Overview of Sealing in Harmonic Gear Reducers for Robots and Some Reflections

01The Role and Performance Requirements of Harmonic Gear Reducers in Robots

The harmonic gear reducer is the core transmission component of humanoid robot joints: it converts the high speed/low torque of the motor into low speed/high torque, providing high precision position control. Its unique structure of “flexible wheel – wave generator – rigid wheel” offers advantages such as high transmission ratio, high transmission accuracy, and ultra-low backlash. In high-degree-of-freedom, lightweight humanoid robots, harmonic gear reducers are commonly integrated into each joint, necessitating the achievement of sufficient output torque, high efficiency, and extremely low backlash within limited volume and mass constraints, while maintaining high dynamic response to accommodate multi-joint coordinated control.

While meeting transmission performance, the sealing design at the joints is crucial. The structure of humanoid robot joints is compact, and the available space for sealing elements is minimal, yet it must balance the following objectives:

Low Friction: To avoid increasing transmission losses and heat generation;
Durability and Wear Resistance: To adapt to long-term swinging/reciprocating small angle movements;
Environmental Protection: To prevent dust, water, and contaminants from entering;
Thermal Stability: To maintain elasticity and sealing performance after temperature rise during operation;
Controlled Size and Mass: To achieve reliable sealing within compact assembly spaces.

Only by achieving a balance between compact size and high reliability can the seal effectively prevent grease leakage and the ingress of external impurities, ensuring the long-term stable operation and lifespan of the harmonic gear reducer and the entire machine from a detailed perspective.

02Key Points in Sealing Structure Design

1) Sealing Layout and Structural Form

Typical harmonic gear reducer joints often have radial shaft seals at the output end to retain grease and block external contamination. Common engineering configurations include:

Double Lip Oil Seal (Contact Type): One lip faces inward to suppress grease overflow, while the other lip faces outward to block dust and moisture.
Non-Contact Protection: In high-speed or extremely low friction demand locations, labyrinth seals, oil slinger/shield, or V-shaped dust rings can be used as the first barrier on the outside, with minimal rotational resistance; an inner contact-type main lip seal retains grease, forming a non-contact outer + contact inner composite solution.
Simplified Input End: At the input end of the high-speed shaft, the motor’s built-in sealed bearings or non-contact shields can replace independent oil seals to simplify the structure and reduce friction.

Design Tips:• Labyrinth components can be pre-greased in small amounts for dust capture in specific applications, but in high-dust environments, the “dirt-absorbing” side effects and maintenance strategies need to be evaluated.• The installation direction of the labyrinth must ensure that the open end faces the main source of contamination.• Low resistance structures should be prioritized for high-speed areas; composite multi-lip seals should be prioritized for low-speed high-torque outputs to ensure sealing reliability.

2) Material and Lubrication Compatibility

Humanoid robot joints typically operate under grease lubrication conditions, with a typical environmental temperature range of -20 to 80 °C (expandable depending on the project). The choice of sealing materials directly affects lifespan and stability:

Rubber: FKM, HNBR, etc., which combine temperature resistance, oil/grease resistance, and wear resistance. Compatibility verification between base oil/additives and elastomers is required; some synthetic ester systems may cause swelling or hardness changes in certain rubber formulations.
Thermoplastic Materials: PTFE and its filled composites have advantages of self-lubrication, low wear, and high temperature resistance (up to 200 °C), suitable for low friction demands, but typically require higher standards for shaft surface (roughness, hardness, cylindricity).
Formula Enhancement: Adding solid lubricants/wear-resistant fillers (such as graphite, molybdenum disulfide) to elastomer formulations can reduce friction and wear.
Framework and Shell: To reduce weight, aluminum alloy frameworks/casings can be used, and anodizing or anti-corrosion coatings can enhance corrosion resistance and surface hardness.

3) Dynamic Performance and Swing Conditions

Humanoid robot joints often involve small angle reciprocating/swinging rather than continuous unidirectional rotation, presenting three types of challenges:

Oil Film Redistribution Difficulty: Under small amplitude swings, the oil film can become uneven, leading to poor local lubrication and accelerated wear.
Stick-Slip: At micro speeds/micro displacements, torque fluctuations and noise can occur due to “stick-slip” effects, affecting micro-movement precision.
Assembly Deviations and Runout: Manufacturing/assembly errors can cause slight radial/axial displacements and runout; if the seal’s compliance is insufficient, it can lead to localized overload wear.

