O-ring extrusion
and nibbling —
what causes it and how to prevent it
If you have pulled out a failed O-ring and found damage concentrated on one edge while the rest looks intact, extrusion is likely the cause. This article explains the mechanism, the conditions that produce it, and the three ways to address it.
What extrusion is — the mechanism
An O-ring seals by sitting under compression between two surfaces. System pressure acts on the O-ring and pushes it against the low-pressure side of the groove. In a well-designed joint at appropriate pressure, the clearance gap between the mating components is small enough that the O-ring cannot enter it — the rubber stays in the groove and the seal holds.
When the clearance gap is too large for the compound hardness at the operating pressure, the rubber begins to be forced into the gap. Each pressure cycle pushes a small amount of material further into the clearance. The sharp edges of the gap then cut or shear the extruded rubber as the joint cycles. This progressive damage is nibbling — the O-ring is literally being nibbled away at the edge exposed to the gap.
How to recognise extrusion damage
The damage pattern from extrusion is distinctive and asymmetric. When you remove a nibbled O-ring, one edge shows chipping, fraying or a ragged appearance while the rest of the O-ring is intact or shows only normal compression marks. With steady pressure from one direction, damage is typically found toward the low-pressure clearance gap. Reversing or cycling pressure, groove geometry and assembly damage can change the observed pattern, so use direction as a clue rather than a complete diagnosis.
This asymmetry is the key distinguishing feature. Other O-ring failure modes produce different patterns:
- Compression set — the O-ring is flattened symmetrically, both sealing faces show permanent deformation, no edge damage
- Chemical swelling — the O-ring is larger than nominal overall, may show surface tackiness or cracking, damage is not concentrated on one edge
- Installation damage (cutting) — clean cut or nick at a single point, not progressive edge damage around the circumference
- Thermal degradation — hardening and cracking distributed across the surface, not concentrated on one side
- Extrusion nibbling — asymmetric damage on one edge only, chipped or frayed appearance, progressive rather than a single point of failure
The diagnostic question: is the damage on one edge only, while the rest of the O-ring looks intact? If yes, extrusion is the likely cause. If the damage is distributed or affects the sealing faces symmetrically, look at other failure modes first.
What causes extrusion — the four conditions
This is the most common root cause. The gap between mating components is larger than the O-ring compound can bridge under the operating pressure. A softer compound (70 Shore A) requires a tighter clearance than a harder compound (90 Shore A) at the same pressure. As system pressure increases, the maximum allowable clearance for a given hardness decreases.
Clearance gaps can also increase over time through wear, especially in dynamic applications — a joint that was correctly specified at commissioning may develop extrusion problems as the components wear and the gap grows.
An O-ring correctly specified for a given pressure and clearance combination may extrude if system pressure increases beyond the design condition. Pressure spikes — brief excursions above normal operating pressure — are a particular cause of extrusion in otherwise correctly specified seals. The spike opens the clearance gap momentarily and allows the O-ring to be forced into it, with nibbling occurring as the gap closes again.
Pressure spikes are common in hydraulic systems with rapid valve actuation and in pneumatic systems with compressor cycling. Steady operating pressure and spike pressure may need to be considered separately when specifying O-ring hardness and backup ring requirements.
A 70 Shore A compound may be correctly specified for the clearance and steady-state pressure, but insufficient if pressure spikes or if the application is dynamic. Replacing a 70 Shore A O-ring with another 70 Shore A when extrusion has been identified as the failure mode will produce the same result.
The compound hardness must be matched to the pressure and clearance conditions, not just selected as a standard default. If 70 Shore A is consistently extruding, the compound selection is the variable to change — not the size.
In dynamic sealing — reciprocating pistons and rods — the O-ring is subjected to both pressure and relative movement between components. The combination increases the risk of extrusion because the O-ring is repeatedly being pushed against the low-pressure side as pressure cycles with each stroke. Even a correctly sized clearance for static conditions may allow extrusion in dynamic service, particularly at higher pressures.
Dynamic applications at higher pressure typically require either a harder compound or backup rings — and often both.
