An O-ring is a rubber ring with a circular interface shape and is the most widely used seal in hydraulics and pneumatics.

An O-ring is a bidirectional sealing element. During installation, the initial radial or axial compression of the O-ring determines its initial sealing ability. The force exerted by the system pressure on the O-ring is its total sealing force, which increases with increasing system pressure.

Under pressure, the O-ring's shape mimics that of a liquid with high surface tension. Pressure is transmitted equally in all directions.

Features:

1. Small size and easy assembly and disassembly

2. Suitable for both dynamic and static sealing

3. Virtually leak-free static sealing

4. Single-piece bidirectional sealing

5. Low dynamic friction

6. Low price

O-ring Sealing Principle

An O-ring is an extrusion seal. When the seal experiences initial deformation and stress Pseal, and Pw > Pseal, leakage will not occur.

Pm = P0 + Pp, where Pp = K × P.

Pm = P0 + K × P

K is the coefficient of the medium pressure transferred to the O-ring pressure (for rubber, K = 1). Therefore, as long as there is initial pressure in the O-ring, a leak-free, absolute seal is achieved.

O-ring seals are self-sealing.

Selecting the Compression Set Rate for O-ring Seals

Theoretically, sealing can be achieved with zero compression, but this is not practical.

Eccentricity: Under operating load, the O-ring stretches and becomes thinner, potentially causing leakage.

Low temperatures: The rubber shrinks and becomes thinner, potentially causing leakage (low temperatures accelerate rubber aging and loss of compensating capacity).

Generally, the cross-section has a compression set rate of 7%-30%. Static seals require a higher compression rate (15%-30%), while dynamic seals require a lower compression rate (9%-25%).

Selecting O-rings for internal and external pressures

For internal pressure: The O-ring outer diameter should be the same as the groove outer diameter.

For external pressure: The O-ring inner diameter should be the same as the groove inner diameter.

Prevent the O-ring diameter from shrinking under operating pressure.

When installed in a groove, an O-ring is subjected to tension or compression. Excessive tension or compression will result in an excessive increase or decrease in the O-ring's cross-section, as a 1% stretch results in a corresponding decrease in the cross-sectional diameter W by approximately 0.5%. For hole seals (internal pressure), O-rings are best maintained in tension, with a maximum allowable tension of 6%. For shaft seals (internal pressure), O-rings are best compressed along their circumference, with a maximum allowable circumferential compression of 3%.

O-Ring Extrusion Principle

O-Ring Allowable Extrusion Gap

The maximum allowable extrusion gap gmax is related to the system pressure, the O-ring cross-sectional diameter, and the material hardness. Generally, the higher the operating pressure, the smaller the maximum allowable extrusion gap gmax. If the gap g exceeds the allowable range, the O-ring will be extruded or even damaged. When the pressure exceeds 5 MPa, it is recommended to use a backup ring.