O-Ring Failure Modes: Extrusion Damage

While O-ring designs are meant to withstand outside influences within the application environment in which they are used, failure is still possible. A combination of factors, from design to human error, can contribute to the premature failure of an O-ring.

As a technical resource for our customers, we wanted to share our knowledge and experience with potential seal failures and how you can prevent them. Our new series will highlight some of the most common O-ring failure modes.

Extrusion Damage

In this edition, we’ll discuss the causes and signs of extrusion damage, as well as potential solutions and methods for prevention. Extrusion damage failures often occur in dynamic applications such as hydraulic controlled rod and piston seals. These can also occur in static applications when seals are subjected to high pressure pulsing, especially when the gland extrusion gaps are excessive.

Exposure to high-pressure loads is typically the cause for seal extrusions. High pressure creates high stress, forcing the seal material into the clearance gap, resulting in excessive seal deformation. If the O-ring becomes trapped between the sharp edges of the mating surfaces, permanent damage can occur.

In addition to excessive system pressure, extrusion problem sources include:

• Excessive gland clearances (E‑gap)
• Low hardness of O-ring material (low modulus seal materials)
• O-ring degraded by system fluid (chemical attack, excessive volume change)
• Misalignment/side loading
• Irregular clearance gaps
• Irregular O-ring gland manufacturing
• O-ring gland overfill
• Cylinder/rod surfaces too rough or too smooth

Spotting extrusion damage is actually quite easy. Visual indicators include patterns of small nibbles on the downstream edge of an O-ring, producing a surface that appears chipped. More severe cases of extrusion may be present when the surface of an O-ring appears as though it has been peeled away. Extrusion into both the top and bottom of the groove is typically indicative of O-ring gland overfill.

Extrusion in radial seals can be prevented by adding a plastic or metal backup ring or by decreasing the gland clearance gap. The option of a properly-sized, harder O-ring material may also be beneficial. During application design, evaluate the approximate pressure rating for the seal. Additionally, confirm the elastomer compatibility with the temperature range and fluids used in the application.

Worth noting: static face seals often have no extrusion gap, so these applications can withstand much higher pressures than radial seals. Absence of this gap significantly reduces the risk of extrusion.

Nibbling

In some instances, O-ring extrusion damage is described as “nibbling,” where small pieces are torn away from the seal. Nibbling typically occurs as the result of pressure fluctuations in the system; increased pressure creates a larger clearance gap, allowing the seal to extrude. After the pressure reduces, the seal returns to its original shape, but small pieces are eroded away as the seal escapes the gap. As the cycles continue, nibbling repeats and the seal eventually fails.

This is part of a series of educational articles to aid in understanding common O-ring field failures. Click here to read additional articles in the series.