Extrusion is the primary failure mode for O-rings in high-pressure systems. When system pressure exceeds the elastomer's resistance to flow, the O-ring material deforms into the clearance gap between mating components. The result — a ragged, nibbled edge on the low-pressure face — is almost entirely avoidable with correct clearance design or backup rings.
Quick answer: Use backup rings when dynamic operating pressure exceeds 150 bar with standard clearance and a 70 Shore A compound, or when any combination of pressure + temperature + clearance gap places the design above the limits in the table below.
The Extrusion Mechanism
Extrusion happens when three conditions occur together:
- Compressive hydraulic load: System pressure acts radially on the O-ring cross-section, transmitting load in all directions — the O-ring behaves as a confined fluid under pressure.
- Unsupported clearance gap: The gap between piston/rod OD and bore/gland ID is the only region where the O-ring is not supported by metal on both sides.
- Insufficient elastomer stiffness: If the material's elastic resistance to flow (governed by hardness and modulus) is lower than the extrusion force, material flows into the gap.
Under pressure, the contact stress at the O-ring/metal interface rises with system pressure. For a 70 Shore A NBR O-ring at 13% squeeze in a groove, the contact stress without pressure is approximately 1.0–1.5 MPa; at 150 bar (15 MPa) system pressure, contact stress rises to 17–20 MPa. The portion of the O-ring facing the clearance gap is unsupported — pressure drives material toward the gap proportionally to the gap area and inversely to the material's elastic modulus.
Approximate extrusion force per unit gap width (3.53 mm CS, 70 ShA NBR):
| System Pressure | Extrusion Force/mm Gap Width | Observation |
|---|---|---|
| 50 bar | ~2.5 N/mm | Elastomer deforms elastically into gap; recovers on depressurization |
| 100 bar | ~6 N/mm | Permanent tongue forms at gap; very thin (< 0.1 mm) |
| 150 bar | ~10 N/mm | Visible tongue 0.2–0.4 mm; may shear in dynamic service |
| 200 bar | ~15 N/mm | Progressive tongue growth; shearing in dynamic service removes material |
| 300 bar | ~25 N/mm | Rapid nibbling; failure within dozens of cycles in dynamic service |
In static seals without motion, extrusion produces a progressive leak path as the extruded tongue grows under sustained pressure. In dynamic reciprocating seals, each stroke shears the extruded tongue, gradually reducing cross-section — leakage begins when the remaining cross-section falls below the minimum contact stress threshold.
Pressure Thresholds by Hardness and Clearance
The combination of pressure, clearance gap, and hardness governs whether extrusion will occur. Use this table as a first-pass filter:
| Operating Pressure | Extrusion Risk | Design Action |
|---|---|---|
| < 50 bar | Low | Standard clearance; no backup rings required |
| 50–100 bar | Low to moderate | Verify clearance; 70 ShA adequate at standard clearance |
| 100–150 bar | Moderate | 80 ShA preferred for dynamic; check clearance table |
| 150–250 bar | High | 80–90 ShA required, OR backup rings on low-pressure side |
| 250–400 bar | Very high | Backup rings required; 90 ShA or PEEK backup recommended |
| > 400 bar | Extreme | Double backup rings; PEEK or metal backup; tight clearance critical |
Thresholds assume: operating temperature ≤ +80°C; standard radial clearance 0.10–0.20 mm. At elevated temperature, effective hardness drops and the threshold pressure decreases — see temperature correction section below.
