Working Principle of O-ring Seals

Overview

An O-ring, named for its circular cross-sectional shape, is a widely used rubber sealing component in hydraulic and pneumatic applications. This O-shaped seal features bidirectional sealing capability, where the initial compression—both radial and axial—directly influences its initial sealing performance. When system pressure acts on the O-ring, it generates a sealing force that increases with the system pressure. Under pressure, the O-ring behaves similarly to a liquid with high surface tension, uniformly transmitting pressure in all directions.

Sealing Principle and Process

Sealing Principle

O-ring sealing belongs to the extrusion-type seal category. When the seal produces initial deformation and stress, if the sealing pressure (Pw) exceeds the initial stress (Pseal), a leak-free seal is achieved. The total sealing pressure (Pm) consists of the system pressure (P0) and the medium pressure (Pp), where Pp is related to the medium pressure coefficient (K, typically 1 for rubber). Thus, as long as the O-ring has an initial pressure, reliable absolute sealing is feasible.

Self-Tightening Sealing Effect

O-rings also feature a self-sealing structure. When installed between a groove and a sealing surface, proper compression generates a rebound force, applying initial compressive stress to the sealed smooth surface and groove bottom for pre-tightening or initial sealing. Due to this pre-sealing property, the O-ring maintains close contact with both surfaces. When fluid attempts to enter the groove through gaps, it acts on one side of the O-ring. As the working pressure increases, the fluid’s internal pressure pushes the O-ring toward the opposite side of the groove, causing extrusion deformation into a "D" shape. This deformation generates pressure transmitted to the sealing contact surface, ensuring the actual sealing pressure exceeds the applied fluid pressure based on the initial interference pressure.

Compression Set Rate

While theoretically possible to seal with 0% compression, practical implementation is challenging. Factors like eccentricity and low temperature can affect sealing effectiveness. Thus, in real-world applications, selecting an appropriate compression set rate is crucial to ensure optimal sealing performance.

Selection for Internal vs. External Pressure

The choice of O-ring for internal or external pressure applications relates to maintaining proper diameter to prevent shrinkage under working pressure. When installed in a groove, an O-ring is subjected to tension or compression. Excessive tension or compression can cause over-increase or decrease in cross-sectional diameter (e.g., 1% tension reduces diameter W by ~0.5%). Therefore:

For internal pressure (bore) sealing, the O-ring should be in tension, with a maximum allowable tension of 6%.

For internal pressure (shaft) sealing, the O-ring should be compressed along its circumference, with a maximum allowable circumferential compression of 3%.