By VNOVO Technical Support Team
In recent years, more and more shoulder joint solutions are choosing planetary reducers.
Planetary reducers have compact structure, can bear heavy loads, and have good impact resistance. They are quite suitable for shoulder joints that must both lift arms and bear loads. Unitree G1’s joints use a two-stage planetary reducer mechanism. Schaeffler also made planetary gear actuators for shoulder, knee, and hip at CES 2026. The planetary solution is changing from alternative to preferred.
But something easily overlooked is the lubricating grease. Planetary reducers on shoulder joints face many lubrication challenges. Choose wrong, and after a few thousand cycles, tooth surfaces develop pitting and teeth break.
Gate 1: High-load impact, oil film must not collapse
The shoulder joint lifts things, raises arms, suddenly changes direction. Peak torque easily exceeds 100Nm, and tooth surface contact stress spikes. At this point, whether the grease extreme pressure capability is sufficient depends on whether the oil film can withstand it. If the extreme pressure value is insufficient and the oil film breaks, tooth surfaces make hard contact, micro-pitting appears within hundreds of impacts. Micro-pitting develops into spalling pits, and precision is gone.
Gate 2: Shear stability, cannot become progressively softer
In planetary reducers, planet wheels simultaneously rotate on their own axis and orbit. On the same tooth surface, the shear rate at the tooth tip and tooth root can differ by several times. At the tooth tip where speed is high, the grease must become slightly thinner, otherwise resistance is too high. At the tooth root where speed is low, the grease must be thicker to support the oil film. If the grease shear thinning characteristics are unsuitable, or the thickener is sheared apart, the grease softens, runs away, and the tooth surface directly grinds dry. After running a few thousand hours, noise increases, and backlash doubles — all for this reason.
Gate 3: Both low-speed heavy load and high-speed direction change must be covered
When the shoulder joint supports something, the rotation speed is very low and torque is very high. At this time, the dynamic pressure oil film is basically unreliable, entirely relying on base oil viscosity and extreme pressure film. The grease requires high viscosity at low shear. But when the arm swings quickly, the grease must quickly thin, otherwise starting torque is too high, motor response is slow, and force control oscillates. This is not about viscosity being high or low — the same grease, the tooth root wants thick, the tooth tip wants thin. Choose poorly, and either heavy-load wear or high-speed seizure occurs.
The pit of sealed maintenance-free: Oil runs out, then it dries
Most shoulder joint reducers are sealed, designed to fill once and use until报废. But grease with too high an oil bleeding rate (for example, exceeding 10%), after a few thousand hours, the base oil slowly seeps and runs out. The remaining thickener becomes a hard cake stuck in tooth gaps. That is not lubrication; that is adding sand inside. The hard cake rolls around, micro-pitting becomes spalling, spalling becomes tooth breakage.
On another path, the extreme pressure film fails first, tooth surfaces make direct contact, cracks at the tooth root initiate and propagate, and finally snap. On high-load, high-frequency direction-change shoulder joints, this process is much faster than on ordinary reducers.
Three numbers to watch for grease selection
Selecting grease for shoulder joint planetary reducers, focusing on three indicators is basically sufficient.
One is operating temperature range. At least -30 to 180 degrees. It cannot freeze during winter warehouse standby, and it cannot melt when it runs hot.
One is oil bleeding rate. After 100 degrees for 24 hours, separated oil cannot exceed 5%. For sealed maintenance-free applications, 5% is the red line. The dexterous hand requirement can be slightly looser (10%), but shoulder joints have heavy loads and more heat generation, more sensitive to oil loss. Must be pressed below 5% to confidently claim years without drying.
One is extreme pressure anti-wear performance. Four-ball welding load should reach above 800kg, or wear scar diameter should not exceed 0.5mm (measured at 396N). Shoulder joints frequently have instantaneous impacts, the extreme pressure film must be tough. Smaller wear scar, better protection.
Taking VNOVO SYN150 shoulder bearing dedicated grease as an example: operating temperature -30 to 180 degC, oil bleeding rate 5% or less, evaporation loss less than 1.0%, wear scar diameter 0.46mm. These numbers precisely hit the core range for shoulder joint planetary reducers.
A final note
Planetary reducers used in shoulder joints — good load-bearing, good impact resistance — is recognized by more people. But the lubricating grease part is often dragged to the end before being looked at. By the time tooth surfaces turn black and joints make abnormal noise, it is already too late to look back.
If your shoulder joint planetary reducer is heavy-load, sealed, maintenance-free for years — hold to three numbers: temperature range can handle it, oil bleeding rate low enough, extreme pressure film tough enough.
When you are doing shoulder joint selection or testing, have you encountered failures caused by grease drying? Where do oil bleeding rate and extreme pressure indicators rank in your selection checklist? Measured data welcome to share.