Clearance Gap: The Critical Design Parameter
Radial clearance is the single-sided gap between piston/rod OD and bore/gland ID:
Radial clearance = (Bore ID_max − Rod/Piston OD_min) / 2
Diametral clearance = Bore ID_max − Rod/Piston OD_minMaximum recommended radial clearance — static seals:
| O-Ring Hardness | < 70 bar | 70–150 bar | 150–250 bar | > 250 bar |
|---|---|---|---|---|
| 50–60 Shore A | 0.10 mm | 0.07 mm | Not recommended | Not recommended |
| 70 Shore A | 0.20 mm | 0.15 mm | 0.10 mm | Not recommended |
| 80 Shore A | 0.25 mm | 0.20 mm | 0.15 mm | 0.10 mm |
| 90 Shore A | 0.35 mm | 0.28 mm | 0.20 mm | 0.12 mm |
Maximum recommended radial clearance — dynamic reciprocating seals:
Reduce static allowable by ~30% for reciprocating service — each stroke shears any extruded tongue, accelerating cumulative material loss.
| O-Ring Hardness | < 70 bar | 70–150 bar | 150–250 bar | > 250 bar |
|---|---|---|---|---|
| 70 Shore A | 0.14 mm | 0.10 mm | 0.07 mm | Not recommended |
| 80 Shore A | 0.18 mm | 0.14 mm | 0.10 mm | 0.07 mm |
| 90 Shore A | 0.24 mm | 0.20 mm | 0.14 mm | 0.08 mm |
Worked example — clearance gap check:
A hydraulic cylinder has:
- Bore ID: 50.00 mm nominal, H7 tolerance (+0.025 / 0 mm) → max bore = 50.025 mm
- Piston OD: 49.94 mm nominal, h6 tolerance (0 / −0.019 mm) → min piston = 49.921 mm
- Maximum radial clearance: (50.025 − 49.921) / 2 = 0.052 mm
- Maximum diametral clearance: 0.104 mm
- System pressure: 180 bar; O-ring: 80 Shore A NBR
From the dynamic table, 80 ShA at 150–250 bar allows 0.10 mm radial clearance. This system at 0.052 mm is well within limits — no backup ring required for this design.
Change to a worn bore (50.05 mm after wear) with same piston min OD 49.921 mm:
- New max radial clearance: (50.05 − 49.921) / 2 = 0.065 mm — still within 0.10 mm limit.
- At 0.090 mm (further bore wear): approaching the limit; schedule inspection.
Temperature Effect on Extrusion Risk
Elastomers soften with increasing temperature. The Shore A hardness measured at +23°C does not represent service hardness at elevated operating temperatures. As hardness drops, the material flows more readily into clearance gaps.
Temperature-corrected effective Shore A hardness (approximate, varies by compound formulation):
| Nominal Shore A at +23°C | Effective at +80°C | Effective at +100°C | Effective at +120°C | Effective at +150°C |
|---|---|---|---|---|
| 90 ShA | 82–85 | 78–82 | 73–78 | 65–72 |
| 80 ShA | 72–75 | 68–72 | 63–68 | 55–62 |
| 70 ShA | 62–65 | 58–62 | 52–58 | 45–52 |
| 60 ShA | 52–55 | 48–52 | 43–48 | 35–42 |
Practical implication: A seal designed for 200 bar at +23°C using 70 Shore A NBR at 0.12 mm radial clearance will approach extrusion risk at +120°C — the effective hardness has dropped to approximately 55 Shore A, which is not rated for 200 bar dynamic service at 0.12 mm clearance. Many hydraulic cylinder manufacturers specify 80 Shore A as the minimum hardness for elevated-temperature systems above 100 bar.
Temperature correction for clearance limits:
| Operating Temperature | Adjustment to Allowable Clearance |
|---|---|
| Up to +80°C | No adjustment required; use table values directly |
| +80°C to +120°C | Reduce allowable clearance by 20% |
| +120°C to +150°C | Reduce allowable clearance by 35% |
| Above +150°C | Reduce by 50%; strongly recommend backup rings regardless of pressure |
Backup Ring Types
Solid (Continuous) Backup Ring
A seamless ring with no cut — must be installed by sliding over the end of the rod or piston before assembly.
