An O-ring is the primary seal. A backup ring is an anti-extrusion support element that prevents the primary seal from being destroyed by the pressure it is sealing against. They perform fundamentally different functions: the O-ring creates the fluid barrier through elastic contact stress; the backup ring bridges the clearance gap that the O-ring would otherwise be forced into under high pressure. Understanding when extrusion risk crosses the threshold that justifies adding backup rings — and which backup ring type is correct for each application — is the engineering decision this guide addresses.
Definition Block
O-ring: An elastomeric ring with a circular cross-section that creates a seal through compression between two mating surfaces. Sealing force comes from initial squeeze (elastic deformation) plus pressure-energized contact (system pressure pushes the O-ring against the sealing surfaces, increasing contact stress). The O-ring is the fluid barrier — without it, there is no seal.
Backup ring (anti-extrusion ring): A rigid ring — typically PTFE, PEEK, or a hard engineering polymer — installed adjacent to the O-ring in the groove on the low-pressure side of the assembly. The backup ring does not seal — it provides no elastic recovery, no contact stress, and no fluid barrier. Its sole function is to bridge the clearance gap between the piston/rod and the bore, physically preventing the elastomeric O-ring from extruding into that gap under pressure. Without the O-ring, a backup ring in a groove is inert and non-functional.
Extrusion Mechanism: How and Why It Occurs
Extrusion occurs when system pressure pushes the O-ring toward the clearance gap between mating metal surfaces faster than the elastomer's resistance to deformation can prevent it. The physical mechanism:
- System pressure builds and loads the O-ring against the sealing faces
- Simultaneously, the pressure gradient across the O-ring pushes the low-pressure-side portion of the O-ring cross-section toward the clearance gap
- The elastomer flows into the clearance gap, extruding a thin ribbon of material through the gap
- On a dynamic (reciprocating) seal, each stroke shears the extruded ribbon — the sheared material cannot return to the O-ring cross-section
- Cumulative material loss reduces O-ring CS, reducing effective compression, until contact stress falls below the sealing threshold
- Leakage begins while the O-ring is still in position — the O-ring has been consumed by extrusion from the outside in
Visual identification of extrusion damage: A failed O-ring that shows extrusion damage has a distinctive appearance. The high-pressure side of the O-ring cross-section is typically smooth and undamaged. The low-pressure side (the gap side) shows:
- A frayed, nibbled, or shredded edge
- Material pulled away in strips or flakes
- One localized area of damage on a specific arc of the circumference (where the gap is largest)
- Damage confined to one side only — not distributed around the full cross-section
This pattern distinguishes extrusion from chemical attack (damage distributed throughout), compression set (uniform flattening without material loss), and installation cuts (clean, straight cuts at installation points).
Variables That Control Extrusion Risk
Four variables interact to determine whether extrusion occurs for a given O-ring:
| Variable | Effect | Design Response |
|---|---|---|
| Diametral clearance gap (mm) | Largest single influence — exponential effect; doubling gap increases extrusion rate ~4× | Minimize by tightening bore/rod tolerances; specify fit class |
| System pressure (bar) | Linear effect on extrusion force | Add backup rings above threshold; increase hardness |
| Material hardness (Shore A) | Inverse — harder = more extrusion resistance; 90A requires ~3× more force to extrude than 70A | Specify 80–90 Shore A for high pressure |
| Temperature (°C) | Elevated temperature reduces modulus — effectively reduces hardness | Account for operating temp, not just ambient |
Temperature interaction: An NBR 70 Shore A compound at +120°C may perform as an effective 50–55 Shore A compound — the same elastomer has substantially lower extrusion resistance at operating temperature than at ambient. Always evaluate extrusion risk at the maximum operating temperature, not room temperature.
Pressure and Clearance Gap Thresholds
These guidelines assume standard 70 Shore A elastomers in standard machined metal glands. Tighter clearances or harder compounds shift the thresholds.
