An O-ring may look correctly installed and still fail after the system begins operating under pressure. One possible cause is extrusion.
Extrusion occurs when part of the rubber ring is forced into the clearance between mating components. Under sufficient pressure, the material can gradually deform into this gap. As the deformation continues, the exposed section may become damaged, cut, or torn.
This type of failure is particularly relevant in hydraulic and pneumatic systems.
How Does Extrusion Happen?
An O-ring seals by being compressed between two surfaces. When system pressure increases, the fluid pushes the ring against the low-pressure side of its groove.
If the clearance gap is too large for the pressure, material hardness, and operating conditions, part of the O-ring may be forced into the opening.
Repeated pressure changes can make the problem worse. The rubber may become progressively damaged until leakage occurs.
Extrusion is not determined by pressure alone. It also depends on the size of the clearance gap, O-ring hardness, temperature, compound properties, and mechanical deformation of the surrounding components.
Why Does the Same O-Ring Perform Differently in Different Equipment?
Consider two hydraulic assemblies using O-rings of the same nominal size and material.
One has a closely controlled clearance and suitable groove dimensions. The other has a larger gap due to manufacturing tolerances or component movement under pressure.
Although the O-rings are identical, their operating conditions are not. The
second assembly may be more vulnerable to extrusion.
Temperature can introduce another complication. A rubber compound may become softer under certain conditions, reducing its resistance to deformation. Fluid compatibility also matters because swelling or material deterioration can affect mechanical strength.
How Can Extrusion Risk Be Reduced?
There is no single solution for every application. The design should be reviewed as a complete system.
Several measures may help:
Reduce excessive clearance between mating components.
Select an appropriate O-ring hardness and compound.
Check groove dimensions and manufacturing tolerances.
Consider component deformation under operating pressure.
Use a backup ring when the pressure and clearance conditions require additional extrusion resistance.
A backup ring is a separate, harder component installed beside the O-ring to help prevent rubber from entering the clearance gap. Its material, position, and configuration must match the sealing arrangement.
Simply choosing a harder O-ring is not always enough. Excessive hardness or an unsuitable design can introduce other sealing problems.
Diagnose the Failure Before Changing the Material
When an O-ring shows a damaged or shaved edge on its low-pressure side, extrusion should be considered as a possible cause.
Inspect the damaged ring, groove, mating components, and clearance dimensions. Also check the actual pressure and temperature rather than relying only on the equipment's nominal operating rating.
A reliable O-ring design depends on the relationship between pressure, clearance, groove geometry, and material properties. Replacing the ring without checking these factors may lead to repeated failures.