- Extrusion resistance: Maximum — 360° continuous support with zero gap
- Best for: Static seals, slow reciprocating seals where full disassembly is acceptable at installation
- Limitation: Cannot be installed on an assembled rod without disassembly; impractical for field service or retrofits
- Effective max pressure (PTFE solid): ~280 bar dynamic / unlimited static when clearance is within design limits
Scarf-Cut Backup Ring
A scarf-cut backup ring has a single angled cut (typically 45°) that allows the ring to be opened for installation. This is the most common field-installable style. It is sometimes confused with a spiral-cut backup ring, which has a continuous helical cut and behaves more like a retaining ring; spiral-cut backup rings are rarely used for O-ring anti-extrusion service because the helical gap is difficult to seal.
Scarf-cut backup ring characteristics:
A single angled cut — typically 30° or 45° to the ring axis — allows the ring to be opened for installation around an assembled rod or piston.
- Most common type: Standard for hydraulic cylinder field service
- Cut gap at 45°, in PTFE, 3.53 mm CS: Approximately 0.15–0.30 mm gap after installation and compression; the gap narrows under system pressure as the ring is radially loaded
- Effective max pressure: ~250 bar dynamic with good clearance control; O-ring extrusion at the cut gap becomes measurable above 300 bar
- Installation: Spread the cut, position in groove, release — ring self-centers
Step-Cut (Z-Cut) Backup Ring
A two-step overlapping cut that closes the installation gap. The two steps overlap each other, providing near-zero effective gap at the cut line after assembly.
- Gap at cut (installed): 0.03–0.08 mm (vs 0.15–0.30 mm for scarf-cut at equivalent conditions)
- Effective max pressure: ~350–400 bar dynamic — approximately 40–60% higher than scarf-cut at equivalent clearance
- Still field-installable: Steps flex apart for installation without disassembly
- Best for: Pressures above 200 bar dynamic; systems where scarf-cut gap results in O-ring nibbling at the cut location
Scarf-cut vs step-cut comparison:
| Parameter | Scarf-Cut (45°) | Step-Cut (Z-Cut) |
|---|---|---|
| Installed gap at cut | 0.15–0.30 mm | 0.03–0.08 mm |
| Effective max pressure (dynamic) | ~250 bar | ~350–400 bar |
| Installation without disassembly | Yes | Yes |
| Field replaceability | Easy | Slightly more complex |
| Cost premium over scarf-cut | Baseline | +20–35% |
| O-ring extrusion at cut > 200 bar | Possible | Rare |
| Preferred application | Standard hydraulics ≤ 200 bar | High-pressure > 200 bar dynamic |
Thermoplastic (PEEK, UHMWPE) Backup Rings
For extreme pressure service above 350 bar, harder thermoplastic materials replace PTFE. Higher compressive strength and lower cold flow allow larger clearance bridging without self-extruding.
- PEEK: Compressive strength 200 MPa (vs ~12 MPa for virgin PTFE); max pressure 500+ bar; temperature range −60°C to +250°C
- UHMWPE: Good impact resistance; max pressure ~250 bar; temperature limit +80°C (limited in hot hydraulic oil)
- Nylon PA6/66: Very low cost; max ~150 bar; not suitable for water or steam service (hydrolysis)
Backup Ring Material Comparison
| Material | Compressive Strength | Max Pressure (Dynamic) | Temp Range | Cold Flow Rate | Chemical Notes | Relative Cost |
|---|---|---|---|---|---|---|
| Virgin PTFE | ~12 MPa | ~200 bar | −200°C to +260°C | Highest | Near-universal chemical resistance | 1× |
| 15% Glass-filled PTFE | ~20 MPa | ~280 bar | −200°C to +260°C | Moderate | Glass inert; avoid HF service | 1.2× |
| 25% Carbon-filled PTFE | ~25 MPa | ~320 bar | −200°C to +260°C | Low | Conductive; slight galvanic risk | 1.4× |
| 40% Bronze-filled PTFE | ~35 MPa | ~400 bar | −200°C to +260°C | Low | Avoid oxidizing acids; bronze may leach | 1.6× |
| PEEK (unfilled) | ~200 MPa | 500+ bar | −60°C to +250°C | Very low | Excellent chemical resistance | 4–6× |
| UHMWPE | ~22 MPa | ~250 bar | −150°C to +80°C | Moderate | Not for aromatics or oxidizers | 0.9× |
| Nylon (PA6/66) | ~80 MPa | ~150 bar | −40°C to +120°C | Moderate | Absorbs water; poor in steam | 0.6× |
Material selection guidance:
- Industrial hydraulics 150–250 bar: 15–25% glass-filled PTFE — best balance of cost and performance
- Oil & gas downhole, 250–400 bar: Bronze-filled PTFE or PEEK
- Chemical process (no metallic filler): Virgin PTFE or carbon-filled PTFE
- Pharmaceutical/food contact: Virgin PTFE only (glass-filled acceptable for non-contact surfaces)
- Cryogenic service (< −100°C): PTFE performs well; PEEK acceptable; avoid UHMWPE below −150°C
Groove Design for Backup Ring Assemblies
Adding a backup ring requires additional groove width to accommodate both the O-ring and the backup ring cross-section. Backup rings are typically 1.0–2.5 mm thick depending on O-ring CS and pressure class.
