By VNOVO Technical Support Team
In May 2026, Zhiyuan Robot’s G2 completed an 8-hour production line livestream at the Nanchang Longcheer factory. One process step every 18 to 20 seconds, with an end-to-end success rate of 99.9%—this figure is impressive, but a closer look at the operating conditions behind it reveals a wear mechanism that has not been fully discussed.
At a 20-second cycle, the single G2 shoulder joint experiences approximately 1,440 complete start-stop cycles during 8 hours of continuous operation. This means the planetary reducer must endure a ‘zero speed → acceleration → high-speed operation → deceleration → stop’ periodic shock every 20 seconds. During continuous rotation of an industrial robot, tooth surfaces are always in a hydrodynamic lubrication state; whereas for the humanoid robot’s shoulder joint, every startup instant is a ‘reset’ of lubrication conditions.
Hydrodynamic oil film establishment and failure
In hydrodynamic lubrication theory, whether a complete hydrodynamic oil film can form between two relatively moving tooth surfaces depends on three variables: speed, viscosity, and clearance. Specifically, oil film thickness is proportional to the relative velocity of tooth surfaces and inversely proportional to base oil viscosity.
This reveals a stark reality: at the instant of startup, the oil film does not exist. When the motor drives the shoulder joint to accelerate from a stationary state, the relative velocity of tooth surfaces climbs from zero, the hydrodynamic effect gradually builds, and during this velocity ramp period, the tooth surfaces are actually in a boundary lubrication or even short-term dry friction state. This is an inherent defect of all start-stop reducers, not the fault of the grease.
Oil film recovery time: the key variable
The real question is: how long does it take for the oil film to fully establish after startup?
This recovery time is determined by the grease’s rheological characteristics, primarily influenced by three factors:
Base oil viscosity. Taking PAO (polyalphaolefin) as an example, its viscosity index can reach 120 to 200, far superior to mineral oil’s 90 to 100 range. A higher viscosity index means the base oil’s viscosity changes less with temperature, maintaining sufficient film-forming capability during cold starts. However, on the other hand, excessively high viscosity increases starting resistance and causes oil supply lag—this is a trade-off that needs careful consideration.
Thickening agent structural strength. Different thickening agents such as complex lithium, polyurea, and calcium sulfonate exhibit significant differences in structural stability under centrifugal force fields. The function of the thickening agent is analogous to a ‘sponge’—adsorbing base oil within the fiber framework, releasing it through shear during tooth surface motion, and re-absorbing and recovering after shutdown. At high speeds, if the thickening agent is continuously sheared and structural recovery is too slow, the ‘bare grinding’ window before oil film establishment during the next startup cycle further expands.
Thixotropic recovery characteristics. After shear thinning, the grease requires a certain amount of time to rebuild its viscosity structure at rest—this process is called thixotropic recovery. If recovery is too fast, starting resistance significantly increases, affecting shoulder joint response speed; if recovery is too slow, each start-stop cycle is accompanied by a longer boundary lubrication exposure time. For a 20-second cycle production line scenario, ‘moderate thixotropic recovery’ is the core criterion for grease selection.
The cumulative effect of start-stop cycles: micro-pitting as a slow variable
In a single start-stop event, the boundary lubrication window duration may be only milliseconds—sounding trivial. But in 1,440 cycles per 8 hours, continuously over days or even weeks on a production line, this number accumulates linearly to the tens of thousands of cycles range.
Each instant of short-term dry friction during startup causes repeated welding and tearing between tooth surface micro-asperities. The peaks of microscopic tooth surface irregularities repeatedly bear contact stress, gradually initiating micro-cracks, which then develop into micro-pitting—a mode of early failure that conventional vibration detection struggles to capture, yet significantly reduces tooth surface precision and NVH performance after thousands of operating hours.
Unlike catastrophic tooth fracture, micro-pitting develops as a ‘slow variable’: individual events are undetectable, but cumulative effects are substantial. In humanoid robot joint designs with extremely high maintenance-free cycle requirements, this is precisely the most easily overlooked failure path.
Engineering reference
Common indicators for measuring a grease’s start-stop protection capability are low-temperature starting torque (reflecting starting resistance level) and mechanical stability (reflecting thixotropic recovery consistency). For example, VNOVO SYN150, a shoulder joint grease specifically optimized for low-temperature starting torque and shear stability, can shorten oil film recovery time and reduce the duration of the boundary lubrication window. Performance data will not be elaborated here; it serves only as an engineering selection reference point.
Conclusion
Lubrication design for humanoid robot shoulder joints cannot simply apply continuous-operation experience from industrial robots. The relationship between oil film recovery time under start-stop shock, thixotropy, and maintenance-free service life is an engineering blind spot worthy of in-depth research. When conducting shoulder joint start-stop durability tests, have you paid attention to the grease’s starting torque and oil film recovery time curves? Have you encountered early precision degradation caused by start-stop wear? Feel free to share actual test data in the comments.