Three ways to address extrusion
Harder compound
Moving from 70 Shore A to 90 Shore A increases extrusion resistance by making the rubber stiffer and harder to deform into the gap. This is the simplest intervention and is appropriate when the clearance and pressure conditions are close to the limit for 70 Shore A. It does not close the clearance gap — it makes the O-ring more resistant to being pushed into it.
Tighter clearance
Reducing the diametral clearance between components directly addresses the root cause. A smaller gap is harder for the O-ring to enter. This may require machining the components to tighter tolerances, or replacing worn components where the gap has grown through service. It is the most effective fix where the joint geometry allows it.
Backup ring
A backup ring is a rigid ring — typically PTFE or nylon — placed on the low-pressure side of the O-ring in the groove. It fills the clearance gap without sealing, preventing the O-ring from entering it. Backup rings are used in high-pressure and dynamic applications where neither compound hardness nor clearance alone is sufficient. A single backup ring is used for unidirectional pressure; two backup rings (one each side) for bidirectional pressure.
Choosing the right approach
| Situation | Typical approach |
|---|---|
| 70 ShA extruding at steady pressure | Move to 90 ShA — compound selection is the variable |
| Correctly specified compound extruding due to pressure spikes | Consider backup ring — spike pressure exceeds compound capacity |
| Extrusion in dynamic application | 90 ShA compound plus backup ring — dynamic duty typically requires both |
| Clearance gap grown through wear | Replace worn components or add backup ring — compound change alone may not be sufficient |
| High-pressure static seal with controlled clearance | 90 ShA compound — backup ring may not be required if clearance is tight |
| Very high pressure, large clearance | Backup ring regardless of compound — clearance-driven extrusion exceeds compound hardness solution |
Selection guidance is indicative. Actual O-ring and backup ring specification for high-pressure or dynamic applications should follow the relevant design standard and the equipment manufacturer's requirements.
What extrusion is not
Extrusion is driven primarily by pressure, clearance and hardness, but it can be aggravated by swelling or softening, overfilled glands, incorrect squeeze, sharp edges, eccentricity and installation damage. Changing to a chemically superior O-ring material will not prevent extrusion if the clearance and pressure conditions remain unchanged. Fitting a new O-ring of exactly the same specification into a joint that has already nibbled one O-ring will produce the same result.
Diagnosing extrusion correctly before replacing the O-ring saves time and material. The damaged O-ring tells you what happened — one-sided, progressive edge damage with intact sealing faces means the seal design or operating conditions need to change, not just the O-ring.
Asymmetric edge damage on one side of the O-ring is the signature of extrusion. The O-ring is not the problem — the clearance gap, the pressure, or both are.
Address extrusion through harder compound, tighter clearance, or backup ring — or a combination where the application demands it. Replacing the same compound into the same joint at the same pressure produces the same failure.
FAQ
What is O-ring extrusion?
O-ring extrusion occurs when system pressure forces the O-ring material into the clearance gap between two mating components. The rubber is pushed into the gap on the low-pressure side of the groove, where it is trapped and damaged as the joint cycles under pressure. The resulting damage is called nibbling — a chipped, frayed or ragged appearance on the low-pressure edge of the O-ring. Extrusion is typically caused by excessive clearance gap, insufficient O-ring hardness for the pressure, or pressure spikes that exceed the design capacity of the seal.
What does O-ring nibbling look like?
Nibbling produces a characteristic damage pattern: the O-ring shows chipping, fraying or a ragged edge on one side — the low-pressure side of the seal. The damage is asymmetric, affecting only the edge that was exposed to the clearance gap. The opposite side and the sealing faces of the O-ring typically appear undamaged or only normally compressed. This asymmetric damage pattern distinguishes extrusion nibbling from other O-ring failure modes such as compression set, swelling or installation damage.
How do I prevent O-ring extrusion?
Extrusion is usually addressed through one or more of three approaches: selecting a harder O-ring compound where the application allows, reducing the clearance gap in the joint design, or adding a backup ring on the low-pressure side of the O-ring. Backup rings fill the clearance gap without being the primary seal, reducing the tendency for the O-ring to be pushed into the gap. In high-pressure or dynamic applications, backup rings are often required rather than relying on hardness alone.