When to use back-up rings —
why an O-ring alone
fails at higher pressure
A back-up ring supports the O-ring at that low-pressure gap. It is not the primary sealing element. It supports the O-ring mechanically at the low-pressure clearance gap, helping keep its geometry intact at pressures where the O-ring alone might extrude and be cut.
Why O-rings extrude at higher pressure
An O-ring installed in a groove is compressed between the groove base and the mating surface. That compression creates the sealing contact stress. But the groove is not perfectly closed — there is always a small clearance gap between the groove walls and the mating surface, present due to assembly clearance and operating tolerances.
At low pressure, this clearance gap does not cause problems. The O-ring is compressed against its seating surfaces and stays in place. As system pressure increases, the pressure acts on the O-ring from the high-pressure side and tries to push the elastomer toward the low-pressure side. If the clearance gap on the low-pressure side is large enough relative to the O-ring hardness and the system pressure, the O-ring begins to deform into and through the gap.
Once extrusion starts, damage is usually progressive. The extruded portion is cut and nibbled by the edge of the gap as pressure cycles. The O-ring loses material. Cross-section is reduced. Sealing contact is degraded. Eventually the O-ring fails — not from chemical attack or thermal degradation, but from mechanical destruction at the clearance gap.
Without back-up ring — extrusion into gap
With back-up ring — gap closed
What determines whether a back-up ring is needed
Whether an O-ring can seal without a back-up ring at a given pressure depends on several interacting factors. No single pressure threshold applies universally — the combination of these factors determines the extrusion risk.
System pressure
Higher pressure increases the force pushing the O-ring toward the clearance gap. At low pressure, the O-ring's own stiffness is sufficient to resist extrusion. As pressure increases, this balance shifts. Published O-ring design guides typically indicate approximate pressure thresholds above which back-up rings are recommended for a given clearance gap and O-ring hardness — these thresholds vary significantly with the other factors below.
Diametral clearance gap
The clearance gap between the groove and the mating surface is the space the O-ring can potentially extrude into. A smaller gap requires higher pressure to force the O-ring through. A larger gap — as may occur with worn components, loose tolerances or soft materials — allows extrusion at lower pressure. Reducing the diametral clearance by tightening machining tolerances can raise the pressure at which back-up rings become necessary.
O-ring hardness (Shore A)
A harder O-ring compound resists extrusion better than a softer one at the same clearance and pressure, because it requires more force to deform through the gap. Softer O-ring compounds — which provide better sealing at low contact stress — are more susceptible to extrusion at elevated pressure. Using a harder compound can shift the extrusion threshold, but harder compounds may require higher assembly force and may be less effective at sealing on imperfect surfaces.
Temperature
At elevated temperature, most elastomers soften — their effective Shore A decreases. An O-ring that resists extrusion adequately at ambient temperature may begin to extrude at operating temperature because the material is softer. When selecting whether a back-up ring is needed, the operating temperature and its effect on the O-ring hardness should be considered alongside the ambient condition.
One back-up ring or two — pressure direction determines the answer
A back-up ring works by closing the clearance gap on the low-pressure side. Pressure pushes the O-ring toward the low-pressure side — the back-up ring is between the O-ring and that gap, preventing the O-ring from entering it.
One back-up ring
When pressure acts predominantly in one direction — as in many static pressure connections — a single back-up ring on the low-pressure side is typically sufficient. The back-up ring must be placed on the correct side: the side the O-ring is being pushed toward. A back-up ring on the high-pressure side provides no benefit for extrusion prevention.
Two back-up rings
When pressure may act in either direction — as in pressure-cycling applications, or when the system design does not guarantee unidirectional pressure — two back-up rings are used, one on each side of the O-ring. This supports the O-ring at the clearance gap on both sides, regardless of which direction pressure acts. The O-ring sits between the two back-up rings.
The back-up ring must be on the low-pressure side — not the high-pressure side. System pressure pushes the O-ring away from the pressure source, toward the low-pressure side. The back-up ring must be between the O-ring and the low-pressure clearance gap to prevent extrusion. Installing a back-up ring on the high-pressure side does not prevent extrusion and may interfere with correct O-ring seating.
In a face seal groove with a back-up ring, the groove width must be increased to accommodate both the O-ring and the back-up ring. The groove depth, O-ring squeeze and gland fill calculations must account for the presence of the back-up ring as a rigid element in the groove.
What back-up rings do not do
- Seal. The O-ring remains the sealing element. A back-up ring provides no sealing function — it is a mechanical support for the O-ring geometry under pressure.
- Compensate for incorrect groove geometry. A back-up ring does not compensate for fundamentally incorrect groove geometry. Groove depth, width, squeeze and fill must still be correct for the combined O-ring and back-up ring arrangement.
- Replace the need for correct O-ring material selection. If the O-ring material is chemically incompatible with the medium, or if the thermal range exceeds the material's capability, a back-up ring does not address these failures.
