By VNOVO Technical Support Team
From May 11 to 12, the first Chengdu-Chongqing Regional Embodied Intelligence Robot Application Scenario Challenge was held in Mianyang. Thirty teams competed, with Yushu, Zhiyuan, Ubtech, and Leju on the same stage; all six real-scenario events were live-fire tests—multi-precision hole matching, intelligent sorting and transfer, and intelligent fire reconnaissance—each a genuine industrial challenge.
However, while following the competition, another thought kept nagging me: if on day 3 of the competition, a robot’s dexterous hand suddenly developed abnormal noise or malfunction, how would everyone react?
This is actually not an unfounded worry. Under high-intensity operation grease, fatigue issues in gear transmission systems accelerate their exposure. And this type of problem is often already present before the competition even begins.
How tooth fractures occur
Planetary reducer tooth fractures grease primarily fall into two categories: fatigue fracture and overload fracture.
Under the conditions of frequent forward-reverse operation, start-stop, and shock at the dexterous hand, the tooth root of each gear tooth endures alternating bending fatigue stress; cracks gradually propagate and eventually fracture—this accounts for 42% of all tooth fracture cases. Overload fractures from short-term shock or severely worn and thinned gear teeth account for approximately 31%. The remaining ~27% are related to material defects.
However, the key to understanding dexterous hand tooth fractures lies in the gear dimensions.
The gear module of dexterous hand planetary reducers is typically only 0.3–0.8 mm, far smaller than ordinary industrial gears. At the tooth root fillet of these micro gears, the stress concentration factor can reach 2.8–3.5, much higher than ordinary gears. The shock load at each start-stop is greatly amplified at the tooth root. High stress concentration significantly accelerates fatigue crack initiation and propagation—this is one fundamental reason why small-module gears face higher failure risk under dexterous hand operating conditions.
Fatigue fracture is not a single ‘snap’—it is a gradual process.
Stage 1: Micro-cracks quietly initiate at the tooth root. There are no symptoms at this stage, the equipment cannot detect any abnormality, but the metal’s microstructure has already begun accumulating damage.
Stage 2: Under the continual alternating stress, cracks stably propagate outward, and fracture surfaces show shell-like marks. Crack propagation rate is typically in the 10⁻⁸ to 10⁻⁶ m/cycle range—for the high-frequency dexterous hand operating conditions, this speed means the effective service life window is quite limited.
Stage 3: The remaining cross-section can no longer withstand the load, and instantaneous fracture occurs. The gear loses transmission capability, the dexterous hand joint locks up, and the entire machine requires shutdown for repair.
Before crack initiation, the tooth surface often already shows detectable precursors—micro-pitting. During long-term gear operation, tooth surface micro-pitting is the earliest failure signal. Tiny cracks continuously propagate, lubricating oil enters the crack interior, and after multiple repeated mesh cycles, small metal fragments spall off to form pitting pits. This pit becomes a new stress concentration source, which in turn accelerates tooth root fatigue crack initiation. Even more troublesome, spalled metal particles mix into the lubrication system, forming an abrasive wear cycle—small particles accelerate tooth surface wear, further increasing stress concentration. This creates a vicious cycle.
So in high-intensity competitions, if a robot’s dexterous hand exhibits abnormal noise, start-stop hesitation, or torque fluctuation, it is often not a software problem but a gear system sending an SOS signal.
Material is of course important, but the most underestimated factor is grease
I must first acknowledge: material, heat treatment, and design are crucial for dexterous hand planetary reducer reliability. Carburized layer depth, retained austenite control, and tooth root strengthening—this is what raises the fatigue resistance ceiling of gears.
But these are the foundation, necessary conditions, not sufficient conditions.
Against the high-intensity conditions of batch entry and competition, the differences brought by different lubrication approaches truly come to the surface. The same gear set, under different lubrication approaches, can show fatigue crack initiation cycles differing by several times. The most easily underestimated critical variable is grease.
Conventional greases face three core challenges under dexterous hand operating conditions.
First, oil film is difficult to establish stably. Dexterous hand planetary reducer gears have extremely small modules and small tooth surface curvature radii, so lubricating oil film thickness is typically only 1–3 μm. With frequent forward-reverse operation of the dexterous hand and extremely short contact time, conventional greases under non-continuous high-temperature conditions have greatly increased probability of oil film repeatedly rupturing and rebuilding, leading to direct metal contact.
Second, EP additive failure. Sulfur- and phosphorus-containing EP additives need to form protective films of FeS and FePO₄ on metal surfaces; this reaction requires a certain contact time and temperature to complete effective film formation. However, under the conditions of frequent direction reversals and extremely short contact time at the dexterous hand, the additives simply do not have sufficient reaction time, and film formation efficiency is greatly reduced.
Third, thickening agent structure failure. Ordinary lithium-based greases under high-frequency shear gradually destroy the three-dimensional network structure of the thickening agent, viscosity decreases, and softening and increased oil separation occur. Effective lubricating oil volume decreases, and the oil film thins further.
How X500 addresses these challenges
VNOVO’s X500 dexterous hand joint grease uses perfluoropolyether (PFPE) base oil combined with a PTFE thickening agent system. This formulation addresses not just a single parameter but an entire logic of operating condition adaptability.
EP load-bearing capacity is the key indicator under start-stop and shock loads for planetary reducers. X500’s EP load exceeds 800 kg, capable of withstanding the Hertz pressure generated under high contact stress in planetary gears and preventing metal direct contact caused by oil film rupture. This is the physical foundation for delaying fatigue crack initiation.
Low-temperature starting torque affects the consistency of response during frequent forward-reverse operation of the dexterous hand. The low-viscosity base oil design reduces starting torque, allowing motor output power to be more effectively converted into joint motion, reducing start-up impulse load—and this is precisely one of the trigger conditions for tooth root fatigue crack initiation.
Wide temperature stability covers a -50 °C to 220 °C operating temperature range. The high bond energy of C-F bonds in the PFPE base oil molecular structure makes it resistant to oxidative degradation at high temperatures, fundamentally reducing oil film failure caused by grease deterioration.
Anti-loss performance ensures long-term maintenance-free operation. Oil-bleeding loss rate at 100 °C/24 h is less than 10%; under high-speed centrifugal force and shear, the grease is not prone to loss or drying out. For the enclosed-space, limited-grease-fill dexterous hand planetary reducer scenario, this directly determines whether the grease can continuously function throughout the entire lubrication cycle before replacement.
Fellow practitioners, have you encountered tooth fracture problems on dexterous hand planetary reducers? Before the fracture occurred, were there signs of micro-pitting or significant wear? How can the lubrication approach and other protective measures (such as tooth root strengthening and carburized layer control) work together to truly push dexterous hand life to a level acceptable for production lines?
Feel free to exchange perspectives in the comments. If you need to discuss lubrication approaches for specific operating conditions, private messages are also welcome.