Static seals
| System Pressure | Maximum Safe Diametral Gap (70 Shore A) | Maximum Safe Gap (90 Shore A) | Backup Ring Requirement |
|---|---|---|---|
| < 70 bar | 0.40 mm | 0.50 mm | Not required |
| 70–150 bar | 0.20 mm | 0.30 mm | Recommended |
| 150–250 bar | 0.10 mm | 0.15 mm | Required (single) |
| 250–500 bar | 0.05 mm | 0.08 mm | Required (single or dual) |
| > 500 bar | 0.03 mm | 0.05 mm | Required (dual solid) |
Dynamic (reciprocating) seals
For dynamic seals, the threshold is lower — each stroke shears extruded material, accelerating cumulative loss:
| System Pressure | Maximum Safe Diametral Gap (70 Shore A) | Maximum Safe Gap (90 Shore A) | Backup Ring Requirement |
|---|---|---|---|
| < 40 bar | 0.20 mm | 0.25 mm | Not required |
| 40–100 bar | 0.10 mm | 0.15 mm | Recommended |
| 100–200 bar | 0.05 mm | 0.10 mm | Required (single) |
| 200–400 bar | 0.03 mm | 0.06 mm | Required (single or dual) |
| > 400 bar | < 0.03 mm with dual | — | Required (dual; consider PEEK) |
Combined approach: At pressures above 150 bar dynamic or 250 bar static, specify both 90 Shore A hardness AND backup rings. Each countermeasure alone is less effective than both together: 90 Shore A raises the safe gap threshold; backup rings bridge the remaining gap.
Backup Ring Types and Selection
Flat solid PTFE backup ring
The most common backup ring — a flat ring with rectangular cross-section machined from PTFE, cut to fit the O-ring groove width with the backup ring present.
Characteristics:
- Highest extrusion resistance per unit thickness (no gap or joint)
- Cannot be installed without disassembly (solid ring cannot be expanded over the piston/rod without split gland)
- PTFE is chemically inert to essentially all industrial fluids and has −200°C to +260°C service range
- Some cold flow (creep) under sustained compressive load at elevated temperature
Best for: Static seals; moderate-speed reciprocating service; installations where disassembly is acceptable for seal maintenance.
Spiral-cut PTFE backup ring
Cut from a strip of PTFE and wound into a coil — the spiral geometry allows the ring to expand during installation (snap-and-slide over piston or rod without disassembly).
Characteristics:
- Installation without gland disassembly — the coil can be opened and slid into place
- Small continuous gap at the coil turn — allows pressure equalization across the backup ring (reduces chance of O-ring being hydraulically pinched between backup ring and groove wall)
- Slightly lower maximum pressure capability than solid ring (gap allows small amount of O-ring to extrude at the joint under very high pressure)
- Suitable to approximately 300–400 bar dynamic; solid ring preferred above this
Best for: Large-diameter seals where disassembly is not practical; face seals; applications with significant thermal cycling where pressure equalization is beneficial.
Step-cut PTFE backup ring
A single-piece ring with an interlocking step joint rather than a straight diagonal cut. The step geometry closes the joint more completely than a spiral cut, providing better extrusion resistance while still allowing installation without complete disassembly.
Characteristics:
- Better joint seal than spiral-cut — step interlock leaves no through gap
- Smaller gap than scarf-cut ring
- Still installable over a rod or piston without full gland disassembly (two-step snap-in)
- Effective to approximately 400–500 bar dynamic; preferred over spiral-cut for > 300 bar
Best for: Moderate-to-high pressure (200–500 bar) dynamic seals where disassembly is impractical; better than spiral-cut at the same installation convenience.
PEEK (polyetheretherketone) backup ring
PEEK is a semi-crystalline thermoplastic with Shore D hardness of ~85–90 (equivalent to approximately 95+ Shore A on the elastomer scale). Much harder than PTFE — PEEK resists extruding itself at extreme pressures where PTFE would deform.
Characteristics:
- Continuous service temperature to +250°C (higher than PTFE's reliable creep-resistant range)
- At > +150°C, PTFE backup rings develop significant creep — PEEK maintains dimensional stability
- Higher hardness bridges clearance gaps that PTFE backup rings cannot reliably bridge at extreme pressure (> 500 bar)
- More brittle than PTFE — installation requires care to avoid cracking
- Higher material cost (PEEK is 3–5× more expensive than PTFE for equivalent backup ring)
Best for: Very high pressure (> 500 bar) static and dynamic seals; high-temperature service (> 150°C) where PTFE creep reduces backup ring thickness over time; oil and gas downhole tools where HPHT conditions combine both factors.
