Body-on-frame construction remains the reference architecture for off-road vehicles, and electrification has done little to change that. A ladder frame delivers the torsional stiffness and impact tolerance that a unitised body struggles to match, which is why hybrid and battery-electric off-roaders keep it. The same frame, however, dictates the packaging. Two longitudinal rails leave little room for a transversely mounted engine–motor–transmission stack, so the powertrain is laid out along the vehicle centreline: engine, damper, electric motor, transmission and final reduction all on one axis, with a single pair of spiral bevel gears turning the drive through 90° toward the axles. In this longitudinal hybrid layout the bevel pair is the only geared path between the power sources and the wheels. Every unit of traction torque and every unit of regenerative braking torque passes through it.
Bevel gears in conventional rear-drive axles and transfer cases work under well-understood duty: a dedicated oil sump, moderate pinion speeds, torque dominated by the engine curve, and a housing exposed to airflow. The longitudinal hybrid powertrain changes all four of those boundary conditions at once, and the case discussed here — a tooth flank spalling failure in the bevel pair of an off-road hybrid programme at Dongfeng Motor Corporation — illustrates how the change propagates into the gear mesh.
Duty profile imposed by the hybrid off-road application
Three characteristics of the application set the load collective seen by the bevel pair.
Energy management. Off-road hybrids carry comparatively small battery capacity relative to vehicle mass and rolling resistance, so the hybrid controller works the electric and engine paths hard and frequently. The bevel pair therefore sees rapid torque steps, repeated start–stop events, short-distance duty with little steady-state cruising, and — most importantly — frequent reversals between traction torque and recuperation torque. Combined with the high torque demanded at crawling speeds, this produces an alternating high-load spectrum rather than the quasi-static spectrum typical of a road-going axle drive.
Thermal environment. Motor, reduction gearing and parallel-shaft bearings share a single integrated housing, which removes the separate oil sump and the external surface area that a stand-alone axle would offer. Heat generated by the transmission accumulates in the same oil volume that lubricates the bevel pair, and the bevel assembly sits at the engine exhaust end of the housing, where the heat-rejection path is shortest in geometry but longest in practice. Under the repeated climbing and acceleration cycles characteristic of off-road use, the bevel assembly heats faster than the rest of the transmission.
Lubrication. One oil must cover creeping at low load, sustained climbing at high load, and a wide ambient and operating temperature range. The formulator has to trade oxidation resistance and thermal stability against churning losses and drag, because viscosity that protects the mesh at 120 °C costs efficiency at 30 °C.
The mesh itself combines high contact pressure with high sliding velocity — the product that governs flash temperature at the flank. In a spiral bevel pair the contact is essentially pure rolling along the pitch line and increasingly sliding toward the toe and heel, so the same torque that is tolerable at moderate speed becomes a thermal problem at high speed, and the same speed that is tolerable at part load becomes a film-collapse problem at high torque.
Failure mode observed in durability testing
During durability testing of the longitudinal hybrid system, an abnormal noise developed in the bevel assembly. Teardown revealed flaking on the tooth flanks. Physicochemical and metallographic inspection classified the damage as contact fatigue: pitting concentrated along the meshing line, coalescing into spalls, with fracture surfaces and pit morphology indicating that both the cracks and the final spalls propagated from the pits. The flanks also carried lighter wear marks.
The sequence is consistent with mixed or boundary lubrication. Oil film thickness scales with entrainment velocity, and at low vehicle speed with high torque — crawling, launch, gradient start — the film collapses and asperities carry the load. Cyclic contact stress then initiates micro-pits at or near the pitch line. Once formed, a pit is a stress raiser: the trailing edge of the pit concentrates shear, the crack grows a little on each pass, and eventually a fragment of the hardened case detaches. Because the damage started at the pitch line and spread toward the edges, the failure points to a contact pattern problem rather than to a single overload event.
Root causes
Contact pattern displacement under load and temperature
The contact pattern is the primary acceptance criterion for a spiral bevel set. It is checked at assembly with marking compound and must sit inside the flank with a margin at toe and heel, and must not run off either edge. A pattern that is displaced toward the toe or the heel, or that is shorter than specified, concentrates load on a fraction of the available face width. Local contact stress and local sliding velocity both rise, and both are inputs to pitting.
Input torque above the assumed profile. Comparison of logged vehicle data with the bench profile used during development showed that peak torque at the bevel pair in real service exceeded the bench input. Part of the discrepancy traced to the energy management strategy: recuperation was allocated entirely to the front drive, with rear-axle regenerative braking disabled. The bevel pair consequently carried the full recovery torque in addition to traction torque, including rapid sign reversals during lift-off and descents. A control-strategy decision had silently rewritten the mechanical load case.
