Dimensions and applications of the O-ring


 An O-ring, also known as a seal or toric joint, is a torus-shaped mechanical seal, which is a loop of round cross-section designed to fit into a groove and compress during assembly between two or more parts, forming a seal on the surface.

The O-ring can be used in static or dynamic applications where there is relative movement between the parts and the O-ring. Dynamic examples include rotating pump shafts and hydraulic cylinder pistons. Static applications of O-rings may include liquid or gas sealing applications where:

(1) the O-ring is compressed resulting in zero clearance,

(2) the O-ring material is solid vulcanized so that it is impermeable to liquid or gas, and

(3) O-ring material resists degradation by liquid or gas.

The wide range of potential liquids and gases that need to be sealed has necessitated the development of a wide variety of materials.

O-rings are one of the most commonly used types of seals in machinery design because they are inexpensive, easy to manufacture, reliable, and have simple installation requirements. The recommended maximum pressure for an O-ring seal depends on the hardness of the seal and the gland clearance.

O-rings can be produced by extrusion, injection molding, injection molding, or transfer molding.

 

Dimensions and applications of the O-ring

 

History

 

The O-ring was invented by the Swedish JO Lundberg on May 12, 1896. The American patent for the O-ring was filed in 1937 by a then 72-year-old Danish machinist, Niels Christensen. His earlier application, filed in 1933, begins: "This invention relates to new and useful improvements in hydraulic brakes, particularly to improved sealing of pistons in power transmission cylinders."


Sizes


O-rings are available in a variety of metric and inch standard sizes. The dimensions are determined by the inner diameter and the diameter (thickness) of the cross section. In the United States, the most common standard sizes are according to the SAE AS568C specification. ISO 3601-1:2012 contains the most commonly used standard sizes worldwide, metric sizes. In the UK there are also standard sizes known as BS sizes, which generally range from BS001 to BS932. There are many other size specifications.

The world's largest O-ring was produced by Trelleborg Sealing Solutions in a successful Guinness World Record attempt in Tewkesbury, in collaboration with 20 students from Tewkesbury School. This O-ring was placed around the medieval Tewkesbury Abbey, measuring 364 m in circumference, approximately 116 m in internal diameter, and 7.2 mm in cross-section.

Typical applications


A successful O-ring connection requires a rigid mechanical attachment that causes predictable deformation of the O-ring. This results in a calculated mechanical stress on the O-ring contact surfaces. As long as the pressure of the fluid inside does not exceed the contact stress of the O-ring, leakage cannot occur. The pressure of the fluid inside it passes through the material of the essentially incompressible O-ring, and the contact stress increases as the pressure increases. Because of this, the O-ring can easily seal high pressure until it mechanically fails.

The seal is designed to make point contact between the O-ring and the sealing surfaces. This allows for a high local stress that can hold high pressure without exceeding the yield stress of the O-ring body. The flexible nature of O-ring materials eliminates fasteners imperfections. But it is still important to maintain a good surface quality of mating parts, especially at low temperatures when the sealing rubber reaches its glass transition temperature and becomes increasingly inflexible and glassy. Surface treatment is especially important in dynamic applications. A surface that is too rough will rub the surface of the O-ring, and a surface that is too smooth will not allow proper lubrication of the seal.

Vacuum applications


In vacuum applications, the permeability of the material makes point contacts unusable. Instead, higher mounting forces are applied and the O-ring fills the entire groove. In addition, protective rings are used to protect the O-ring from excessive deformation. Since the O-ring only senses ambient pressure and gas partial pressure at the seal, their gradient will be steep near the seal and shallow in the bulk.


High temperature applications


In some high temperature applications, O-rings may need to be installed tangentially compressed to compensate for the Gow-Joule effect.

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