Comparison table
| Backup Ring Type | Max Effective Pressure (dynamic) | Seal Type | Installation | Relative Cost |
|---|---|---|---|---|
| Solid flat PTFE | > 700 bar | Static and low-speed dynamic | Disassembly required | 1.0× |
| Spiral-cut PTFE | 300–400 bar | Dynamic (reciprocating) | No disassembly | 1.2× |
| Step-cut PTFE | 400–500 bar | Dynamic and static | Minimal disassembly | 1.5× |
| PEEK (solid) | > 700 bar | Static and dynamic | Disassembly required | 4–6× |
Single vs Dual Backup Ring Configuration
Single backup ring (unidirectional pressure)
Install one backup ring on the low-pressure side of the O-ring — the side that system pressure is pushing the O-ring toward. In a hydraulic cylinder with pressure on the left, the backup ring goes on the right side of the O-ring groove (the low-pressure side).
Use single backup ring when:
- Pressure direction is fixed and known
- Single-acting cylinders, valves with unidirectional pressure
- Static seals where pressure direction is consistent
Dual backup ring (bidirectional or uncertain pressure)
Install one backup ring on each side of the O-ring — one on each side of the O-ring in the groove.
Use dual backup rings when:
- Pressure can reverse direction: double-acting hydraulic cylinders, bidirectional pumps, pressure-cycling systems
- Pressure direction is uncertain (valve seats, manifold junctions)
- Pressure excursions may occur from either side (transient pressure spikes)
- System pressure exceeds 400 bar regardless of direction
Groove width for backup rings: Adding backup rings requires a wider groove. Standard O-ring groove width (1.15–1.25 × CS) does not accommodate backup rings:
| Configuration | Groove Width |
|---|---|
| O-ring only, static | 1.15–1.25 × CS |
| O-ring + 1 backup ring | 1.45–1.65 × CS |
| O-ring + 2 backup rings | 1.80–2.10 × CS |
Installing a backup ring in an unmodified O-ring groove over-compresses the O-ring (pushes the O-ring against the sealing surface harder than intended), increasing friction and accelerating compression set. Always verify groove width accommodates the selected backup ring configuration before ordering.
Material Compatibility: Backup Ring and O-Ring Independence
The backup ring does not contact the process fluid under normal conditions — the O-ring is the fluid contact component. The backup ring contacts the O-ring and the groove walls. However, if the O-ring fails (extrusion has already begun or the O-ring has been removed for maintenance), the backup ring may contact the process fluid. Select backup ring material for chemical compatibility with the process:
| Service Environment | Recommended Backup Ring | Avoid |
|---|---|---|
| General petroleum hydraulics (mineral oil) | PTFE (any type) | Nylon (oil absorption) |
| Phosphate-ester hydraulic fluid | PTFE | Nylon (incompatible) |
| Water and glycol-based hydraulics | PTFE | Nylon (swells in water service) |
| High-temperature petroleum (> 150°C) | PEEK | PTFE (creep at temperature) |
| Sour gas (H₂S/CO₂) | PTFE or PEEK | Carbon-filled PTFE (if H₂S attacks filler) |
| Chemical process (aggressive chemicals) | PTFE | PEEK (limited chemical resistance vs PTFE) |
| Cryogenic service (< −100°C) | PTFE or PEEK | Nylon (embrittlement at cryogenic) |
| Food and pharmaceutical contact | FDA-compliant PTFE | — |
| Semiconductor process | Virgin PTFE (UHP grade) | Filled PTFE (filler contamination) |
Groove Design Verification Procedure
When adding backup rings to an existing groove design:
- Measure existing groove: Record groove width (W), groove depth (d), bore/rod diameter
- Calculate current O-ring compression rate: Squeeze = (CS − d) / CS × 100
- Determine backup ring thickness: Typically 1.0–1.5 mm thick for standard PTFE flat rings; manufacturer provides specific dimensions
- Calculate new groove width required: W_new = W_o-ring + backup ring width(s) + 0.05–0.10 mm assembly clearance per ring
- Verify O-ring compression is unchanged: With backup ring installed in wider groove, groove depth for O-ring contact is identical — compression rate does not change when only groove width is modified
- Verify fill rate with backup ring in place: Fill rate calculation includes backup ring cross-section area in groove area — ensure total fill does not exceed 90%
When O-Rings Alone Are Sufficient
A backup ring is not always needed — and adding one without sufficient justification increases cost, complicates groove geometry, and can cause problems if the groove width is not properly adjusted. O-ring alone is the correct design when:
- System pressure is < 70 bar static or < 40 bar dynamic
- Diametral clearance gap is controlled to < 0.10 mm at all tolerance combinations
- O-ring hardness is appropriate for the pressure (80–90 Shore A for 70–150 bar)
- The application is truly static with no pressure cycling or pressure spikes
- Temperature is within the compound's normal service range (no modulus reduction at operating conditions)
For a typical process instrumentation fitting at 20–30 bar with good machining tolerances (clearance < 0.10 mm), a correctly specified 70 Shore A O-ring in a standard groove is the complete, correct, and over-designed-if-backup-ring-added solution.
