The O-Ring is one of the simplest, most economical and commonly used types of seal. O-rings are widely used to prevent leakage across a variety of applications, from pumps and valves to cylinders and connectors. They can be used in both static and dynamic operations, and in pneumatic and hydraulic applications.
An O-ring is a gasket in the form of a ring with a circular cross section. It is designed to be seated in a groove and compressed during assembly between two or more parts, Groove design is relatively straightforward, and there are well-developed rules for groove geometry. These result in an economical and reliable seal.
The O-ring’s tendency to return to its original shape when the cross-section is compressed is one of the key factors that it make an excellent seal.
The force required to compress an O-ring is dependent on the hardness and cross-section diameter. Stretching an O-ring affects the seal compression by reducing the cross-section. This, in turn, reduces the sealing potential of the O-ring.
When the two surfaces contact, they form a gland, and compress the O-ring, which deforms the round cross section. This diametrically squeezes the seal, and the resulting force ensures surface contact with the inner and outer walls of the gland.
With little or no pressure, the natural resilience of the elastomer compound creates the seal and keeps fluid from passing by. Increasing the squeeze (for example, by using a larger diameter O-ring in the same-size groove) increases deformation and sealing force, but this can lead to problems in high pressure dynamic applications.
Applying fluid pressure pushes the O-ring against the groove wall on the low-pressure side, and increases the sealing force. Interference between the seal and mating surfaces allows the O-ring to continue operating leak-free.
At higher pressures, the O-ring deforms to a “D” shape, and contact area between elastomer and gland surfaces can increase significantly from initial zero-pressure conditions (see above diagram). Due to the elastomer’s resilience, release of pressure allows the O-ring to return to its original shape.
Extreme pressures can force elastomer material into the small clearance between the mating surfaces just beyond the groove (see above diagram). The O-ring material shears and flows into the extrusion gap, and the seal can fail. Seal extrusion can occur more quickly in dynamic applications, however, even in static applications, high pressure can open the extrusion gap sufficiently to allow leakage.
Although O-rings are relatively straightforward seals, there are still a number of factors to be considered when specifying them. O-rings are available in a wide range of materials and elastomers. It is crucial that the correct type of material and hardness is selected for each application. This allows the O-rings to offer excellent fluid compatibility, withstand various operating environments and handle extreme temperatures.
Other considerations include static or dynamic (rotary or axial) conditions, operating pressure, and whether the system sees pressure spikes. These, in turn, allow engineers to specify design parameters. Properly designed, O-rings can provide long, trouble-free life in countless applications.
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