Tooth flank temperature. Thermal simulation and measurement both showed the bevel assembly warming faster than the rest of the transmission. In a sustained 120 km/h run on a 3 % gradient the oil temperature at the bevel assembly climbed continuously over roughly 20 minutes, whereas the bench duty cycle used during development had only exercised the assembly up to 90–95 °C. The consequence is geometric: differential thermal expansion of gear blanks, shaft and housing alters backlash and shifts the contact pattern along the face width. Bench observation at constant torque shows the pattern migrating toward the toe and shrinking in area as oil temperature rises from ambient to 120 °C, so that at the top of the range the pattern approaches the toe boundary. Because the shift depends on the thermal state of the whole assembly, it cannot be tuned out at the design stage by adjusting cutting parameters alone; it has to be measured.
Insufficient surface hardness
Case hardness governs whether the flank can keep an effective oil film under extreme pressure and resist metal-to-metal contact when the film is thin. Hardness traverses on the failed gear showed surface hardness below that of production gears of comparable class, and chemical analysis showed lower nickel and molybdenum contents than a candidate material used in equivalent applications. Both elements matter here: nickel raises toughness, hardenability and case/core support, while molybdenum raises temper resistance and hardenability and helps retain hardness and wear resistance at elevated temperature. A shortfall in either leaves the flank softer exactly where the application is hottest. Lower hardness in turn promotes localised adhesive wear, which roughens the surface, further reduces film thickness, and accelerates the pitting that leads to spalling.
Lubricant degradation
Oil condition at the time of failure is summarised against the new-oil baseline and the change-out criteria of GB/T 30034-2013 in Table 1. Viscosity had fallen well outside the permitted band and the acid number had risen above the trigger value, while water content remained below its limit.
| Parameter | Unit | Failed oil vs. new oil | Change-out limit |
|---|---|---|---|
| 100 °C kinematic viscosity change | % | −27.46 | −15 to +10 |
| Acid number increase | mg KOH/g | 2.75 | 1.5 |
| Base number change | mg KOH/g | 1.7 | — |
| Water content | wt% | 0.071 | 0.5 |
A 27 % drop in kinematic viscosity directly thins the film at operating temperature and pushes the mesh further into boundary contact. The simultaneous rise in acid number indicates oxidation and additive depletion, and the acidic products attack the metal surface, adding a corrosive component to the wear process. Water, even below the change-out limit, lowers the load-carrying capacity of the oil and raises friction between the flanks. The lubricant was therefore not an innocent bystander: it degraded along the same path as the mechanical damage and amplified it.
Countermeasures
Relocating the contact pattern
Since the pattern migrates toward the toe as the assembly heats, the practical remedy is to start it further toward the heel when cold. Cutting parameters — cutter head geometry, spiral angle, pressure angle and ease-off — were adjusted so that at assembly temperature the pattern sits toward the large end, leaving enough margin that the thermally induced shift toward the small end still terminates inside the safe flank area at 120 °C. The approach costs no hardware and can be implemented within an existing cutting process, but it consumes part of the available face width at the heel and makes the set more sensitive to manufacturing tolerance and to assembly backlash, so the cold pattern has to be re-qualified rather than simply shifted.
Material and process
Two levers act on surface hardness. The first is chemistry: the gear steel was changed to raise nickel and molybdenum, lifting case hardness, temper resistance and wear resistance at the temperatures the assembly actually reaches. The second is residual stress. Shot peening bombards the flank with small steel or ceramic shot, plastically deforms the surface layer and leaves a compressive residual stress field that suppresses crack initiation and growth. The conventional sequence — peen, then grind — is self-defeating, because grinding removes part of the layer that peening created. Reversing the order to grind-then-peen lets the finished flank reach its geometric accuracy and surface roughness first, with burrs and grinding defects removed, so that the peening energy acts on a smooth, defect-free substrate and produces a uniform, dense compressive layer. Comparative fatigue testing of the two sequences shows a clear life advantage for grind-then-peen.
Lubricant selection and thermal management
An oil with a higher viscosity index and a stronger antioxidant package was evaluated against the original in a 150 °C thermal oxidation test. The upgraded oil retained viscosity and acid number substantially better over the test duration. The benefit is not free, however. Higher viscosity increases the circulating oil volume and the pump duty, and it raises churning losses: drag torque at the bevel assembly measured higher with the high-viscosity oil than with the existing fill across the normal speed range. Seal compatibility, air-release and foaming behaviour all require re-verification for any new fluid.
The practical conclusion is that viscosity grade should be selected from measured duty — speed, load and temperature distributions from real usage — together with an antioxidant package matched to the thermal environment, rather than by simply moving up a grade. Where the bevel assembly’s heat rejection remains the binding constraint, an auxiliary cooler or air-cooling provision on the housing addresses the cause rather than the symptom, by lowering the oil’s working temperature and slowing the oxidation that produced the viscosity loss in the first place.
