In recent years, planetary reducer solutions for shoulder joints have become increasingly common.
Planetary reducers are compact, can handle heavy loads, and resist impact well — suitable for shoulder joints that must both lift arms and bear loads. Unitree G1’s joints use two-stage planetary reducer mechanisms; Schaeffler presented planetary gear actuators for shoulder, knee, and hip at CES 2026. Planetary solutions are transitioning from “alternative” to “preferred.”
But one thing is easily overlooked: grease. Planetary reducers in shoulder joints have many Grease challenges. Choose wrong, and after thousands of cycles, tooth surfaces develop pitting and tooth breakage — both seen before.
Challenge 1: High Load Impact — Oil Film Must Not Collapse
When shoulder joints lift objects, raise arms, or suddenly change direction, peak torque easily exceeds 100 N·m, and tooth surface contact stress surges. At this point, whether the grease’s EP capability can withstand the oil film is critical.
If EP value is insufficient, oil film collapses, tooth surfaces hard-contact, micro-pitting appears after hundreds of impacts. Micro-pitting develops into spalling pits, and precision is gone — never recovering.
Challenge 2: Shear Stability — Cannot Become “Softer” During Operation
In planetary reducers, planetary gears both spin and orbit. On the same tooth surface, the shear rate that tooth tip and tooth root experience can differ several times.
At the tooth tip where speed is high, grease needs to become thinner, otherwise resistance is excessive; at the tooth root where speed is low, grease must remain thicker to support the oil film. If grease shear-thinning characteristics are unsuitable, or thickener is sheared apart, the grease softens, migrates, and tooth surfaces directly dry-grind. After thousands of hours, noise increases and return clearance doubles — all from this cause.
Challenge 3: Low-Speed Heavy Load and High-Speed Direction Change — Both Must Be Addressed
When shoulder joints support objects, rotational speed is very low and torque is very high. At this point, hydrodynamic oil film basically cannot be relied upon — everything depends on base oil viscosity and EP film, requiring grease to have high viscosity under low shear.
But when arms quickly swing, grease must rapidly thin — otherwise start-up torque is too high, motor response slows, and force control wobbles. This is not a matter of viscosity being high or low — the same grease needs to be thick at the tooth root and thin at the tooth tip. Choose poorly, and either heavy-load wear or high-speed seizure occurs.
The Sealed Maintenance-Free Trap: When Oil Escapes, It Dries
Shoulder joint reducers are mostly sealed, intended to be filled once and used until end of life. But greases with high oil separation rates (say, over 10%) will have base oil slowly seep and escape after thousands of hours, leaving thickener caked into hard chunks that jam in tooth gaps.
That is not lubrication — it is like pouring sand inside. Hard chunks grind back and forth, micro-pitting becomes spalling, spalling becomes tooth breakage.
Another path: EP film fails first, tooth surfaces directly contact, cracks at tooth root germinate and propagate, then snap — on a shoulder joint with high load and high-frequency direction change, this process is much faster than in ordinary reducers.
Three Numbers for Selection
For shoulder joint planetary reducer grease selection, focusing on three indicators is basically sufficient.
Operating temperature range: At least -30°C to 180°C. Cannot freeze in winter cold storage, cannot melt when running hot.
Oil separation rate: After 100°C for 24 hours, separated oil must not exceed 5%. For sealed maintenance-free applications, 5% is the red line. Dexterous hands can be slightly looser (10%), but shoulder joints have heavier loads and more heat — more sensitive to oil loss, must be kept within 5% to ensure years of no dry-out.
EP anti-wear: Four-ball welding load should reach 800 kgf or above, or wear scar diameter not exceeding 0.5 mm (tested at 396 N). Shoulder joints frequently have instantaneous impacts — EP film must be tough.
Taking VNOVO SYN150 shoulder bearing dedicated grease as example: operating temperature -30~180°C, oil separation rate ≤5%, evaporation loss <1.0%, wear scar diameter 0.46 mm. These numbers fall precisely in the shoulder joint planetary reducer core range.
Which letter applies to you? A. Have done shoulder joint planetary reducer selection and encountered grease dry-out or tooth breakage. B. Oil separation rate and EP indicators rank in top three of selection checklist. C. No measured data yet, but paying close attention to sealed maintenance-free scenarios. D. Already addressing heavy-load plus high-speed direction change through lubrication optimization.