Standard backup ring thickness by O-ring cross-section (PTFE and filled PTFE):
| O-Ring CS (mm) | Backup Ring Thickness (typical) | Backup Ring Width (axial) | Single Backup Groove Width | Dual Backup Groove Width |
|---|---|---|---|---|
| 1.78 mm | 1.0 mm | 1.5 mm | CS + 1.8 mm | CS + 4.0 mm |
| 2.62 mm | 1.2 mm | 1.8 mm | CS + 2.2 mm | CS + 5.0 mm |
| 3.53 mm | 1.5 mm | 2.2 mm | CS + 2.8 mm | CS + 6.5 mm |
| 5.33 mm | 2.0 mm | 3.0 mm | CS + 3.5 mm | CS + 8.5 mm |
| 6.99 mm | 2.5 mm | 3.5 mm | CS + 4.2 mm | CS + 10.0 mm |
Groove depth (gland depth) does not change when backup rings are added — the gland depth is set by the O-ring CS and the required squeeze percentage. The backup ring occupies the same gland depth as the O-ring but in a separate axial region of the groove.
Groove wall between O-ring and backup ring: Maintain a minimum land width of 0.5 mm between the O-ring groove and backup ring groove. Without this land, the O-ring can migrate axially into the backup ring region and reduce effective squeeze.
Single backup ring (unidirectional pressure):
- Backup ring on low-pressure side of O-ring
- Groove layout: [High-pressure side] → [O-ring groove] → [land ≥ 0.5 mm] → [backup ring groove] → [low-pressure side]
Dual backup ring (bidirectional pressure):
- Backup ring on each side of O-ring
- Groove layout: [backup ring groove] → [land] → [O-ring groove] → [land] → [backup ring groove]
- Total groove width: O-ring contact zone + 2× backup ring zones + 2× lands
Bidirectional pressure worked example (3.53 mm CS, 200 bar, double-acting cylinder):
Given:
- O-ring CS = 3.53 mm → gland depth = 3.05 mm (13.5% squeeze, standard)
- O-ring groove axial width = 4.7 mm (standard for 3.53 CS, ~1.33× CS)
- Backup ring thickness = 1.5 mm; backup ring axial width = 2.2 mm each
- Land width = 0.6 mm each
Total groove width = 2.2 + 0.6 + 4.7 + 0.6 + 2.2 = 10.3 mm
Groove gland depth remains 3.05 mm (unchanged from O-ring-only design). The dual backup assembly is installed as: backup ring (left) → O-ring → backup ring (right), all within a single continuous groove of 10.3 mm axial width.
PEEK Backup Ring Design for Extreme Pressure
PEEK backup rings are used when PTFE cold flow or pressure exceeds PTFE limits:
| Parameter | PTFE (Bronze-filled) | PEEK (Unfilled) |
|---|---|---|
| Compressive strength | ~35 MPa | ~200 MPa |
| Max dynamic pressure | ~400 bar | 500+ bar |
| Temperature range | −200°C to +260°C | −60°C to +250°C |
| Cold flow | Low | Very low |
| Cost | 1.6× virgin PTFE | 4–6× virgin PTFE |
When to specify PEEK:
- Pressure > 400 bar
- Large clearance gap that PTFE cannot bridge
- High temperature where PTFE creep is excessive
- Chemical environment compatible with PEEK (avoid concentrated sulfuric/nitric acid)
PEEK backup rings are usually solid and require split-gland installation.
