By VNOVO Technical Support Team
Over the past week I came across a figure: more than 60% of humanoid robot operational failures are related to joint module vibration and noise.
Looking at the specifics, there are four root causes: transmission accuracy deviation, improper structural design, assembly process deviation, and incorrect application matching. Lubrication failure is classified under the last category — sounds like just “one of four,” which may seem light in weight. But the problem is: once lubrication fails, it does not act alone. It amplifies all three of the other problems simultaneously.
Inside a sealed joint, when lubricating grease fails, gear tooth surfaces make direct metal-to-metal contact, and transmission accuracy instantly drops from the micron level to the millimeter level. Impact loads are no longer absorbed by the oil film, and the cushioning margin built into the structural design is consumed entirely. The microscopic coaxiality deviation from assembly, under conditions of lost oil film lubrication, rapidly evolves into local overload. These four failure causes intertwine — noise is simply the audible outcome.
Therefore, increasing joint noise is never as simple as “time to change the grease.” It is the last-warning signal of lubrication grease that has already failed.
Lubrication grease failure contributes to noise primarily through three pathways
Pathway 1: Oil film rupture — hard collision between tooth surfaces.
In a planetary reducer, the planet gear simultaneously performs orbital revolution and self-rotation. The relative sliding velocity along the tooth profile varies, with the tooth tip typically running 2 to 3 times faster than the tooth root. The same lubricating grease entering the mesh zone experiences completely different shear rates at different positions.
If viscosity is too low, the oil film cannot be maintained under high shear, and gear tooth surfaces make direct metal impact, producing impulsive abnormal noise. If viscosity is too high, the oil film in the low-speed heavy-load zone becomes excessively thick, raising starting torque and also triggering noise. Both extremes point to the same issue: oil film integrity does not match operating conditions — noise is the direct result.
Pathway 2: Base oil loss — dry friction emerges.
Shoulder joint planetary reducers mostly employ sealed structures, and the lubricating grease receives no supplemental maintenance for months or even years. If the grease separation rate is high, base oil continuously exudes and evaporates, the thickener framework shrinks and hardens, and gears and bearings enter boundary friction or dry friction conditions lacking base oil lubrication. The friction coefficient jumps from 0.02 to above 0.08, accompanied by sharp frictional noise.
This process often advances silently. The appearance of the grease shows no obvious change, yet performance has already degraded.
Pathway 3: Temperature rise and lubrication failure fuel each other.
This is the most destructive pathway — and the most easily overlooked.
Once the lubricating grease oil film ruptures, gear tooth surfaces make direct contact, and frictional heat multiplies. Temperature rises, grease viscosity drops, the oil film thins further, and lubrication efficiency continues to decline — oxidation accelerates, base oil evaporation intensifies, and the grease separation rate increases. Temperature rise causes lubrication failure; lubrication failure in turn accelerates temperature rise, forming a positive feedback loop. Noise increases sharply in the short term, accompanied by joint housing temperature rise warnings.
In a sealed joint, once this cycle initiates, the structural recovery rate of the grease cannot keep pace with heat accumulation — failure becomes irreversible.
Five key indicators for selection
Based on the above three pathways, lubricating grease selection for shoulder joint planetary reducers centers on two objectives — “noise contribution resistance” and “application matching” — with five key indicators requiring focused attention.
Indicator 1: Shear stability and oil film persistence.
The cone penetration change rate after high-frequency directional reversals should be controlled within 15%. This is the baseline capability for the oil film to maintain integrity under high shear.
Indicator 2: Grease separation rate — not exceeding 5% under 100°C/24h conditions.
This is the red line for sealed, maintenance-free operation. Exceeding this value means base oil continuously depletes within the sealed cavity, and the thickener framework hardens within months, bringing noise along with it.
Indicator 3: Evaporation loss — not exceeding 1.0% under 99°C/22h.
High evaporation loss means effective lubrication components decrease silently. Performance degradation follows a chronic disease path — by the time it is discovered, it is often already late.
Indicator 4: Operating temperature range, covering -30°C to 180°C.
During high-frequency shoulder joint operation, internal temperatures can reach 80°C to 120°C. Combined with ambient temperature variation, the 180°C upper limit is the necessary margin for coping with thermal accumulation.
Indicator 5: Noise reduction and damping characteristics are easily overlooked.
The thickener system in wide-temperature-range dedicated greases typically offers superior viscoelasticity, which dissipates part of the vibration energy through viscoelastic damping before metal impact reaches the housing — reducing the vibration amplitude transmitted to the shell. This is also one of the technical pathways by which leading projects such as Unitree and DeepRobotics achieve noise levels of 45 dB or below. The key is not optimizing a single indicator to extremes, but simultaneously meeting multiple indicators to generate a synergistic effect.
VNOVO SYN150’s noise reduction logic
VNOVO SYN150 is a wide-temperature-range lubrication solution for shoulder joint planetary reducer bearings and gears, with the following key parameters:
Operating temperature: -30°C to 180°C; Grease separation rate 100°C/24h ≤ 5%; Evaporation loss 99°C/22h < 1.0%; Wear scar diameter ≤ 0.46mm.
This parameter set corresponds precisely to the core requirements of the five indicators above: the wide-temperature range covering -30°C to 180°C ensures viscosity can adapt to oil film thickness requirements across the full temperature span during high-frequency directional reversal operation; the dual control of grease separation rate and evaporation loss guarantees no loss or evaporation during long-term sealed operation; the extreme-pressure anti-wear wear scar diameter controlled within 0.46mm addresses medium-to-high load impacts in the shoulder joint.
From this logic, SYN150’s design approach aligns with the “wide-temperature-range dedicated grease” concept in noise reduction strategies from leading industry projects — not excelling in any single indicator, but simultaneously achieving multi-dimensional compliance, thereby reducing joint vibration noise overall.
Why selection is an engineering lever, not a procurement footnote
Among the four major causes of joint vibration noise, transmission accuracy is locked at the design stage, assembly processes are constrained by supply chain management, and structural design is largely standardized. All three are pre-production variables with extremely high modification costs once in mass production.
Only lubricating grease selection is a variable that engineers can actively optimize during the production phase. Without changing structures, without re-certification, without involving supply chain re-qualification — through one precise selection, you can directly influence noise levels and operational lifespan.
This is also why lubricating grease selection deserves to be treated as an engineering decision, rather than a footnote at the bottom of a procurement list.
When you are conducting shoulder joint noise testing or failure investigations, have you tracked the physicochemical changes of the grease at different operational stages? Have you regularly tested the grease separation rate? In the temperature-rise-to-noise-increase curve, how large a weight does the grease contribute? Feel free to share your measured data in the comments — let’s discuss together.
