Introduction
Extrusion is one of the most common O-ring failure modes in high-pressure systems. When pressure forces the elastomer into the clearance gap between mating metal parts, the O-ring nibbles away, leading to leakage and eventual blowout. The solution is often a PTFE backup ring installed on the low-pressure side of the seal. This guide explains how extrusion happens, how to calculate risk, and when backup rings are mandatory.
What Is O-Ring Extrusion?
Extrusion occurs when the clearance gap between the piston (or rod) and the housing is large enough to allow the O-ring to deform into the gap under pressure. The deformed material is then sheared off during motion, creating a characteristic "nibbled" or "chewed" edge. In severe cases, the seal can extrude completely into the gap and fail instantly.
Extrusion is most common in:
- Hydraulic cylinders operating above 150 bar
- Pneumatic systems with worn pistons or oversized bores
- High-temperature applications where the elastomer has softened
- Systems with large manufacturing tolerances
Factors That Increase Extrusion Risk
Several factors work together to increase extrusion risk:
- High pressure: Risk increases significantly above 150 bar (2,200 psi). At 300 bar, even small clearance gaps can cause failure without backup rings.
- Large clearance gap: Manufacturing tolerances, bearing wear, and metal deformation increase the gap over time.
- Soft O-ring: Lower hardness (50–60 Shore A) deforms more easily under pressure and extrudes at lower pressures.
- High temperature: Elevated temperatures soften the elastomer, reducing its resistance to extrusion. A compound that resists 200 bar at 20°C may fail at 120 bar at 120°C.
- Dynamic motion: Reciprocating motion repeatedly shears the extruded material, accelerating failure compared to static seals.
Recommended Clearance Gaps
The maximum allowable radial clearance depends on O-ring hardness, system pressure, and whether the seal is static or dynamic.
| Hardness | Pressure < 70 bar | Pressure 70–150 bar | Pressure > 150 bar |
|---|---|---|---|
| 70 Shore A | 0.20 mm | 0.15 mm | 0.10 mm |
| 80 Shore A | 0.25 mm | 0.20 mm | 0.12 mm |
| 90 Shore A | 0.30 mm | 0.25 mm | 0.15 mm |
Note: Values are typical for radial clearance in static applications. Reduce by 50% for dynamic applications or add backup rings above 150 bar.
When to Use Backup Rings
Install PTFE (or harder thermoplastic) backup rings when any of the following apply:
- Operating pressure exceeds 150 bar (2,175 psi).
- Radial clearance exceeds 0.10 mm and cannot be reduced economically.
- The O-ring hardness is below 80 Shore A.
- Temperature is continuously above 100°C, softening the elastomer.
- The application involves frequent pressure spikes or shock loading.
- The seal is in a dynamic reciprocating application above 100 bar.
Backup Ring Types and Selection
| Type | Construction | Best For | Installation |
|---|---|---|---|
| Solid PTFE | Uncut continuous ring | Static, slow-speed reciprocating | Requires piston/rod disassembly |
| Spiral (scarf-cut) PTFE | Cut at 45° angle | General hydraulic use, field service | Wraps around rod or piston easily |
| Split (step-cut) PTFE | Two-step overlapping cut | Medium pressure dynamic | Easy installation, good sealing |
| Continuous molded | Seamless molded PTFE | Critical high-pressure static | Requires full disassembly |
Solid PTFE Backup Rings
Best for static or slow-speed reciprocating applications where maximum extrusion resistance is needed. Because they are uncut, they provide 360° support but must be slid over the end of the rod or piston during assembly.
Spiral (Scarf-Cut) PTFE Backup Rings
Easiest to install on rods and pistons without full disassembly. The 45° spiral cut allows the ring to expand for installation while maintaining good extrusion resistance. This is the most common type for hydraulic cylinders.
Split (Step-Cut) Backup Rings
Feature a stepped or Z-cut overlap that provides better sealing than a simple scarf cut. Often used in medium-pressure dynamic applications where some leakage past the backup ring must be minimized.
Installation Rules
Position
Always place the backup ring on the low-pressure side of the O-ring. The backup ring blocks the clearance gap, preventing the O-ring from being forced into it by system pressure.
Bidirectional Pressure
In bidirectional pressure systems (where pressure can come from either side), use two backup rings (one on each side) to protect the O-ring regardless of pressure direction.
Stacking Multiple Rings
For extremely high pressures (above 300 bar), some designs use two backup rings on each side to bridge larger clearance gaps. This is common in oil & gas downhole equipment and high-pressure test fixtures.
Backup Ring Material Options
While PTFE is the most common backup ring material, other options exist for specialized applications:
| Material | Temperature Range | Best For |
|---|---|---|
| Virgin PTFE | -200°C to +260°C | General purpose, chemical resistance |
| Glass-filled PTFE | -200°C to +260°C | Higher pressure, reduced cold flow |
| Bronze-filled PTFE | -200°C to +260°C | Highest pressure, wear resistance |
| PEEK | -60°C to +250°C | Ultra-high pressure, minimal extrusion |
| Nylon (PA) | -40°C to +120°C | Low cost, good for pneumatic systems |
Troubleshooting Extrusion Failures
| Symptom | Likely Cause | Fix |
|---|---|---|
| Nibbled edge on low-pressure side | Extrusion into clearance gap | Add backup ring or reduce clearance |
| Nibbling on both sides | Bidirectional pressure, single backup ring | Add second backup ring on opposite side |
| Backup ring extrudes too | Clearance too large for PTFE | Use harder backup ring material (PEEK) or reduce clearance |
| Rapid wear in dynamic app | Backup ring material too soft | Switch to glass-filled PTFE or bronze-filled PTFE |
Conclusion
Extrusion is preventable. Tighten clearances, increase hardness, or add PTFE backup rings whenever pressures climb above 150 bar or clearances exceed safe limits. A small investment in backup rings can eliminate costly unplanned maintenance and extend the service life of your hydraulic and pneumatic seals.
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Frequently Asked Questions
Can I reuse PTFE backup rings? Spiral-cut PTFE backup rings can often be reused if they show no signs of scoring, cracking, or permanent deformation. Solid rings should be inspected carefully before reuse.
Do backup rings replace the need for tight clearances? No. Backup rings bridge the clearance gap, but excessive clearance still increases the risk of backup ring extrusion. Always design for the tightest practical clearance.
Can I use a backup ring with any O-ring material? Yes. PTFE backup rings are chemically compatible with virtually all elastomers including NBR, FKM, EPDM, VMQ, HNBR, and FFKM.
What is the difference between a backup ring and a guide ring? A backup ring prevents O-ring extrusion. A guide ring (or wear ring) supports the piston or rod laterally and prevents metal-to-metal contact. Some designs combine both functions.
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Need backup rings for your application? Request a quote for PTFE anti-extrusion rings in standard and custom sizes.