By VNOVO Technical Support Team
In recent years, the approach of choosing planetary reducers for shoulder joints has been gaining momentum.
Planetary reducers feature a compact structure, high load capacity, and excellent shock resistance, making them well-suited for shoulder joints that must both lift the arm and bear external forces. The Yushu G1 robot’s joints use a two-stage planetary reducer mechanism, and Schaeffler showcased planetary gear actuators for shoulder, knee, and hip joints at CES 2026. The planetary solution is evolving from a ‘fallback option’ to a ‘preferred choice’.
However, there is one aspect that is easy to overlook—lubricating grease. When planetary reducers are used in shoulder joints, the grease must pass through quite a few checkpoints. Choose the wrong one, and after a few thousand cycles you’ll see tooth surfaces developing pits and even broken teeth—things we’ve all witnessed.
Checkpoint 1: High-load shock—oil film must not collapse
During lifting, arm raising, and sudden direction changes at the shoulder joint, peak torque can easily exceed 100 Nm, causing contact stress on the tooth surfaces to spike dramatically. At this point, whether the grease’s extreme-pressure (EP) capability can withstand the test depends entirely on whether the oil film holds up. Insufficient EP rating and the oil film ruptures, leaving tooth surfaces in direct metal-to-metal contact—micro-pitting appears within a few hundred impacts. As micro-pitting progresses into spalling pits, precision is lost for good.
Checkpoint 2: Shear stability—must not progressively ‘soften’
Inside a planetary reducer, each planet gear simultaneously spins on its own axis while orbiting the sun gear. On the same tooth surface, the shear rate experienced at the tooth tip can differ by several times from that at the tooth root. At the tooth tip, the speed is higher, requiring the grease to thin out somewhat to reduce drag; at the tooth root, speed is lower, requiring the grease to stay thicker to maintain the oil film. If the grease’s shear-thinning characteristics are unsuitable, or if the thickening agent is sheared apart, the grease softens and migrates, leaving tooth surfaces running dry. After a few thousand hours, noise increases and return clearance doubles.
Checkpoint 3: Low-speed heavy load and high-speed direction reversal—both ends of the spectrum must be addressed
When the shoulder joint is supporting a load, rotation speed is low and torque is high. At this point, hydrodynamic oil film formation is essentially unreliable, and everything depends on base oil viscosity and EP films, requiring the grease to maintain high viscosity under low shear. But when the arm swings rapidly, the grease must thin out quickly, otherwise starting torque becomes too high, the motor responds sluggishly, and force control oscillates. This is not simply a matter of high or low viscosity—the same grease must be thick at the tooth root and thin at the tooth tip. Choose poorly, and you face either heavy-load wear or high-speed stalling.
The sealed maintenance-free trap: when the oil escapes, everything dries out
Shoulder joint reducers are mostly sealed, designed to be lubricated once and forgotten. But greases with high oil-bleeding rates (e.g., exceeding 10%) will slowly seep and lose base oil over a few thousand hours, leaving the thickening agent dried into hard clumps that lodge between teeth. That is not lubrication—it is like pouring sand inside. Hard particles roll and crush, micro-pitting becomes spalling, spalling becomes tooth fracture.
Another path is EP film failure, direct tooth surface contact, cracks at the tooth root that initiate and propagate, and finally—snap—a broken tooth. On shoulder joints with high load and high-frequency direction reversals, this process occurs much faster than in ordinary reducers.
Selecting Grease: Just Watch Three Numbers
For planetary reducers in shoulder joints, focusing on three indicators is essentially sufficient for grease selection.
First is operating temperature range. It should cover at least -30 to 180 °C. It must not freeze during standby in a cold warehouse in winter, nor melt when things heat up.
Second is oil-bleeding rate. When held at 100 °C for 24 hours, separated oil should not exceed 5%. For sealed maintenance-free applications, 5% is the red line. Dexterous hand requirements can be slightly looser (10%), but shoulder joints have higher loads and more heat generation, making them far more sensitive to oil loss—must be kept within 5% to confidently claim years of no drying out.
Third is EP anti-wear performance. Four-ball weld load should preferably reach above 800 kg, or wear scar diameter should not exceed 0.5 mm (measured at 396 N). Shoulder joints frequently experience instantaneous shocks—an EP film that is not tough enough simply will not do. The smaller the wear scar, the better the protection.
Take the VNOVO SYN150 shoulder joint bearing grease as an example: operating temperature -30 to 180 °C, oil-bleeding rate ≤5%, evaporation loss <1.0%, wear scar diameter 0.46 mm. These numbers fall precisely within the core range for planetary reducers in shoulder joints.
A final note
Planetary reducers in shoulder joints offer excellent load-bearing and shock resistance—more people are recognizing this. But the grease aspect is often left to the very end, and by the time tooth surfaces turn dark or the joint starts making abnormal noises, it is already too late.
If your shoulder joint planetary reducer is heavy-duty, sealed, and designed for years of maintenance-free operation—three numbers to watch: temperature tolerance, low enough oil-bleeding rate, and tough enough EP film.
When selecting or testing shoulder joint configurations, have you encountered failures caused by grease drying out? Where do oil-bleeding rate and EP indicators rank on your selection checklist? Feel free to share actual test data.
