How Can Skeleton Oil Seals Adapt to ±180 Degree Reciprocating Rotation in Robot Joint Shafts?

Number of hits：362026-01-08 15:12:21　

The ±180° reciprocal motion of industrial robot joint shafts presents a unique and severe challenge to traditional sealing concepts. The core issue lies in the high-frequency, limited-angle directional motion, which easily disrupts the lubricant film, causing the seal lip to frequently operate in a boundary lubrication state against the shaft surface. This leads to significant wear, fluctuating frictional torque, and thermal accumulation, which accelerates material degradation. Therefore, a sealing design for this operating condition needs to transition from addressing static leakage to building a low-friction, wear-resistant system that can actively maintain lubrication.

Material Selection: PTFE Composites as Primary Seals, High-Performance Rubber as Secondary Seals

Material choice is the foundation for addressing friction and wear problems. In this operating condition, material selection should adhere to the principle of "low friction for the primary seal and high elasticity for the secondary seal."

Primary Seal Material: PTFE Composites

PTFE composites or filled-modified PTFE should be the first choice for the primary sealing lip. PTFE offers:

Extremely low friction coefficient (as low as 0.02-0.1)

Excellent self-lubrication

Good wear resistance

Secondary Seal Material: FKM and HNBR

FKM and HNBR are ideal for secondary seals due to their:

Excellent elasticity

Superior sealing ability

Resistance to temperature and oils (operating temperature range: -50°C to +150°C)

These materials are mainly used for dust lip seals, static O-rings, or as elastic supports for PTFE primary seals. They complement PTFE functionality, such as FKM is dust lip preventing external contaminants from damaging the primary seal.

Special Materials: FFKM

FFKM is unmatched in chemical resistance and extreme high-temperature performance (up to 325°C), but due to its high cost, it is generally only used in specialized, high-demand applications such as chemical processing or semiconductor robotics.

Lip Design: From “Passive Barrier” to “Dynamic Seal with Backflow Capability”

Lip design needs to go beyond simple geometry and focus on actively managing the lubrication interface during oscillation.

Fluid Dynamic Profile

The primary sealing lip should adopt an asymmetric profile (such as Z-shaped, K-shaped, or S-shaped). This design generates a slight directional pumping effect during shaft rotation, continuously "pumping" the small amount of extruded grease back into the sealing cavity, ensuring a dynamic and active seal rather than a passive barrier.

Precision in Dual-Lip Function

Typical rotary shaft seals utilize a dual-lip structure:

Primary lip (inward-facing) seals the lubricant, with the profile optimized based on fluid dynamic principles.

Secondary lip (outward-facing) is usually made of more elastic rubber, designed to block external contaminants such as dust and water.

The materials and functions of the two lips are distinct but work in unison.

Spring Preload System: The “Stabilizer” for Sealing Performance

In reciprocal motion, maintaining a constant and moderate radial force is key to stable sealing performance and controlling frictional torque.

Core Function

The internal spring provides initial contact pressure and compensates for lip wear over time, ensuring reliable sealing throughout the component's life cycle. The precise design and stability of this force directly influence the startup torque and backlash.

Key Requirements

The spring must have excellent fatigue resistance and media compatibility to ensure it does not loosen or crack under long-term dynamic loading, with minimal loss of force.

Wear Resistance and Low Friction Design: A System-Level Solution

Wear resistance is not just a material attribute but the result of the overall friction system design.

Self-Lubricating Materials

As mentioned, PTFE is self-lubricating properties are fundamental in reducing wear. Additionally, integrating solid lubrication coatings such as molybdenum disulfide (MoS2) on the lip surface can further optimize initial break-in and long-term running performance.

Structural Friction Reduction Innovations (High-End Solutions)

For extremely demanding conditions, a "composite rolling seal" design can be used. This principle involves embedding a series of rolling elements (such as balls or similar components) within the sealing ring, which converts sliding friction between the seal and shaft into rolling friction, significantly reducing frictional torque (by over 70%) and virtually eliminating wear. However, this solution is high-cost and typically only used in highly reliable scenarios.

Temperature Adaptability and Thermal Management

The sealing system must be capable of withstanding the heat generated inside the joint, while its design should help minimize thermal generation.

Material Temperature Stability

Materials like FKM, HNBR, and PTFE maintain stable performance within a wide temperature range from -50°C to +150°C (with short-term resistance to higher temperatures).

Design for Low Thermal Build-up

By utilizing low-friction materials and optimizing contact pressure, frictional heat generation can be minimized from the outset, thus preventing aging failure of the seals due to high temperatures. This approach is more effective than relying solely on material temperature resistance.

Installation and System Integration

Even the best design relies on precise manufacturing and installation.

Installation Precision

The surface hardness of the shaft (usually ≥ HRC 55), roughness (Ra 0.2 - 0.8 μm), and tolerance must meet the sealing requirements. Special installation tools should be used to ensure concentricity and prevent uneven wear of the lip.

Modular Trend

Leading suppliers now offer pre-assembled, pre-lubricated sealing modules. Users only need to press-fit these modules, simplifying the installation process and reducing reliance on on-site procedures, ensuring consistent performance.

Structural Optimization and Long-Term Durability

Skeleton Rigidity: The metal skeleton should provide sufficient support to prevent deformation during press-fitting or under load, ensuring the accuracy of the seal lip's geometry and positioning.

Elastic Design: The elastic portion of the seal should balance between following the shaft's eccentric motion (dynamic runout) and providing stable sealing force, ensuring effective contact throughout the robot’s entire motion range.

To adapt a skeleton oil seal for the ±180° reciprocal motion of industrial robot joint shafts, the key lies not in simply upgrading materials, but in constructing a system where:

PTFE composites form the core primary seal,

Fluid dynamic lips actively manage lubrication return,

Spring preload controls contact pressure,

High-performance rubber provides secondary sealing support.

For general industrial robots, this is a reliable, cost-effective, and mature technical path. For extreme conditions, further structural innovations or specialized material solutions may be evaluated.