Anti-Extrusion Ring vs Backup Ring
The terms are often used interchangeably, but technically:
- Backup ring: A thin ring placed adjacent to the O-ring to block extrusion
- Anti-extrusion ring: Any ring (including thicker machined rings) that prevents seal extrusion
For O-ring service, both refer to the same function: bridging the clearance gap to prevent the elastomer from flowing out of the groove.
Installation Procedure
Correct installation order is critical — an O-ring installed after the backup ring is seated, or a backup ring on the wrong side, results in immediate failure.
- Verify orientation: Identify the high-pressure and low-pressure sides of the seal groove. The backup ring goes on the low-pressure side (toward the pressure-free side).
- Lubricate both components: Apply a thin film of system-compatible grease (e.g., silicone grease for dry or hydraulic systems) to both the O-ring and backup ring. PTFE's low friction helps installation but lubricant prevents rolling and pinching during assembly.
- Install backup ring first (if single-sided): Place the scarf-cut or step-cut backup ring into the low-pressure side of the groove. For scarf-cut rings, orient the cut at approximately 90° from the highest-pressure loading zone if possible.
- Install O-ring: Roll the O-ring into the groove without twisting — a twisted O-ring creates a helical leak path even under compression. Use a smooth, round insertion tool if the groove is recessed.
- Assemble mating component carefully: Avoid sharp edges that could nick the O-ring or backup ring during insertion. Use a cone-shaped assembly mandrel for rod seals, or chamfer the bore entry (15–20° chamfer, smooth finish) for piston seals.
Inspection after installation: Before closing the assembly, verify that the O-ring and backup ring are both seated (not spiraled or rolled), the scarf-cut gap is closed (not gaping), and no nicks or cuts are visible on either component.
Troubleshooting Extrusion Failures
| Symptom | Root Cause | Corrective Action |
|---|---|---|
| Nibbled edge, low-pressure side only | Unidirectional extrusion into clearance gap | Add scarf-cut PTFE backup ring on low-pressure side |
| Nibbled edge on both sides | Bidirectional pressure with no backup ring, or single backup ring | Add backup ring on both sides of O-ring |
| Nibbled edge at one circumferential location | Localized wear, possible bore out-of-round | Check bore roundness and surface finish |
| Backup ring has also extruded | Clearance gap too large for PTFE backup; gap exceeds bridging capacity | Use PEEK or bronze-filled PTFE; reduce clearance by bore/rod rework |
| Extrusion despite backup ring (backup ring undamaged) | Backup ring on wrong side (pressure side, not low-pressure side) | Reverse installation orientation |
| Extrusion occurs hot but not at ambient | Thermal softening drops effective hardness below threshold | Specify higher Shore A compound; add backup ring regardless of ambient result |
| Rapid O-ring wear in dynamic service after backup ring addition | Backup ring too hard, abrasive contact at gap | Check backup ring surface finish (≤ Ra 0.4 µm on sealing face); switch to softer PTFE grade |
| Cracking in backup ring after short service | Groove too narrow; over-constraint | Verify groove width against backup ring dimensions; allow 0.05–0.10 mm axial clearance |
FAQ
Q1: At exactly what pressure do I need backup rings?
There is no single pressure threshold — the requirement depends on pressure, clearance gap, and O-ring hardness together. As a practical rule: always use backup rings when dynamic operating pressure exceeds 150 bar with a 70 Shore A O-ring at standard clearance (0.10–0.20 mm). For 80 Shore A, the threshold rises to approximately 200–250 bar at standard clearance. Below 100 bar with tight clearances (< 0.10 mm radial), backup rings are generally not required for well-compounded standard elastomers. Use the clearance tables above to verify your specific combination before deciding.