Design Countermeasures:• Select low-friction materials + light preload structures to mitigate stick-slip.• Use non-directional micro-textures/waves or oil grooves to promote oil film redistribution during reciprocation; directional helical grooves need special verification to avoid pumping effects during reciprocation.• Employ spring lip seals for swinging seals to better accommodate eccentricity and runout.• At the system control level, periodic large-angle movements can be planned to “refresh” the lip oil film coverage.

03Common Failure Modes and Countermeasures

1) Grease Leakage

Phenomenon: Oil seepage/grease fling appears at the joints.Causes: Temperature rise causes gas and grease expansion in the cavity, creating pressure differentials; improper matching of lip structure and rotation direction causes external pumping effects; grease is too thin or overfilled, etc.Countermeasures::

Control temperature rise (reduce meshing losses, optimize grease shear characteristics);
Set up micro-breathing channels/breathing valves, if necessary, in combination with labyrinths or waterproof breathable membranes to balance pressure differentials;
Optimize lip geometry to create inward pumping of the oil film;
Select greases with good anti-separation properties and fill according to specifications to avoid accumulation in the sealing area.

2) Foreign Object Ingress

Phenomenon: Dust particles/sand particles/liquids entering cause wear or corrosion.Countermeasures::

Use a dual protection system with an outer non-contact barrier + inner contact main lip;
Select sealing structures with dust lips to clear dirt;
Conduct regular inspections and surrounding clean-ups in dusty conditions, and promptly replace aging sealing elements.

3) Lip Wear/Aging

Phenomenon: Friction accumulation leads to wear, while temperature rise and environmental oxidation reduce elasticity and cause cracking.Countermeasures::

Select self-lubricating/low-friction materials or add wear-resistant and anti-oxidation additives to the formulation;
Incorporate oil grooves/reservoirs in the structure to maintain lubrication;
Control operating temperature within the allowable range of materials through heat dissipation design.

4) Assembly and Precision Issues

Phenomenon: Surface roughness/scratches on the shaft, deviations in coaxiality/inclination, installation of flipped lips/spring detachment, etc., lead to immediate or early failure.Countermeasures::

Strictly control the geometric precision and surface quality of the shaft and casing (chamfering and smoothing to avoid scratching the lip);
Use specialized tooling to achieve uniform pressing and positioning;
Conduct airtight/leakage tests on critical joints after assembly to screen for defects.

04Acceptance and Testing Recommendations

Friction Torque – Angle Characteristics: Record starting and steady-state friction torques, forward and reverse symmetry, and stick-slip characteristics.
Leakage and Protection: Regularly weigh leakage mass; conduct dust/water tests in conjunction with target IP ratings.
Durability: Cumulative hours and performance retention under swinging conditions (amplitude, frequency, temperature).
Environmental and Thermal Aging: Changes in elasticity and sealing performance after high and low temperature and thermal aging.
Surface and Material Compatibility: Verification of shaft roughness, hardness, coating, and sealing material coupling (especially for PTFE lip seals).

05Surface and Assembly Key Points

Shaft/Sleeve Surface: Control Ra and Rz, eliminate steps and burrs; critical surfaces may consider hardening or thin film coatings to enhance wear resistance and fit stability.
Chamfering and Guiding: Set guiding chamfers, use lubrication/assembly tools to avoid lip flipping and scratching.
Positioning and Pressing: Vertical and uniform pressing to avoid eccentricity; temperature differences/tooling fit may be necessary.
Breathing and Venting: Provide controlled breathing paths to relieve pressure differentials without compromising target waterproof and dustproof capabilities.

06Some Thoughts on Sealing Design for Harmonic Gear Reducers

The sealing of harmonic gear reducers, due to their swinging/small angle reciprocating motion mode, grease dominated lubrication method, and stringent requirements for ultra-low friction torque, necessitates a coordinated optimization of materials, structures, and system control. Different companies have developed their design philosophies and trade-offs regarding spring lip seals (rubber), filled PTFE lip seals, and labyrinth + contact composite structures.

Avoid the path dependency of “holding a hammer and seeing every problem as a nail”; choose solutions based on data rather than preferences, significantly enhancing the reliability and communicability of sealing design.