O-Ring Sealing: Principles and Features

The O-ring is a circular rubber ring, which is the most widely used seal in hydraulic and pneumatic systems. O-rings can have circular, square, X-shaped, and Y-shaped cross-sections, each shape having different properties and applicability.

1. The Sealing Principle of O-rings

The O-ring is a bidirectional sealing element. When installed, the initial compression of the O-ring in the radial or axial direction determines its initial sealing capability. The force generated by the system pressure acting on the O-ring is its total sealing force; this sealing force increases as the system pressure rises.

Under pressure, the shape of the O-ring and the liquid with high surface tension are similar. The pressure is transmitted equally in all directions.

The self-sealing of the O-ring is limited. When the internal pressure is too high, the “rubber extrusion” phenomenon of the O-ring will occur. That is, due to the existence of a gap in the sealed part, the O-ring under high pressure will generate stress concentration at the gap.

When the stress reaches a level that the rubber material of the O-ring cannot bear, the rubber material will be squeezed out. At this time, although the O-ring can still temporarily maintain the seal, it has actually been damaged.

Therefore, it is necessary to strictly select the type.

2. Features of O-rings

1. Advantages of O-rings

• Simple design, compact structure, easy to install and remove;
• Suitable for various sealing forms, both dynamic and static seals can be used;
• The cross-sectional structure of the O-ring seal is extremely simple, and it has a self-sealing function. The sealing performance is reliable, and there is almost no leakage in static sealing;
• The frictional resistance during movement is small, making it suitable for situations with alternating pressure;
• Single use bidirectional sealing;
• The size and groove have been standardized, the product is readily available, easy to use and purchase, and the price is relatively low.

2. Disadvantages of O-rings

• During compression dynamic sealing, the starting friction resistance is high;
• In the process of air pressure and water pressure sealing, lubricating oil needs to be added to prevent wear. In some situations, it is often necessary to add dust-proof and protective retaining rings;
• The processing dimensions and precision requirements of the mating parts (such as motion surfaces, grooves, gaps, etc.) are strict.

3. Precautions for O-ring installation

1. Basic requirements for O-ring installation

Before installing the O-ring, check the following:

• Whether the chamfering is processed according to the drawing, check if the sealed surface has any defects;
• Whether the inner diameter has been deburred, whether the sharp edges have been rounded, and ensure that all edges or transitions have been chamfered;
• Whether the processing burrs and iron filings have been completely removed, ensuring all processing residues are cleared;
• Whether the thread tips on the installation path have been covered to prevent the thread tips from scratching the O-ring;
• Whether the seals and parts have been coated with a small amount of lubricating grease or oil compatible with the O-ring;
• Do not use lubricating grease containing solid additives, such as molybdenum disulfide, zinc sulfide.

2. For manual installation, note the following:

• Do not use sharp tools;
• Ensure the O-ring is not twisted, use auxiliary tools to ensure correct positioning.
• Avoid overstretching O-rings;
• For O-rings adhered with a sealant strip, do not stretch at the connection point;
• O-rings are generally not reusable;
• When installing an O-ring into a groove, be careful not to twist the O-ring.

3. When installing over threads, splines

• After stretching an O-ring, when passing through threads, splines, keyways, etc., an installation mandrel must be used. This mandrel can be made of a softer metal or plastic, and should not have burrs or sharp edges.
• When installing tightening screws, the screws should be tightened symmetrically, not in a directional sequence.