed for high ground clearance push the drive unit into a longitudinal arrangement: engine and electric machine in line, with a single spiral bevel pair turning the torque through 90° to the axles. That one gear pair therefore sits at the intersection of every load path in the powertrain, and its failure mode — tooth flank spalling along the meshing line — is not a generic gear problem but a consequence of how this architecture is used.
The bevel pair as the structural bottleneck of a longitudinal hybrid drive
In a longitudinal hybrid system the bevel gearset is the only component that converts longitudinal input into transverse output, so it carries the summed torque of the combustion engine and the traction motor. Compared with bevel gears in a conventional transfer case or a driven axle, the operating envelope is different in three ways.
- Load composition. Off-road use is dominated by low-speed, high-torque operation: frequent starts and stops, abrupt torque reversals, and high wheel-end torque demand. The motor’s millisecond-scale torque response superimposes impact peaks on top of the quasi-static engine torque, and those peaks concentrate at the front drive.
- Thermal environment. The P2 motor, the gearbox and the parallel-shaft geartrain share a single housing, so the bevel assembly both generates heat and absorbs heat from neighbouring components, with little surface available for convection. It also sits close to the exhaust side of the engine, which restricts the heat-rejection path. During the long climbs and repeated accelerations typical of off-road driving, the oil temperature of the bevel assembly rises faster than steady-state thermal models predict.
- Lubricant duty. One oil must simultaneously cover high load, high speed and high temperature, while satisfying oxidation resistance, thermal stability and power-economy targets. These requirements pull in opposite directions, and the compromise is where degradation begins.
Failure appearance and mechanism
A longitudinal hybrid drive unit run on a durability cycle developed an abnormal noise, and vibration readings exceeded the acceptance limit. Inspection of the bevel gearset showed pitting concentrated along the meshing line, forming a band of pits on the flank; severe cracking and spalling propagated from those pits, while the rest of the tooth surface showed only slight wear. Physicochemical examination of the gear identified the damage mode as contact fatigue spalling rather than overload fracture or case crushing.
The mechanism follows the contact kinematics of a spiral bevel pair. The two flanks are loaded by normal force and tangential friction, and the local sliding velocity varies along the line of action. Near the pitch line the sliding velocity approaches zero, so the entraining action that builds the oil film weakens and film thickness reaches a minimum. Under that condition asperities on the two surfaces interact directly, the film collapses locally, and cyclic contact stress nucleates pits. Once a pit forms it acts as a stress raiser, cracks extend along the flank, and adjacent pits link into a spall. Three factor groups control this sequence: contact pattern, surface hardness and material chemistry, and lubricant condition.
Contact pattern: optimistic bench assumptions and thermal distortion
Input torque outside the bench envelope
The contact pattern is the primary acceptance criterion for a bevel gearset; an uneven pattern or insufficient contact area shifts the load to the tooth ends and produces the stress concentration that initiates pitting. Comparison of vehicle road data against bench load data showed that the regenerative braking torque on the front drive exceeded the bench conditions used for validation. Because braking energy recovery was assigned entirely to the front axle, with rear-axle recovery deactivated, the bevel pair saw recovery torque that never appeared in the original bench profile. High torque of this kind pushes the contact pattern toward the tooth edges, raising edge stress and starting pitting at the toe or heel.
Tooth temperature
Thermal measurements showed a rapid rise in bevel assembly temperature, whereas the corresponding bench test stabilized at only 90–95 °C oil temperature even for a 120 km/h + 3% grade condition. The gap matters because heating causes differential thermal expansion of gear and pinion: the tooth geometry changes, the contact pattern changes size and position, and the effective contact area may drop below the design value. Temperature-dependent geometry is difficult to capture in design-stage simulation, so the pattern had to be mapped experimentally across the operating temperature range rather than predicted.
Surface hardness and alloy chemistry
Higher surface hardness allows the flank to resist wear and to maintain an effective oil film under extreme pressure, suppressing direct metal-to-metal contact. Hardness measurements on the failed gear were lower than those of gear material used in comparable vehicles of the same class. With insufficient hardness, the asperity contact fraction rises, adhesive wear and micropitting accelerate, and the damaged layer can no longer support the contact stress — a self-reinforcing degradation.
Elemental analysis of the failed material against a reference gear from a comparable application showed markedly lower nickel and molybdenum contents. Both elements are added to gear steels to raise hardness, hardenability, temper resistance and toughness, and their shortfall is consistent with the measured hardness deficit.
Lubricant condition at the time of failure
Oil condition was assessed against the change-over criteria of GB/T 30034-2013, the Chinese standard for gear oil service limits. The measured values are summarized below.
| Parameter | Unit | Failed oil vs. fresh oil | Reference limit |
|---|---|---|---|
| 100 °C kinematic viscosity change | % | −27.46 | −15 to +10 |
| Acid number increase | mg KOH/g | 2.75 | — |
| Base number change | mg KOH/g | 1.7 | — |
| Water content | wt% | 0.071 | 0.5 (condemnation threshold) |
The viscosity change of −27.46% lies well outside the permissible window and indicates shear degradation of the viscosity index improver or dilution of the base oil. The acid number rise points to oxidation products and inorganic acids that corrode metal surfaces and accelerate wear, while the base number change reflects depletion of the detergent and anti-wear additive package. Water at 0.071 wt% is below the condemnation threshold but still reduces film strength, increases friction between the flanks and promotes the pit-to-spall transition. Because the acidic additive chemistry is central to gear protection, acid number deserves equal weight with viscosity when oil condition is monitored.
