What is the Sealing Principle of Skeleton Oil Seals and How is the Lip Interference Fit Determined?

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The sealing performance of a skeleton oil seal (also known as a rotary shaft seal) depends not only on the material and structural design but also closely relates to the sealing mechanism at the lip and the appropriateness of the interference fit. This article provides a systematic explanation of the sealing mechanism and the method for determining lip interference from an engineering application perspective.

Sealing Structure Parameters

Lip Configuration: Single lip / Double lip (with dust lip)

Spring Inclusion: With spring (e.g., TC, TB types) / Without spring

Lip Angle and Thickness: These influence oil film formation and oil return capability

Lip Interference

Definition: The dimensional difference between the oil seal’s inner lip diameter in its free state and the actual operating shaft diameter

Typical Reference Range (for standard rotary oil seals):

Radial interference (single side): approx. 0.2–0.4 mm

Corresponding diameter interference: approx. 0.4–0.8 mm

Related Fit Parameters

Shaft Surface Roughness: Typically Ra 0.2–0.8 μm

Shaft Diameter Tolerance and Concentricity;

Operating Speed and Temperature Range;

Common Applications of skeleton oil seals;

Automotive engines, transmissions, and electric drive systems

Industrial motors, pumps, and fans

Gearboxes, drive shafts, and mechanical shaft ends

In these applications, oil seals must prevent lubricant leakage while allowing a stable micro oil film to form between the lip and shaft, reducing friction and wear.

Core Principles and Common Challenges

Sealing Mechanism of skeleton oil seals

Contrary to the idea of achieving sealing solely through tight lip compression, the sealing function relies on a combination of mechanisms:

Initial contact pressure generated by lip interference;

Formation of a thin oil film between the rotating shaft and the lip;

Oil return effect created by the oil film, which helps retain lubricant;

Spring-loaded designs provide consistent contact pressure over time;

This “dynamic oil film sealing” is the foundation for long-term, stable performance.

Common Issues from Improper Interference

Excessive Interference:

Increased friction and heat generation;

Accelerated lip wear;

Early failure under high-speed conditions;

Insufficient Interference:

Inadequate contact pressure;

Unstable oil film;

Oil leakage or flinging;

Achieving the right balance between sealing effectiveness and service life requires careful interference design.

Key Factors Influencing Interference Selection

Shaft Speed: Higher speeds often require reduced interference

Operating Temperature: Elevated temperatures affect rubber elasticity

Sealing Medium: Low-viscosity fluids demand more stable contact

Rubber Material:

NBR: Suitable for standard interference designs

FKM: Higher hardness and temperature resistance; requires more precise interference control

Spring Inclusion: Seals with springs rely less on interference for maintaining contact pressure

By combining lip interference, dynamic oil film behavior, and optional spring compensation, skeleton oil seals establish a reliable and stable sealing interface on rotating shafts. The lip interference is not a fixed value—it must be determined based on shaft dimensions, operating conditions, and material properties. A well-engineered interference design is key to balancing sealing reliability and service life.