Backup Rings: Types, Materials, and Installation

What are Backup Rings?

Backup rings are anti-extrusion devices installed adjacent to O-rings or other primary seals in high-pressure applications. Their primary function is to bridge the clearance gap between mating parts, preventing the primary seal from being forced (extruded) into the gap under pressure.

When to Use Backup Rings

Backup rings should be used when:

• System pressure exceeds 10 MPa (1500 PSI) for static applications
• System pressure exceeds 7 MPa (1000 PSI) for dynamic applications
• Clearance gaps are larger than recommended for the O-ring cross-section
• Pressure spikes or pulsations are present in the system
• The application requires extended seal service life

Types of Backup Rings

Solid PTFE Backup Rings

Solid (uncut) PTFE backup rings provide the best anti-extrusion protection. They are manufactured as continuous rings without any split or cut.

Advantages: Maximum anti-extrusion protection, no gap for seal extrusion Limitations: Require stretch installation over the shaft or into the bore Applications: Critical high-pressure applications, aerospace, and precision hydraulics

Spiral-Cut PTFE Backup Rings

Spiral-cut backup rings have a helical cut that allows them to be easily installed without stretching. The spiral design maintains continuous support around the circumference.

Advantages: Easy installation, good anti-extrusion protection Limitations: Slight gap at the spiral cut, not suitable for the highest pressures Applications: General industrial hydraulics, field maintenance, and standard applications

Scarf-Cut PTFE Backup Rings

Scarf-cut (bias-cut) backup rings have a single angled cut. They are simpler than spiral-cut rings but provide adequate protection for many applications.

Advantages: Simple design, easy installation, low cost Limitations: Single gap at the cut point, limited to moderate pressures Applications: Standard hydraulic applications, pneumatic systems

Rubber Backup Rings

Rubber backup rings are made from elastomer materials (typically NBR or polyurethane) and provide a flexible anti-extrusion barrier.

Advantages: Elastic, self-energizing, good for dynamic applications Limitations: Limited temperature range, may extrude at very high pressures Applications: Dynamic sealing, moderate pressure applications

Material Selection

PTFE (Recommended for most applications):

• Temperature range: -200°C to +260°C
• Excellent chemical resistance
• Low friction
• Good dimensional stability

Filled PTFE:

• Glass-filled PTFE for improved wear resistance
• Carbon-filled PTFE for better thermal conductivity
• Bronze-filled PTFE for maximum load-bearing capacity

Rubber (NBR, Polyurethane):

• Good elasticity and conformability
• Self-energizing under pressure
• Limited temperature and chemical resistance

Installation Guidelines

Single Backup Ring Configuration

For pressures up to 20 MPa, install one backup ring on the low-pressure side of the O-ring. The backup ring should be positioned between the O-ring and the clearance gap.

Double Backup Ring Configuration

For pressures above 20 MPa or alternating pressure directions, install backup rings on both sides of the O-ring. This provides protection regardless of pressure direction.

Installation Steps

1. Ensure the groove is clean and free of burrs
2. Install the backup ring first, positioning it on the low-pressure side
3. Install the O-ring next to the backup ring
4. For spiral-cut rings, ensure the spiral is oriented correctly
5. Verify that all components are properly seated in the groove
6. Apply light lubrication to facilitate assembly

Groove Design Considerations

When using backup rings, the groove must be wider to accommodate both the O-ring and the backup ring(s). Consult the seal manufacturer's groove dimension tables for the correct groove width. The groove depth remains the same as for an O-ring-only installation.