Countermeasures and their trade-offs
Compensating thermal contact-pattern drift
If the pattern migrates toward the small end as the gearset heats, the design pattern at ambient temperature can be deliberately biased toward the large end so that the hot pattern remains inside the safe zone. With optimised design parameters, the ambient contact pattern was shifted approximately 6 mm toward the large end of the flank; at elevated temperature the pattern moves back toward the small end but stays within the acceptable contact area. The constraint is the cold end of the range, where the biased pattern must not run off the edge of the tooth.
Hardness: alloy content and the sequence of peening
Two levers act on surface hardness. The first is chemistry: raising nickel and molybdenum content improves hardness, wear resistance and core toughness. The second is process sequence. Shot peening induces a residual compressive stress layer that raises fatigue strength, wear resistance and corrosion resistance, but the conventional route — peen first, then grind — removes part of that layer during grinding. Inverting the sequence to fine grinding followed by shot peening lets the peening act on a smooth, defect-free surface, producing a uniform and dense compressive layer; deburring and surface-defect removal before peening further improves the result. Reported experience on comparable products shows a measurable gain in contact fatigue life for the grind-then-peen route.
Lubricant: the viscosity trap
A lubricant with better thermal-oxidative stability was evaluated against the production oil in a 150 °C oxidation test. The optimized oil showed smaller viscosity and acid number changes, as expected from a higher viscosity grade. The penalty appeared elsewhere: higher viscosity increased churning loss and the required oil quantity, and the drag torque of the bevel assembly worsened by about 26% across the normal speed range, with knock-on effects on efficiency and cost. Seal compatibility, air release and foaming behaviour also require separate qualification. Raising viscosity is therefore not a free fix; the practical route is to select the viscosity grade from measured road-use data on speed, load and temperature, add anti-wear chemistry within that grade, and address the underlying problem by improving heat rejection — for example with a dedicated oil cooler or air-cooling feature for the bevel assembly — so that the oil spends less time at temperatures that drive oxidation.
Validation: converting a road spectrum into bench conditions
Because a bench condition that does not reproduce the real torque history validates nothing, the verification programme was built on a measured vehicle road spectrum. Data were logged for a hybrid off-road usage profile covering extreme conditions that occupy less than 1% of operating time, ABS braking events, gradient climbs and long uphill runs, recording torque and speed in the time domain. The raw signals were preprocessed to remove drift and spike artefacts, and the record was segmented by operating mode of the longitudinal hybrid system.
The torque history was then reduced with the Palmgren–Miner linear damage rule, which treats accumulated damage as the sum of the damage contributed by each load level:
$$D=\sum_{i}\frac{n_{i}}{N_{i}}$$
where $n_i$ is the number of cycles applied at stress level $i$ and $N_i$ is the fatigue life at that level. This equivalent-life consolidation converts a complex torque–time history into a set of constant-torque blocks, which were then mapped onto bench operating points — principally a high-speed, medium-torque condition and a low-speed, high-torque condition — chosen to reproduce the road-duty mix.
Contact pattern testing
Pattern checks were performed before durability running, since they are non-destructive and can be repeated cheaply. To establish temperature dependence, the complete gearset was soaked in a chamber at 25 °C, 60 °C, 90 °C and 120 °C for 0.5 h so that the temperature was uniform throughout the assembly, and the output shaft was then rotated slowly with a torque wrench at approximately ±10 r/min under about 30 N·m, a near-no-load condition representative of light-duty operation. Torque dependence was measured separately at ambient temperature using a dynamometer at 25%, 50% and 100% of maximum torque. The acceptance requirement is a pattern that stays distributed across the flank without edge loading at every combination of temperature and load.
Durability testing
The bench durability run used oil circulation cooling so that the thermal environment of the vehicle could be approximated, allowing both the gear flanks and the shaft and bearing components to be assessed under realistic heat rejection. Load points derived from the road spectrum were applied through the high-speed, medium-torque and low-speed, high-torque conditions described above. The combination of contact-pattern optimization and material optimization was carried through to bench and vehicle-level validation, and the design changes were confirmed by both.
Design implications
The sequence observed here — a contact pattern that drifts out of its safe zone as the gearset heats, a hardness and alloy shortfall that removes the margin to accommodate it, and a lubricant that has already lost a quarter of its viscosity — illustrates why a single-factor fix is unreliable in a longitudinal hybrid drive.
- Thermal distortion of the tooth geometry belongs in the design loop from the start. Contact pattern should be specified as a function of temperature, not as a single ambient value, and the design pattern biased so that the hot pattern still lands inside the safe zone.
- Nickel and molybdenum content and the position of shot peening in the process chain are the two most direct levers on surface durability; grind-then-peen preserves the compressive layer that the conventional sequence removes.
- Lubricant viscosity grade is an efficiency-versus-durability decision, measurable as roughly 26% drag torque at the bevel assembly for the higher grade tested. Where thermal-oxidative margin is the real requirement, dedicated cooling of the bevel assembly is a more efficient answer than raising viscosity.
- Bench conditions should be derived from measured road spectra covering the rare high-load events — regenerative braking torque, extreme gradients — that the nominal duty cycle omits, since those events set the contact pattern and the pitting initiation site.
- Further work lies in closing the gap between simulation and test: predicting temperature-dependent contact pattern numerically would shorten the iteration now performed experimentally.