- Eliminate extrusion risk at any pressure. At extremely high pressures, even back-up rings can extrude into clearance gaps if the gap is large enough. For very high-pressure applications, engineering design — including clearance gap specification, back-up ring material selection and groove geometry — should follow the applicable pressure vessel or sealing standard.
- Correct for damaged or oversized clearance gaps. If the clearance gap is larger than the design specification due to wear, corrosion or manufacturing error, a back-up ring designed for the original gap may not close the oversized gap effectively.
Back-up ring materials and profile types
Back-up rings are most commonly made from PTFE or filled PTFE compounds. PTFE is chemically resistant to most media, has a wide temperature range, and is rigid enough to close the clearance gap without extruding itself under normal O-ring service pressures. Filled PTFE compounds — with glass fibre, bronze or graphite — are used where higher stiffness or wear resistance is required.
Two common profile types are used:
- Flat (solid) back-up rings: a simple flat ring, typically rectangular in cross-section. Easy to install on assemblies where the groove is accessible. Cannot be installed on closed grooves without a spiral cut or segmented design.
- Spiral-cut back-up rings: a flat ring with a helical cut through the cross-section, allowing it to be opened up and installed on grooves that are not accessible from the end. The spiral cut introduces an additional design consideration and should be installed in accordance with the manufacturer's guidance.
Groove dimensions change when a back-up ring is added. Adding a back-up ring to an existing groove designed for an O-ring alone is not straightforward. The groove width must increase to accommodate the combined width of the O-ring and back-up ring. The available space for O-ring squeeze and gland fill must still be within the correct ranges. If you are retrofitting back-up rings into an existing groove, verify that the resulting groove geometry is still within the design limits for the O-ring and back-up ring combination.
Identifying when a back-up ring might be needed
- O-ring shows nibbling or cut edges after service: classic indicator that extrusion is occurring at the clearance gap. The extruded material is being cut by the gap edge during pressure cycling. Back-up ring on the low-pressure side is the typical engineering response.
- O-ring has developed a flat face on one side: the O-ring has permanently deformed under unidirectional pressure, partially extruding into the gap without being cut. May indicate that the gap is at the limit of what the O-ring can handle without support.
- Joint leaks only at elevated pressure, not at lower pressure: may indicate that extrusion is occurring at higher pressure, reducing sealing contact. At lower pressure the O-ring holds; at higher pressure it deforms into the gap.
- System pressure is in the range where published design guidance recommends back-up ring evaluation: published O-ring design guides — from O-ring manufacturers and standards bodies — typically include pressure-clearance charts that indicate when back-up rings should be considered for a given combination of clearance, O-ring hardness and pressure. These charts are the correct starting point for the evaluation.
A back-up ring is a mechanical support, not a seal. The O-ring still seals — the back-up ring keeps it intact under pressure.
When pressure, clearance gap, O-ring hardness and temperature combine to create extrusion risk, a back-up ring supports the O-ring at the low-pressure clearance gap and helps keep its geometry intact. In general, one back-up ring is used where pressure is predominantly unidirectional, and two where pressure may reverse or cycle. The groove geometry must account for the back-up ring — it is not a drop-in addition to an existing groove design. For specific applications, consult the O-ring manufacturer's design guide and the applicable sealing standard.
FAQ
What does a back-up ring do?
A back-up ring is a rigid or semi-rigid ring placed in the groove on the low-pressure side of the O-ring. It supports the O-ring at the low-pressure clearance gap, effectively blocking the O-ring from extruding into it under system pressure. The back-up ring does not seal — the O-ring remains the sealing element. The back-up ring provides mechanical support to keep the O-ring geometry intact at higher pressures than the O-ring alone could withstand without extruding.
When do you need one back-up ring versus two back-up rings?
A single back-up ring is placed on the low-pressure side of the O-ring — the side the O-ring is being pushed toward by system pressure. This is appropriate when pressure acts predominantly in one direction, which is the case in many static applications. Two back-up rings — one on each side of the O-ring — are used when pressure direction may reverse, or when the application involves pressure cycling that alternates the direction of force on the O-ring. Using two back-up rings prevents extrusion in either direction. Using only one back-up ring on the wrong side — the high-pressure side — provides no benefit.
What material are back-up rings made from?
Back-up rings are commonly made from PTFE (polytetrafluoroethylene) or filled PTFE compounds, nylon, or leather in some older applications. PTFE is the most widely used material because it has low friction, is chemically resistant to a wide range of media, and is sufficiently rigid to close the clearance gap without extruding itself. Filled PTFE compounds — with glass fibre, bronze or graphite filler — are used where additional stiffness or wear resistance is required. The back-up ring material must be compatible with the fluid medium and operating temperature, and must be harder and less deformable than the O-ring elastomer.