FAQ
Q1: Can a backup ring replace an O-ring in a sealing application?
No — a backup ring provides zero sealing force and creates no fluid barrier. It is a rigid mechanical support ring that prevents the O-ring from extruding, not a seal itself. If the O-ring is removed and only the backup ring remains in the groove, leakage is immediate and total at any system pressure. The O-ring + backup ring combination is a seal stack where each component performs a distinct function.
Q2: At what pressure should I start considering backup rings?
For dynamic (reciprocating) seals with standard 70 Shore A elastomers and a typical machined clearance gap (0.10–0.15 mm diametral), consider backup rings above 40 bar and specify them above 100 bar. For static seals with tighter clearance control, the threshold is higher — typically 100–150 bar before backup rings become necessary. The clearance gap dimension is as important as the pressure: a very tight gap (< 0.05 mm) extends the pressure range where O-ring alone is reliable; a large gap (> 0.20 mm) requires backup rings at much lower pressure.
Q3: What is the correct material for a backup ring?
PTFE is correct for the majority of industrial applications — it is chemically inert to essentially all industrial process fluids, covers −200°C to +260°C, and provides reliable extrusion support. Specify PEEK when operating temperature exceeds +150°C (PTFE creep becomes significant above this temperature) or when system pressure exceeds 500 bar (PTFE itself may deform at extreme pressure; PEEK's higher hardness prevents this). Nylon/polyamide backup rings are less expensive but absorb moisture in water service (causing dimensional change) and have limited chemical resistance — specify only for dry pneumatic systems.
Q4: Should I use a single or double backup ring?
Single backup ring when pressure direction is fixed and known: single-acting cylinders, valves with one-directional pressure, static seals with consistent pressure direction. Double backup rings (one on each side of the O-ring) when: pressure can reverse direction (double-acting cylinders), pressure direction is uncertain (bidirectional pumps, manifold connections), or system pressure exceeds 400 bar regardless of direction (the extrusion protection margin requires support on both sides at extreme pressure).
Q5: Do I need to change the groove dimensions to add a backup ring?
Yes — installing a backup ring in a standard O-ring groove (designed for O-ring only) overfills the groove. The O-ring + backup ring combination occupies more axial width than the O-ring alone. This excess fill over-compresses the O-ring against the sealing surface, increases friction, and accelerates compression set. Groove width must be increased by approximately 1.0–1.5× the backup ring thickness per ring added. The groove depth (which controls O-ring compression rate) remains unchanged — only the groove width must be modified.
Q6: How do I identify extrusion failure from a failed O-ring?
Extrusion damage has a distinctive visual pattern: frayed, nibbled, or shredded material on one edge of the O-ring cross-section — always the low-pressure side of the groove, the side facing the clearance gap. The opposite (high-pressure) side of the cross-section is typically smooth and undamaged. The damage is localized to where the gap is largest around the circumference. Compare to: compression set failure (uniform flattening, no material loss); chemical attack (surface cracking or swelling distributed throughout the full O-ring surface); installation cuts (clean, straight cuts at specific points on the circumference corresponding to installation over sharp edges).
Q7: Do PTFE backup rings increase friction in dynamic (reciprocating) service?
With correctly sized groove dimensions, PTFE backup rings do not significantly increase running friction. PTFE has a very low coefficient of friction (0.04–0.10 against steel) — lower than most elastomers. In a properly specified groove, the backup ring rides against the bore or rod with minimal friction contribution. However, if the groove width is insufficient for the O-ring + backup ring combination, the over-compression increases O-ring contact stress and friction proportionally. Always verify groove width before adding backup rings to a dynamic seal.
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