Trade-offs between the options
| Countermeasure | Primary mechanism | Main penalty |
|---|---|---|
| Contact pattern relocation | Compensates thermal shift by repositioning the cold pattern toward the heel | Consumes heel margin; sensitive to tolerance and backlash; requires cut-and-check iterations |
| Material upgrade (Ni, Mo) | Higher case hardness, temper resistance and wear resistance | Higher material and heat-treatment cost; long qualification cycle |
| Process change (grind then peen) | Uniform compressive residual stress on a defect-free flank | Additional process step; grinding parameters must be re-optimised |
| Lubricant upgrade | Thicker film, better oxidation stability, longer oil life | Higher drag torque, larger oil volume, seal and foaming re-verification |
Validation based on the vehicle load spectrum
For the case at hand, contact pattern relocation was combined with the material upgrade and taken forward to verification. The combination addresses both sides of the failure: peak stress at the flank edge and resistance of the flank itself.
Road spectrum acquisition
Torque and speed time-domain data were logged at the bevel assembly over a route defined by the usage profile of the hybrid off-road vehicle, covering low-speed creeping, hard acceleration, ABS braking, gradient climbing and long-distance climbing. The raw channels were pre-processed to remove drift and spike artefacts, then segmented by operating mode of the longitudinal hybrid system so that each mode could be weighted separately when the bench profile was built.
Contact pattern tests
Contact pattern verification precedes durability testing, because it is cheap, fast and diagnostic. The requirement is a pattern that is uniformly distributed across the flank under every combination of temperature and load, with no edge contact and no concentrated stress band. The test matrix is given in Table 2.
| Variable | Levels | Conditions held constant |
|---|---|---|
| Assembly temperature | 25 °C, 60 °C, 90 °C, 120 °C | 0.5 h soak for thermal uniformity; ~30 N·m applied torque; ±10 r/min output rotation |
| Applied torque | 30 N·m, 25 %, 50 %, 100 % of maximum | Ambient temperature (~25 °C); output speed held at the low-speed bound |
For the temperature sweep the assembly is soaked in a chamber at each set point for 0.5 h so that the whole gear system reaches a uniform temperature, then the output shaft is turned slowly with a torque wrench at ±10 r/min under approximately 30 N·m, a near-no-load torque that reproduces the light-load state without masking the pattern. For the torque sweep a dynamometer loads the assembly at 25 %, 50 % and 100 % of maximum torque at ambient temperature, with output speed at the lower bound of the operating range.
Durability profile derived from the load spectrum
Reproducing the road spectrum directly on a bench is impractical for a long-duration test. The variable-amplitude torque history is instead reduced to a block cycle using the Palmgren–Miner cumulative damage rule, which states that under cyclic loading at several stress levels the total damage equals the sum of the damage contributed by each level:
$$D=\sum_{i=1}^{k}\frac{n_i}{N_i}$$
where $n_i$ is the number of cycles applied at level $i$ and $N_i$ is the number of cycles that level would sustain to failure. Failure is predicted when $D$ reaches unity. Applying the rule converts the measured torque–time history into a small set of constant-torque blocks with equivalent damage, an equal-life load compilation that can be mapped onto bench operating points in the speed–torque plane.
The resulting profile was organised into two dominant conditions — high torque at low speed, representing climbing and crawling, and high speed at moderate torque, representing sustained road and gradient cruise — each weighted according to its share of the measured spectrum. Oil circulation cooling was applied during the durability run so that the worst-case heat-rejection condition of the installed powertrain, rather than a favourable laboratory condition, is reproduced, and so that the bevel gear and its shaft bearings are validated under the temperatures they will actually see.
Design implications
The case yields several conclusions that generalise beyond the specific gear set.
Temperature belongs in contact pattern development, not in validation. A pattern qualified at room temperature is not qualified for a longitudinal hybrid powertrain, whose bevel assembly runs against a shared housing, an exhaust-adjacent mounting position and a duty cycle dominated by climbing. Design and manufacturing should explicitly account for the thermally induced migration of the pattern, so that the flank at 120 °C is still loaded inside its safe area rather than on its boundary.
Control strategy is a mechanical load case. Routing all regenerative braking through the front drive changed the torque history at the bevel pair, in magnitude and in sign reversals, relative to the assumptions on which the original bench profile was built. Torque channels at the bevel input should be logged and reconciled with the profile whenever the energy management strategy changes.
Bench conditions should be derived from measured vehicle usage rather than from generic specifications. Building the durability profile from a logged road spectrum, and reducing it to bench blocks through cumulative damage equivalence, keeps the test representative of the real load collective and keeps the results defensible. The same applies to the contact pattern tests, which are only meaningful when the temperature and torque levels bracket the in-service range.
Countermeasure selection is a cost–benefit exercise, not a ranking. Contact pattern relocation is inexpensive but tolerance-sensitive; material chemistry and grind-then-peen raise durability at the cost of material, process time and qualification effort; lubricant upgrading buys film thickness and oxidation life at the price of drag torque and system-level re-verification. Which combination is appropriate depends on whether the dominant driver is peak stress, surface resistance or thermal exposure. Other surface-strengthening routes, laser shock peening among them, remain available where conventional shot peening cannot deliver the required residual stress field. Combining higher-fidelity thermal–mechanical simulation with the design and manufacturing chain is the obvious next step for pushing bevel gear reliability further in electrified off-road powertrains.