Q2: Which is better — scarf-cut or step-cut backup rings?
Step-cut rings provide better protection above 200 bar because the installed gap at the cut is 0.03–0.08 mm versus 0.15–0.30 mm for scarf-cut. At ≤ 150 bar, the smaller gap of the step-cut provides no measurable benefit — scarf-cut is adequate and less expensive. Above 200 bar dynamic, or in applications where O-ring nibbling at the scarf-cut position has been observed, step-cut is the correct choice. Both are field-installable without disassembly.
Q3: Can I reuse PTFE backup rings after disassembly?
Scarf-cut and step-cut PTFE backup rings can often be reused if they show no scoring, cracking, cold-flow deformation, or visible gap widening at the cut. Measure the cut gap: if it has opened beyond 0.5 mm from permanent set, replace the ring. Solid (uncut) backup rings that have been under high pressure may have cold-flowed into the groove profile and should be dimensionally inspected before reuse. For high-pressure critical applications, replace backup rings at every seal replacement interval — the cost difference is negligible versus the risk of returning a deformed backup ring to service.
Q4: Do backup rings replace the need for tight clearances?
No. Backup rings bridge the clearance gap and prevent O-ring extrusion, but the backup ring itself must be hard enough to bridge the gap without self-extruding. PTFE backup rings at excessive clearance (> 0.35 mm radial) can themselves extrude — in which case, PEEK or a reduced clearance is required. Backup rings are a complement to proper clearance design, not a substitute for it.
Q5: Can I use PTFE backup rings with FKM, EPDM, or VMQ O-rings?
Yes — PTFE backup rings are chemically compatible with all common elastomers (NBR, FKM, EPDM, VMQ, HNBR, FFKM) and do not affect elastomer chemical performance. The backup ring does not contact system fluid in normal service; its function is purely mechanical. For aggressive chemical environments where PTFE itself might be attacked (fuming fluorine, alkali metals), use carbon-filled PTFE or PEEK backup rings.
Q6: How do I calculate the groove width for a dual backup ring assembly?
Total groove width = (O-ring axial groove width) + 2 × (backup ring axial width) + 2 × (land width between O-ring and backup ring). For a 3.53 mm CS O-ring with 2.2 mm axial-width backup rings and 0.6 mm lands: 4.7 + 2(2.2) + 2(0.6) = 10.3 mm. Gland depth remains the same as a standard O-ring groove — only the axial dimension changes. Always add 0.05–0.10 mm axial clearance per backup ring to prevent over-constraint and cracking.
Q7: Why does my backup ring fail even though the O-ring looks intact?
Backup ring failure without O-ring failure typically indicates: (1) insufficient compressive strength in the backup ring material for the operating pressure; (2) excessive clearance gap that exceeds the backup ring's bridging capability; or (3) the backup ring was installed on the pressure side instead of the low-pressure side, loading it in the wrong direction. Inspect the failure pattern — nibbled edges on the backup ring mean the clearance gap is too large for the material; axial cracking means the groove is too narrow (over-constraint); no visible damage but ongoing leakage means the O-ring is the failure point, not the backup ring.
Q8: Can backup rings be used in rotary seal applications?
Yes, with modifications. In rotary applications, the backup ring faces circumferential wear from the rotating surface rather than reciprocating shear. PTFE backup rings in rotary service should be specified as solid (uncut) to eliminate the cut gap, which becomes a wear site under rotation. Surface finish of the shaft is more critical in rotary service: specify ≤ Ra 0.2 µm (8 µin) on the shaft OD. For high-speed rotary (> 1 m/s surface velocity), consult with the backup ring manufacturer — PTFE cold flow rate increases at elevated contact pressure and temperature generated by frictional heat.
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Need backup rings for your application? Request a quote with your groove dimensions, O-ring cross-section, operating pressure, and temperature — we will recommend the correct backup ring type, material, and dimensions. MOQ starts at 1 piece; stocked PTFE backup rings in AS568 and metric sizes ship in 3–5 business days.